The light-dependent and light-independent stages of photosynthesis, the stages of aerobic respiration, anaerobic respiration, and the role of ATP and electron carriers.

What this dot point is asking

CCEA wants you to describe the light-dependent and light-independent reactions of photosynthesis, the stages of aerobic respiration, the products of anaerobic respiration, and the role of ATP and electron carriers in both processes.

Photosynthesis

In the light reactions, light excites electrons in chlorophyll; these pass along an electron transport chain, releasing energy that pumps protons to drive ATP synthesis by chemiosmosis. Photolysis of water ($2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}_2$) replaces the lost electrons, releases oxygen and provides protons to reduce NADP. The Calvin cycle then uses these products in the dark stroma to build sugars; it is light-independent but depends on the supply of ATP and reduced NADP.

Aerobic respiration

Reduced NAD and FAD carry electrons from glycolysis, the link reaction and the Krebs cycle to the electron transport chain on the inner membrane (the cristae). As electrons pass along the chain, protons are pumped into the intermembrane space, then flow back through ATP synthase, driving ATP synthesis by chemiosmosis. Oxygen accepts the electrons and protons at the end of the chain to form water; without it, the chain backs up and stops.

Anaerobic respiration and ATP

When oxygen is absent, only glycolysis runs. In animals, pyruvate is reduced to lactate; in yeast and plants it becomes ethanol and carbon dioxide. This regenerates NAD so glycolysis can continue, but yields far less ATP (a net of about 2 per glucose, against about 30 to 38 in aerobic respiration). ATP is the universal energy currency in both processes, made and hydrolysed to release small, usable amounts of energy.

Examples in context

Example 1. Lactate build-up during a sprint. During an intense sprint, oxygen cannot be delivered fast enough, so muscle cells respire anaerobically, converting pyruvate to lactate to regenerate NAD and keep glycolysis going. The lactate causes the burning sensation and contributes to fatigue, and afterwards is taken to the liver and reconverted to glucose using oxygen (the oxygen debt repaid by heavy breathing). This is a direct, examinable example of anaerobic respiration in animals.

Example 2. Brewing and bread making with yeast. In brewing, yeast respires anaerobically, converting sugars to ethanol and carbon dioxide; the ethanol gives the alcohol and the carbon dioxide the fizz. In bread, the same anaerobic respiration produces carbon dioxide that makes the dough rise (the ethanol evaporates during baking). This shows the ethanol pathway of anaerobic respiration applied commercially, a favourite CCEA context.

Try this

Q1. State the products of the light-dependent reactions of photosynthesis. [3 marks]

• Cue. ATP, reduced NADP and oxygen.

Q2. Explain why oxygen is essential for aerobic respiration to continue. [2 marks]

• Cue. Oxygen is the final electron acceptor at the end of the electron transport chain, allowing oxidative phosphorylation to keep making ATP.

Q3. A muscle cell makes a net of 2 ATP per glucose anaerobically and about 36 ATP per glucose aerobically. Calculate how many times more ATP is made aerobically. [1 mark]

• Cue. $36 \div 2 = 18$ times more.