A level-readyAll subjects · Biology · WBI14 (Unit 4)
The light-dependent reactions
In the thylakoid membranes, light energy excites electrons in chlorophyll. Their energy is used to make ATP and reduced NADP, and water is split to replace them.
Exciting electrons
Chlorophyll and other pigments are arranged in photosystems (photosystem I and photosystem II) in the thylakoid membranes. Light energy absorbed by the pigments is passed to a chlorophyll a molecule at the reaction centre.
This excites electrons, raising them to a higher energy level so they leave the chlorophyll. They are picked up by an electron acceptor and passed along an electron transport chain of carrier proteins in a series of redox reactions.
Non-cyclic photophosphorylation
1. Electrons excited in photosystem II pass along the electron transport chain to photosystem I.
2. The energy they release is used to pump hydrogen ions (H+) into the thylakoid space. H+ flows back out through ATP synthase, which makes ATP (chemiosmosis).
3. In photosystem I, light excites the electrons again. They are passed to NADP, which also takes up H+, forming reduced NADP.
4. Photosystem II replaces its lost electrons by photolysis of water: 2H2O → 4H+ + 4e- + O2.
Products: ATP, reduced NADP and oxygen.
Cyclic photophosphorylation
Only photosystem I is involved. Excited electrons pass along the electron transport chain and then return to photosystem I instead of reducing NADP.
The energy they release still makes ATP, but no reduced NADP is made and no water is split, so no oxygen is given off. Cyclic photophosphorylation tops up the extra ATP the Calvin cycle needs.
| Non-cyclic | Cyclic | |
|---|---|---|
| Photosystems | PSII and PSI | PSI only |
| ATP made | Yes | Yes |
| Reduced NADP made | Yes | No |
| Photolysis and O2 | Yes | No |
Worked example
Explain why photolysis of water is needed for non-cyclic photophosphorylation to continue.
- Electrons leave photosystem II and end up reducing NADP, so they do not return.
- Photosystem II must replace them.
- Photolysis of water supplies electrons (and H+ and O2).
Answer: Photolysis replaces the electrons lost from photosystem II; without it the chlorophyll could not keep passing electrons on.
Check yourself
- Where do the light-dependent reactions take place?
- Stroma of the chloroplast
- Thylakoid membranes of the chloroplast
- Cytoplasm of the cell
- Inner membrane of the mitochondrion
Show answer
Thylakoid membranes of the chloroplast. Photosystems and the electron transport chain are in the thylakoid membranes.
- How does photosystem II replace the electrons it loses?
- From reduced NADP
- From photosystem I
- From carbon dioxide
- From the photolysis of water
Show answer
From the photolysis of water. Splitting water gives electrons, H+ and O2.
- Which product is made in cyclic photophosphorylation?
- Reduced NADP
- ATP
- Oxygen
- Glucose
Show answer
ATP. Electrons cycle back to PSI, so only ATP is made.
- How many molecules of oxygen are released when 8 molecules of water are split by photolysis?
Show answer
4. 2H2O → 4H+ + 4e- + O2, so 8 water give 4 O2.
- What happens to electrons in chlorophyll when it absorbs light energy?
- They are excited to a higher energy level and leave the chlorophyll
- They combine with oxygen to make water
- They are used to split carbon dioxide
- They are stored in the stroma
Show answer
They are excited to a higher energy level and leave the chlorophyll. Excited electrons are passed to an electron acceptor.
- How does the electron transport chain lead to ATP synthesis?
- Electrons combine directly with ADP
- Oxygen reacts with ADP
- Energy released pumps H+ into the thylakoid space; H+ flows back through ATP synthase
- Reduced NADP is hydrolysed to give ATP
Show answer
Energy released pumps H+ into the thylakoid space; H+ flows back through ATP synthase. This is chemiosmosis: a proton gradient drives ATP synthase.
- What is the role of reduced NADP made in the light-dependent reactions?
- It supplies hydrogen to reduce GP in the Calvin cycle
- It absorbs light
- It splits water
- It releases oxygen
Show answer
It supplies hydrogen to reduce GP in the Calvin cycle. Reduced NADP carries hydrogen (and electrons) to the light-independent reactions.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.