AQA Categories Archives: A Level



  1. The two photosystems are involved in non-cyclic photophosphorylation, PSII comes before PSI. The photosystems are found in the thylakoid membranes and are linked by electron carriers (proteins that transfer electrons). The photosystems and electron carriers form an electron transport chain.
  2. Light energy is absorbed by PSII and this excites electrons/reaction centres in the photosystem. The electrons move to a higher energy level (have more energy) and the high energy electrons are passed along the electron transport chain to PSI.
  3. When excited electrons move from PSII to PSI they need to be replaced – this is done by photolysis. Here, light energy splits water into protons (h+), electrons and oxygen. (the o2 in photosynthesis comes from water).
  4. Excited electrons lose energy as they move along the electron transport chain, and the energy is used to transport protons into the thylakoid through proton pumps. This gives the thylakoid a higher proton concentration than the stroma (forming a proton gradient). The protons will move down their concentration gradient (into the stroma) through ATP synthase. The energy from the movement combines ADP and Pi to form ATP. The process of a proton gradient driving ATP synthesis is called chemiosmosis.
  5. Light energy is also absorbed by PSI, exciting the electrons again to a higher energy level.
  6. The electrons are transferred to NADP with a proton (H+) from the stroma, forming reduced NADP (NADPH).
  7. Cyclic photophosphorylation only produces ATP. It only uses PSI. The electrons aren’t passed onto NADP, they’re passed back onto PSI by electron carriers. The electrons are basically recycled by PSI. No NADPH or O2 is produced.



  • Also called the Calvin cycle. It takes place in the stroma of the chloroplasts. The reactions are linked in a cycle, so the starting compound, ribulose bisphosphate, is regenerated. Sometimes called carbon dioxide fixation (carbon from CO2 is fixed into an organic molecule)
  1. Carbon dioxide is combined with ribulose bisphosphate to form 2 molecules of glycerate 3-phosphate. The CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast. When it combines with RuBp it gives an unstable 6 carbon compound, which breaks down into 2 molecules of the 3 carbon compound glycerate 3-phosphate (GP). RuBisCO (ribulose bisphosphate carboxylase) catalyses the reaction between CO2 and ribulose bisphosphate.
  2. ATP and NADPH (reduced NADP) are required to reduce GP into triose phosphate (TP). 2 lots of ATP and 2 lots of NADPH are used from the light independent reaction. The NADP can then go back to the light-dependent reaction. The triose phosphate can then be converted into many useful organic compounds. (e.g. glucose)
  3. 5 out of every 6 TP molecules produced in the cycle are used to regenerate RuBP. This uses the rest of the ATP produced by the light-dependent reaction
  • TP and glycerate 3-phosphate (GP) are used to make:
    1. Carbohydrates – hexose sugars (like glucose) are made by joining 2 triose phosphate molecules together.
    2. Lipids – made using glycerol, which is synthesised from triose phosphate and fatty acids, which are synthesised from GP.
    3. Amino acids – some are made from GP.
  • 3 turns of the Calvin cycle produce 6 molecules of TP (2 molecules of TP are made for every CO2 molecule used. 5/6 of the molecules are used to regenerate RuBP in ach cycle, so in 3 turns of the cycle, only 1 TP is produced to make a hexose sugar. Hexose sugars have 6 carbons, and 2 TP molecules are used to make 1 hexose sugar. Six turns of the cycle require 18 ATP (3 are used in each cycle) and 12 NADPH (2 used in each cycle)



Light intensity·         Lights needed to give energy for the light-dependent reaction. The higher the intensity, the more energy provided.

·         Only certain wavelengths of light are used for photosynthesis.

·         The photosynthetic pigments (chlorophyll a, chlorophyll b and carotene) only absorb red and blue light (green is reflected, which is why plants look green)

Temperature·         Photosynthesis involves enzymes (e.g. ATP synthase, RuBisCO). If temp. Falls below 10°C they become inactive, and they denature at temps above 45°C.

·         High temps have an effect on :

o   Stomata -They close at high temps to avoid losing too much water. This slows down photosynthesis because less CO2 enters the leaf when the stomata are closed.

o   Thylakoid membranes – may be damaged, reducing the rate of the light-dependent stage by reducing the number of sites available for electron transfer.

o   Chloroplasts – membranes around them could be damaged, possibly causing enzymes important in the Cycle to be released into the cell. This would lessen the rate of the light-independent stage.

o   Chlorophyll – could be damaged, reducing the amount of pigment that can absorb light energy, reducing the rate of the light-dependent reactions.

Carbon dioxide·         Makes up 0.04% of the gasses in the atmosphere.

·         Increasing this to 0.4% gives a higher rate of photosynthesis. Any higher than this and the stomata will close.

Water stress·         When plants don’t have enough water, their stomata will close to save the little water that they have.

·         Less CO2 will enter the leaf for the Calvin cycle

·         Slows photosynthesis down.


Saturation point = where increasing the factor after this point makes no difference because something else has become the limiting factor. A graph levels off here.

Light intensity, temperature and CO2 concentration all affect the rate of photosynthesis, so all affect the levels of GP, RuBP, and TP in the Calvin cycle.

Light intensity:

  • When its low the products of the light-dependent reaction will be in short supply (ATP and NADPH)
  • So the conversion of GP into TP and RuBP will be slow.
  • The level of GP will rise (still being made) and levels of TP and RuBP will fall (as they’re being used to make GP)


  • The reactions in the Calvin cycle are catalysed by enzymes (e.g. RuBisCO)
  • At low temperatures, all the reactions will be slower as the enzymes work more slowly.
  • So levels of RuBP, GP, and TP will fall.
  • They’re all affected in the same way at high temperatures because the enzymes will start to denature.

Carbon dioxide concentration:

  • When its low, conversion of RuBP to GP is slow (there’s less CO2 to combine with RuBP to make GP)
  • The level of RuBP will rise (still being made)and levels of GP and TP will fall (used to make RuBP)
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Energy and Ecosystems

Energy and Ecosystems

5.1 Food Chains and Food Webs

  • Ultimate source of energy is sunlight converted to chemical energy by photosynthesising organisms then passed as food to other organisms


  • Photosynthetic organisms that manufacture organic substances using light energy, water and CO2
  • PHOTOSYNTHESIS EQUATION: 6CO2 + 6H2O + energy à C6H12O6 + 6O2
  • ­Green plants are producers


  • All organisms that obtain energy by consuming other organisms
  • Animals are consumers
  • Those that directly eat producers are PRIMARY consumers – first in the chain
  • Those that eat the primary consumers are secondary consumers
  • Those that eat the secondary consumers are tertiary consumers
  • Secondary and tertiary are usually predators but can also be scavengers or parasites


  • Producers and consumers die and the energy they contain can be used when the complex materials are broken down into single components again
  • In breaking these materials down they can be absorbed by plants and be recycled
  • Majority of this is done by decomposers (fungi and bacteria) and some by detritivores (earthworms)

Food Chains

  • Food chain – relationship in which producers are eaten by primary, who are eaten by secondary, who are eaten by tertiary.
  • Longer chains may contain quaternary consumers
  • Each stage in the chain is a trophic level

Food Webs

  • Within a habitat many food chains will be linked together to form a food web
  • Problem is their complexity
  • A producer can be eaten by several different organisms etc.
  • Charting all feeding inter-relationships is not feasible
  • Also, these relationships are not fixed, they depend on times of the year



5.2 Energy Transfer Between Trophic Levels

  • As little as 1% of the Sun’s energy is captured by green plants and so made available to organisms in the food chain
  • These organisms then pass only a small fraction of the energy on that they receive to each successive trophic level

Energy Losses in Food Chains

  • Plants normally convert 1-3% of the Sun’s energy into organic matter.
  • Most of the Sun’s energy isn’t converted because:
    • Over 90% of the Sun’s energy is reflected back into space by clouds and dust or is absorbed by the atmosphere
    • Not all wavelengths of light can be absorbed and used for photosynthesis
    • Light mightn’t fall on a chlorophyll molecule
    • Might be limiting factor, e.g. CO2, stopping the energy being converted
  • The total quantity of energy that plants produce in a community convert to organic matter is called the gross production
  • However, 20-50% of this energy is used in the plant’s respiration, leaving little to be stored
  • The rate at which plant’s store energy is called the net production

Net production = gross production – respiratory losses

  • Even then only about 10% of this food stored in plants is used by primary consumers for growth
  • Secondary and tertiary consumers are slightly more efficient, transferring about 20% of energy available into their own bodies
  • The low percentage of energy transferred at each stage is the result of:
    • Some of the organism isn’t eaten
    • Some parts are eaten but can’t be digested so is lost in faeces
    • Some of the energy is lost in excretory materials, e.g. urine
    • Some energy losses occur as heat from respiration and directly from the body into the environment. These losses are higher in mammals and birds because of their high temps. Much more energy is needed to maintain their body temp.
  • It is the relative inefficiency of energy transfer between trophic levels that explain why:
    • Most food chains have only 4 or 5 trophic levels because insufficient energy is available to support a large enough breeding population at trophic levels higher than these
    • The total mass of organisms in a particular place (biomass) is less at higher trophic levels
    • The total amount of energy stored is less at each level


Calculating the Efficiency of Energy Transfers

  • Energy available is usually measured in kilojoules per square metre per year (KJm­-2year-1)

5.3 Ecological Pyramids

Pyramids of Number

  • Usually, the numbers of organisms at lower trophic levels are greater than those at higher levels –shown by bars proportional to the number at each trophic level
  • Significant drawbacks to using a number pyramid
    • No account is taken of size – one tree is treated the same as an aphid and each parasite has the same numerical value as its larger host. This means sometimes it is not a pyramid at all
    • The number of individuals can be so great that it’s impossible to represent them accurately on the same scale as other species in the food chain. E.g. one tree may have millions of greenfly living of it.


Pyramids of Biomass

  • More reliable, quantitative description of a food chain where biomass is measured not numbers
  • Biomass is the total mass of the plants and/or animals in a particular place
  • Fresh mass is easy to asses but differing amounts of water present makes it unreliable – use dry mass instead BUT animals therefore have to be killed
  • Because they need to be killed only a small sample is taken and so may not be representative
  • Biomass is measured in grams per square metre (gm-2) where an area is being sample. When a volume is being sampled (in a pond or ocean) it’s measured in grams per cubic metre (gm-3)
  • In both PoN and PoB only the organisms present at a particular time are shown, seasonal differences are not apparent
  • This is particularly apparent when the biomass of some marine ecosystems arre measured
  • Over the course of a whole year the mass of phytoplankton (plants) must exceed that of zooplankton (animals), but this is not seen at certain times of the year. E.g. in early spring around the British Isles, zoo plankton consume phytoplankton so rapidly that the biomass of zooplankton is greater than that of phytoplankton

Pyramids of Energy

  • Most accurate representation of the energy flow through a food chain
  • Measures the energy stored in organisms
  • Collecting data can be difficult and complex
  • Data are collected in a given area for a set period of time ( 1m2 for a year)
  • Results are more reliable than those for biomass because two organism of the same dry mass may contain different amounts of energy. E.g. 1g of fat stores twice as much as 1g of carbohydrate
  • An organism with more fat therefore has more energy even though their biomass is equal
  • Energy flow is usually measured in KGm-2




5.4 Agricultural Ecosystems

  • Agricultural ecosystems are largely made up of animals and plants used to produce food for mankind
  • There are considerable energy losses at each trophic level and as we are third or even fourth in the chain we receive only a tiny proportion of the Sun’s energy
  • Agriculture tries to ensure that as much of the available energy from the Sun as possible is transferred to humans
  • It is channelling the energy flowing through a food web into the human food and chain and away from other food chains
  • This increases the productivity of the human food chain

What is Productivity?

  • Productivity ids the rate at which something is produced
  • Plants are producers because the produce chemical energy by converting light energy into it during photosynthesis
  • The rate at which plants assimilate this energy is called gross productivity, measured over a period of time for a given area usually in KJ m-2year-1
  • About 20% of this energy is used in the plant’s respiration
  • The remainder is net productivity

Net Productivity = Gross Productivity Respiratory Losses

  • Net productivity is affected by 2 main factors
    • The efficiency of the crop at carrying out photosynthesis. This is improved if all necessary conditions for photosynthesis are supplied (no limiting factors)
    • The area of the ground covered by leaves of the crop

Comparisons of Natural and Artificial Ecosystems

  • Artificial ecosystems are based on the same ecological principles as natural but differ in several ways, the 2 basic ones are energy input and productivity

Energy Input

  • In a natural ecosystem the Sun is the only source of energy
  • Most of Britain’s land would be covered by forest if left to grow – a climax community
  • To maintain an agricultural ecosystem this climax community is prevented from developing, this is done by excluding most of the species in that community, leaving only the crop you want to grow
  • To remove or suppress the unwanted species and to maximise growth requires an additional input of energy
  • This energy is used to plough fields, sow seeds, remove weeds, suppress pests and disease, feed and house animals, transport materials etc. and other tasks of farmers
  • This additional energy comes in two forms
    • Food – Farmers and farmhands expend energy as they work which comes from the food they eat
    • Fossil fuels – As farms become more mechanised, energy increasingly comes from the fuel used to plough and harvest and transport crops, to apply fertilisers and pesticides and to feed, house and transport livestock


  • In natural ecosystems productivity is relatively low
  • Additional energy put into agricultural ecosystems is used to increase the productivity of a crop by reducing the effect of limiting factors on its growth
  • The energy used to remove all other species means the crop has no competition for light, CO2, minerals and the water which are all needed for photosynthesis
  • The ground is therefore exclusively covered by the crop
  • Fertilisers are added to provide essential ions and pesticides are used to destroy pests and prevent disease
  • Together these factors mean that productivity is much higher in an agricultural ecosystem than in a natural one
Natural EcosystemAgricultural Ecosystem
Solar energy only – no additional energy inputSolar energy plus energy from food (labour) and fossil fuels (machinery and transport)
Lower productivityHigher productivity
More species diversityLess species diversity
More genetic diversity within a speciesLess genetic diversity within a species
Nutrients are recycled naturally within the ecosystem with little addition from outsideNatural recycling is more limited and supplemented by the addition of artificial fertilisers
Populations are controlled by natural means such as competition and climatePopulations are controlled by both natural means and by use of pesticides and cultivation
Is a natural climax communityIs an artificial community prevented from reaching its natural climax



5.5 Chemical and Biological Control of Agricultural Pests

What Are Pests and Pesticides?

  • Pest – organism that competes with humans for food and resources or it could be a danger to health
  • Pesticides – poisonous chemicals that kills pests. They’re named after the pests they kill, herbicides kill plants, fungicides destroy fungi and insecticides kill insects
  • An effective pesticide should:
    • Be specific – so it’s only toxic to the organisms it is trying to destroy. It should be harmless to humans and other organisms, especially the natural predators of the pest, to earthworms and pollinating insects
    • Biodegrade – so, once applied, it will breakdown into harmless substances in the soil. At the same time, it need to be chemically stable to last long
    • Be cost-effective – because development costs are high and new pesticides are only useful for a limited time due to pests developing resistance, making the pesticide useless
    • Not accumulate – so it doesn’t build up, either in specific parts of an organism or as it passes along the food chain

Biological Control

  • It’s possible to control pest by using organism that are predators or parasites to the pest organism
  • The aim is to control not eradicate the pest as it could be counter-productive
  • If the pest is reduced too much, predators of the pest have insufficient food and so deplete
  • Surviving pests would then multiply with barely any or no predators to eat them
  • Ideally, the control agent and the pest should exist in balance with one another, at a level where the pest has little, or no, adverse effect
Biological DisadvantagesBiological AdvantagesChemical DisadvantagesChemical Advantages
Don’t act quickly – interval between introduction and reductionVery specificAlways have some effect on non-target speciesEffective straight away
Control organism may become a pest itself when there are no natural predatorsOnce introduced, the control organism reproduces itselfMust be reapplied at intervals, therefore expensive 
 Pests do not become resistantPests develop genetic resistance, and new pesticides must be developed 



Integrated Pest-Control Systems

  • Aim to integrate all forms of pest control instead of relying on one
  • Emphasis is on finding an acceptable level of the pest rather than eradicating it. Eradication is costly, counter-productive and almost impossible
  • Integrated control involves:
    • Choosing animal or plant varieties that suit the local area and are as pest-resistant as possible
    • Managing the environment to provide suitable habitats, close to the crop, for natural predators
    • Regularly monitoring the crop for signs of pests so early action can be taken
    • Removing the pests mechanically (hand-picking, vacuuming, erecting barriers) if the pest exceeds an acceptable population level
    • Using biological agents if necessary and available
    • Using pesticides as a last resort if pest populations start to get out of control

How Controlling Pests Affects Productivity

  • Pests reduce productivity in agricultural ecosystems
  • Weeds compete with plants for light, space, water, mineral ions and CO2
  • As these are often in limited supply, any amount taken in by the pest means less is available for the crop plant
  • One or more of them may become the limiting factor in photosynthesis, thus reducing the rate of photosynthesis and hence productivity
  • Insect pests may damage the leaves of crops, limiting their ability to photosynthesise and again reduces productivity
  • Alternatively, they may be in direct competition with humans, eating the crop itself
  • Many crops are now grown in monoculture, and this enables insect and fungal pests to spread rapidly
  • Pests of domesticated animals may cause disease
  • The animals may not grow as rapidly, be unfit for human consumption or die – all of these lead to reduced productivity
  • The aim of pest control is to limit the effect of pests on productivity to a commercially acceptable level
  • In other words, to balance the cost of pest control with the benefits it brings
  • The problem is that at least 2 different interests are involved: the farmer who has to satisfy our demand for cheap food while still making a living, and the conservation of natural resources, which will enable us to continue to have food in the future
  • The trick is to balance these two, often conflicting, interests

5.6 Intensive Rearing of Domestic Livestock

Intensive rearing and energy conversion

  • As energy passes through a food chain only a small percentage passes from one organism to the next
  • This is because much of the energy is lost as heat during respiration
  • IR of domestic livestock is converting the smallest possible amount of food energy into the greatest quantity of animal mass
  • One way to achieve this is to minimise the energy losses from domestic animals during their lifetime
  • This means more food energy will be converted into body mass to be passed on to the next link and eventually us
  • Energy conversion can be made more efficient by ensuring that as much energy from respiration as possible goes into growth rather than other processes
  • This is done by keeping animals in confined spaces, such as small enclosures, barns or cages – factory farming
  • This increases the energy-conversion rate because:
    • Movement is restricted – less energy is used in muscle contraction
    • Environment can be kept warm in order to reduce heat loss from the body
    • Feeding can be controlled so animals receive optimum amount and type of food for maximum growth with no wastage
    • Predators are excluded so no loss to other organisms
  • Other means of improving the energy-conversion rate include:
    • Selective breeding of animals to produce varieties that are more efficient at converting the food they eat into body mass
    • Using hormones to increase growth rates

Features of Intensive Rearing of Livestock

Food is essential for life and with an ever-expanding human population there is pressure to produce more food intensively. Main features include:

  • Efficient energy conversion – by restricting wasteful loss of energy, more energy id passed along the food chain to humans
  • Low cost – food such as meat, eggs and milk can be produced more cheaply than by other methods
  • Quality of food – often argued that the taste of foods produced by intensive rearing is inferior to foods produced less intensively
  • Use of Space – Intensive rearing uses less land while efficient production means that less of the countryside is required for agriculture, leaving more as natural habitats
  • Safety – smaller concentrated units are easier to control or regulate. The high density animal populations are more vulnerable to the spread of disease but it’s easier to prevent infections being produced from the outside and to isolate the animals if this happens
  • Disease – large numbers of animals living in close proximity means that infections can spread easily among them. To control this antibiotics are used
  • Use of drugs – over-use of antibiotics to prevent disease has lead to antibiotic resistance. This can be transferred to bacteria that cause human disease, making their treatment with certain antibiotics ineffective. Other drugs may be given to animals to improve their growth or reduce aggressive behaviour and these can alter the taste of the food or pass into the foods and then into humans, affecting their health
  • Animal welfare – the larger intensive farms have the resources to maintain a high level of animal welfare and are more easily regulated. However, animals are kept unnaturally and this may cause stress resulting in aggressive behaviour. This may cause them to harm themselves or others, which is why battery chickens are de-beaked. Restricted movement leads to osteoporosis and joint pain. The well-being of the animal is sacrificed for financial gain
  • Pollution – Intensively reared animals produce large concentrations of waste in a small area. Rivers and ground water may become polluted. Pollutant gases may be dangerous or smell D:. Large intensive farms may have their own disposal facilities that enable them to treat waste more effectively than smaller non-intensive farms
  • Reduced genetic diversity – selective breeding is used to develop animals with high energy-conversion rates and a tolerance of confined conditions. This reduces the genetic diversity of domestic animals, resulting in the loss of genes that might later prove to be beneficial
  • Use of fossil fuels – High energy-conversion rates are possible because fossil fuels are used to heat the buildings that house the animals, in the production of the materials in the buildings (especially cement) and in the production and transportation of animal feeds. The CO2 emitted increases global warming.

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The Carbon Cycle

Summary Questions p.90

  • The carbon dioxide concentration of the atmosphere is less on a summers’ day than on a winters’ day because more plants are photosynthesising in summer. This is because light is required for photosynthesis, and more light is available in summer, therefore the light dependent reaction can take place, providing ATP which transitions to the light independent reaction. In the Calvin Cycle, ribulose bisphosphate combines with atmospheric CO2 to create a 6-carbon compound. This is why more CO2 is taken from the atmosphere in summer.
  • A – Combustion

B – Respiration

C – Feeding

D – Photosynthesis

  • D best represents the carbon cycle.
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Nutrient Cycle

Nutrient Cycles

All nutrient cycles have once sequence:

  • Nutrient is taken up by producers as simple, inorganic molecules.
  • Producer incorporates nutrient into complex organic molecules.
  • When the producer is eaten, the nutrient passes into consumers.
  • Nutrient then passes onto secondary, tertiary or quaternary consumers.
  • When the producers and consumers die, their complex molecules are broken down by decomposers that release the nutrient in its original simple form.

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  1. Glycolysis – making pyruvate from glucose
  2. Link reaction – converting pyruvate to acetyl coenzyme A
  3. Krebs cycle – producing reduced coenzymes and ATP
  4. Oxidative phosphorylation – producing a large amount of ATP



  • Splitting one glucose (6C) →2 pyruvates (3C)
  • Doesn’t need oxygen to take place – is anaerobic.
  • In cytoplasm of cells.
  • Stage 1 Phosphorylation: Glucose phosphorylated – 2 phosphates added from 2ATP. This creates 2 molecules of triose phosphate and 2ADP
  • Stage 2 Oxidation: Triose phosphate oxidised – loses hydrogen, forming 2 molecules of pyruvate. NAD collects H ions, forming 2reducedNAD. 4ATP produced, but 2 used in phosphorylation, so net gain of 2ATP.
  • The two molecules of reduced NAD go to oxidative phosphorylation.
  • The 2 pyruvate molecules go into the matrix of the mitochondria for link reaction



  • Decarboxylases (removeCO2) pyruvate.
  • Reduces NAD – collects hydrogen from pyruvate, changing pyruvate to acetate.
  • Combines acetate with coenzyme A to form acetyl coenzyme A.
  • No ATP produced
  • In mitochondria
  • Occurs twice for every glucose molecule -2 pyruvate made for every glucose that enters glycolysis. Means link r.&Krebs cycle happen 2x for every glucose
  • For each glucose: 1)Two molecules of acetyl coenzyme A go into Krebs cycle. 2)Two CO2 released 3)Two reduced NAD are formed and go to oxididative phosphorylation



  • Series of oxidation-reduction reactions
  • In the matrix of the mitochondria
  • Happens once for every pyruvate molecule(2x for every glucose)
  1. Acetyl CoA from link reaction combines with oxaloacetate to form citrate. Coenzyme A goes back to link reaction to be used again.
  2. 6C citrate molecule is converted to 5C molecule. When this happens decarboxylation and dehydrogenation occur. The hydrogen is used to produce reduced NAD from NAD.
  3. 5C molecule is then converted into a 4C molecule. Decarboxylation and dehydrogenation occur between intermediate compounds, producing one reduced FAD and two reduced NAD.
  • ATP is produced by the direct transfer of a phosphate group from an intermediate compound to ADP (substrate-level phosphorylation). Citrate has now been converted to oxaloacetate.
  • From Krebs Cycle: 1CoA reused in link reaction,
  • oxaloacetate regenerated for use in Krebs Cycle,
  • 2CO2 released as waste product,
  • 1ATP used for energy,
  • 3reducedNAD and 1reduced FAD used in oxidative phosphorylation



  • Where energy carried by electrons from reduced coenzymes (reduced NAD & reduced FAD) is used to make ATP.
  • Involves two processes – the electron transport chain and chemiosmosis.
  • Hydrogen atoms are released from reduced NAD and reduced FAD as they are oxidised to FAD and NAD. H atoms split into protons (H+) and e-‘s.
  • Electrons move along electron transport chain, losing energy at each carrier.
  • This energy is used by e- carriers to pump protons from mitochondrial matrix to the intermembrane space.
  • Concentration of protons now higher in intermembrane space than in mitochondrial matrix – electrochemical gradient formed (conc. Gradient of ions).
  • Protons move ↓electrochemical gradient, back into mitochondrial matrix, via ATP synthase. This movement drives the synthesis of ATP from ADP and Pi.
  • Chemiosmosis – Movement of H+ ions across a membrane, generating ATP.
  • In the mitochondrial matrix, protons, electrons and O2 (from the blood) combine to form water. O2 said to be final electron acceptor.


32 ATP can be made in total from one glucose molecule with aerobic respiration.

  • In oxidative phosphorylation, ATP made from reduced coenzymes. 2.5ATP made from each reduced NAD and 1.5ATP made from each reduced FAD.
  • Link reaction and Krebs cycle happen 2x for each glucose (2 pyruvate produced)
  • So 8×2.5 (2reducedNAD in link reaction, 6 in Krebs cycle) + 2×1.5 (2reducedFAD in Krebs Cycle) + 2ATP (made in Krebs Cycle) = 25ATP from link reaction & Krebs Cycle alone.
  • + 2ATP and 2×2.5ATP (from reduced NAD) produced in glycolysis = total of 32 ATP.


  Anaerobic respiration doesn’t involve the link reaction, Krebs cycle or oxidative phosphorylation. Products of glycolysis are converted to ethanol(plant, yeast) or lactate(animal).

The production of lactate regenerates NAD, so glycolysis can continue in the absence of oxygen. A small amount of ATP can still be produced.


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Production of ATP in Respiration

Summary of respiration to see how much ATP is made from each glucose molecule. ATP is made in two different ways:

  • Some ATP molecules are made directly by the enzymes in glycolysis or the Krebs cycle. This is called substrate level phosphorylation (since ADP is being phosphorylated to form ATP).
  • Most of the ATP molecules are made by the ATP synthase enzyme in the respiratory chain. Since this requires oxygen it is called oxidative phosphorylation. Scientists don’t yet know exactly how many protons are pumped in the respiratory chain, but the current estimates are: 10 protons pumped by NADH; 6 by FADH; and 4 protons needed by ATP synthase to make one ATP molecule. This means that each NADH can make 2.5 ATPs (10/4) and each FADH can make 1.5 ATPs (6/4).

Two ATP molecules are used at the start of glycolysis to phosphorylate the glucose, and these must be subtracted from the total.

The table below is an “ATP account” for aerobic respiration, and shows that 32 molecules of ATP are made for each molecule of glucose used in aerobic respiration. This is the maximum possible yield; often less ATP is made, depending on the circumstances. Anaerobic respiration only produces the 2 molecules of ATP from the first two rows.

Other substances can also be used to make ATP. Glycogen of course is the main source of glucose in humans.

Triglycerides are broken down to fatty acids and glycerol, both of which enter the Krebs Cycle. A typical triglyceride molecule might make 50 acetyl CoA molecules, yielding 500 ATP molecules. Fats are thus a very good energy store, yielding 2.5 times as much ATP per g dry mass as carbohydrates. Proteins are not normally used to make ATP, but in starvation they can be broken down and used in respiration.

They are first broken down to amino acids, which are converted into pyruvate and Krebs Cycle metabolites and then used to make ATP.

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Anaerobic Resipiration

If there is no oxygen (anaerobic conditions) then the final reaction to make water cannot take place. Electrons can’t leave the respiratory chain, and so NADH cannot unload any hydrogens to the respiratory chain.

This means that there is no NAD in the cell; it’s all in the form of NADH. Without NAD as a coenzyme, some of the enzymes of the Krebs cycle and glycolysis cannot work, so the whole of respiration stops.

For glycolysis to continue its products of pyruvate and hydrogen must be constantly removed. In particular, the hydrogen must be released from the reduced NAD in order regenerate NAD.

Without this, the already tiny supply of NAD in cells will be entirely converted to reduced NAD, leaving no NAD to take up the hydrogen newly produced from glycolysis. Glycolysis will then stop.

The replenishment of NAD is achieved by the pyruvate molecule from glycolysis accepting the hydrogen from reduced NAD.

Some cells can get round this problem using anaerobic respiration. This adds an extra step to the end of glycolysis that regenerates NAD, so allowing glycolysis to continue and some ATP to be made. Anaerobic respiration only makes 2 ATPs per glucose.

There are two different kinds of anaerobic respiration:


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Electron Transport Chain

Hydrogen atoms and the electrons they possess are a valuable source of energy. These hydrogen atoms are carried by coenzymes NAD and FAD into the next stage of the process: the electron transport chain.
This is the mechanism by which the energy of the electrons within the hydrogen atoms is converted into a form that cells can use – ATP.

The Electron Transport Chain and Mitochondria

Mitochondria are rod-shaped organelles that are found in eukaryotic cells. Each mitochondria is bounded by a smooth outer membrane and an inner one that is folded into extensions called cristae.

The inner space, or matrix, of the mitochondrion is made up of a semi-rigid material of protein, lipids and traces of DNA.

Mitochondria play an active role in respiration and the release of energy.

The Electron Transport Chain and the Synthesis of ATP

1. The hydrogen atoms produced during glycolysis and the Krebs cycle combine with the coenzymes NAD and FAD that are attached to the cristae of the mitochondria

2. The reduced NAD and FAD donate the electrons of the hydrogen atoms they are carrying to the first molecule in the electron transport chain

3. This releases the protons from the hydrogen atoms and these protons are actively transported across the inner mitochondrial membrane

4. The electrons pass along a chain of electron transport carrier molecules in a series of oxidation-reduction reactions

The electrons lose energy as they pass down the chain and some of this is used to combine ADP and inorganic phosphate to make ATP
The remaining energy is lost in the form of heat

5. The protons accumulate in the space between the two mitochondrial membranes before they diffuse back into the mitochondrial matrix through special protein channels Energy is used to pump the protons across

6. At the end of the chain the electrons combine with these protons and oxygen to form water Oxygen is therefore the final electron transport chain

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Krebs Cycle

The Krebs cycle involves a series of oxidation-reduction reactions that take place in the matrix of the mitochondria.

1. A 2 carbon acetylcoenzyme A from the link reaction combines with a 4 carbon molecule to produce a 6 carbon molecule

2. This 6 carbon molecule loses carbon dioxide and hydrogen to give a 4 carbon molecule and a single molecule of ATP produced as a result of substrate-level phosphorylation

3. The 4 carbon molecule can now combine with a new molecule of acetylcoenzyme A to begin the cycle again

Coenzymes are molecules that some enzymes require in order to function. Coenzymes play a major role in photosynthesis and respiration where they carry hydrogen atoms from one molecule to another.

NAD important in respiration FAD important in the  Krebs cycle NADP important in photosynthesis

The Significance of the Krebs Cycle

  • It breaks down macromolecules into smaller ones; pyruvate is broken down into carbon dioxide
  • It produces hydrogen atoms that are carried by NAD to the electron transport chain for oxidative phosphorylation This leads to the production of ATP that provides metabolic energy for the cell
  • It regenerates the 4 carbon molecule that combines with acetyl coenzyme A which would otherwise accumulate
  • It is a source of intermediate compounds used by cells in the manufacture of other important substances such as fatty acids, amino acids and chlorophyll


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Link Reaction

The pyruvate molecules produced during glycolysis possess potential energy that can only be released using oxygen in a process called the Krebs cycle.
Before they enter the Krebs cycle, these pyruvate molecules must first be oxidised in a procedure known as the link reaction take place exclusively inside mitochondria.

The Link Reaction

The pyruvate molecules produced in the cytoplasm during glycolysis are actively transported into the matrix of mitochondria.

Here, pyruvate undergoes a series of reactions during which the following changes take place:

1. The pyruvate is oxidised by removing hydrogen This hydrogen is accepted by NAD to form reduced NAD which is later to produce ATP

2. The 2 carbon molecule called an acetyl group, that is thereby formed combines with a molecule called coenzyme A to produce a compound called acetyl coenzyme A

3. A carbon dioxide molecule is formed from each pyruvatev


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