Chemiosmosis, reduction of NADP and thylakoids

Molecules (Interaction and interdependence) · Photosynthesis · note 7 of 9

Spec C1.3.12, C1.3.13, C1.3.14

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Chemiosmosis, reduction of NADP and thylakoidsSpec C1.3.12, C1.3.13, C1.3.14

In short

In the light-dependent reactions, excited electrons pass along a chain of carriers in the thylakoid membrane, pumping protons into the thylakoid lumen. Protons diffuse back through ATP synthase, making ATP by chemiosmosis. Electrons come from photosystem II in non-cyclic or photosystem I in cyclic photophosphorylation. Photosystem I passes electrons to NADP, which also takes a hydrogen ion from the stroma.

ATP production by chemiosmosis

Excited electrons emitted by a photosystem pass along a chain of electron carriers in the thylakoid membrane.
Energy released as the electrons flow is used to pump protons from the stroma into the thylakoid lumen.
Protons (also released into the lumen by photolysis) build up a proton gradient: high concentration in the lumen, low in the stroma.
Protons diffuse back into the stroma through ATP synthase, which uses the energy to phosphorylate ADP, releasing ATP into the stroma. This is chemiosmosis.

Two ways of supplying electrons for ATP production
	Non-cyclic photophosphorylation	Cyclic photophosphorylation
Source of electrons	Photosystem II	Photosystem I
Path of electrons	PSII → carrier chain → PSI → NADP	PSI → carrier chain → back to PSI
Products	ATP, reduced NADP and oxygen	ATP only
Photolysis of water	Yes	No

Reduction of NADP by photosystem I

Photosystem I absorbs light and emits excited electrons. NADP is reduced by accepting two electrons that have come from photosystem I. It also accepts a hydrogen ion from the stroma, forming reduced NADP. Electrons lost from photosystem I are replaced by electrons arriving along the carrier chain from photosystem II.

NADP + 2e⁻ + H⁺ → reduced NADP

Exam tip:

Pair the terms consistently: write 'NADP and reduced NADP' or 'NADP⁺ and NADPH', never a mixture such as 'NADP and NADPH'.

Thylakoids as systems for the light-dependent reactions

Where each light-dependent process happens in a thylakoid
Process	Location
Light absorption by photosystems	Thylakoid membrane
Photolysis of water	Photosystem II, on the lumen side of the membrane; protons released into the lumen
Proton pumping	Electron carriers in the membrane, from stroma into lumen
ATP synthesis by chemiosmosis	ATP synthase in the membrane; ATP released into the stroma
Reduction of NADP	Stroma side of photosystem I

The small volume of the thylakoid lumen means a steep proton gradient builds up quickly, and stacking thylakoids into grana gives a large membrane area for photosystems. ATP and reduced NADP are both released into the stroma, where the Calvin cycle uses them.

Thylakoid membrane cross-section with stroma above and thylakoid lumen below: light excites photosystem II, where photolysis of water releases H⁺ into the lumen; electrons pass along a chain of electron carriers that pump H⁺ from the stroma into the lumen, then to photosystem I, which reduces NADP in the stroma; a cyclic path returns electrons from photosystem I to the carrier chain; H⁺ flows from the lumen through ATP synthase into the stroma, forming ATP. — The light-dependent reactions: electrons flow from photosystem II to photosystem I to NADP, building a proton gradient that drives ATP synthase.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why do plants absorb red and blue light but not green?

Chlorophyll absorbs mainly red and blue light because only photons with energy matching the gaps between its electron energy levels can excite its electrons. Green light does not match well, so most of it is reflected or transmitted, which is why leaves look green. Accessory pigments such as carotenoids absorb some other wavelengths.

What is the difference between an absorption spectrum and an action spectrum?

An absorption spectrum shows the percentage of light a pigment absorbs at each wavelength, while an action spectrum shows the rate of photosynthesis at each wavelength. Both peak in blue and red light. The action spectrum stays above zero in green light because accessory pigments absorb some of it, and the similar shapes show absorbed light drives photosynthesis.

How do you calculate Rf values in chromatography?

Divide the distance moved by the pigment by the distance moved by the solvent front, both measured from the origin line. Rf values lie between 0 and 1 and have no units. Pigments are identified by their colour and by comparing their Rf values with reference values for the same solvent and medium.

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