Photosynthesis

Photosynthesis overview

Photosynthesis uses light energy to make glucose from carbon dioxide and water, in the chloroplast. It has two stages: the light-dependent reactions (on the thylakoid membranes) and the light-independent reactions / Calvin cycle (in the stroma).

Chloroplast structure (linked to function)

• Thylakoid membranes (stacked into grana) hold chlorophyll and the electron transport chain — large surface area for the light-dependent reactions.
• Stroma — fluid containing the enzymes for the Calvin cycle.

The light-dependent reactions (thylakoid membranes)

1. Light is absorbed by chlorophyll, exciting electrons (photoionisation).

2. Photolysis of water: light splits water → 2H⁺ + 2e⁻ + ½O₂. Oxygen is released as a by-product.

3. Excited electrons pass down an electron transport chain, releasing energy that pumps H⁺ and drives ATP synthase (photophosphorylation → ATP, by chemiosmosis).

4. H⁺ and electrons reduce NADP to form reduced NADP (NADPH).

Products passed to the Calvin cycle: ATP and reduced NADP.

The light-independent reactions — the Calvin cycle (stroma)

1. Fixation: CO₂ combines with a 5C acceptor RuBP, catalysed by the enzyme rubisco, forming two molecules of the 3C glycerate-3-phosphate (GP).

2. Reduction: GP is reduced to triose phosphate (TP) using ATP and reduced NADP from the light-dependent stage.

3. Regeneration: most TP is used (with ATP) to regenerate RuBP; some TP is used to make glucose and other organic molecules.

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

Limiting factors

The rate is limited by whichever is in shortest supply: light intensity, CO₂ concentration, and temperature (enzymes denature if too high). On a graph the rate rises then plateaus when another factor limits.

Worked example

Where do the ATP and reduced NADP used in the Calvin cycle come from?

• They are produced by the light-dependent reactions on the thylakoid membranes (ATP from photophosphorylation, reduced NADP from reducing NADP with electrons/H⁺ from photolysis). ✓

Common mistakes

• Mixing up the locations — light-dependent on thylakoids, Calvin cycle in the stroma.
• Forgetting oxygen comes from the photolysis of water, not CO₂.
• Confusing the three Calvin-cycle steps (fixation → reduction → regeneration).

Exam tips

• Learn the location, inputs and outputs of each stage.
• Emphasise the link: light-dependent supplies ATP + reduced NADP to the Calvin cycle.
• Interpret limiting-factor graphs (which factor limits the plateau).

Key facts to remember

• Light-dependent (thylakoids): chlorophyll excites electrons; photolysis of water → O₂ + H⁺ + e⁻; makes ATP (photophosphorylation) and reduced NADP.
• Calvin cycle (stroma): CO₂ + RuBP →(rubisco) GP → reduced to TP using ATP/reduced NADP → RuBP regenerated; TP → glucose.
• Rate limited by light, CO₂ and temperature.