Photosynthesis as an energy transfer process

Biology A2 · Photosynthesis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Photosynthesis transfers light energy into chemical energy in organic molecules. It has two stages inside the chloroplast.

The light-dependent stage takes place on the thylakoid membranes, which are stacked in grana. Pigments absorb light, electrons in chlorophyll are raised to a higher energy level (photoactivation) and their energy is used to make ATP and reduced NADP. Water is split and oxygen is released.

The light-independent stage (Calvin cycle) takes place in the stroma. It uses the ATP and reduced NADP to turn carbon dioxide into triose phosphate, from which carbohydrates, lipids and amino acids are made.

Rules to remember

Cyclic photophosphorylation: photosystem I only; makes ATP only
Non-cyclic photophosphorylation: photosystems I and II; makes ATP, reduced NADP and oxygen
Photolysis at photosystem II, by the oxygen-evolving complex: 2H₂O → 4H⁺ + 4e⁻ + O₂
Electrons passing along the electron transport chain release energy that moves protons into the thylakoid space; protons return to the stroma through ATP synthase, making ATP
Calvin cycle: CO₂ + RuBP (5C) → 2 GP (3C), catalysed by rubisco; GP is reduced to TP using reduced NADP and ATP; RuBP is regenerated from TP using ATP
Absorption spectrum: how much light a pigment absorbs at each wavelength. Action spectrum: the rate of photosynthesis at each wavelength
R_f = distance moved by pigment ÷ distance moved by solvent front

Common mistake

Students often say the oxygen released comes from carbon dioxide. It comes from the photolysis of water in the light-dependent stage; carbon dioxide is used only in the Calvin cycle.

Worked example

Isolated chloroplasts in bright light are given a chemical that lets protons leak freely across the thylakoid membrane. Electron flow continues. What happens to the Calvin cycle?

Protons pumped into the thylakoid space leak straight back out, so no proton gradient builds up.
With no gradient, protons do not flow through ATP synthase, so no ATP is made, although reduced NADP is still formed.
The Calvin cycle needs ATP to reduce GP to TP and to regenerate RuBP, so both steps stop.

Answer: The Calvin cycle stops because there is no ATP, and GP builds up.

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