IB Categories Archives: Options

Option G.5 – Population Ecology


Option G.5 – Population Ecology

G.5.1 – Distinguish between r-strategies and K-strategies

Population ecology focuses on the numbers and structures in populations. Part of this study involves reproduction strategies in relation to habitat.

Because K-strategists are larger, they consequently require a larger habitat. Their later maturity also means that they need more protection from predators. They will perform better in environments where competition is high and there are no vacant niches. Examples include elephants, whales and tortoise.

G.5.2 – Discuss the environmental conditions that favour either r-strategies or K-strategies.

In a more predictable environment, K-strategies will be used to maximise fitness, by investing resources in long-term development and longer life.

When the environment is unstable, it is more beneficial to produce a greater number of offspring as quickly as possible, thus employing r-strategies. Ecological disruption favours r-strategists, which include pathogens and pest species.

It is not always possible for biologists to agree on which strategies organisms display. For example, if we look at human populations, the strategies seem to depend on

For example, if we look at human populations, the strategies seem to depend on the circumstances. In developed parts of the world, most people will conform to K-strategies. In less developed parts, however, more r-strategies are used. Some organisms display

Some organisms display extreme r- or K- strategies, however most organisms have life histories that are on the intermediate of the continuum. Some species can even switch strategies depending on the environmental conditions.

G.5.3 – Describe one technique used to estimate the population size of an animal species based on a capture-mark-release-recapture method

In the study of populations, the collection of accurate information on the size of populations present in a habitat is very important. A total count of all the members of a population is called a census. This gives the

A total count of all the members of a population is called a census. This gives the most accurate data, however it is usually impractical. This is because the population may be located in a very large area, fast moving or only active for short periods of time.

A Lincoln index, or the capture, mark, release, recapture (MRR) technique, is a more practical method for estimating the size of a population when they are small and mobile. This can be done with rings, tags, or dabs of paint or nail varnish.

The first sample is caught and marked (n1). The method of marking should be resistant to moisture or the deliberate actions of the animal. It also should not harm the animal, such as increased visibility to predators. For more significant results, are larger sample should be caught.

After their release, the marked individuals should be free to distribute themselves randomly among the whole population. Once this has happened, the second sample is taken (n2). Any from sample 2 that were marked (therefore belonging also to sample 1) are classified into sample 3 (n3). Once this data has been collected, the following formula is used.

A number of assumptions are made when using this technique:


G.5.4 – Describe the methods used to estimate the size of commercial fish stocks

Since fish are an important source of food for many human populations, they tend to be exploited commercially, more often marine species. Populations are commonly shared among a number of harvesting countries. As a result, over-fishing is becoming an increasing problem. In this case, there is conflict between the economic and conservation interests. The demand for harvest and the need for economic returns for fishermen must be balanced with our need to sustain these resources. In recent years, the scale of commercial fishing in Western Europe has seriously depleted stocks.

If these populations are consistently over-fished, stocks will be rapidly depleted to point where they collapse and can no longer support a commercial fishery.

Fishery research services of national governments measure and assess the four parameters in order to gain information about the state of fish stocks. These include:

Fishing mortality – the proportion of fish stocks taken each year during commercial fishing

Spawning stock biomass – the total mass of mature fish in the population

Recruitment – the number of young fish produced each year which survive to enter the spawning stock

Landings – the total annual tonnage of fish landed by the fishing fleet.

Ecological computer models are used with this data to create a picture of the total biomass of living fish across the area studied, and also track changes in trends

G.5.5 – Outline the concept of maximum sustainable yield in the conservation of fish stocks

The maximum sustainable yield (or MSY) of a stock represents the maximum average catch that a stock can sustain over an indefinite period of time. This will correspond to the optimum balance between the reproductive rate and the growth rate of the stock, as well as the death due to harvesting and natural mortality. The MSY should be exactly half the carrying capacity of the species, since it is at this stage that the population growth is the highest. Harvesting at MSY will usually result in lower harvest rate at many fisheries. The maximum sustainable yield is usually higher than the optimum sustainable yield.


For the MSY to be calculated, a good knowledge of the relationship between the size of the stock and the number of juveniles produced each year is required. However, there is a natural variation in the production of juveniles, which makes establishing this relationship difficult. This may lead to setting the maximum sustainable yield too high or low. Data needs to be collected over at least 20 years. There are still reliable methods for obtaining an adequate approximation of the MSY

G.5.6 – Discuss international measures that would promote the conservation of fish

Fishing nations have had to implement various control measure to cope with their over exploiting of their fishing resources. These have included bans on the fishing of certain species. If these are respected, breeding stocks are allowed to recover.

The European Union is regulating fishing around the shores of the member states through various measures:

At the same time, funding is maintained for national marine biology station to carry out research to gain a clear picture of the ecology in that area, and of the key stages of the life cycles organisms in the relevant food chain. Other measures are:

  • Monitoring of stocks and reproduction rates
  • Quotas for catches
  • Minimum net sizes to stop immature fish being caught
  • Banning drift nets, which catch many different species indiscriminately
  • Moratoria (temporary suspension) on catching endangered species


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Option G.4 – Conservation of Biodiversity

Option G.4 – Conservation of Biodiversity

G.4.1 – Explain the use of biotic indices and indicator species in monitoring environmental change

One of the major threats to biodiversity is the destruction of rainforest. Human activity is causing changes to many biomes worldwide, such as through pollution, which may not always be self-evident. In order for conservation of natural environments to happen, there must be early detection of environmental change. As some organisms are particularly sensitive or vulnerable to environmental change, their numbers and conditions can function as biological indicators (or biotic indices) of environmental change and the health of an ecosystem. Lichens and mosses are indicator species of environmental change.


Lichens are dual organisms. Their body contains fungal and algal components living together for mutual benefit. There are many species in existence, and they often occur in hostile habitats. The compact fungal hyphae absorb water and retains water and ions, while the algal part carries out photosynthesis.

Lichens, along with mosses, are susceptible to air-borne pollutants dissolved in rainwater, as their surfaces are not protected by a waxy cuticle. Since the thallus will absorb various pollutants, they can be analysed for heavy metal ions. Their growth rate is affected by pollution, which can serve as a valuable indicator.

Fresh Water Pollution

Aquatic plant growth becomes abnormally high in water enriched with inorganic ions. This may be due to pollution from raw sewerage and stockyard effluent – manure or silage liquors. Excessive or incorrect use of fertilizers on farm crops results in it leaking from the soil by rain. Increase in concentration of ammonium, nitrate and phosphate ion increases plant growth as they are beneficial to plants.

Waters that are ion-enriched in summer with raised temperatures undergo a plant population explosion. This is often referred to as an algal bloom because algae are involved and, as many are unicellular, their growth rates are spectacular. When plant life is in excess in polluted, it is an example of eutrophication.

In marine habitats, the extensive seasonal algal blooms of the oceans can be observed in satellite images.

After the algal bloom has died back, saprotrophic, aerobic bacteria will decay the organic remains of the plants. The water becomes deoxygenated, causing anaerobic decay and hydrogen sulfide formation. While some organisms can survive these conditions, many aquatic organisms like fish will die due to absence of dissolved oxygen, as well as the presence of hydrogen sulfide. Changing populations of invertebrate aquatic animals are indicators of the degree of pollution and recovery. Such species may be classes according to how well they tolerate pollution and given a corresponding value. This allows a quantitative value to be given to the state of the waterway.

The effect of pollution of a river can be observed by eye and measured by chemical analysis. This may include testing dissolved oxygen concentration, the conductivity and concentration of organic matter.

Red Data Books

The rate of extinctions is very high. Environmentalists will seek the survival of endangered species by initiating and maintaining local, national and international action. The updating of Red Data Books is co-ordinated by the International Union for the Conservation of Nature and Nature Reserves. The Red Data Books list endangered species, and identify those which need special conservation efforts. Health and general well-being of populations of these organisms are indicators of environmental change. Practical conservation steps may include the designation and maintenance of

Practical conservation steps may include the designation and maintenance of nature reserves, botanical and zoological gardens (with captive breeding programmes) and the establishment of viable seed banks. These will combat the loss of endangered species and the habitats which support them.

G.4.2 – Outline the factors that contributed to the extinction of one named species

New species evolve, while other species that are less suited to their environment will become extinct

Tasmanian tiger

Tasmanian Tiger (Thylacinus cynocephalus) became extinct after the arrival of European settlers in Tasmania, Australia, in tragic period of conflict due to the introduction of sheep. European settlers were puzzled by it and feared, and would kill it when they could Tasmanian tigers were shy creatures and avoided contact with humans. They would usually give up without a struggle when captured, many dying suddenly of apparent shock.


They were widespread throughout Tasmania, and would feed on the introduced sheep. As a result, the parliament placed a price on the head of the tiger, which caused numbers to rapidly decline. The population dwindled to the point that there were only a few remaining in zoos.

However, the Tasmanian Tiger did not breed well in zoos. They lived in captivity for about 9 years. The last captive tiger died in Hobart in 1936 and Tasmanian Tigers were declared extinct by international standards in 1986.



Aboriginal rock paintings and fossils suggest that they once lived on mainland Australia and New Guinea, and are believed to have become extinct there due to predation and competition with dingos.




The dodo was a forest-dwelling bird that is related to modern species of pigeons. They were large birds that grew to about a metre long and weighed approximately 20kg. They nested on the ground, where they would rear their young. Dodos fed on fruit and seeds that fell to the ground from forest trees.


The dodo was extinct by 1681, which came about as a result of the spice trade. The European explorers secretly used Mauritius to grow spice plants, stop over port for restocking and farming, people migrated there and smuggling seeds and plants.

G.4.3 – Outline the biogeographical features of nature reserves that promote the conservation of diversity

Biological conservation can be attempted by setting aside land for restricted access and controlled use to allow the local maintenance of biodiversity. Nature reserves, game parks and National Parks serve as a solution to extinction pressures on wildlife. They represent habitats of different descriptions, some of their conservation work being carried out by volunteers all over the world.

A nature reserve will control any alien species, removing any that are not originally supposed to be there. Areas degraded by human impact are restored through reforestation and species reintroduction. Threatened species are helped to recover, and human exploitation is controlled.


The area enclosed in a nature reserve is important. There is usually an optimum size for reserves, with an area too small being ineffective, and one too large securing no greater diversity. This will vary according to species, size and lifestyle of the threatened species it is trying to protect. Larger nature reserves tend to promote better conservation of diversity than small ones.

Edge Effect

There is also an edge effect for a given reserve. A compact reserve with minimal perimeter is more effective than one with an extensive interface between its perimeter and surroundings. The uses of the surroundings of the reserve are also important, as it may indirectly support the reserve’s wildlife. The ecology of the edges of a reserve is different from the central areas as a result of these edge effects.


Geographical isolation is another important feature. Reserves in close proximity of each other are more effective than those at great proximity. Corridors of land may also overcome the disadvantage of small reserves in need of a larger provision as they increase the total size of the reserve. For example, in agricultural areas, hedgerows may be protected from contact with the pesticide treatments that nearby crops receive. Another example is tunnels under busy roads, allowing for movement of organisms between different parts of a fragmented habitat. This way, organisms can move around for feeding and breeding, maintaining genetic diversity.

G.4.4 – Discuss the role of active management techniques in conservation

Conservation is an active process, involving using knowledge of ecology of habitats (both biota and abiotic factors), in order to manage the environment of the nature reserve, and maintain biodiversity.

Active management helps to maintain an endangered species or habitat. Wildlife can be secured under favourable conditions for contemplation, education or research. These methods counterbalance unchecked exploitive management in natural resources, such as deforestation and agriculture. Degraded areas must be restored and succession beyong the desired level must be prevented

Proper conservation relies on a number of factors:


G.4.5 – Discuss the advantages of in situ conservation of endangered species (terrestrial and aquatic nature reserves)

Endangered species tend to have very low population numbers, and are in danger of becoming extinct. They are conserved for our own benefit, including a larger gene pool for genetic engineers, as well as sources of compounds with medical value. Some wild organisms are also more efficient energy converters that existing crops and herbs in particular ecosystems. Diversity of both flora and fauna is also essential for the continuation of the process of evolution of new
species in response to the changing environments of the Earth.

These reserves can often have commercial use, as they provide tourist venues.



G.4.6 – Outline the use ex situ conservation measures, including captive breeding of animals, botanic gardens and seed banks

Ex situ measures may take place complementary to in situ conservation, as they have a valuable role to play in the recovery of endangered species.

Captive Breeding

When a habitat is destroyed, the species will need to be moved to another location to preserve it. Captive breeding programmes now make good use of the resources available at zoos, maintaining the genetic stock of rare and endangered species. Through cooperation between zoos, and sometimes artificial insemination, the genetic problems arising from limited numbers to act as parents are overcome. The animals in zoos are also available to participate in breeding programmes for longer as they have a longer life expectancy.

Artificial propagation of plants may also take place, with them subsequently being returned to the wild.

These programmes are usually highly successful. However, the young do not grow up in the wild, so they do not have as much opportunity to learn from their parents and peers.

Healthy individuals are produced in good numbers, and advantage against natural predators during re-introduction. Zoos have sites for the public to visit, which contributes to public education and awareness on the environmental crisis.

Botanic Gardens

These can also be used when a habitat has been destroyed. Botanic gardens have sites for the public to visit, which contributes to public education and awareness on the environmental crisis. In a similar way, zoos and aquaria will promote public awareness and facilitate research whilst conserving the species.

Gene Banks

Seed banks are a convenient and efficient way of maintaining the genetic material of endangered plants, with similarities to the ways seeds may survive long periods in nature.

Similar methods are sperm and ova banks and field banks. With these collections, the seeds can be used if the species becomes endangered. These seeds are carefully stored so that they may remain viable for up to 100 years.

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Option G.3 – Impact of Humans on Ecosystems

Option G.3 – Impact of Humans on Ecosystems

G.3.1 – Calculate the Simpson diversity index for two local communities

During early succession, diversity is low. The populations are usually dominated by abiotic factors. In populations that are larger and more diverse, abiotic changes do not have such a dramatic effect. As dominant plants provide nutrients and determine the habitats that exist, they set the way of life for other inhabitants. They will also modify and reduce the effects of abiotic factors.

From this, we see that the diversity of species present in a habitat is an indicator of the stability of the community. We measure species richness with the Simpson Diversity index:

N = total number of organisms of all species found
n = number of individuals of each species

A high value of D would indicate an ancient, stable site, while a low value indicates recent colonisation, pollution or agricultural management.

G.3.2 – Analyse the biodiversity of the two local communities using the Simpson index

When the values have been calculated, they can be compared. The higher value would suggest that the site is healthier, with greater diversity of species.

G.3.3 – Discuss reasons for the conservation of biodiversity using rainforests as an example

Unfortunately, tropical rainforests are rapidly destroyed. We know this through satellite imaging, with the only real ones left being in South America, West Africa and the Far East.

It is estimated that the current rate of destruction is about one hectare every second. As a result, the extinction of many species is coming about more rapidly than ever.

Conservation involves applying the principles of ecology to manage the environment.

G.3.4 – List three examples of the introduction of alien species that have had significant impacts on ecosystems

Alien species are plants and animals that have been accidentally or deliberately transferred from their habitat to a new environment where the abiotic conditions are still suitable for them. Should they take over in an aggressive way, which is detrimental to food chains of their new habitat, then they will be described as an invasive species.


These were deliberately introduced to Australia from Europe. The myxoma virus from South America is a disease of rabbits, and was introduced as a biological control. The rabbit numbers have declined since this introduction.

Once in Australia, the rabbits spread very rapidly. There were no natural predators, so interspecific competition failed, so they very soon over-ran large parts of the continent. The rabbits caused a great deal of damage to grassland for cows and sheep.

Out of desperation, the myxoma virus was introduced from South America in 1950, which was found in rabbits. It caused a parasitic disease called myxomatosis. The native rabbits of South America were only mildly affected by it. Over a few years, the virulence of the virus changed, indicating that a small number of the population already had immunity. As the others died, they thrived on the weakened competition, and soon the bulk of the population were immune.

Japanese Knotweed

This was deliberately introduced to northern Europe as an ornamental plant for garden ponds and lakes in the early nineteenth century.

When Knotweed was introduced to Britain, the natural parasites and predators it had come to exist in balance with were not present. Natural herbivores did not browse it, so it was able to escape into natural waterways. Eventually, it grew to the point that it crowded out native species, in dense, submerged thickets. It also blocked waterways and public access to stream banks.

In Japan, it lived amongst its natural population of predators. However, in Britain, the chemicals in its leaves and roots discouraged predation from leaf browsers or root parasites, which was not the case in Japan.

American Grey Squirrel

This was accidentally introduced to Britain in the nineteenth century.

The indigenous red squirrel of Britain is under serious threat of extinction from the American grey squirrel’s invasive pattern of spread. The grey squirrel consumes a wide range of growing nuts, more than the red squirrel, causing faster breeding. The grey squirrel now competes for hazel nuts and pine cones, causing the diet of the red squirrel to be very limited.

G.3.5 – Discuss the impacts of alien species on ecosystems

G.3.6 – Outline one example of biological control of invasive species

Once in Australia, the rabbits spread very rapidly. There were no natural predators, so interspecific competition failed, so they very soon over-ran large parts of the continent. The rabbits caused a great deal of damage to grassland for cows and sheep.

Out of desperation, the myxoma virus was introduced from South America in 1950, which was found in rabbits. It caused a parasitic disease called myxomatosis and acted as a biological control. The native rabbits of South America were only mildly affected by it. Over a few years, the virulence of the virus changed, indicating that a small number of the population already had immunity. As the other died, they thrived on the weakened competition, and soon the bulk of the population were immune. Any that didn’t develop immunity were soon taken out.

G.3.7 – Define biomagnification

Biomagnification is a process in which chemical substances become more concentrated at each trophic level.

It is the bioaccumulation of a substance up the food chain by transfer of residues if the substance in smaller organisms. If the accumulation of the substance comes only through contact with water, then it is known instead as bioconcentration.

G.3.8 – Explain the cause and consequences of biomagnification, using a named example

The use of pesticides to control harmful organisms has greatly improved agriculture, but at the same time causes problems in the environment.

The chemical DDT (dichlorodiphenyltrichloroethane) was found to be an effective insecticide. It is a molecule with chlorine atoms attached to hydrocarbon rings. It acts as a nerve poison to insects, causing rapid death, even in low concentrations. It remains lethal for a long time, as it is stable when dispersed into the biosphere. It did not do any harm to vertebrates, so it was liberally used. DDT was especially effective against mosquitoes, which were a vector in the spread of malaria. Many insect predators of pest species were also killed by DDT.

Since DDT is fat soluble, and is selectively retained in fatty tissues of animals instead of being excreted by the kidneys, it becomes more concentrated at each stage of the food chain.

In non-vertebrates such as fish and birds, DDT concentrations can reach toxic levels. In top carnivores, it was concentrated with devastating consequences. While DDT is not a nerve poison for birds and mammals, it does inhibit the deposition of calcium in the eggshell of breeding birds. The eggs are thin-shelled and easily crack, causing a rapid decline in numbers of birds of prey.

Once these effects were realised, a ban was placed on DDT, and the quality of stability was renamed ‘persistence.’

Now, insecticides that are biodegradable are sought, and are more specific in their actions. Organophosphates were tried, but are suspected to be harmful in humans. They act as synapse blockers in insects. As a result, synthetic derivatives of pyrethrum have been developed, as they are much less toxic in mammals, and are biodegradable. However, they are lethal to fish, and must not be used near waterways.

G.3.9 – Outline the effects of ultraviolet (UV) radiation on living tissues and biological productivity

UV radiation is absorbed by organic bases (adenine, guanine, thymine, cytosine and uracil) of nucleic acids (DNA and RNA), which causes them to be modified.

UV radiation kills phytoplankton, which form a significant portion of net photosythesis in the biosphere. The whole oceanic food web is affected by their death.

Many terrestrial plants suffer retarded growth as their rate of photosynthesis slows, resulting from radioactive damage and mutation in plant leaves. It may also kill symbiotic bacteria which fix nitrogen in the root nodules of legumes In animals, increases in UV exposure increases rates of skin cancer, sunburn and eye cataracts.

G.3.10 – Outline the effect of chlorofluorocarbons (CFCs) on the ozone layer

Substances made by industry, like chlorofluorocarbons (CFCs), are threatening the ozone layer. They are very unreactive and stable, deliberately manufactured for use as propellants in aerosol cans and as coolants in refrigerators. The gases escape into the atmosphere from time to time, and are slowly carried into the stratosphere, taking up to five years. Once in the stratosphere, they are exposed to high levels of UV light, and a broken down. The highly reactive chlorine atoms are then released, and break down ozone in a cyclic reaction.

The result is that ozone molecules are broken down faster than they can be reformed. Large quantities of CFCs have been released, and despite current steps to replace them with safer chemicals, the time taken for them to reach our atmosphere allows for ozone depletion to continue.

The ozone hole, or thinning of the ozone layer, is a potential problem for all organisms exposed to sunlight on land. Care must be taken for people in Australia, New Zealand, South Africa and Chile that they do not become over-exposed to sunlight and develop skin cancer.

G.3.11 – State that ozone in the stratosphere absorbs UV radiation

Ozone is a molecule of three oxygen atoms, which occurs naturally in the Earth’s atmosphere in an ozone layer, found in the stratosphere.

Ozone is formed by the action of UV radiation in the upper atmosphere on O2


The highest concentration of ozone is at midpoint of the stratosphere, as there is little oxygen at the top where there is much UV radiation, and little radiation closer to the Earth’s surface where there is the highest concentration of oxygen.

As the ceaseless cycle continues, most of the incoming UV light is absorbed. The stratosphere is slightly warmed by the reactions, but the heat is lost to space.

Any UV radiation that reaches Earth is harmful to living things as it is absorbed by the organic bases of nucleic acids, and causes them to be modified. UV radiation may also damage proteins and lipids. Exposure gives rise to increase in cancer rates, glaucoma, cataracts and skin aging. In plant and phytoplankton, exposure may negatively affect productivity.

UV radiation does have beneficial qualities. It is used to treat jaundice in newborn infants, and is used in vitamin D synthesis. During water purification, UV radiation is used to kill microbes. As a result, it is extremely important that the high-level ozone layer is maintained.


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Option G.2 – Ecosystems and Biomes

Option G.2 – Ecosystems and Biomes

G.2.1 – Define gross production, net production and biomass

Biomass is the total dry weight (or volume, or energy equivalent) of living organisms in a given area (e.g. a quadrat.), measured in kJ per square metre per year

Gross Production is the total amount of organic matter produced by plants in an ecosystem measured in kJ per square metre per year

Net Production is organic matter of organisms less the amount needed to fuel respiration

G.2.2 – Calculate values for gross production and net production using the equation: gross production – respiration = net production

The units for this are kJ m-2 yr-1

Values for this may be given in a data table. If you are missing certain variables, rearrange the equation to find it.

G.2.3 – Discuss the difficulties of classifying organisms into trophic levels

Trophic levels are defined by the method the organism uses to obtain food. Within one trophic level, all the organisms are the same number of energy transfers from the original energy source – the Sun and photosynthesis.

The main difficulty is when organisms obtain their energy from food sources that are a variety of transfers from the first energy source. Thus, some of the top carnivores are at trophic levels 2 to 5 at any one time.

This is especially the case with omnivores, which feed on producers and consumers. Thus, they are primary consumers and secondary consumers, making them difficult to place on the food pyramid

Another difficulty arises due to the fact that many animals change their diets over the course of their growth and development.

Also, the detritivores and decomposers are not always featured on food webs, yet up to 80% of plant matter produced may be consumed this way.

It is for this reason that the food web was developed, showing the organisms in relation to each other in a complex network instead of in a hierarchal structure. Also, individual animals themselves are shown, instead of simple hubs of trophic levels.

Another solution is to classify them according to their main food source.

G.2.4 – Explain the small biomass and low numbers of organisms in higher trophic levels

At higher trophic levels, the small biomass and lower number of organisms is due to the energy loss at each level. As we know, this loss comes as a result of respiration, using their energy for different activities. When glucose or another respiratory substrate is oxidised, the energy is released for use then lost as heat.

Also, a portion of the biomass at each level is not eaten by the next. Biomass losses occur in the form of excretion of faeces and urine, which will pass directly to decomposers.

As a result, an average of 10% of the matter and energy at any one level will be passed on to the next. The amount of energy per gram does not decrease in the higher trophic levels, in fact it may increase. However, there the total amount of biomass available to the trophic levels is less. This is why there are fewer organisms at the higher trophic levels, resulting in a low biomass per unit area.

As well as this, top carnivores may hunt for their food, which must then be killed. During the chase, more energy is transferred, prey may be injured and die elsewhere, and possibly become food for other organisms at lower trophic levels.

G.2.5 – Construct a pyramid of energy, given appropriate information

Pyramids of numbers of organisms present, or of biomass, or of energy, are a proportional representation of the organisms present at successive trophic levels. They should be represented on graph paper by areas proportional to the data being used.

The units are kJ m-2 yr-1, so the pyramid should be drawn on a grid.

The bottom bar of the pyramid is always the energy flowing through the producers, or gross production. The trophic level must always be labelled on the pyramid.

G.2.6 – Distinguish between primary and secondary succession, using an example of each

Ecological succession is the process of a sequence of communities developing with time. In this, plant and animal communities increase in complexity. The climax community is when the succession has ended and it has all of its characteristics. It can also be seen as a series of changes to an ecosystem, caused by the interaction between the living organisms and the abiotic factors.

Primary Succession

This is when the succession starts on entirely new land without established soil. This may happen at river deltas, at sand dunes, and from cooled volcanic lava. These also develop in aquatic habitats, such as a pond fed from a spring. The first significant development in this case is the formation of soil. Another example may be a new island that was created through volcanic activity. This occurs when a major catastrophic disturbance takes place. Examples include

This occurs when a major catastrophic disturbance takes place. Examples include glacial retreat and volcanic eruptions. All the plants in the area are destroyed, and new species may colonise.

Secondary Succession

This is a succession that starts from existing soil. Communities may be disrupted and destroyed, such as during bushfires when a lot of vegetation is destroyed. The soil is already formed and present; however, the existing biota has been removed.

There must have been a change in conditions, such as a farm being abandoned, and becoming a forest. Unlike in primary succession, not all the plants are wiped out completely, leaving behind a seedbank or vegetable propagules. Dominance is usually achieved by the fast growing plants.

G.2.7 – Outline the changes in species diversity and production during primary succession

At the initial site of a primary succession, all that may be present is parent rock, from which the bulk of the soils is formed by erosion. This is the breaking of solid rock into smaller particles by the effects of extremes of temperature, the action of wind and water, and chemical reactions, such as when acid rain falls. Mineral particles may also be blown or washed in from elsewhere. The resulting mineral skeleton is of particles of a wide range of sizes from small stones and coarse sand to the finest clay particles. Succession can be seen as a directional change in a community with time. Initially,

Succession can be seen as a directional change in a community with time. Initially, abiotic factors have the greater influence on the survival and growth of organisms. Later, as the numbers of living organisms build up, biotic factors increasingly affect survival too. A xerosere is

A xerosere is succession under dry, exposed conditions where water supply is an abiotic factor limiting growth of plants.

The eventual outcome of the succession is the climax community. An important feature of succession is the progressive increase in the number of species present. As more and more species occur in the habitat, the food webs are likely to be more diverse and complex. As a result, if one population crashed, such as due to a disease, then alternative food chains may be sufficient to supply the higher trophic levels.

If primary succession takes place in aquatic habitats, then the sequence of pioneer plants differs from a dry land primary succession, although it may still result in a woodland community. The succession is called a hydrosere. Plants adapted to aquatic or permanent swamp conditions are known as hydrophytes.

Changes to the abiotic conditions during succession include:

  • Increase in the amount organic matter in the soil
  • Soil becomes deeper
  • Soil structure improves, resulting in better water retention
  • Soil erosion is reduced
  • The amount of mineral recycling increases.

G.2.8 – Explain the effects of living organisms on the abiotic environment, with reference to the changes occurring during primary succession

Soil, when fully formed, has organic matter called humus wrapped around the particles of the mineral skeleton. Humus is a substance derived from dead plant and animal remains, together with animal faeces, that have been decomposed by the actions of microorganisms. Humus is a dark-coloured, sticky substances, that continues to be decayed, releasing mineral nutrients. Humus also helps the soil to hold water. Between mineral particles and

Between mineral particles and humus linings are innumerable pockets of air. Also present in a developed soil is a huge community of microorganisms and small animals, including earthworms, all adapted to life in this habitat.

Humus is first added by plant invaders of the primary succession, known as pioneer plants. These are typically lichen and cushions of moss, and after them, tiny herbaceous plants, many with features that help them survive where water is scarce.

Until the soil is fully formed it retains little water, even when water is freely available. Plants able to survive drought are called xerophytes, and their special features are called xeromorphic features. When a sere starts from dry conditions, then the sere is called a xerosere.

The growth and death of the early plant communities continue to add humus, and so more soil water is retained. Nutrients are added to the soil when organisms die, and the range of nutrients available to plants increases steadily. Nutrients are the ions essential for plant growth, such as nitrates, phosphates, and a range of macronutrients. Different plants now grow – various herbaceous weeds may start to shade out the pioneers. The conditions in the soil are increasingly favourable to microorganisms and soil animals, which continue to invade the habitat from the surroundings. Herbaceous plants are followed by shrubs and small trees, all growing from seeds that are carried in the wind, water or activities of animals. The first-formed soil accumulates relatively

The first-formed soil accumulates relatively slowly, because it is vulnerable to erosion and dissipation by wind and heavy rains. Increasingly, as plant life becomes established, the growth of plant roots and the cover provided by plant vegetation prevent or reduce loss by soil erosion.

G.2.9 – Distinguish between biome and biosphere

A major life zone characterised by the dominant plant life present.

Also defined as a climatic and geographically defined area of ecologically similar communities of plants, animals and soil organisms, often referred to as ecosystems. Biomes are defined based on factors such as plant structures (trees, shrubs and grasses), leaf types (broadleaf and needleleaf), plant spacing (forest, woodland, savannah) and other factors like climate. Unlike ecozones, biomes are not defined by genetic, taxonomic or historical similarities. Biomes are often identified with particular patterns of ecological succession and climax vegetation.


The part of the Earth, including air, land, surface rocks and water within which life occurs, and which biotic processes in turn alter or transform. Also defined as the restricted zone which living things can inhabit.

From the broadest biophysiological point of view, the biosphere is the global ecological system integrating all living things and their relationships, interaction with the elements of the lithosphere, hydrosphere and atmosphere. This process is said to have evolved, beginning at a process of biogenesis or biopoesis around 3.5 billion years ago or more.

G.2.10 – Explain how rainfall and temperature affect the distribution of biomes

The main factors that affect distribution of the biomes are rainfall and temperature. The interaction of these factors can vary with latitude, longitude, position within land masses and proximity to the sea.

Temperature is influential because it affects the metabolism of organisms. The phases in the life cycles of many plants and animals require certain temperatures for them to succeed, such as seed germination.

In the same way, the availability of water in the soil is critical for the growth and nutrition of plants. The adaptations of plants to varying water levels influence their ability to grow successfully.

G.2.11 – Outline the characteristics of six major biomes

Although they are found in completely different areas, many places have similar climatic conditions. They will also have comparable ecosystems due to natural selection. These major ecosystems are called biomes. Biomes are identified by the dominant plant found in their communities, as well as the diversity of animal life and subdominant plant forms, which are controlled by the abiotic environment.



Deserts typically consist of shrub covered land where the plants are well dispersed. They are influence by the descending air currents, which limit the formation of precipitation.

The dominant plants include drought resistant shrubs and water-storing succulents like cacti. Other species are short-lived annuals that complete their life cycles during the infrequent, short rainy periods. Deserts are often void of vegetation due to the low rainfall.

Desert mammals are usually nocturnal to avoid the high temperatures, often lizards and snakes as the conditions promote the success of cold-blooded species. Evaporation tends to concentrate salts on the surface of the soil, and the litter layer is limited. The organic content of surface soil layer is low.

Deserts receive very limited and unpredictable rainfall. The plants in this habitat either remain dormant until rain comes, then completing their cycle in days or weeks, or are succulents that survive above ground all year round, as they can store water.

The limited animal life responds to the danger of extreme dehydration in many ways. Camels, for example, have adapted to allow their body temperature to vary by up to 7oC, allowing for conservation of water in daylight hours.


In areas where precipitation is less, buffalo grass and other grasses that are only a few inches tall are common. There are also flowering herbs, which are still very common. Very few trees can be found, usually close to streams and in low-lying areas.

In these areas, the soil is rich and black chernozemic. Chernozems are rich in nutrients and is the most fertile in the world. In drier parts, the soil can be influenced by salinisation. Humans often use these areas to grow grain and other dryland crops because of their fertility.

Smaller, burrowing herbivores such as prairie dogs, jack rabbits, ground squirrels and gophers dominate the grassland mammals, as well as larger running herbivores such as bison, pronghorn, antelope and elk. Carnivores in these areas include badger, coyote, ferret and cougar. Habitat destruction has had a dramatic impact on population size in these areas, many on the edge of extinction.

This can also occur as steppe and pampas, in areas of moderately dry climate, with hot summers and cold winters. The most common plants are perennial grasses, along with broad-leaved flowering plants. Grassland can provide natural pasture for grazing animals, and where rainfall is sufficient, have been converted into grain-crop grassland for human use.

Savannah is tropical grassland with scattered, individual trees, usually with three distinct seasons: hot and dry, cool and dry, and warm and wet. There is typically an abundance of wildlife, with large herds of herbivores and their predators, often from the cat family. A limiting factor for the growth and spread of trees is destruction by herbivores like elephants and fires caused by electrical storms.

Shrubland (chaparral)

This biome has a very specific spatial distribution. These areas have a dry climate because of their subtropical high temperatures during autumn, summer and spring. Precipitation falls during the winter because of seasonal movement of the polar front. As a result, the vegetation has adapted to withstand drought and fire. The trees and shrubs will tend to have hard, evergreen leaves. They do not lose their leaves in autumn because it is expensive to replace them. The dry climate slows the rate of decomposition, so the plant do not have nutrients available to make new leaves, instead producing ones that withstand arid conditions.

The main species in this area include cork oak, olive, eucalyptus, arbutus, acacia, maritime pine, shrub oak and live oak. Many of the plants have thorns to protect them from herbivore damage. These areas have abundant winter rainfall and dry summers.

Temperate Deciduous Forest

This biome is characterised by a moderate climate and deciduous trees. This biome has been affected by human activity, much of it being converted into agricultural fields or urban development. Dominant plant species include maple, beech, oak, hickory, basswood, cottonwood, elm and willow. There are also many well-developed and richly diversified shrubs and herbs. There are many different types of herbivores and carnivores, as well as reptiles and amphibians. Temperate deciduous forests also have brown forest soils. Their surface has a litter layer that is very thin, as decomposition takes place rapidly.

These forests typically have several layers, with enough light reaching the bottom layer of herbaceous plants. The leaves fall at the end of summer, leaving the trees dormant during winter. The leaf loss is only possible if the soil contains enough nutrients for the leaves to regrow in spring.

Tropical Rainforest

Tropical rainforests usually occur near the equator. Rainfall in these areas is usually distributed evenly throughout the year. Temperature and humidity is usually high. Plant species are diverse. The trees in this biome are closer together, forming a canopy 25-35 metres tall. Characteristic species include vines and orchids, as well as ferns and palms. Most plants are evergreen with large, dark green, leathery leaves.

Tropical rainforests also contain a great variety of animals, believed to be 30-50% of all the Earth’s species

The rate of decomposition is high because of high temperatures and abundance of moisture. Frequent rains cause the soils to be subject to extreme chemical weathering and leaching. These soils are acidic and nutrient poor

Because little light can penetrate the canopy to reach the forest floor, much of the animal life is confined to the canopy. The soil below along with the canopy above support a huge fauna of non-vertebrates


Tundra means marshy plain. It is characterised by the absence of trees, and the presence of dwarf plants, as well a ground surface that it wet, spongy and hummocky. Soil in this biome are usually permanently frozen, called permafrost, up to a metre or more deep (though usually only a few centimetres). This line acts as a physical barrier to plant root growth. The amount of air in the soil is also limited because of the permafrost.

Conditions sometimes improve enough in summer to support plant life, along with the brief appearance of insects.

Temperature, precipitation and evaporation all tend to be at a minimum. In summer months of most tundra biomes, the temperature will average at below 10oC. The precipitation would be no higher than 25mm. The ground of tundra tends to be waterlogged because of the low rates of evapotranspiration.

There is a relatively small species diversity of tundra vegetation. The plant communities are typically composed of a few species of dwarf shrubs, some grass species, sedges and mosses. The most characteristic plants are lichens (such as reindeer moss). The principal herbivores would include caribou, musk, ox, arctic hare, voles and lemmings. Most bird species of tundra are able to migrate to warmer places during the cold winter months. The herbivores support a few carnivore species such as wolves, polar bears, snow owls and arctic fox. There are very few reptiles and amphibians because of the low temperatures.

Alpine tundra is very similar to arctic tundra, although without the presence of permafrost and the presence of a better drainage system. It occurs on the highest mountains, well above the tree-line. The night-time temperatures are below freezing, though the day-time temperature will usually rise above freezing, allowing for slow growth of plants.


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Option G.1 – Community Ecology

Option G.1 – Community Ecology

G.1.1 – Outline the factors that affect the distribution of plant species, including temperature, water, light, soil pH, salinity and mineral nutrients

All species are restricted to certain geographical areas and particular habitats. Abiotic factors are conditions caused by non-living elements. The main factors that determine the distribution of green plants are:


This determines the rate of biochemical reactions in organisms, as enzymes have optimum temperatures for function. Temperature also determines the rate of evaporation from plants, called transpiration. Plants have adapted to resist variation in temperature.


This is vital to life, as it is necessary in reactions of photosynthesis and respiration, along with other uses. Rainfall will restrict plant distribution and density. Plants have evolved to retain water according to their environment, include waxy cuticles and extensive root systems, including the xerophytes.


This is necessary for photosynthesis by autotrophic organisms, which provide energy for the whole ecosystem. Light intensity and wavelength vary, in turn affecting temperature and humidity in the area. Also, day length is used to control some daily or seasonal rhythms, including flowering. In this way, light can affect the structure of plant communities, controlling which types of plants can grow in certain places.

Soil pH

This affects the availability of essential ions. Also, highly acidic or alkaline soils may cause the denaturation of proteins, disrupting important physiological processes. It also affects population and soil bacteria activity that decompose organic matter. While diversity is generally greatest at pH7, many plants have adapted or acid or alkaline conditions.


This affects the distribution of water plants. Salts and ions can accumulate to high levels in water. When salt concentration in the soil is high, the plant may lose too much water through osmosis. Salt marshes have a variable salinity based on the tide, and often contain halophytes, which survive high salt conditions by retaining enhanced levels of ions and resisting loss of water to their environment.

Mineral nutrients

These are found dissolved in water of the soil solution, usually at low concentrations, and are essential for the growth of the plant. The essential elements are released by decay of dead plants and animals and their waste matter, including K and Ca, NO3 and PO3. They are absorbed then reused by the plant as ions, and endlessly cycled. Stocks can be added to by the weathering of rocks, but nitrates are generated from atmospheric nitrogen gas by the actions of nitrogen-fixing microorganisms, and as an outcome of lightning. The availability of minerals is affected by pH and bacteria.

G.1.2 – Explain the factors that affect the distribution of animal species, including
temperature, water, breeding sites, food supplies and territory

The dominant plants in an ecosystem will often determine animal distribution as the animals are highly dependent on them for their sources of energy and carbon. It also determines the type of habitat that exists, along with the critical abiotic conditions. This should be kept in mind when investigating animal distribution.


This can influence the metabolism and growth of animals, as well as their behaviour. There is strong correlation between amount of light and temperature. Ectotherms are able to survive very low or high temperatures using behavioural means, but cannot survive if it gets too extreme. Homeotherms are animals that regulate their own temperature.

Poikilotherms maintain less control, and are more dependent on the external temperature.



The bodies of animals are mostly made up of water. It is lost by evaporation during the processes of gaseous exchange, regulation of body temperature and disposal of waste matter. Some animals, such as insects, reptiles, birds and mammals, have adapted to retain more water by having an impervious body covering, and can occupy a wider range of habitats. Some may use it as their permanent habitat, such as fish, or just for breeding, such as amphibians. However, all animals need a reliable water source in their habitat.

Breeding sites

These are sites in which young are fed and reared, typically placed at the heart of the territory. These sites often have special requirements, including temperature and light. Food resources and protection have a large effect on the chances of survival for the young. Without these sites, the species is likely to become extinct.

Food supplies

This affects the existence and size of local populations. They may be dependent on plant matter, other animals or both for meeting their special dietary requirements. When supplies are short, individuals must compete for a limited resource. This can occur between the same species, intraspecific competition, or different species, interspecific competition. This competition is a major factor determining population size. If food supplies run out due to over-predation, the effects are felt all the way up the food chain.


This is a defended area of a habitat, established by individuals, breeding pairs or family groups. Organisms will have specific requirements for their territory. Herds will mark their territory using scent marking and defend their territory through active behaviour, although fighting will be kept to a minimum; instead, ritualised aggression through body language is usually used. They may occupy habitat on either a temporary or permanent basis.

G.1.3 – Describe one method of random sampling, based on quadrat methods, that is used to compare the population size of two plant or two animal species

Random sampling means that every individual of the population has an equal chance of being selected, so a representative sample is assured. A quadrat is a square frame used to outline an area for sampling. The size of the quadrat used will depend on the size of the individuals in the population being analysed. Many quadrats will need to be placed over the area for an accurate population size estimate.

When the running mean stabilises, then enough quadrats have been used.

The optimum size of a quadrat will depend on the size of the area being analysed and the size of the individuals.

G.1.4 – Outline the use of a transect to correlate the distribution of plant or animal species with an abiotic variable.

When there is a gradient in an abiotic factor, such as altitude, salinity or light, then communities will often show a trend in variation. Transects are used to measure this trend. The transect is placed at a right angle to the impact of these factors.

A single transect may not give an adequate sample, so multiple samples should be done. When there are changes such as height of the land, these can be measured and recorded as a profile transect.

A belt transect is when quadrats are placed in a line, or at intervals. The number of individuals within each transect is used.

A line transect is when the number of individuals found along a line is measured at certain intervals. The individuals that are touching the string are counted, along with the distance along the line.

G.1.5 – Explain what is meant by the niche concept, including an organism’s spatial habitat, its feeding activities and its interactions with other species

A niche is an ecological term to define how an organism feeds, where it lives, and how it behaves in relation to other organisms in its habitat. This concept is useful because identifies the precise conditions which a species needs. It is these combinations of environmental conditions are necessary for the species to tolerate their physical environment, obtain energy and nutrients, and to avoid predators. Competition is the interaction between two organisms striving for the same resource in

Competition is the interaction between two organisms striving for the same resource in the same place due to overlap in their niches. Potential competitors in fact have evolved from different niches.

G.1.6 – Outline the following interactions between species, giving two examples of each: competition, herbivory, predation, parasitism and mutualism

As resources are in limited supply, organisms must then compete for them. This includes space, light and mineral ions.

Intraspecific competition – when individuals of the same species compete for resources or a
mate. Interspecific competition – when individuals of a different species compete for resources.

  • Different species of Paramecium compete for plankton
  • Beech and oak trees in England compete for minerals and water


When an animal feeds on plant matter.

  • Leaf-cutter ants in Costa Rica, feeding of leaves
  • Wildebeest in Africa eat grass


When an animal preys on another for food. This will often require patient stalking and a determined final attack because most prey species will help to defend their whole herd.

  • Lions in Africa prey on wildebeest, usually going after young or injured ones as they are easier to catch.
  • Lady beetles feed on aphids inside plant step tissue.


When two organisms lives together, where the relationship is harmful to the host, or at least unhelpful.

  • Plasmodium is a protozoan that lives in the blood and liver of humans, causing malaria. It is transferred via mosquitoes. As it resides inside the host, it is an endoparasite.
  • Ticks are also parasites, which attach themselves to warm-blooded, furry mammals and suck the blood. They are ectoparasites. When they are satiated with blood, they may drop off, though usually somewhere in the vicinity of the host so that they can reattach later.


This is a form of symbiosis. Two organisms of a different specieslive in intimate association to a mutual advantage. This is a form of favourable interaction between organisms.

  • Microorganisms, usually bacteria or fungi, will live in the rumen of ruminant mammals. The rumen is a large, stomach-like fermentation vessel. The microorganisms break down cellulose and other polymers in plant matter, producing organic acids (along with methane). The ruminant is dependent on this enzymic digestion in order to gain nutritional benefit from its diet. The rumen forms the habitat for the microorganisms (until they are digested by ruminant enzymes further into the gut of the host.
  • Lichens are made of a fungal and algal part. The algal part performs photosynthesis, and the fungal part absorbs mineral and other nutrients.

G.1.7 – Explain the principle of competitive exclusion

When two species occupy the same niche or have overlapping niches, only one will be able to exclusively use the resources, and the other will either move away or die off. The overlap leads to competition between the species. If the overlap is small, then they may be able to co-exist.

The competitive exclusion principle is the idea that ecological separation of closely related or otherwise similar species is the inevitable outcome. If two species share the same resource at the same place and same time, the dominant, or stronger species will outcompete the other, which will die out or move away. This may be the reason that closely related species living in close proximity have evolved clearly defined but separate niches, with no competition between them. This occurs over an extended period of time. This is also referred to as Gause’s Law, saying that two species competing for the same resources cannot coexist, as one will always have an advantage.

G.1.8 – Distinguish between fundamental and realised niches

Most species can survive in a range of conditions, so their niches are fairly unrestricted ones. For example, animals can mostly eat a wide range and variety of food sources.

The fundamental niche of a species is the potential mode of existence, given the adaptations of the species. It is the total range of conditions the organism is suited to with influence of intraspecific competition or predation from other species.

It is only when organisms experience resources and conditions totally outside their range are they fatally threatened by their environment or competition, so they will die out.. The portion of their fundamental niche that is available to them becomes the realised niche.

The realized niche of a species is the actual mode of existence, which results from its adaptations and competition with other species. It is the segment of the fundamental niche that the organism actually occupies.

The niche of an organism refers to how an organism or population responds to the distribution of resources and competitors, and how it alters those factors, acting as a food source and a consumer.

G.1.9 – Define biomass

Biomass is the total dry weight (or volume, or energy equivalent) of living organisms in a given area (e.g. a quadrat)

G.1.10 – Describe one method for the measurement of biomass of different trophic levels in an ecosystem

The trophic levels in a community are:

To measure the biomass at different trophic levels, we must first estimate the number of organisms of each type at each trophic level of the community. If the mass is to be expressed as dry mass (as it commonly is) then the dry mass of a representative sample must be found for each type of organism The dry mass can be found by heating a weighed sample to about 80oC, removing water without burning any organic

The dry mass can be found by heating a weighed sample to about 80oC, removing water without burning any organic matter. The cooled samples are weighed and heated, showing that all the water has been evaporated. The heating is repeated until there is no change in the mass of the organism. Unfortunately, to obtain the dry mass of samples they

Unfortunately, to obtain the dry mass of samples they must first be killed. As a result, communities of organisms are destroyed by this technique, reducing their habitat to a ‘desert,’ at least until it is repopulated by surrounding communities.

There are many ethical issues arising from this technique, especially when biodiversity is under threat. To take a less destructive approach, the fresh mass of a small representative sample could be taken, which can then be returned to the environment. However, this approach is less accurate.

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Option E.5 – The Human Brain

Option E.5 – The Human Brain
E.5.1 – Label, on a diagram of the brain, the medulla oblongata, cerebellum,
hypothalamus, pituitary gland and cerebral hemispheres

During the development of the embryo, the brain begins as the neural tube, which then becomes the three main structures of the forebrain, midbrain, and hindbrain. These structures continue to fold and thicken into the other parts of the brain.

In mammals, the brain develops cerebral hemispheres from the forebrain. The majority of the neurons in the human brain are found here. The brain is encased in the cranium for protection. Immediately around it

The brain is encased in the cranium for protection. Immediately around it are the meninges or membranes. For additional protection, the space in between the membranes and the brain is filled with fluid to cushion the movement of the brain.

E.5.2 – Outline the functions of each of the parts of the brain listed in E.5.1

The brain performs many functions, including receiving impulses from sensory receptors, integrating and correlating incoming information in association centres, sending impulses to effector organs, storing information and building up an accessible memory bank and initiating impulses from its self-contained activities.

Medulla Oblongata

This is located in the brainstem. It controls automatic functions, including breathing, blood pressure, swallowing, digestion, vomiting and heart rate. It also controls homeostasis in the body. In this area, the nerve fibres between the brain and spinal column cross over, which means that the left side of the body is controlled by the right side of the brain, and vice versa.


This is located in the hindbrain. It coordinates unconscious functions including movement and balance. This is important for the integration of sensory perception, coordination, and motor control. It has neural connections to link with the cerebral motor cortex, which sends message to the muscles, and the spinocerebellar tract, which gives feedback on the position of the body in space. It has an essential role in coordinating and fine-tuning motor movements.


Located in the forebrain and forms a link between the nervous and endocrine systems. It regulates unconscious systems, such as the impulses to the cardiac muscle, glands, metabolism and smooth muscle. It sends messages to the pituitary gland to control the release of hormones. The hypothalamus also secretes some hormones into the pituitary gland.

This is also the control centre for maintaining homeostasis, including body temperature, blood glucose concentration, fatigue, thirst and blood pH. Feeding and eating reflexes, aggression, fight-or-flight response and reproductive behaviour (sexual desire) are also controlled in the hypothalamus.

Pituitary Gland

This is attached to the hypothalamus, and is the main hormone-producing gland. It regulates bodily functions and releases some hormones produced in the hypothalamus. It regulates homeostasis by releasing hormones to stimulate the endocrine glands. This gland is divided into two sections: the posterior lobe stores and releases hormones that are secreted from the hypothalamus, whilst the anterior lobe produces and secrete the hormones that regulate bodily functions.

Cerebral Hemispheres

These are located in the forebrain and make up most of the brain. This is a distinguishing feature between the human brain and other mammals, showing greater development. They are the integrating centre for the more complex functions of the brain, including learning, memory and emotion. These coordinate the body’s voluntary activities and some involuntary ones. They also form the integrating centre of memory, learning, emotions and other complex functions.

The brain is divided into the right and left cerebral hemispheres, linked by the corpus callosum. The two hemispheres are asymmetrical, although there is some variation between individuals. The hemispheres are folded with deep grooves to extend the surface. The surface is covered in the layer of grey matter called the cerebral cortex. It is made up of densely packed non-myelinated neurons which have a huge number of synaptic connections.

E.5.3 – Explain how animal experiments, lesions and FMRI (function magnetic resonance imaging) scanning can be used in the identification of the brain parts involved in specific functions

Animal Experiments

In these investigations, a part of the animal’s brain is removed or connections with the brain are severed. The animal remains alive so that the functions of the brain can be examined. The effect this has on the animal’s behaviour gives understanding of the role of that area or those connections.

However, these days, with the different attitudes towards ethics, along with the developments in technology, such experiments are less frequently conducted. This is to prevent the suffering or sacrifice of the animals.

Another example includes the testing of the response of animals to certain drugs and recording their behaviour.


The brain can be damaged in a number of ways, including accidents, strokes, and tumors. Many investigations have been conducted on people who have suffered brain damage to establish the effect of it on behaviour and body functions. In addition, this allows us to identify the specific areas of the brain that control certain functions. Post-mortem investigation of stroke victims has also been beneficial for understanding the role of each area of the brain. One example is the study of people with epilepsy who had their corpus callosum cut.

One example is the study of people with epilepsy who had their corpus callosum cut. By experimenting with placing objects in their left and right visual fields, it was determined that the left hemisphere is primarily responsible for language.


Magnetic resonance imaging of the brain can be conducting while the individual is performing certain functions to see which areas have the greatest neural activity at that point. It is a non-invasive technique, and provides information in high resolution. The scans can show activity in any part of the brain. The subject may be given a series of tasks to perform during the scan, and the results are recorded on a computer.

MRI involves measuring the emission of electromagnetic energy from hydrogen atoms using a strong magnet. By sending pulses of radio waves, the location of the hydrogen atoms can be determined. Functional magnetic resonance imaging is an extension of this: it allows us to identify activity in the different areas of the brain. It looks at the supply of red blood cells to these areas, since blood flow is increased in active areas.

E.5.4 – Explain sympathetic and parasympathetic control of the heart rate, movements of the iris and flow of blood to the gut 

The peripheral nervous system is divided into a number of complex parts. Within it is the autonomic nervous system, which controls the unconscious activities and functions of the body. It is controlled by the medulla and hypothalamus, as well as some conscious regions of the brain. Motor neuron nerve fibres run from the brain and spinal cord to specific tissues, organs and glands. These attach to the smooth muscle, which surround the internal organs and glands.

The autonomic nervous system is divided into the sympathetic and parasympathetic nervous system. The two systems are antagonistic.

E.5.5 – Explain the pupil reflex

The iris muscles will contract or relax depending on the amount of light present to control how much light enters the eye. This protects the retina from damage. These muscles are controlled by the parasympathetic nervous system.


E.5.6 – Discuss the concept of brain death and use of the pupil reflex in testing for this

Brain death is when all functions of the brain are irreversibly ceased. The tests for brain death include:

  • Absence of pupil reflex – pupils remain in mid position and do not react to changes
    in light
  • o Bright light is shone on the eyes. The pupils will constrict if the patient is alive. If not, this         indicates brain death. The patient may still be living, but have suffered serious, irreparable     brain damage that will eventually lead to death. Doctors will know that preserving the               patient will not allow for recovery.
  • Eyes do not blink when touched
  • Eyes do not rotate in their sockets when the head is moved
  • No movement of extremities
  • Eyes do not move when iced water is placed in the outer ear canal
  • Gag reflex – No cough or gagging when a suction tube is placed well into the trachea
  • Breathing does not commence when the patient is taken off the ventilator

E.5.7 – Outline how pain is perceived and how endorphins can act as painkillers

The receptors of the nervous system allow us to register and respond to stimuli. These receptors include our sense organs and nerve endings. For example, in our skin, there are a range of small receptors that detect different aspects of touch. Some respond to sensation such as vibrations and flutters, other respond to pressure, and others are specific to pain.

Pain helps us to respond to things that are damaging our bodies. These pain receptors are found everywhere in the body except for the brain, which

These pain receptors are found everywhere in the body except for the brain, which send messages to the cortex of the cerebral hemispheres. Pain comes in two forms:

  • Fast pain – This occurs rapidly after the stimulus is received. The feeling of pain is acute and localised, and is not felt deep within the body.
  • Slow pain – This occurs after a delay with gradually increasing intensity. It will often become chronic, burning or throbbing pain. It can be felt deep in the body and will seem to come from a large area.


The body releases endorphins from the pituitary gland when we experience pain so that normal activity is not inhibited. These will block the release of the neurotransmitters that send pain signals. Endorphins travel through the blood to the brain. Exercising also increases levels of endorphins to give a sense of euphoria.

The same principle is applied to the use of other painkillers to keep the patient aware of the pain, whilst reducing its intensity so that normal functioning can continue.

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Option E.4 – Neurotransmitters and Synapses

Option E.4 – Neurotransmitters and Synapses

Action potentials are passed between neurons across the synapse using chemical transmission in the form of neurotransmitters. These are stored at the end of the axon.

When the action potential arrives, the ion channels open to allow Ca2+ to enter, activating the enzymes that act on presynaptic proteins. These proteins cause the release of the neurotransmitters from the vesicles and across the membrane of the nerve ending. Neurotransmitters are constantly recycled.

It then diffuses across the synaptic cleft and interacts with the receptors of the next neuron. Glial cells around the synaptic cleft contain transporters to remove any remaining neurotransmitters.


E.4.1 – State that some presynaptic neurons excite postsynaptic transmission and others inhibit postsynaptic transmission

Each neurotransmitter must bind to its matching receptor, which then stimulates the opening of the ion channels, exciting the neuron. If it reaches the threshold, an action potential will form and be passed along the post-synaptic neuron. When the process occurs this way, it is called an excitatory synapse.

On the other hand, there are also inhibitory synapses. Instead of triggering the release of Ca2+, the neurotransmitters cause the ion channels to open to allow the entry of Cl- ion or the exit of K+. The gradient changes in the opposite direction, making the overall charge of the neuron more negative or hyperpolarised. An action potential cannot be initiated.

An action potential may be formed from a number of impulses reaching the synapse in quick
succession, or from the combined effected of impulses from different axons.

Presynaptic Potential

E.4.2 – Explain how decision-making in the CNS can result from the interaction between the activities of excitatory and inhibitory presynaptic neurons at synapses

The brain coordinates and controls our body’s functions, as well as storing memories. It can also initiate activity, and allows us to perform abstract reasoning.

Each area of the brain performs a different function, and they are all connected. Using fMRI, these areas can be identified.

Integration is the process of data being taken into the centres and compiled with the existing data, or memories. This is then used in decision making. Each neuron has many synaptic knobs, forming different types of connections.

Decision making is also dependent on the interaction between excitatory and inhibitory synapses and the different connection pathways between neurons to produce a different result.


Divergent connections are when information from one pathway branches out and is passed to a number of others, forming the basis of a variety of responses.


Convergent connections are when information from multiple pathways focuses onto fewer pathways. This leads to stronger excitation or inhibition, and can trigger a response to multiple stimuli.


Circular or Reverbatory connections are when the information returns to its source to reinforce a message or make it last longer


Parallel or After-Discharge connections are when the post synaptic neuron sends out a number of impulses without any feedback, resulting in a precise, strong response.


E.4.3 – Explain how psychoactive drugs affect the brain and personality by either increasing or decreasing postsynaptic transmission

Psychoactive drugs affect the mind by altering the performance of synapses. Some drugs amplify their processes to increase post-synaptic transmission, such as nicotine and atropine. Some inhibit the process of synaptic transmission to decrease transmission, such as amphetamines and beta blocker drugs.


Synaptic transmission is significant in brain function. The brain in integral for the coordination of body functions, storing information, maintaining our memory bank, imagination, create, plan, calculate, predict and for abstract reasoning, but excluding reflexes.

Psychoactive drugs affect behaviour and possibly personality. Many of these drugs are used in medicine as tranquilisers and painkillers to make the brain neurons resistant to excitation. They are also used in horticulture as insecticides, to inhibit enzymic breakdown of transmitter substances after attachment to the post-synaptic membrane. It disrupts the nervous system to the point of death. Nerve gases can be used as weapons.

These are also used in a social and recreational context, but often have dangerous or tragic consequences.

E.4.4 – List three examples of excitatory and three examples of inhibitory psychoactive drugs

E.4.5 – Explain the effects of THC and cocaine in terms of their action at synapses in the brain


This is also called marijuana, pot, grass or weed and is usually smoked. The chief mind altering chemical in it is THC (delta-9-tetrahydrocannabinol), which is typically present in concentration of 1-4%.


It is a mild hallucinogen, showing similar disinhibiting properties to ethanol. It induces a sense of well-being and a dreamy state of relaxation. Also:

  • Encourages fantasies
  • User becomes suggestible and vulnerable
  • Possible paranoia

Emotions peak after 10-30 minutes, and wear off after 2-3 hours.


Affects ability to absorb and retain information, maturity, leads to inappropriate lifestyle choices, gateway to cocaine. Also increases risk of car crashes, STI’s and unwanted pregnancy.


It is a highly addictive stimulant, and is usually ingested, snorted or injected. Crack cocaine is heated to evaporate the water and inhaled from a heated pipe for faster absorption. it affects neurons containing dopamine, which affects pleasure. It has euphoric effects, interfering the reabsorption of dopamine and prolonging stimulation for extended pleasure response.


It induces euphoria and hyperstimulation, reducing fatigue and increasing mental clarity. It causes intense pleasure beyond the normal range of human experience. Faster absorption leads to more intense sensations.

Use of cocaine is then followed by a crash, where the user become restless, irritable and anxious. Some people will binge, and suffer auditory hallucinations, paranoid psychosis and lose contact with reality


Users can quickly become addicted, and the cravings lead to repetitive and compulsive behaviours. It gives sensations of insects crawling under skin, severe depression, agitated delirium and paranoid psychosis. It has destructive social consequences, causing the user’s family to become alienated, with the user becoming isolated and suspicious. They will spend all their time and money obtaining more.

  • Spend time and money obtaining more
  • Often resort to crime
  • Put aside loved ones

E.4.6 – Discuss the causes of addiction, including genetic predisposition, social factors and dopamine secretion

Addiction – State of taking a mood-altering drug habitually, and being unable to give it up without experience unpleasant side-effects. The user is unable to control or abandon their drug use.

Drug addiction is characterised by a pathological desire for drugs. It occupies their thoughts and they can only think of obtaining more. They will spend time seeking drugs, with use occurring at the expense of other activities such as study, work and social activity. They cannot control the frequency of use or stop altogether, even if they want to.

The initial motivation for taking the drug affects the likelihood of addiction. If it is taken for medical purposes, then it rarely leads to addiction. However, if it is taken for pleasure, then they are more likely to get addicted.


Genetic Predisposition

Some people are predisposed to addiction because their brain is changed in complex ways. This specifically happens in the regions including the reward system and areas involved in executive functions and judgement. Their decision-making ability is also affected.

Other personality types are more likely to become addicted, including those who are inclined to risk-taking or hedonistic.

They might also have a metabolic state that makes drugs more effective, such as the absence of an enzyme to dispose of the drug. These conditions may be inherited.

Social Factors

Environmental factors, such as stress, affect people’s response to the drug. Poor diet, high unemployment and limited access to education and training are also contributing factors to susceptibility. Little opportunity for personal fulfilment leads to a sense of hopelessness, so that the drugs become an escape.

Dopamine Secretion

Drug abuse can eventually alter the structure and chemical makeup of the brain. Since they affect the reward system, they can produce please, or remove stress and pain. This area is involved in learning, natural rewards, and causes the body to expect more or repeat the action.

Some drugs mimic dopamine, causing them to want to repeat use. However, they will eventually become less sensitive to or more tolerant of it, and they will need higher doses to get the same effect. Eventually, they may become dependent and feel like they can’t function without it. If they try to stop use, they will suffer from withdrawal symptoms.

Drugs interfere with the dopamine metabolism to produce a state of dependence. This often starts with gateway drugs, then later experimentation with more harmful ones, as more is required to produce the same effect. This drug use is habit-forming and users feel that they cannot live without it.

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Option E.3 – Innate and Learned Behaviour

Option E.3 – Innate and Learned Behaviour

E.3.1 – Distinguish between innate and learned behaviour

Innate Behaviour – Behaviour that develops independently of the environmental context. It is present at or soon after birth.

Learned Behaviour – Behaviour that develops as a result of experience


E.3.2 – Design experiments to investigate innate behaviour in invertebrates, including either a taxis or a kinesis

Experiments can be done to test two types of behaviour in response to a stimulus.

Kinetic Movements

This is when the rate of movement is related to the intensity of the stimulus. However, the direction of this movement is random. For example, you may look at the speed at which an organism moves at higher and lower humidity, proximity to food, temperature, etc.

Example: Woolice and Humidity

In a Petri dish, you should aim to create a gradient of humidity in the atmosphere. In the more humid sections, they should become less active, whilst in the drier areas, their movements would become quicker. These movements would lead to more woodlice being found in the more humid areas. This is because when they are in the drier area, they would be more likely to move in to the more humid area due to their random movements, and would remain there after their movements slowed.

Aim: To investigate the effect of a humidity gradient on the distribution of woolice.

Risks: There are very few risks – good laboratory practice would remove any danger.

Method: Things to consider

  • Petri dish choice chamber design
  • How to maintain the humidity gradient
  • Surface for the free movement of the woodlice
  • Number of woolice used
  • Frequency of recording data
  • Environmental conditions for woodlice before experiment
  • Other variables to control
  • Can a data-logger be used?
  • How long each trial should last
  • How to treat the animals after the experiment

Data Recording, Presentation and Analysis:

  • Use of a table for recording raw data
  • Type of grph to make trends evident
  • Statistical test that can be done

Discussion of Results

  • Behavioural response: kinesis or taxis?
  • Relate to actual living conditions of the woodlice
  • Modifications to improve
  • Other investigations based on the results

Tactic Movements

This is when the direction of the stimulus determines the direction of the response. An organism may move towards food or light

Example: Blowfly Larvae and Light

Some blowfly maggots have a positive response to light, whilst others have a negative one.

Aim: To investigate the response of blowfly larvae to light in different directions

Risks: Safety implications of light intensity (i.e. too bright may be damaging to eyes, too low may make it difficult to perform experiment safely). Possible contraction of infection from handling organisms.

Method: Things to consider include

  • Environmental conditions of larvae before experiment
  • Number of larvae needed
  • How to remove light in the area
  • An appropriate surface for the movement of larvae
  • How to create a unilateral beam
  • How to vary the direction and intensity of light
  • Method for recording the direction of movement
  • Other variables to control
  • How long the investigation should last
  • Can data-loggers be used
  • How to treat the organisms after the experiment

Data Recording, Presentation and Analysis

  • How to pool the data from different experiments
  • Type of graph to make trends evident
  • Statistical tests to be used

Discussion of Results

  • Is the response positively or negatively phototactic?
  • How does it relate to their real way of life?
  • Ways to improve the method
  • Further investigations based on results

Remember to consider how these responses and behaviours can improve the organisms chance of survivable and reproduction.

E.3.3 – Analyse data from invertebrate behaviour experiments in terms of the effect on chances of survival and reproduction


Woodlice are crustaceans, belonging to the same family as crabs and lobsters. They typically live under logs and stones, in bark crevices, among dead leaves and rotting plant material, and usually eat decaying vegetable material. They are nocturnal and forage when it is dark.

Since their skins are not waterproof, they are restricted to living in humid conditions. Their front five pairs of legs act as gills, and can only function when the environment is damp. As a result, if the area is too dry, or if they are submerged, they will suffocate or drown.

These things restrict the possible habitats and behaviours of the woodlice that allow them to survive and reproduce.

  • Remain away from light in a humid environment
  • Forage at night on walls, where they encounter fewer predators or drown under rain.

Bumble Bees

These visit flowers to collect nectar and pollen to be used as food. Evolution of both bees and flowers has provided them with mutual advantage. Flowers need to provide an abundance

Flowers need to provide an abundance of these nutrients at critical times in the bee’s lifecycle, such as during mating and rearing young. They must use foraging techniques that allow for the optimal amount of food to be collected. This involves developing flight paths that allow them to collect food effectively.

For the flowers, the bees are important for pollination and cross-pollination of the flowers.

E.3.4 – Discuss how the process of learning can improve the chances of survival

Learning is the process of changing behaviour in response to a development in the organism’s environment. It is part of the process of adapting to the changing circumstances of the environment, and can only be acquired through experience. As a result, two individuals of the same species may learn different things through having different experiences. Learning comes in a few forms:


This is when the response of the organism is dulled, or decreased, as a result of repeated exposure to the stimulus. For example, a snail will eventually learn that it is not necessary to withdraw into its shell when it touches a leaf, realising that this poses no danger.

However, this is only learned after the snail touches the leaf a number of times, and each time sees that the leaf is not a threat. If the snail did not learn to ignore this stimulus, it would waste a lot of time withdrawing into its shell, which could be spent on more productive activity to increase its chances of survival and reproduction. This can also be applied to flocks of birds that feed on farmer’s crops: the farmers may install a bird scaring device that makes a loud, frightening sound. However, after repeatedly seeing that there is no actual danger with the sound, the birds learn to ignore it and will feed on the crops again.


This is when an organism forms an attachment with another organism during the early, receptive stage in their life. The bond is formed almost instantaneously at birth. In most cases, this would be with their mother, who will then teach them the necessary skills for survival, including communication and feeding.


This is when the learning is associated with a reward or punishment. This was investigated by Ivan Pavlov, as discussed in the next section. He demonstrated that if an unrelated stimulus is given with an unconditioned stimulus, the animal will learn to associate them and eventually have the same response to the unrelated stimulus, called a conditioned reflex.

Trial and Error

This behaviour has been investigated using mazes, where a wrong choice leads to no reward. If the animal can master it, they will find a reward at the end. Some animals learn to navigate them successfully more quickly than others.

The maze is learnt through exploration, with their curiosity maintained by rewards.


This is also called reasoning. It requires the ability recall data from past experiences, and make generalisations. We can also use information that we have learned without experiencing it. We can fall back on previous trial-and-error learning so that the same process does not have to be repeatedly experienced.

E.3.5 – Outline Pavlov’s experiments into conditioning of dogs 

Pavlov’s investigations looked at the dog’s reflex response of salivation in the presence of food. The meat was the unconditioned stimulus, with the secretion of saliva being the unconditioned response.

The bell, on the other hand, was a neutral stimulus and the dog had no response to it. Pavlov combined this neutral stimulus with the unconditioned stimulus of the food by ringing the bell every time he presented the dog with meat. This was repeated over a few days, by which time the dog began to secrete saliva when the bell was rung, even though the meat had not been presented yet. The bell had become the conditioned stimulus and the salivation when the bell rang was the conditioned response.

The conditioned response comes because the dog learned to associate the ringing of the bell with the food, called classical conditioning.

However, unlike the unconditioned response, the conditioned response in not permanent. The learned behaviour will not stay with the animal for the rest of its life, but will eventually wear off once it is no longer paired with the unconditioned stimulus. In other words, the response becomes extinct.

Pavlov also found that if the two stimuli are once again paired together, this leads to the spontaneous recovery of the learned response. The individual does not completely forget the response, but will reinstate it more quickly than during the original conditioning.

These days, there are many laws that prevent unnecessary experimentation on animals, such as the ones undertaken by Pavlov. Despite these ethical issues, the investigations done in the past have still given us valuable insight into many areas.

E.3.6 – Outline the role of inheritance and learning in the development of birdsong in young birds

Birds use songs as their form of communication. They are able to vary their sound through movements of the head and opening and closing their mouth. Most birds are raised by their parents and hear their song. However, some species, like the cuckoo, will never hear their parent’s song. Nevertheless, cuckoos will still develop their own song – they do not simply use that of their adopted parents. This shows that birds do not learn to sing, but that it is inherited.

Despite this, research has shown that a young bird will only inherit a template, and their song will be altered as the listen to their parents and refine it to be more like their parents’.


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Option E.2 – Perception of Stimuli

Option E.2 – Perception of Stimuli

E.2.1 – Outline the diversity of stimuli that can be detected by human sensory receptors, including mechanoreceptors, chemoreceptors, thermoreceptors and photoreceptors

Within the body, there are a number of different, specialised sense cells that can detect and respond to stimuli. They are called receptors.

Sense cells will transfer the energy from the stimulus (such as sound energy, light energy, heat energy, etc) into an action potential to be sent to the rest of the nervous system.

For example, the sensory hair cells in the inner ear respond to sound waves and gravity. On the other hand, the rod and cone cells in the eye respond to light.

E.2.2 – Label a diagram of the structure of the human eye 


The eye sockets provide protection for the eyes. Eyes receive information about the size, colour, shape and movement of other objects in the environment. To produce a three dimensional image, different information is received from each eye and then compiled.




E.2.3 – Annotate a diagram of the retina to show the cell types and the direction on which light moves

The retina can recognise wavelengths of light that are within the visible spectrum. It is made up of rod and cone cells. These cells are elongated, with the outer segment containing the light-sensitive pigment, and the inner segment containing the mitochondria.

Rod Cells

There are more rod cells in the retina, and are present evenly across the retina, except at the fovea.

Cone Cells

These are mainly concentrated around the fovea, where vision is the most accurate. Since the retina is inverted, light will reach the neurons that synapse with the rod and cone cells before reaching the outer segment of the rods or cones.

E.2.4 – Compare rod and cone cells

E.2.5 – Explain the processing of visual stimuli, including edge enhancement and contralateral processing

Since cone cells each have their own neuron, whilst the rod cells all feed into the same one, the amount of detail we can see is affected, or the resolution of our vision. When our cone cells are being used, or in bright light, we can see in greater detail. Each part of the image received by our brain is detected by a different cell, and the boundaries are not blurred. Conversely, in dim light, our vision has a lower resolution. This is because the rod cells all converge to a single neuron and the image is poorly resolved.

Although our vision becomes less detailed in dim light as a result of this arrangement, it does give us the advantage of being able to still see at these light levels. This is because the convergence of visual information makes us more sensitive to stimulus. The collective information from the rod cells allows for enough information to be pooled to create an action potential.

Edge Enhancement

This takes place within the retina. It is demonstrated with the Hermann grid illusion. This illusion is created when we look at a grid of black squares on a white background, and have the impression that there are grey blobs at the intersections of the surrounding white lines. However, they do not remain if we look at them directly. Since the retina is circular, light will either fall directly in the centre, or around it. The ganglion cells of the optic nerve neurons respond differently depending on where they are located on the retina. Thos in the centre have increased activity, whilst those further from it have less. The lower activity from the outer cells is called lateral inhibition.

Contralateral Processing

This is due to the optic chiasma, where the right brain processes information from the left visual field, and vice versa. Our perception of the world around us is processed in the brain based on the visual information we receive. This is illustrated by the abnormal perceptions of patients with brain lesions.

Our perception of visual stimuli is also affected by past experiences and expectations. Therefore, our vision is highly subjective and is not an entirely reliable sense.

E.2.6 – Label a diagram of the ear

E.2.7 – Explain how sound is perceived by the ear, including the roles of the eardrum, bones of the middle ear, oval and round windows, and the hair cells of the cochlea

Our ears are responsible for hearing and balance. Sound waves are channelled into the ear by the outer ear, or pinna.


This is a thin sheet of connective tissue. The sound waves reach the eardrum and cause it to vibrate. These vibrations are small movements towards and away from the middle ear. This allows for sound waves to be transmitted to the middle ear.

Bones of the Middle Ear

The middle ear is an air-filled space, and is the location of the smallest bones of the body. All of these bones are touching. Together they form a lever system, increasing the pressure on the oval window by 20 times. The first bone touches the eardrum, reducing the amplitude of the waves, but increasing their force as they are passed to the other bones until they reach the oval window. If particularly loud sounds are picked up, the muscles around the bones will reduce the vibrations to prevent damage.

Oval Window

This is also a membrane, and serves to pass the vibrations to the cochlea. The sound is magnified by the bones of the middle ear before it reaches the oval window. The third bone, called the stapes, or stirrup, touches it and passes on the vibrations. The cochlea is filled with fluid, with the other window, the round window, allowing for the fluid to vibrate.

Hair Cells in the Cochlea

The cochlea in located in the inner ear, and is a coiled, fluid-filled tube. It is divided into three compartments that are separated by membranes. The upper compartment touches the oval window, and the lower compartment touches the round window. The middle compartment is between the two canals, made up of the basilar membrane and an inflexible membrane.

The basilar membrane support the hair cells of the cochlea, located just below the inflexible membrane, with the hair touching the inflexible membrane. The pressure waves make the hair cells rub or pull against the inflexible membrane. The hair cells are connected to the auditory nerve, and their movement creates an action potential which can be passed down it.

The Cochlea:- 


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Option E.1 – Stimulus and Response

Option E.1 – Stimulus and Response

E.1.1 – Define the terms stimulus, response and reflex in the context of animal behaviour

Stimulus – A change in the environment, internal or external, that is detected by a receptor and elicits a response

Response – The activity of a cell or organism in terms of movement, hormone secretion or enzyme production as a result of a stimulus

Reflex – A rapid, unconscious response to a stimulus E.1.2 – Explain the role of receptors, sensory

E.1.2 – Explain the role of receptors, sensory neurons, relay neurons, motor neurons, synapses and effectors in the response of animals to stimuli


These detect a stimulus, and can be sensory cells or nerve endings of sensory neurons.

Sensory Neurons

These receive messages across synapses from receptors and carry them to the central nervous system (spinal cord or brain). These have a single, long nerve fibre called a dendron to bring impulses towards the cell body and a single axon to take them away.

Relay Neurons

These receive messages, across synapses, from sensory neurons, and pass them to the motor neurons that can cause the appropriate response. These may also be called interneurons, and have many short, nerve fibres

Motor Neurons

These receive messages across synapses from relay neurons and carry them to an effector. These have dendrites which bring impulses towards the cell body, and a single long nerve fibre called an axon which carries impulses away from the cell body.

The Schwann cells wrap around the axon and form the myelin sheath, which provides protection and support. The gaps between the cells are called nodes of Ranvier. This increases the speed at which the impulses are conducted. Motor neurons form part of the nerve fibres, which send rapid and precise impulses.


The synapse is the small space between neurons. Since electrical impulses cannot pass across this space, chemical messengers called neurotransmitters are used to send messages from neurons.




These carry out a response after a message from a motor neuron. These can be muscles, which respond by contracting, or glands, which respond by secreting hormones or enzymes

The brain and spine form the central nervous system, whilst the nerve fibres to the rest of the body form the peripheral nervous system.


E.1.3 – Draw and label a diagram of a reflex arc for a pain withdrawal reflex, including spinal cord and it spinal nerves, the receptor cell, sensory neuron, relay neuron, motor neuron and effectors

The reflex arc allows us to respond to a stimulus in the same way every time. The response is an immediate and unconscious one – the message is not sent to the brain, but to the spinal cord. For example, the pain withdrawal reflex, such as when we touch something hot, means that we immediately pull back from the heat source. Some reflex arcs also send messages to the brain.

E.1.4 – Explain how animal responses can be affected by natural selection, using two examples

Many inherited traits are beneficial to the survival of an animal, and will increase its chances of being passed on. It may help the organism’s ability to feed, breed and survive. This contributes to natural selection. Such characteristics are the result of genetic variation and the expression of genetic variation in the phenotype. Not only do these alter the structure and appearance of the organism, but they can also contribute to its behaviour.

Example 1 – Erinaceus europaeus (European hedgehog)

Hedgehogs are nocturnal and omnivorous. The hedgehog exhibits self-defensive behaviour by curling up into a ball to expose its spines when it senses danger. The spines create a shield for its skin, body and limbs.

Across different populations, this response can vary:

  • Raising spines
  • Periodically peering out of spines to gauge danger levels
  • Running away if possible

In areas where there are vehicles and roads, the hedgehogs that run away when faced with danger will be favoured over others. They are better able to escape danger and only use rolling up as a last resort and cannot escape. In the situation of motor vehicles, a hedgehog will receive no protection from its spines. Those that roll up in a ball will be at a disadvantage, and are not selected for in the population.

Example 2 – Sylvia atricapilla (blackcap)

This is a bird that used to breed in Germany summer, and then go to Spain and the Mediterranean in winter. Studies show that 10% of migrating blackcaps will fly to the UK

Studies show that 10% of migrating blackcaps will fly to the UK instead. To show that this behaviour was genetic, and not learned, a sample of eggs was taken from birds that flew to the UK and birds that flew to Spain. They were raised independently of their parents so that there was no opportunity for them to learn the behaviour. Once the time came to migrate, they were not able to follow their parents. The data showed that birds whose parents had flown to the UK also instinctively flew west, whilst those with parents who flew to Spain, flew south.

This shows that the direction the birds flew in was genetic, indicating that their response was the result of natural selection. The number of blackcaps flying to the UK is increasing, as the birds have a greater chance of survival due to warmer conditions.

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