Energy Transfers in and between Organisms
Energy Transfers in and between Organisms is the A-level Biology topic covering photosynthesis, aerobic and anaerobic respiration, and the transfer of energy and nutrients through food chains, ecosystems and biogeochemical cycles. It requires linking biochemical pathways to whole-ecosystem energy budgets, and includes ATP-yield and productivity calculations that are frequently tested.
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Method
- Split photosynthesis into the light-dependent reactions (thylakoid membrane, produce ATP and reduced NADP) and the light-independent reactions (stroma, the Calvin cycle uses ATP and reduced NADP to fix carbon dioxide).
- Split respiration into its four linked stages (glycolysis, link reaction, Krebs cycle, oxidative phosphorylation) and know where each occurs (cytoplasm vs mitochondrial matrix vs inner mitochondrial membrane).
- For ATP-yield calculations, use the given ATP-per-NADH and ATP-per-FADH2 values, multiply by the number of each coenzyme produced, then add the substrate-level phosphorylation ATP.
- For ecosystem energy questions, apply the correct equation for each term (GPP - R = NPP; percentage efficiency = energy transferred / energy available x 100) and always show units.
- For nutrient cycle questions (nitrogen or carbon), name the correct type of bacteria or process at each stage (nitrogen-fixing, nitrifying, denitrifying) rather than describing the cycle vaguely.
- In required-practical questions (e.g. respirometers, algal balls), identify the independent, dependent and controlled variables before attempting any calculation.
Worked example
Per molecule of glucose, aerobic respiration produces 10 reduced NAD (NADH) and 2 reduced FAD (FADH2) in total. Oxidative phosphorylation yields 2.5 ATP per NADH and 1.5 ATP per FADH2, and a further 4 ATP are produced directly by substrate-level phosphorylation (2 in glycolysis and 2 in the Krebs cycle). Calculate the total number of ATP molecules produced by the complete aerobic respiration of 3 molecules of glucose.
- Calculate ATP from NADH for one molecule of glucose: 10 x 2.5 = 25.
- Calculate ATP from FADH2 for one molecule of glucose: 2 x 1.5 = 3.
- Add the substrate-level phosphorylation ATP: 2 (glycolysis) + 2 (Krebs cycle) = 4.
- Add all sources together for one glucose molecule: 25 + 3 + 4 = 32.
- Multiply by the number of glucose molecules respired: 32 x 3 = 96.
- Final answer: 96 ATP molecules from the complete aerobic respiration of 3 molecules of glucose.
Practice questions
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Q1Name the two molecules made during the light-dependent stage of photosynthesis that are then used by the light-independent stage (the Calvin cycle).Show answer
Answer: ATP and reduced NADP.
Q2Which stage of aerobic respiration takes place in the cell's cytoplasm rather than inside a mitochondrion?Show answer
Answer: Glycolysis.
Q3For every molecule of glucose broken down, how many ATP molecules are gained overall (net) from glycolysis?Show answer
Answer: 2 ATP (net gain, after 2 ATP are invested and 4 ATP are produced).
Q4Define the term gross primary productivity (GPP).Show answer
Answer: The total amount of energy (or biomass) fixed by producers through photosynthesis per unit area/time.
Q5In a grassland ecosystem, producers had a gross primary productivity of 21400 kJ per m^2 per year and used 13600 kJ per m^2 per year in respiration. Calculate the net primary productivity (NPP).Show answer
Answer: 7800 kJ per m^2 per year (21400 - 13600 = 7800)
Q6The primary consumers in the same grassland had a net productivity of 858 kJ per m^2 per year. Calculate the percentage of the producers' NPP (from question 5) transferred to primary consumers, to 2 significant figures.Show answer
Answer: 11% (858 / 7800 x 100 = 11.0)
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
Explain how ATP synthase uses a proton (H+) gradient across the thylakoid membrane to produce ATP during photosynthesis.
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Required practical. A student investigated the effect of light intensity on the rate of photosynthesis using algal balls in hydrogencarbonate indicator solution, timing how long the indicator took to change colour from red to purple as a measure of rate. At a distance of 15 cm from the lamp, the colour change took 30 seconds. The rate of photosynthesis is directly proportional to light intensity, and light intensity follows the inverse square law with distance from the lamp. Calculate the time predicted for the colour change to occur at a distance of 30 cm from the lamp, assuming light remains the limiting factor throughout.
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This question is about the nitrogen cycle. (a) Name the type of bacteria responsible for converting ammonium ions into nitrite ions, and the type responsible for converting nitrate ions into atmospheric nitrogen gas. (b) Explain the mutualistic relationship between nitrogen-fixing bacteria and leguminous plants growing in root nodules.
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Free printable worksheet
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