Stage-by-stage breakdown — A-Level Biology
Light energy is absorbed by chlorophyll in photosystems II and I (PSii first, then PSi). This energy excites electrons, which pass along the electron transport chain embedded in the thylakoid membrane.
Water is photolysed (split by light): 2H₂O → 4H⁺ + 4e⁻ + O₂. The H⁺ ions accumulate in the thylakoid space, creating a proton gradient. They flow back through ATP synthase (chemiosmosis), generating ATP.
NADP⁺ is reduced to NADPH at the end of PSi, using the electrons and H⁺ ions. Both ATP and NADPH move to the stroma for the next stage.
Carbon fixation: CO₂ combines with ribulose bisphosphate (RuBP, a 5C compound) using the enzyme RuBisCO, forming two molecules of glycerate-3-phosphate (GP, 3C).
Reduction: GP is reduced to glyceraldehyde-3-phosphate (GALP/G3P, 3C) using ATP and NADPH from the light-dependent reactions.
Regeneration: Most G3P is used to regenerate RuBP (using ATP). One in six G3P molecules leaves the cycle to form glucose (two G3P → one glucose).
Glucose (6C) is phosphorylated using 2 ATP, then split into two molecules of triose phosphate (3C). Each triose phosphate is then oxidised to pyruvate (3C), producing 4 ATP (net gain: 2 ATP) and 2 NADH.
Glycolysis is anaerobic — it doesn't require oxygen. If oxygen is absent, pyruvate enters anaerobic pathways (fermentation) instead of the link reaction.
Each pyruvate (3C) is decarboxylated (loses CO₂) and combined with coenzyme A to form acetyl CoA (2C). NAD⁺ is reduced to NADH. This happens twice per glucose.
Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Through a series of reactions, citrate is gradually decarboxylated and oxidised, regenerating oxaloacetate.
Per turn: 1 ATP (via GTP), 3 NADH, and 1 FADH₂ are produced. Two turns per glucose molecule.
NADH and FADH₂ donate electrons to the electron transport chain on the inner mitochondrial membrane. As electrons pass through protein complexes, energy is released and used to pump H⁺ ions from the matrix into the intermembrane space.
H⁺ ions flow back through ATP synthase (chemiosmosis), generating approximately 28-34 ATP. Oxygen is the final electron acceptor, combining with H⁺ and electrons to form water.
Glycolysis: 2 | Link: 0 | Krebs: 2 | Oxidative Phosphorylation: 28-34
Photosynthesis and respiration are complementary processes. Photosynthesis removes CO₂ from the atmosphere and produces glucose + O₂. Respiration consumes glucose + O₂ and releases CO₂ + H₂O. They form a closed loop of carbon cycling.
Photosynthesis converts light energy into chemical energy (stored in glucose). Respiration converts chemical energy in glucose into ATP — the universal energy currency cells actually use. Without photosynthesis, there's no glucose. Without respiration, there's no usable ATP.
Both processes use NAD⁺/NADH and involve electron transport chains with chemiosmosis. In photosynthesis, NADP⁺ is reduced to NADPH. In respiration, NAD⁺ is reduced to NADH. Both processes rely on proton gradients driving ATP synthase — the mechanism is essentially the same.
The compensation point is when the rate of photosynthesis equals the rate of respiration — net gas exchange is zero. Above this light intensity, the plant is a net producer of O₂. Below it, the plant is a net consumer. This concept links both processes quantitatively.
"Explain why plants still need to respire even though they photosynthesise." — Plants only photosynthesise in light; they respire 24/7. Also, not all cells contain chloroplasts (root cells, xylem), so they depend entirely on respiration for ATP. Even in light, respiration provides ATP for processes photosynthesis doesn't directly fuel.