Energy transfers in and between organisms
ATP, glycolysis and anaerobic respiration
ATP (adenine + ribose + 3 phosphates) is the immediate energy source: hydrolysis to ADP + Pᵢ by ATP hydrolase releases a small, usable amount of energy; it is resynthesised by ATP synthase. Glycolysis (cytoplasm): glucose is phosphorylated (2 ATP used), split into 2 triose phosphate, then oxidised to 2 pyruvate, producing 4 ATP (net 2) and 2 reduced NAD. Without oxygen, NAD must be regenerated: in mammals pyruvate → lactate (lactate dehydrogenase); in yeast/plants pyruvate → ethanol + CO₂. Anaerobic respiration yields only 2 ATP per glucose. Respiratory quotient RQ = CO₂ produced ÷ O₂ used: carbohydrate 1.0, protein ~0.9, lipid ~0.7. Lipids release the most energy per gram (more H atoms).
≈ 50 minLight-dependent reactions
Chloroplasts have thylakoids (stacked into grana) where the light-dependent reactions occur, surrounded by stroma (light-independent). Photosystems contain chlorophyll a and b and accessory pigments (carotenoids) that absorb different wavelengths; the absorption spectrum matches the action spectrum. Non-cyclic photophosphorylation: light excites electrons in chlorophyll (photoionisation) of PSII; they pass along an electron transport chain to PSI, then to NADP, forming reduced NADP; energy pumps H⁺ into the thylakoid space and ATP synthase makes ATP by chemiosmosis. Photolysis of water replaces PSII's electrons: . Cyclic photophosphorylation (PSI only) makes ATP but no reduced NADP or O₂. Pigments are separated by chromatography ().
≈ 50 minKrebs cycle and oxidative phosphorylation
Link reaction (matrix): pyruvate (3C) is decarboxylated (CO₂) and dehydrogenated (reduced NAD), forming acetyl (2C) which joins coenzyme A. Krebs cycle (matrix): acetyl CoA + oxaloacetate (4C) → citrate (6C); citrate is decarboxylated twice and dehydrogenated four times back to oxaloacetate. Per turn: 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP (substrate-level). Two turns per glucose. Oxidative phosphorylation (inner membrane, cristae): reduced NAD/FAD release electrons to the electron transport chain; energy pumps H⁺ into the intermembrane space; H⁺ flow back through ATP synthase (chemiosmosis) makes ATP. Oxygen is the final electron acceptor, forming water. Total ≈ 32 ATP per glucose. Redox indicators (DCPIP, methylene blue) accept electrons and lose colour.
≈ 55 minCalvin cycle and limiting factors
The Calvin cycle (stroma) uses ATP and reduced NADP from the light-dependent stage. Fixation: RuBisCO combines CO₂ with ribulose bisphosphate (RuBP, 5C) to form two glycerate 3-phosphate (GP, 3C). Reduction: GP → triose phosphate (TP) using ATP and reduced NADP. Regeneration: most TP (5 of every 6) is used to remake RuBP using ATP. TP is used to make glucose, sucrose, starch, cellulose, lipids and amino acids. Six turns fix six CO₂ to make one hexose. Limiting factors: light intensity, CO₂ concentration and temperature; the factor in shortest supply limits the rate. In the dark, GP accumulates and RuBP falls; with low CO₂, RuBP accumulates and GP falls. C4 plants (maize) fix CO₂ in mesophyll cells and release it to bundle-sheath cells, keeping RuBisCO away from O₂.
≈ 50 minEcosystems, energy and nutrient cycles
Producers capture light energy: gross primary production (GPP) is the total chemical energy fixed; net primary production (respiratory losses) is available to consumers. For consumers, (ingested minus faeces/urine and respiration). Only about 10% passes between trophic levels: energy is lost as heat in respiration, in uneaten parts and in faeces. Farming increases efficiency by reducing respiratory losses (warm, restricted animals) and removing competitors and pests. Nitrogen cycle: nitrogen fixation (Rhizobium in root nodules; lightning), ammonification (saprobionts), nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻, aerobic bacteria), denitrification (anaerobic bacteria → N₂). Phosphorus cycles through rocks, soil and organisms; mycorrhizae aid uptake. Fertilisers that leach cause eutrophication: algal bloom → light blocked → plants die → decomposers use O₂ → fish die. Succession: pioneer species → climax community. Rising CO₂ and methane drive climate change.
≈ 50 min