Photosynthesis: connect light to carbon fixation

1. Chlorophyll absorbs light and electrons are excited.
2. Excited electrons pass to acceptors and along an electron-transfer chain.
3. Electron transfer supports proton movement and a gradient across the thylakoid membrane.
4. Protons move down the gradient through ATP synthase.
5. ATP synthase catalyses ATP production from ADP and inorganic phosphate.

Light excitation initiates this electron-flow route. Electron transfer helps establish the proton gradient. Return through ATP synthase couples that gradient to ATP synthesis. Photolysis also contributes protons and replaces electrons lost from photosystem II.

Electrons do not pass through ATP synthase in place of protons.

1. Rubisco catalyses the reaction of carbon dioxide with RuBP in the stroma.
2. The unstable six-carbon product forms two molecules of GP.
3. ATP and reduced NADP support conversion of GP to triose phosphate.
4. Some triose phosphate remains in the cycle for regeneration.
5. RuBP is regenerated, allowing further carbon dioxide fixation.

Carbon dioxide is fixed into an unstable product that yields GP. ATP and reduced NADP support GP reduction. Some triose phosphate supplies useful organic products and some regenerates RuBP, with further ATP use.

The Calvin cycle uses ATP; it is not the stage that directly produces the light-dependent ATP supply.

Correct link: Photolysis of water supplies electrons, protons and oxygen.

Photolysis of water provides replacement electrons for photosystem II and produces protons and oxygen. The oxygen released in photosynthesis derives from water, not carbon dioxide.

Do not confuse the carbon source for organic molecules with the source of released oxygen.

Correct link: ATP supplies energy and reduced NADP supplies reducing power.

ATP and reduced NADP from the light-dependent reactions support GP reduction to triose phosphate. NADP and NAD have different roles in the course pathways; do not swap their names.

The light-independent reactions are not reactions that must occur only in darkness.

Without light, the usual light-dependent supply of ATP and reduced NADP is not maintained. The Calvin cycle requires those products for GP reduction and ATP for RuBP regeneration. Existing pools may allow brief continuation, but carbon dioxide alone cannot sustain the previous rate indefinitely.

Do not say that the light-independent reactions require darkness.

• Darkness removes the normal light-dependent replenishment of ATP and reduced NADP.
• The Calvin cycle needs these products for reduction and regeneration.
• Brief use of existing pools is distinguished from indefinite activity at the earlier rate.

The alternative proton route reduces the gradient available across the membrane. Fewer protons return through ATP synthase with the required driving force, reducing ATP production. Electron transfer alone is not sufficient if the membrane cannot maintain the gradient that couples it to ATP synthesis.

A proton leak is not automatically evidence that every electron-transfer protein is denatured.

• The leak reduces the proton gradient.
• Proton flow through ATP synthase is linked to ATP production.
• Electron transfer is distinguished from maintaining a usable gradient.