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The light-dependent reactions

Energy Flow, Ecosystems and the Environment · Photosynthesis · note 2 of 6

Spec 5.3
A Level only (what this means)A Level only: only in the full A Level, not the AS Level. AS Level students can skip it. What the labels mean
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The light-dependent reactionsSpec 5.3

In short

The light-dependent reactions take place on the thylakoid membranes. Light energy excites electrons in chlorophyll, and these electrons pass along an electron transport chain, which makes ATP. In non-cyclic photophosphorylation, NADP is also reduced and water is split by photolysis, releasing oxygen. Cyclic photophosphorylation makes ATP only.

The light-dependent reactions take place on the thylakoid membranes of the chloroplast. Chlorophyll and other pigments are arranged in two groups called photosystems: photosystem I (PSI) and photosystem II (PSII). Each photosystem traps light energy and passes it to a reaction centre containing chlorophyll a.

How light energy is trapped

When a photon of light hits chlorophyll, an electron in the chlorophyll is raised to a higher energy level. The electron is said to be excited. It leaves the chlorophyll molecule and is passed to an electron carrier. The chlorophyll has been oxidised (it has lost an electron).

Non-cyclic photophosphorylation

  1. Light energy excites two electrons in chlorophyll in photosystem II. They leave the chlorophyll and are taken up by the first electron carrier in the electron transport chain.
  2. The electrons pass down the electron transport chain from carrier to carrier, losing energy at each step in a series of redox reactions.
  3. The energy lost is used to pump protons (H⁺) from the stroma into the thylakoid space. This builds up a proton gradient across the thylakoid membrane.
  4. Protons flow back down their concentration gradient into the stroma through ATP synthase. This is chemiosmosis. The flow drives the synthesis of ATP from ADP and Pi (photophosphorylation).
  5. Light also excites electrons in photosystem I. These electrons are passed to NADP, which also accepts a proton (H⁺) from the stroma, and is reduced to reduced NADP.
  6. The electrons lost from photosystem II are replaced by electrons from water. The enzyme-catalysed photolysis of water splits it: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons replace those lost from chlorophyll. The H⁺ ions contribute to the proton gradient and are used to reduce NADP. The oxygen is a waste product, and diffuses out of the leaf or is used in respiration.

Because electrons flow in one direction, from water through the two photosystems to NADP, this is called non-cyclic photophosphorylation. Its products are ATP, reduced NADP and oxygen.

Cyclic photophosphorylation

In cyclic photophosphorylation only photosystem I is used. Excited electrons leave chlorophyll, pass along an electron transport chain and return to the same chlorophyll molecule. Energy lost along the chain is used to make ATP by chemiosmosis. No NADP is reduced, no water is split and no oxygen is made. The electrons are recycled, so no replacement electrons are needed.

Cyclic and non-cyclic photophosphorylation
FeatureNon-cyclicCyclic
Photosystems usedPhotosystem II and IPhotosystem I only
Source of electronsWater (photolysis)The same chlorophyll (electrons return)
ATP madeYesYes
Reduced NADP madeYesNo
Oxygen madeYesNo
Z-scheme: electrons raised in energy by light at photosystem II, passing down an electron transport chain that pumps protons and drives ATP synthase, then raised again at photosystem I to reduce NADP; photolysis of water feeds photosystem II and a dashed loop shows cyclic photophosphorylation. (opens full size in a new tab)
The Z-scheme of the light-dependent reactions: electrons are raised to a higher energy level twice, with ATP made as they pass down the chain.
Common mistake:

Photolysis is not the splitting of carbon dioxide. Oxygen released in photosynthesis comes from water.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between cyclic and non-cyclic photophosphorylation?

Non-cyclic photophosphorylation uses photosystems I and II, takes electrons from water, and produces ATP, reduced NADP and oxygen. Cyclic uses only photosystem I and recycles its electrons, so it makes ATP only: no reduced NADP and no oxygen are produced.

What happens in the Calvin cycle?

Carbon dioxide combines with RuBP, catalysed by rubisco, to form two molecules of GP. GP is reduced to GALP using ATP and reduced NADP. Most GALP regenerates RuBP, and the rest forms glucose and other organic molecules. It takes place in the stroma of the chloroplast.

Why does photosynthesis need light if the Calvin cycle is light-independent?

The Calvin cycle does not use light directly, but it needs ATP and reduced NADP, which are made in the light-dependent reactions. Without light, these run out, GP cannot be reduced, and the cycle stops. So light is needed indirectly.

All 4 questions on Photosynthesis