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3.5 Energy transfers in and between organisms

5 dot points across 5 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

Why does so little of the Sun's energy reach a top predator?

The transfer of biomass and energy through trophic levels in food chains and food webs; producers, primary, secondary and tertiary consumers, decomposers and saprobionts; the reasons why biomass and energy decrease at successive trophic levels; the calculation of the efficiency of energy transfer between trophic levels.
A focused answer to the AQA 3.5 dot point on energy and ecosystems. Explains trophic levels and food webs, why biomass and energy fall between levels, the role of decomposers, and how to calculate the percentage efficiency of energy transfer.
11 min answer →

How are nitrogen and phosphorus recycled through an ecosystem?

The nitrogen cycle and the roles of saprobionts, nitrogen-fixing, nitrifying and denitrifying bacteria; the phosphorus cycle and the role of mycorrhizae in phosphorus uptake; the role of microorganisms in recycling nutrients; the use of natural and artificial fertilisers and the environmental consequences of using nitrogen-containing and phosphorus-containing fertilisers, including leaching and eutrophication.
A focused answer to the AQA 3.5 dot point on nutrient cycles. Sets out the nitrogen cycle and its four bacterial groups, the phosphorus cycle and mycorrhizae, the role of microorganisms in recycling, and how fertilisers cause leaching and eutrophication.
12 min answer →

How do plants capture light energy and use it to build organic molecules?

Photosynthesis as a two-stage process: the light-dependent reactions in the thylakoid membranes (photoionisation of chlorophyll, photolysis of water, the production of ATP by photophosphorylation, the production of reduced NADP, and the role of the electron transport chain); the light-independent reactions in the stroma (the Calvin cycle: fixation of carbon dioxide by RuBP to form GP, reduction of GP to TP using reduced NADP and ATP, and regeneration of RuBP); the effect of light intensity, carbon dioxide concentration and temperature as limiting factors.
A focused answer to the AQA 3.5 dot point on photosynthesis. Covers the light-dependent reactions (photoionisation, photolysis, the electron transport chain and photophosphorylation), the Calvin cycle in the stroma, and how light, carbon dioxide and temperature act as limiting factors.
13 min answer →

How do we measure how much new biomass an ecosystem produces?

Biomass as the mass of living material, measured as dry mass or as the chemical energy stored in dry biomass using calorimetry; gross primary production (GPP) as the chemical energy store in plant biomass; net primary production (NPP) as GPP minus respiratory losses; the calculation and units of GPP, NPP and net production of consumers; the ways in which farming practices increase the efficiency of energy transfer in food production.
A focused answer to the AQA 3.5 dot point on productivity and biomass. Defines biomass and dry mass, explains how calorimetry measures energy content, sets out GPP, NPP and net production of consumers with their units, and reviews farming practices that raise energy-transfer efficiency.
11 min answer →

How do cells release the energy stored in glucose to make ATP?

Aerobic respiration as four stages: glycolysis in the cytoplasm (phosphorylation of glucose, oxidation to pyruvate, net yield of ATP and reduced NAD); the link reaction and the Krebs cycle in the mitochondrial matrix (decarboxylation, dehydrogenation, production of reduced NAD, reduced FAD, ATP and carbon dioxide); oxidative phosphorylation on the inner mitochondrial membrane (the electron transport chain, chemiosmosis, ATP synthase and the role of oxygen as the final electron acceptor); anaerobic respiration in animals (lactate) and in microorganisms and plants (ethanol).
A focused answer to the AQA 3.5 dot point on respiration. Covers glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation by chemiosmosis, the role of oxygen as the final electron acceptor, and anaerobic respiration producing lactate or ethanol.
13 min answer →