Edexcel Categories Archives: B2

Plant transport systems

Plant transport systems

  • Plants have two specialised tissue: xylem and phloem which carry minerals, salts and water up and down the plant.
  • Xylem tissue consisting of long dead cells that die and form hollow tubes. These tubes also give support to the plant. Xylem transports water and minerals.
  • Phloem tissue is living. This transports food from the leaves which is used to make food at the tip.


  • This is caused by the evaporation and diffusion of water from inside the leaves.
  • This creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it. This means more water is drawn up from the roots so there is a constant stream of transpiration through the plant.
  • Transpiration is a side effect of the way leaves are adapted for photosynthesis. They have stomata for easier diffusion which means the water can get out more easily too, as well as due to the fact there is less water outside than inside the plant.
  • But the transpiration stream does provide the plant with a constant supply of water for photosynthesis.
  • A potometer can be used to measure the rate of transpiration.
  • As the stem takes up water, the air bubble moves along the capillary tube and the length of which it moves can be recorded.

Organisms and their environment

  • A habitat is a place where an organism lives. The distribution of an organism is where the organism is found.
  • The conditions in an environment determine what kind of animal will live there. Different environments may present different challenges (such as the Artic vs the rainforest) and the organisms must adapt to their environment in order to survive there.
  • Ecologists study the biodiversity of life found in an ecosystem or habitat and where particular organisms are found. The data they collect on their population are used to monitor changes in population or test hypotheses about what sort of organisms exist in a certain place.

Sampling techniques

  • Sampling means looking at a small portion of an area. In random sampling every point within an area has an equal chance of being selected. This means it is more likely to be representative of the whole area.


  • This is used to catch small invertebrates through an inlet tube by sucking sharply on a second tube connected to the container.



Sweep net

  • This is used for catching insects in long grass. It is a net lined with strong cloth.
  • It should be quickly swept through the grass and the insects collected should be put into a container.

Pond net

  • This is a net used for catching insects, water snails etc. from ponds.
  • The net should be swept along the bottom of the pond or river to collect the organisms.

Pitfall traps

  • These are useful for trapping small animals, such as spiders, beetles and woodlice. They can be set up and left overnight so it is possible to catch organisms that might not be active during the day.


  • These are square frames of a known size, which is typically used to sample plant species in a habitat.
  • The quadrat is placed in random locations numerous times, and each time the number of plants desired to be counted are recorded. For spreading plants like clover, the percentage of the quadrat area that is covered by the plant is estimated.
  • There are different reasons for a varying number of the same organisms due to the different conditions. For example, observing how light intensity varies might help to explain the distribution of different species of plants.
  • Other factors such as temperature, soil or water pH may also have an effect in determining which organisms can survive in any given part of a habitat.
  • Systematic sampling along a line can be useful when investigating changes in a habitat caused by one environmental factor. The quadrat could be placed at regular intervals along a straight line.

Distribution of organisms in an ecosystem

  • It is important to know the distribution of living organisms in an area. This is particularly true if something in the environment is going to change such as houses being built or a road developed.
  • In these cases, local environment impact surveys are carried out. They involve collecting data about the numbers and distribution of several species in an area which will be affected.
  • The use of sampling techniques is extremely important in this kind of work.
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  • Osmosis: The movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential.
  • Osmosis is a special type of diffusion as it only involves water particles.
  • In the context of a partially permeable membrane, water molecules move randomly in between the membrane. However, because there are more water molecules on one side than the other, there’s a steady net flow of water in the region with fewer water molecules.
  • The previously concentrated solution will become more dilute; it’s as if the water molecules are trying to even up on both sides.

Water transport

Root hair cells

  • Root hair cells are found on the surface of plant roots. They have millions of long thin extensions which reach into the surrounding soil to get all the mineral salts and water possible.
  • They take in water by osmosis. This gives the plant a big surface area for absorbing water from the soil.
  • However, they take in minerals by active transport, which is the movement of particles against the concentration gradient. They have to do this as the concentration of minerals in the soil is usually lower than the concentration in the plant.
  • Active transport requires energy from respiration.
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  • This is the process for plants that produces their ‘food’.
    • Also needs sunlight and chlorophyll.
  • Plants need to respire as well as photosynthesise. Photosynthesis is their way of getting glucose (which is stored as starch) for respiration.
  • Chloroplasts in plant cells are where the reactions for photosynthesis takes place.
  • Light is absorbed by a green substance inside chlorophyll. Chlorophyll transfers the light energy into the stored chemical energy glucose and without it, photosynthesis cannot occur.

Leaf adaptions

  • The leaves of plants have been adapted for their function in many ways. These are:
    • Leaves, the main organ of a plant, are broad so there’s a large surface area exposed to light.
    • Leaves contain lots of chlorophyll in chloroplasts to absorb light.
    • Leaves are full of holes called stomata on their underside. They open and close to allow gases such as oxygen and carbon dioxide to diffuse in and out. They also allow water vapour to escape, which is transpiration.
    • It is thin to allow a shot space for Carbon dioxide to diffuse into.
    • The epidermis is thin and transparent to allow more light to reach the palisade cells.
    • As the palisade layer is at the top of the leaf it gets more sunlight than the layers below and to take full advantage of this, it has more chloroplasts in each cell to ensure all light available is absorbed.
    • The cuticle protects the leaf with its wax but it doesn’t block out sunlight.

Limiting factors

  • Many factors affect the rate of photosynthesis: the concentration of carbon dioxide, light intensity, temperature and water availability.
  • The rate of reaction varies, dependent upon the conditions. These conditions are known as limiting factors.
  • Limiting factors limit the rate at which a reaction can take place.
  • Limiting factor: These stop a reaction from occurring any faster.
  • If there is a variable in low availability this will slow or reduce the rate of reaction. For example, if the amount of carbon dioxide is low, there is a limited amount of carbon dioxide to react with water. Therefore, the rate of photosynthesis is determined by how much carbon dioxide is available.
  • If this concentration is increase, eventually carbon dioxide will no longer be a limiting factor and something else may limit the rate of reaction e.g. the light intensity.
  • Which factor is limiting at a particular time depends on the environmental conditions:
    • at night it’s pretty obvious that light is the limiting factor.
    • in winter it’s often the temperature.
    • if it’s warm enough and bright enough, the concentration of carbon dioxide is usually limiting.
  • As enzymes catalyse most of the reactions in living organisms, if their internal temperature becomes too high the biological reactions stop as enzymes become denatured.
  • There is an optimum temperature for all biological reactions which is the maximum temperature at which a reaction involving enzymes can take place before they begin to denature.
  • Once they begin to denature the rate of reaction slows.
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  • Respiration: this is the process used by all living organisms to release energy from organic molecules. This energy is used for:
    • Building up larger molecules like proteins
    • contract muscles (exercise)
    • maintain a steady body temperature
  • Respiration is the process of breaking down glucose to release energy which goes on in every living cell.

Aerobic respiration

  • During aerobic respiration, oxygen is used to release energy from molecules such as glucose. The word equation is:

Delivering oxygen and glucose and removing waste gases

  • In humans, oxygen and glucose needed for respiration are carried around the body and into tissues by the bloodstream. The blood must also carry the waste carbon dioxide from respiring cells.
  • All these substances move between respiring cells and tiny blood vessels called capillaries by a process called diffusion.
  • Diffusion: The movement of particles from an area of higher concentration to an area of lower concentration.
  • In respiring cells, oxygen and glucose levels fall as they are used up in aerobic respiration. At the same time, carbon dioxide levels in the cells rise. As a result, the carbon dioxide diffuses out of the cells and into the capillaries and oxygen and glucose diffuse out of the capillaries and into the cells.

Gas exchange

  • Lung tissue is spongy and full of tiny air sacs called alveoli. These are surrounded by capillaries and oxygen that enters the body through the lungs moves into the blood by diffusion.
  • Carbon dioxide also leaves the blood by diffusion into the air spaces. As one gas is entering the bloodstream, another one is leaving it and thus it is called gas exchange.

Anaerobic respiration

  • As you work harder your need for energy increases and thus you respire more. During exercise, muscles use up oxygen and glucose very quickly so the blood supply to the muscles must increase (as that carries glucose and oxygen).
  • The amount of blood circulated by the heart depends on heart rate and stroke volume. If we multiply heart rate by stroke volume we get the cardiac output.
  • The cardiac output is the volume of blood circulated by the heart in a given time.
    • cardiac output = stroke volume x heart rate
  • Heart rate and stroke volume increase with exercise so cardiac output also increases. However, this extra blood being pumped around the body is useless if it doesn’t contain enough oxygen for respiration. To make sure it does, our breathing rate increases to increase the rate of oxygen uptake in the lungs.
  • However, during intense exercise we can’t supply oxygen to our muscles quickly enough. This is when anaerobic respiration comes in – respiration without oxygen.

  • Anaerobic respiration releases less energy than aerobic respiration. The lactic acid produces gathers in your muscles which gets painful and gives you cramps.
  • Lactic acid is broken down using oxygen into carbon dioxide and water.
  • After exercise, oxygen is required to break down lactic acid to produce energy for other processes such as cell repair. This requirement for additional oxygen after exercise is called ‘excess post-exercise oxygen consumption’ EPOC.
  • This extra oxygen can be obtained quickly by maintaining a high breathing rate and high heart rate. The time taken for the pulse rate to return to the normal or resting rate after exercise is the recovery time.
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  • Enzymes help chemical reactions going on in the body to work well and quickly. Enzymes are biological catalysts; they help a reaction go faster without itself being changed by the reaction.
  • The chemical reactions in our body will still happen without enzymes, but will be too slow to keep us alive.
  • Enzymes can do different things. Some help break down large molecules into smaller ones and some help smaller chemicals to make large ones.

Enzymes inside cells

  • During DNA replication in mitosis or meiosis, the DNA double helix is unwound and the weak hydrogen bonds are separated by a particular enzyme.
  • As new bases line along each half, so complementary base pairings match, a different enzyme joins them together. As a result, two complete and identical DNA molecules are made. The enzymes are unchanged so can repeat the same action when needed.
  • During protein synthesis, many different reactions occur which are all catalysed by different enzymes.

Enzymes outside cells

  • Enzymes help break down large food molecules such as carbohydrates, proteins and fats as they are much too big to pass across the cell membranes of the gut wall. Digestion is the name of the process that breaks them down. Different enzymes are released into the mouth, stomach and small intestine to help digest these molecules so they can be absorbed into the cells.
  • Microorganisms and fungi also release digestive enzymes but they grow on and through the food they are digesting because they don’t have a gut. After the enzymes have digested the food, the smaller molecules are absorbed through the microorganism’s cell walls.

Enzyme action

  • Molecules that enzymes work on are called substrate molecules.
  • Enzymes work best at a particular temperature, called the ‘optimum’ temperature which in the human body, is usually around 40 degrees Celsius. However, some enzymes have adapted to work efficiently at very low temperatures, such as in ikaite rock in deep water.
  • When temperature increases, the substrate particles move faster as they have more energy. This means they are more likely to meet and react with enzymes.
  • Also, enzymes work best at the optimum pH. Most enzymes in our cells work best at pH 7 but enzymes that function in the digestive system have to be good at working at low or high pH levels.
  • When the temperature is too high or the pH is not right, the bonds holding the enzyme together break and thus make it lose its shape. This slows down the reaction.
  • The rate of reaction for an enzyme is also changed by the concentration of the substrate. With the increase of substrate concentration so does the rate of reaction, but only to a point. Beyond that concentration, the enzyme is incapable of working on the substrate any faster (all the active sites are full). Therefore, adding more substrate molecules will make no difference.

Specific enzymes

  • Enzymes only work with a particular substrate, so we say they have a specificity for their substrate. Enzymes are therefore named according to the substrate they catalyse:
    • Carbohydrases catalyse the breakdown of carbohydrates.
    • Proteases catalyse the breakdown of proteins.
  • All substrate molecules for one particular enzyme have the same 3-D shape in some part of their molecules. This suggests shape is incredibly important in enzyme reaction.
  • The substrate for a particular enzyme fits neatly into the active site of that enzyme. The active site takes a different shape in different enzymes. Since the shape of the substrate fits tightly into the hole of the active site, this model is called ‘lock-and-key’ hypothesis.
  • Changing the pH or temperature a little changes the shape of the active site, so the substrate doesn’t fit so well. Too much change will break the bonds within the enzyme and therefore can change the shape of the active site so much that is denatures the enzyme and destroys the active site.



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Stem Cells

Stem Cells

  • Stem cell: An unspecialised cell that can divide to produce more stem cells or different kinds of specialised cell.
  • In human embryos that are a few days old they have embryonic stem cells. These cells can be differentiated into almost any human cell such as neurones, muscle cells etc. However, one the stem cells have differentiated, they cannot change again.
  • Stem cells found in differentiated body tissues (e.g. bone marrow), which are called adult stem cells, can only differentiated into a few types of cell while embryonic stem cells are the opposite; they are known to be pluripotent as they can transform into any type of cell.
  • In many plant cells they don’t lose the ability to differentiate unlike animal cells.

Stem Cell treatments

  • Since the 1960s, adult stem cells from bone marrow have been used to treat leukaemia, a cancer of certain white blood cells. To do this, the patient’s white blood cells are destroyed and replaced with adult stem cells from someone else. These cells multiply to produce healthy white blood cells.
  • This is known as a bone marrow transplant. However, the treatment doesn’t always work because if the cells are too different, the patient’s body destroys it.
  • This problem can be solved by cloning. If a cloned embryo is created by the skin cell of the patient with leukaemia the embryonic stem cells on the embryo can be taken and used to produce the cells that make white blood cells. These will survive in the patient as their body would recognise the new white blood cells as their own cells.
  • This technique could treat many more problems than adult stem cells could as embryonic stem cells can differentiate into almost every kind of cell. However, that means that this could be used illegally by people to produce human clones.
  • Scientists have also been investigating ways of reprogramming specialised cells into stem cells. This would avoid the ethical problem with using embryos if it works. But further research is required to make any treatment with stem cells safe as if injected into the body they may produce the wrong kind of cell or even create cancer cells.

People don’t like using embryos as they believe it is a form of murder as they are destroying a potential human life. However, some say that curing patients who are suffering is more important than potential life.

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  • Clones are individuals which are genetically identical. Making plant clones – bit of leaf, stem or root is planted to form a clone of the original plant – is far easier than cloning animals.
  • Clone: These are individuals that genetically identical.

Stages in the production of cloned mammals

  1. The removal of the diploid nucleus from a body cell (sheep 1)
  2. Enucleation of egg cell (removal of nucleus) (sheep 2)
  3. Insertion of diploid nucleus into enucleated egg cell.
  4. Electrical stimulation of the diploid nucleus to divide by mitosis.
  5. Implantation of embryo into surrogate mammal. (sheep 3)
  6. The offspring born by the surrogate mammal is a clone of sheep 1 not sheep 2.
  • Making plant clones can be relatively easy; you start with a bit of leaf, stem or root of the original plant. The plant cells divide and produce new cells, which grow into a clone of the original plant. This is an example of asexual reproduction.

Advantages of cloning

  • Cloning mammals could help with the shortage of organs for transplants. For example, genetically-modified pigs are being bred that could provide suitable organs for humans. If this is successful, then cloning these pigs could help meet the demand for organ transplants.
  • The study of animal clones could mean a greater understanding of embryos, ageing and age related disorders.
  • Cloning could be used to preserve an endangered species.

Disadvantages of cloning

  • Cloning mammals could lead to a reduced gene pool – fewer different alleles in the population:
    • If a population are all closely related and a new disease appears, they could all die as none of them may have the allele of resistance to the disease.
  • Cloned mammals do not live as long. Dolly the sheep only lived for 6 years which is half as long as many sheep.
    • She had lung disease and arthritis and she was put down. These diseases are more common in older sheep.
    • She had been cloned from an older sheep, so it may have been her ‘true’ age was older.
    • She may have been just unlucky – her illnesses were not linked with her being a clone.
  • Other risks and problems:
    • Cloning process often fails; it took 237 attempts to clone Dolly.
    • Clones are often born with genetic defects.

Cloned mammals’ immune systems are sometimes unhealthy, so they suffer from more diseases.

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  • This is the type of cell division used to make gametes. Gametes are sex cells, which are ova in females and sperm in males. During sexual reproduction, two gametes combine to form a new cell which will grow to become a new organism.
  • Gametes are haploid, which means they only have one copy of each chromosome. This is so when the two gametes fuse during fertilisation, the zygote has the correct number of chromosomes.


  • Meiosis only occurs in reproductive organs. Four haploid nuclei are produced whose chromosomes are not identical.
  • Two divisions occur during this process.

Division 1

  • As with mitosis, the cell duplicates its DNA.
  • The chromosome pairs line up in the centre of the cell.
  • The pairs of chromosomes are pulled apart so each new cell only has one copy of each chromosome. Some of the father’s chromosomes and some of the mother’s chromosomes go into each new cell.
  • Each new cell will have a mixture of the mother’s and father’s chromosomes. Mixing up the alleles in this way creates variation in the offspring. This is a huge advantage of sexual reproduction over asexual reproduction.

Division 2

  • In the second division the chromosomes line up again in the centre of the cell. The arms of the chromosomes are pulled apart.

Four haploid gametes are produced, each with only a single set of chromosomes in it.



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  • This is the division of somatic/body cells. These cells are diploid which means they have the full number of chromosomes – 46.
  • When a cell divides by mitosis it makes two cells identical to the original cell, each with a nucleus containing the same 46 chromosomes.
  • This type of cell division is for growing/replacing cells that have been damaged, or simply growth.
  • Asexual reproduction also uses mitosis, such as strawberry plants.

The process

  1. Before a cell divides, the DNA is spread out in long strings.
  2. When a cell divides, its DNA needs to be duplicated so there is one copy for each new cell. Each DNA is copied and forms an X-shaped chromosome. Each ‘arm’ of the chromosome is an exact duplicate of the other.
  3. These chromosomes line up at the equator of the cell. Spindle fibres attached to these, at either side of the cell, are pulled apart. As a result, the two arms of each chromosomes go to opposite ends of the clel.
  4. Membranes being to form around each set of chromosomes. These become the nuclei of the two ne cells.
  5. The cytoplasm divides.
  6. Two new diploid cells have been formed which are genetically identical.


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Genetic engineering

Genetic engineering

  • Scientists can now remove a gene from one organism and insert it into the DNA of another. This is called genetic engineering.
  • This technique produces a genetically modified organism.

Benefits of genetic engineering

  1. Reducing vitamin A deficiency
    1. Beta-carotene is used by our bodies to make vitamin A. Vitamin A deficiency is a big problem in parts of south Asia and Africa. Many children go blind due to this deficiency.
    2. Golden rice is a variety of GM rice which contains two genes from other organisms which together enable the rice to produce beta-carotene.
    3. Growing golden rice in these places will mean the people will eat them and fewer people will suffer from vitamin A deficiency.
  2. Producing human insulin
    1. The human insulin gene can be inserted into bacteria to produce human insulin (see later).
    2. This means, as bacteria reproduce quickly, a lot of human insulin can be produced quickly and cheaply to treat diabetes.
  3. Increasing crop yield
    1. GM crops have had their genes modified to make them resistant to herbicides, for example.
    2. Fields of these crops can be sprayed with herbicide and all the plants except the GM crop are killed. This increases the yield of the crop as there is less competition from other plants.


  • Some people think GM crops will affect biodiversity – affecting the number of wildlife that usually lives in and around crops.
  • Not everyone thinks GM crops are safe. Some people think people may develop allergies to the food, but there is probably no increased risk than eating normal foods.
  • Another concern is that transplanted genes may get out into the natural environment. An example is herbicide resistance being picked up by weeds creating a new superweed variety.

Genetic engineering of human insulin

  • Scientists have been able to insert the gene for human insulin into bacterial plasmid DNA.
  • The production of human insulin by GM bacteria has many advantages. Insulin used to be extracted from dead pigs and cattle but now GM bacteria can be used with vegans.
  • Another advantage is that the production of insulin is not affected by animal diseases or the numbers of animals killed for meat.
  • Insulin can be made in vast quantities by using fermenters and can be made more cheaply.
  • However, one disadvantage is that as bacteria produce the insulin slightly differently there are small differences which means it does not suit everyone.

Genetically modifying bacteria to make human insulin

  1. To make insulin, Scientists first have to use a cutting enzyme to cut the insulin
    gene out of a human chromosome.
  2. In bacteria, plasmids are removed and isolated.
  3. A restriction enzyme is used to cut open the plasmids and the human insulin genes are then inserted (DNA ligase enzyme).
  4. The genetic engineers encourage the bacteria to adopt the genetically modified plasmids. Bacteria who adopt this GM plasmid are then multiplied.

By culturing the bacteria a limitless supply of insulin can be produced.

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