How is section 3.5 weighted and where does it appear in the exams?

Section 3.5 is one of the largest second-year topics and is examined across Paper 1 (sections 3.1 to 3.4 plus 3.5 to 3.8 depending on the series) and Paper 3. Expect synoptic questions that combine photosynthesis with respiration (both use chemiosmosis), and ecology questions that combine energy transfer with the maths skills (percentage efficiency, NPP calculations). Several marks each year are pure calculation, so practise the GPP, NPP and efficiency equations until they are automatic.

What is the single biggest source of lost marks in this topic?

Muddling the two phosphorylation pathways and the two electron transport chains. Photophosphorylation happens in the chloroplast thylakoid using light; oxidative phosphorylation happens in the mitochondrial inner membrane using respiratory substrates. Both use an electron transport chain, a proton gradient and ATP synthase (chemiosmosis), but the energy source and the final products differ. Always state the organelle and the energy source.

Do I need to learn the exact ATP yields?

Know that glycolysis gives a net 2 ATP, that the Krebs cycle gives a small amount of ATP directly by substrate-level phosphorylation, and that the great majority of ATP comes from oxidative phosphorylation. The often-quoted total is about 38 ATP per glucose (30 to 32 in practice). AQA rewards understanding which stage makes most ATP and why, more than a memorised number.

How do I tell nitrifying and denitrifying bacteria apart?

Nitrifying bacteria build nitrate up (ammonium to nitrite to nitrate) and need oxygen (aerobic). Denitrifying bacteria break nitrate down to nitrogen gas and work in anaerobic, waterlogged soil. A simple memory hook: deNITRIFYING removes nitrate from the soil; good drainage favours the nitrifiers that crops depend on.

What are the must-know calculations?

Percentage efficiency of energy transfer (energy in a level divided by energy in the level below, times 100); net primary production (NPP = GPP minus respiration); and net production of a consumer (N = I minus F minus R). Always carry the units kJ per m squared per year through the working.

Why is energy flow one-way but nutrient flow cyclical?

Energy enters as light, is transferred between organisms, and is progressively lost as heat from respiration, so it cannot be reused and must be constantly replaced by the Sun. Nutrients such as nitrogen and phosphorus are chemical elements that change form but are not destroyed, so microorganisms can recycle them between the abiotic environment and living organisms indefinitely.

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AQA A-Level Biology 3.5 Energy transfers in and between organisms: full deep dive

A deep-dive guide to AQA A-Level Biology section 3.5. Connects photosynthesis, respiration, energy flow through ecosystems, productivity (GPP and NPP), and the nitrogen and phosphorus cycles, with the exact mark-scheme language and calculation skills AQA repeats.

Generated by Claude Opus 4.822 min readAQA-7402-3.5Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What section 3.5 actually demands
Photosynthesis: capturing energy in two stages
Respiration: releasing energy in four stages
Energy and ecosystems: why so little reaches the top
Productivity and biomass: measuring what an ecosystem makes
Nutrient cycles: recycling the elements
How section 3.5 is examined
Check your knowledge

What section 3.5 actually demands

Section 3.5 is where AQA pulls together the biochemistry of the first half of the course and the ecology of the second. It rewards two distinct skills. The first is precise biochemical recall: the named intermediates of the Calvin cycle and the Krebs cycle, the location of every stage, and the language of chemiosmosis. The second is quantitative ecology: calculating efficiencies, NPP and consumer production, and interpreting limiting-factor graphs. Strong candidates train both, then learn to spot the synoptic links the examiners love, especially the shared mechanism of chemiosmosis in photosynthesis and respiration.

The topic divides cleanly into five clusters, which map onto the five dot points in this section: photosynthesis, respiration, energy and ecosystems, productivity and biomass, and nutrient cycles. This guide takes each in turn and then draws the threads together.

Photosynthesis: capturing energy in two stages

Photosynthesis converts light energy into chemical energy. It runs in two linked stages inside the chloroplast.

The light-dependent reactions occur on the thylakoid membranes. Light photoionises chlorophyll, ejecting high-energy electrons that pass along an electron transport chain. The energy released pumps protons into the thylakoid space, and as they flow back through ATP synthase they drive photophosphorylation (the synthesis of ATP). Meanwhile photolysis splits water into protons, electrons and oxygen: the electrons replace those lost by chlorophyll, and the oxygen is released as waste. At the end of the chain, NADP is reduced. The two products that matter for the next stage are ATP and reduced NADP.

The light-independent reactions (the Calvin cycle) occur in the stroma. Carbon dioxide is fixed onto the 5-carbon RuBP by the enzyme rubisco to make two 3-carbon GP molecules. GP is reduced to triose phosphate (TP) using hydrogen from reduced NADP and energy from ATP. Most TP is then used, with more ATP, to regenerate RuBP; the rest is used to build glucose and other organic molecules. Six turns of the cycle, fixing six carbon dioxide molecules, are needed for one glucose.

The rate of photosynthesis is set by limiting factors: light intensity, carbon dioxide concentration and temperature. The classic exam task is to read a graph and identify which factor is limiting where the curve plateaus, then explain how a greenhouse grower would remove that limit.

Respiration: releasing energy in four stages

Respiration releases energy from glucose to make ATP, in four stages.

Glycolysis in the cytoplasm phosphorylates glucose using 2 ATP, then splits and oxidises it to two pyruvate, yielding a net 2 ATP and 2 reduced NAD. It needs no oxygen.

The link reaction in the mitochondrial matrix decarboxylates and dehydrogenates each pyruvate to form acetyl coenzyme A, releasing carbon dioxide and reducing NAD.

The Krebs cycle, also in the matrix, processes the acetyl group through decarboxylation and dehydrogenation steps, producing per turn 2 CO2, 3 reduced NAD, 1 reduced FAD and 1 ATP. It turns twice per glucose.

Oxidative phosphorylation on the inner mitochondrial membrane is where most ATP is made. Reduced NAD and reduced FAD are oxidised, donating electrons to the electron transport chain. The energy released pumps protons into the intermembrane space, and they flow back through ATP synthase to drive ATP synthesis by chemiosmosis. Oxygen is the final electron acceptor, forming water; without it the chain stops.

When oxygen is absent, only glycolysis runs, and the cell uses fermentation to regenerate NAD: lactate in animals, ethanol and carbon dioxide in plants and microorganisms. The yield is only the net 2 ATP from glycolysis.

Energy and ecosystems: why so little reaches the top

Energy enters ecosystems as light, is fixed by producers, and flows through primary, secondary and tertiary consumers, with decomposers (saprobionts) breaking down dead material.

At each transfer, most energy is lost: not all of an organism is eaten, not all that is eaten is digested (faeces), energy is released as heat from respiration, and some is lost in excretion. Only roughly 10 percent of the energy at one level reaches the next, which is why food chains rarely exceed four or five levels and why pyramids of energy taper sharply.

The key skill is calculating percentage efficiency: the energy in a trophic level divided by the energy in the level below, times 100. Always work from the data given rather than assuming exactly 10 percent.

Productivity and biomass: measuring what an ecosystem makes

Biomass is the mass of living material, measured as dry mass (heated to constant mass, because water content varies and stores no usable energy) or as energy content using calorimetry (burning a known dry mass and measuring the heat released).

Gross primary production (GPP) is the total energy plants fix by photosynthesis. Net primary production (NPP) is what remains after the plants respire: NPP = GPP minus respiration. NPP is the energy available to consumers. For a consumer, net production N = I minus (F plus R), where I is ingested energy, F is energy lost in faeces and urine, and R is respiratory loss. All carry units of energy per area per time, usually kJ per m squared per year.

Farmers raise the efficiency of energy transfer to humans using fertilisers and pesticides to boost plant productivity, and by restricting animal movement and keeping animals warm to cut respiratory losses so more energy is stored as biomass. These methods raise yield but carry welfare and environmental costs.

Nutrient cycles: recycling the elements

Energy is lost as heat and must be replaced by the Sun, but nutrients are recycled by microorganisms.

In the nitrogen cycle, four bacterial groups do the work. Nitrogen-fixing bacteria (such as Rhizobium) convert nitrogen gas to ammonia. Saprobionts decompose dead matter to ammonium (ammonification). Nitrifying bacteria oxidise ammonium to nitrite then nitrate (aerobic), the form plants absorb. Denitrifying bacteria convert nitrate back to nitrogen gas (anaerobic), which is why waterlogged soils lose fertility.

The phosphorus cycle has no gaseous phase. Phosphate is released by weathering of rock, absorbed by plants, passed along food chains, and returned by saprobionts; over geological time it forms sedimentary rock. Mycorrhizae, mutualistic fungus-root associations, greatly increase the surface area for absorbing phosphate, especially in poor soils.

Fertilisers replace nitrogen and phosphorus removed at harvest. Overuse leads to leaching and eutrophication: leached nitrate and phosphate trigger an algal bloom that blocks light; submerged plants die; saprobiotic bacteria decompose the dead material and respire, depleting dissolved oxygen; and aerobic organisms such as fish suffocate.

How section 3.5 is examined

A typical exam profile for this section:

Multiple choice and short answer. Locating stages of respiration or photosynthesis, identifying the limiting factor on a graph, naming the bacteria in the nitrogen cycle.
Calculations. Percentage efficiency, NPP from GPP and respiration, net production of a consumer. These are reliable, learnable marks.
Extended prose (5 to 6 marks). The eutrophication sequence, the role of the electron transport chain in oxidative phosphorylation, or how the products of the light-dependent reaction are used in the Calvin cycle.
Synoptic links. Comparing chemiosmosis in chloroplasts and mitochondria; linking fertiliser use (productivity) to eutrophication (nutrient cycles).

Check your knowledge

A mix of recall, calculation and exam-style questions covering this section. Answer under timed conditions, then check against the solutions block.

State where in a chloroplast each stage of photosynthesis occurs and name the two products of the light-dependent reaction used in the Calvin cycle. (3 marks)
Explain why the concentration of GP rises and the concentration of RuBP falls when a photosynthesising plant is suddenly placed in darkness. (3 marks)
Describe the role of oxygen in aerobic respiration and explain the consequence of its absence for ATP production. (4 marks)
A producer trophic level contains 18 000 kJ per m squared per year. The primary consumers contain 1440 kJ and the secondary consumers contain 130 kJ. (a) Calculate the percentage efficiency of energy transfer at each step. (b) Suggest one biological reason why the values are below 100 percent. (4 marks)
A crop has a GPP of 28 000 kJ per m squared per year and loses 10 500 kJ in respiration. (a) Calculate the NPP. (b) Cattle grazing it ingest 1900 kJ, losing 1300 kJ in faeces and urine and 480 kJ in respiration; calculate the net production of the cattle. (4 marks)
Name the four groups of bacteria in the nitrogen cycle and state the conversion each carries out and whether it needs oxygen. (4 marks)
Explain why energy must be continually supplied to an ecosystem whereas nitrogen and phosphorus are not. (3 marks)
Describe how the overuse of phosphorus-containing fertiliser on farmland can lead to the death of fish in a downstream lake. (5 marks)

Solutions

Q1: The light-dependent reactions occur on the thylakoid membranes; the light-independent reactions (Calvin cycle) occur in the stroma. The two products of the light-dependent reaction used in the Calvin cycle are ATP (which provides energy for the reduction of GP and the regeneration of RuBP) and reduced NADP (which provides hydrogen to reduce GP to TP).
Q2: In darkness the light-dependent reaction stops, so no more ATP or reduced NADP is made. GP is still produced by the fixation of carbon dioxide onto RuBP, but it cannot be reduced to TP because that step requires ATP and reduced NADP, so GP accumulates and its concentration rises. RuBP concentration falls because it is still being used in fixation but cannot be regenerated, since regeneration requires ATP.
Q3: Oxygen is the final electron acceptor at the end of the electron transport chain in oxidative phosphorylation; it accepts electrons and protons to form water. This keeps the chain running, allowing protons to be pumped and ATP to be made by chemiosmosis through ATP synthase. Without oxygen the chain backs up, reduced NAD and reduced FAD cannot be reoxidised, oxidative phosphorylation stops, and the cell makes only the net 2 ATP from glycolysis via anaerobic respiration.
Q4: (a) Producer to primary consumer: (1440 / 18000) times 100 = 8 percent. Primary to secondary consumer: (130 / 1440) times 100 = 9.03 percent (about 9 percent). (b) Energy is lost between levels through respiration (heat), egestion of indigestible material in faeces, and excretion of nitrogenous waste, so less than 100 percent is passed on.
Q5: (a) NPP = GPP minus respiration = 28000 minus 10500 = 17 500 kJ per m squared per year. (b) Net production of cattle N = I minus (F plus R) = 1900 minus (1300 plus 480) = 120 kJ per m squared per year.
Q6: Nitrogen-fixing bacteria convert nitrogen gas to ammonia (ammonium compounds); they may be free-living or in legume root nodules. Saprobionts (decomposers) carry out ammonification, converting nitrogen in dead matter to ammonium ions. Nitrifying bacteria oxidise ammonium to nitrite then to nitrate and require oxygen (aerobic). Denitrifying bacteria convert nitrate back to nitrogen gas in anaerobic (waterlogged) conditions.
Q7: Energy enters as light and is transferred between organisms, but at each transfer a large proportion is lost as heat from respiration; this heat cannot be reused by organisms, so energy flows one way and must be continually replaced by the Sun. Nitrogen and phosphorus are chemical elements that are not destroyed; microorganisms convert them between different forms, so they are recycled between the environment and living organisms and do not need continual external replacement.
Q8: Excess soluble phosphate is leached from the soil into the lake. The added phosphate removes the limiting factor on algal growth, so algae multiply rapidly and form an algal bloom at the surface. The bloom blocks light, so submerged plants cannot photosynthesise and die. Saprobiotic bacteria decompose the dead plants and algae, multiplying and respiring aerobically, which depletes the dissolved oxygen in the water. With too little dissolved oxygen, fish and other aerobic organisms cannot respire sufficiently and die.