This mini-lesson covers AQA 3.5 — Energy transfers in and between organisms: photosynthesis (light-dependent reaction, photophosphorylation, chemiosmosis and the Calvin cycle); respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation, plus anaerobic pathways); and energy transfer through ecosystems — GPP, NPP, percentage efficiency, and the nitrogen and phosphorus cycles.
Work through each screen, answer the questions as you go (some are extended-recall, some are calculations) and collect ⭐ stars. This is A-level content — expect quantitative work and mechanism-level detail. Press Start when you are ready.
Photosynthesis
The light-dependent reaction
Location: the thylakoid membranes of the chloroplast. Purpose: to make ATP and reduced NADP for the Calvin cycle, and to release oxygen as a by-product.
Photoionisation: light energy is absorbed by chlorophyll, exciting a pair of electrons to a higher energy level so that they leave the molecule.
Electron transport chain: the electrons pass along a chain of carriers embedded in the thylakoid membrane. At each transfer they lose energy, and that energy is used to actively pump H⁺ from the stroma into the thylakoid space.
Chemiosmosis: a steep H⁺ gradient builds up. H⁺ diffuse back into the stroma through ATP synthase channels, and the energy released drives photophosphorylation: ADP + Pi → ATP.
Photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂. This replaces the electrons lost by chlorophyll, supplies the H⁺, and releases oxygen as waste.
At the end of the chain, H⁺ and electrons are accepted by NADP to form reduced NADP.
Why the thylakoid is a stack: grana give an enormous surface area for the pigments, electron carriers and ATP synthase, and the thylakoid space is small, so the proton gradient builds quickly.
Photosynthesis
The Calvin cycle and limiting factors
Location: the stroma. It is light-independent only in the narrow sense that light is not used directly — it stops within seconds in the dark because ATP and reduced NADP run out.
Fixation: CO₂ combines with the 5-carbon RuBP, catalysed by rubisco. The unstable 6-carbon intermediate splits immediately into two molecules of GP (3-carbon).
Reduction: GP is reduced to triose phosphate (TP), using reduced NADP (the reducing agent) and energy from ATP.
Regeneration:five out of every six TP molecules are used, with more ATP, to regenerate RuBP. Only one in six leaves the cycle to build glucose, amino acids, lipids and so on.
6 turns → 6 CO₂ fixed → 12 TP → 2 TP out → 1 hexoseper hexose: 18 ATP and 12 reduced NADP are consumed
Limiting factors: light intensity, CO₂ concentration and temperature. Interpreting a graph: if the curve plateaus and adding more light does nothing, light is not the limiting factor at that point — something else is. If light intensity suddenly falls, GP rises and TP and RuBP fall, because GP can no longer be reduced. If CO₂ falls, GP falls and RuBP rises. Learn to reason it out rather than memorising it.
Quick check
A sudden change in the light
?A photosynthesising plant is moved abruptly from bright light into darkness. What happens to the concentrations of GP and RuBP in the next few seconds?
Calculate
Your turn — turns of the cycle
1How many turns of the Calvin cycle are required to produce enough triose phosphate for one molecule of hexose sugar?
turns
Hint: One CO₂ is fixed per turn; a hexose contains 6 carbons.
Quick check
What photolysis is actually for
?Photolysis of water occurs in the light-dependent reaction. Which set of products does it supply, and what are they used for?
Respiration
Glycolysis and the link reaction
Glycolysis — in the cytoplasm, and it does not need oxygen.
Phosphorylation: glucose is phosphorylated using 2 ATP, making it more reactive, and forming hexose bisphosphate.
Lysis: it splits into two triose phosphate molecules.
Oxidation: each TP is oxidised to pyruvate, reducing 2 NAD in total and producing 4 ATP by substrate-level phosphorylation.
Net yield per glucose: 2 ATP, 2 reduced NAD, 2 pyruvate.
The link reaction — in the mitochondrial matrix (pyruvate is actively transported in):
pyruvate + NAD + CoA → acetyl CoA + reduced NAD + CO₂per glucose this happens twice: 2 CO₂ and 2 reduced NAD · no ATP is made
Pyruvate is decarboxylated (losing CO₂) and dehydrogenated (reducing NAD); the resulting 2-carbon acetyl group is carried by coenzyme A into the Krebs cycle.
Respiration
The Krebs cycle and oxidative phosphorylation
Krebs cycle — mitochondrial matrix. Acetyl CoA (2C) combines with a 4-carbon molecule to form a 6-carbon molecule. A series of oxidation and decarboxylation steps regenerates the 4-carbon acceptor.
per turn: 2 CO₂ · 3 reduced NAD · 1 reduced FAD · 1 ATPtwo turns per glucose — so 6 reduced NAD, 2 reduced FAD, 2 ATP, 4 CO₂
Oxidative phosphorylation — the inner membrane (cristae). This is where almost all of the ATP is actually made.
Reduced NAD and reduced FAD are oxidised, releasing their electrons (and H⁺) to the electron transport chain.
Electrons pass from carrier to carrier, losing energy at each transfer. That energy actively pumps H⁺ from the matrix into the intermembrane space, creating an electrochemical gradient.
Chemiosmosis: H⁺ diffuse back into the matrix through ATP synthase, and the energy released drives ADP + Pi → ATP.
Oxygen is the final electron acceptor. It combines with the electrons and H⁺ to form water. Without oxygen the chain backs up: reduced NAD cannot be re-oxidised, so the Krebs cycle and link reaction stop.
Anaerobic respiration exists purely to regenerate NAD so that glycolysis (and its net 2 ATP) can continue. In animals, pyruvate + reduced NAD → lactate + NAD. In yeast and plants, pyruvate is decarboxylated to ethanal, then reduced to ethanol + CO₂. Lactate can later be oxidised back to pyruvate in the liver; ethanol production is irreversible.
Respiratory substrates: lipids yield more energy per gram than carbohydrates because they contain more hydrogen atoms per unit mass, so more reduced NAD/FAD is generated per gram. Proteins are deaminated first and enter as pyruvate or Krebs intermediates.
Quick check
Cyanide and the chain
?Cyanide binds irreversibly to the final carrier in the electron transport chain. Why does this stop the Krebs cycle, which is not directly inhibited?
Calculate
Your turn — counting the coenzymes
2One turn of the Krebs cycle produces 3 molecules of reduced NAD. How many molecules of reduced NAD are produced by the Krebs cycle from one molecule of glucose?
reduced NAD
Hint: One glucose gives 2 pyruvate, so 2 acetyl CoA, so the cycle turns twice.
Energy & ecosystems
GPP, NPP and percentage efficiency
Only about 1–3% of the light energy falling on a plant is converted into organic matter. Most is reflected, passes straight through the leaf, is of the wrong wavelength, or is lost as heat.
NPP = GPP − Rnet primary production = gross primary production − respiratory losses
N = I − (F + R)net production of a consumer = chemical energy of ingested food − (energy lost in faeces and urine + respiration)
Energy transfer between trophic levels is inefficient (typically 10%, and only about 10–20% even between consumers). Energy is lost because: not all of the organism is eaten or digestible; some is excreted or egested; and a great deal is used in respiration and lost as heat — especially in endotherms, which spend energy on maintaining body temperature. This is why food chains rarely exceed four or five trophic levels.
% efficiency = (energy at this trophic level ÷ energy at the previous level) × 100
Farming is applied thermodynamics: simplifying food webs (removing pests and competitors) and restricting animal movement or keeping livestock warm reduces respiratory loss, so more energy goes into biomass — increasing net productivity, at an ethical cost.
Calculate
Your turn — net primary production
3A field of wheat has a gross primary production of 32 000 kJ m⁻² yr⁻¹ and loses 12 500 kJ m⁻² yr⁻¹ in respiration. Calculate the net primary production.
kJ m⁻² yr⁻¹
Hint: NPP = GPP − R = 32 000 − 12 500.
Calculate
Your turn — net production of a consumer
4A cow ingests 5000 kJ of chemical energy. It loses 3200 kJ in faeces and urine and 1000 kJ in respiration. Calculate its net production, N.
kJ
Hint: N = I − (F + R) = 5000 − (3200 + 1000).
Calculate
Your turn — percentage efficiency
5Producers in a grassland have a net production of 87 500 kJ m⁻² yr⁻¹. The primary consumers have a net production of 7000 kJ m⁻² yr⁻¹. Calculate the percentage efficiency of energy transfer.
%
Hint: (7000 ÷ 87 500) × 100.
Nutrient cycles
Nitrogen, phosphorus and fertilisers
Energy flows through an ecosystem and is lost as heat. Nutrients cycle. Both cycles depend on saprobionts (saprobiotic microorganisms), which secrete enzymes and digest dead organic matter extracellularly.
Ammonification — saprobionts convert nitrogen-containing compounds in dead organisms, faeces and urine into ammonia/ammonium.
Nitrification — aerobic nitrifying bacteria oxidise ammonium → nitrite (Nitrosomonas) → nitrate (Nitrobacter). Nitrate is what plants absorb, by active transport.
Nitrogen fixation — Rhizobium in root nodules of legumes (a mutualistic relationship) reduces atmospheric N₂ to ammonia; free-living fixers do the same in soil.
Denitrification — anaerobic denitrifying bacteria in waterlogged soils convert nitrate back to N₂ gas. Hence ploughing and draining soil (aeration) improves fertility.
Phosphorus cycle — phosphate ions are released by weathering of rock, absorbed by plants, passed along food chains, and returned by saprobionts. Mycorrhizae — fungal associations with roots — greatly increase the surface area for absorbing water and phosphate.
Eutrophication, in order: excess nitrate leaches into a river or lake → algae multiply and form an algal bloom → the bloom blocks light → plants below die → saprobiotic bacteria multiply as they decompose them → the bacteria respire aerobically and deplete the dissolved oxygen → fish and aerobic organisms die.
Quick check
Eutrophication
?Fertiliser leaches into a lake and, weeks later, the fish die. What is the direct cause of the fish deaths?
Sort it
Which stage is it?
Tap a statement, then tap the stage it belongs to.
💡 Light-dependent
🌱 Calvin cycle
🔥 Respiration
Match it
Stage and its yield
Tap an item on the left, then its partner on the right.
Stage
What happens
Recap
The big ideas to take away
Light-dependent reaction: thylakoid membranes: photoionisation → electron transport chain → proton gradient → chemiosmosis → ATP; photolysis of water gives electrons, H⁺ and O₂; reduced NADP is made
Calvin cycle: stroma: rubisco fixes CO₂ to RuBP → 2 GP → TP (using ATP and reduced NADP) → 5/6 of TP regenerates RuBP. Six turns and 6 CO₂ per hexose
Glycolysis: cytoplasm; 2 ATP used, 4 made (net 2), 2 reduced NAD, 2 pyruvate. Anaerobic in the strict sense — no oxygen required
Link & Krebs: matrix: pyruvate → acetyl CoA + CO₂ + reduced NAD. Each Krebs turn: 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP (substrate-level)
Oxidative phosphorylation: cristae: electrons from reduced NAD/FAD pass down the chain, pumping H⁺ into the intermembrane space; H⁺ diffuse back through ATP synthase (chemiosmosis). Oxygen is the final electron acceptor, forming water
Anaerobic: pyruvate → lactate (animals) or ethanol + CO₂ (yeast) to regenerate NAD so glycolysis can continue
Ecosystems: NPP = GPP − R; net production of consumers N = I − (F + R); efficiency = (energy at this level ÷ energy at previous level) × 100