Respiration and photosynthesis compared: the four stages of each, without the confusion

Why learning them together works

Photosynthesis and respiration both use a proton gradient across a membrane to make ATP by chemiosmosis, and both use coenzymes to shuttle electrons. In photosynthesis the coenzyme is NADP and the membrane is the thylakoid; in respiration the coenzymes are NAD and FAD and the membrane is the inner mitochondrial membrane. Once you can state that parallel, half the recall is shared.

Respiration stage by stage

• Glycolysis (cytoplasm, anaerobic): glucose is phosphorylated using 2 ATP, split into two triose phosphate, then oxidised to two pyruvate, producing 4 ATP (net 2) and 2 reduced NAD.
• Link reaction (mitochondrial matrix): pyruvate is decarboxylated and oxidised to acetate, reduced NAD is formed, and acetate combines with coenzyme A to give acetyl CoA. No ATP is produced.
• Krebs cycle (matrix): acetyl CoA joins a 4-carbon molecule to form a 6-carbon one; per turn, 2 CO2 are released, 3 reduced NAD, 1 reduced FAD and 1 ATP by substrate-level phosphorylation. Two turns per glucose.
• Oxidative phosphorylation (inner membrane): reduced coenzymes are oxidised, electrons pass along carriers, energy released pumps protons into the intermembrane space; protons return through ATP synthase, making ATP. Oxygen is the final electron acceptor, forming water.

Photosynthesis stage by stage

• Light-dependent reaction (thylakoid membrane): light excites electrons in chlorophyll (photoionisation); electrons pass along carriers, protons are pumped into the thylakoid space, ATP is made by chemiosmosis, NADP is reduced, and photolysis of water replaces the lost electrons, releasing oxygen.
• Light-independent reaction (stroma): CO2 combines with ribulose bisphosphate (RuBP), catalysed by rubisco, producing two glycerate 3-phosphate (GP).
• GP is reduced to triose phosphate (TP) using reduced NADP and ATP from the light-dependent reaction.
• Most TP regenerates RuBP using ATP; the rest forms glucose, amino acids and lipids.

Anaerobic respiration and its purpose

Without oxygen the electron transfer chain stops, reduced NAD is not oxidised, and glycolysis would halt. In animals pyruvate is reduced to lactate, and in yeast to ethanol and CO2 — in both cases the point is to regenerate NAD so that glycolysis, and its small ATP yield, can continue. Say 'to oxidise reduced NAD so glycolysis continues' and the mark is secure.

Limiting factors and inhibitor questions

• Light intensity, CO2 concentration and temperature each limit photosynthesis; on a graph, the plateau shows another factor has become limiting — name a specific one.
• Sudden darkness: GP rises and TP falls, because reduced NADP and ATP are no longer available to reduce GP.
• Sudden CO2 shortage: GP falls and RuBP rises, because RuBP is no longer being used up.
• Respiratory inhibitors: state which stage stops, then which coenzyme accumulates in its reduced form, then the effect on ATP yield.

Handling the numbers

Examiners like yield questions: about 32 ATP per glucose in theory, but less in practice because some protons leak back across the membrane, and some ATP is used to actively transport pyruvate and reduced NAD into the mitochondrion. Quoting a reason is worth more than quoting a number.