Photosynthesis: chloroplast structure; the light-dependent stage (photolysis of water, photophosphorylation and the reduction of NADP); the light-independent stage (the Calvin cycle); and the effect of limiting factors.

What this dot point is asking

Eduqas wants you to describe chloroplast structure, explain the light-dependent stage (photolysis, photophosphorylation and the reduction of NADP), explain the light-independent stage (the Calvin cycle), and explain how limiting factors affect the rate of photosynthesis.

Chloroplast structure

The chloroplast is adapted to its two stages:

• Thylakoids (flattened sacs, stacked into grana) contain chlorophyll and the electron carriers, and are the site of the light-dependent stage. Their stacking gives a large surface area for light absorption.
• The stroma is the fluid surrounding the thylakoids, containing the enzymes for the light-independent stage (the Calvin cycle).

The light-dependent stage (in the thylakoid membranes)

Light energy is converted into chemical energy in ATP and reduced NADP:

1. Light is absorbed by chlorophyll, exciting electrons that leave the chlorophyll and pass along an electron transport chain in the thylakoid membrane.
2. As electrons move, protons are pumped into the thylakoid space; they flow back out through ATP synthase, making ATP (photophosphorylation by chemiosmosis).
3. Photolysis of water splits water using light energy: $\text{H}_2\text{O} \rightarrow 2\text{H}^+ + 2\text{e}^- + \tfrac{1}{2}\text{O}_2$. This replaces the electrons lost from chlorophyll, releases oxygen as a by-product and provides protons.
4. At the end of the chain, electrons and protons reduce NADP to reduced NADP.

ATP and reduced NADP then pass to the stroma for the next stage.

The light-independent stage (the Calvin cycle, in the stroma)

The Calvin cycle fixes carbon dioxide into sugars using the ATP and reduced NADP from the light-dependent stage:

1. Carbon fixation. Carbon dioxide combines with the 5-carbon RuBP (ribulose bisphosphate), catalysed by the enzyme rubisco, forming two molecules of the 3-carbon GP (glycerate 3-phosphate).
2. Reduction. GP is reduced to TP (triose phosphate) using reduced NADP and energy from ATP.
3. Regeneration. Most TP is used to regenerate RuBP (using more ATP), so the cycle can continue; some TP is used to make glucose and other organic molecules.

It takes several turns of the cycle to make one glucose molecule.

Limiting factors

The rate of photosynthesis is limited by whichever factor is in shortest supply:

• Light intensity: more light means more photolysis and more ATP and reduced NADP, up to a point.
• Carbon dioxide concentration: more carbon dioxide means more fixation in the Calvin cycle (often the limiting factor in the field).
• Temperature: the reactions are enzyme-controlled, so the rate rises with temperature up to an optimum, then falls as enzymes (such as rubisco) denature.

On a graph, the rate rises with the factor then plateaus when another factor becomes limiting.

Examples in context

Example 1. Greenhouse carbon dioxide enrichment. Commercial growers raise the carbon dioxide concentration (and light and temperature) in greenhouses to lift the limiting factor and increase the rate of photosynthesis, boosting crop yield. This is a favourite Eduqas applied context.

Example 2. The Calvin lollipop experiment. Calvin traced radioactive carbon-14 through the cycle, showing the order GP then TP then RuBP, which is why we know the structure of the light-independent stage.

Try this

Q1. State the two products of the light-dependent stage that are used in the Calvin cycle. [2 marks]

• Cue. ATP and reduced NADP.

Q2. Explain where the oxygen released in photosynthesis comes from. [2 marks]

• Cue. From the photolysis (splitting) of water in the light-dependent stage, which produces protons, electrons and oxygen.

Q3. Name the enzyme that catalyses the fixation of carbon dioxide onto RuBP. [1 mark]

• Cue. Rubisco (RuBP carboxylase).