IB Biology HLPhotosynthesisPaper 1 & 2~13 min read
Light-Dependent Reactions
Follow one electron. It starts in a chlorophyll molecule, is kicked upstairs twice by light, and ends up on NADP. Every product of this stage — including the oxygen you are breathing right now — is a consequence of that journey.
📚 What you need to know
These reactions occur on the thylakoid membranes and require light directly.
Light excites an electron in PSII, which passes it to an electron acceptor (photoactivation).
PSII replaces the lost electron by photolysis: light splits water into electrons, protons and oxygen.
The oxygen is a waste product and diffuses out of the leaf.
Electrons pass along an electron transport chain of carriers in the membrane, releasing energy at each step.
That energy is used to pump protons into the thylakoid space, driving ATP synthesis (covered fully on the next page).
Light excites an electron in PSI too; that electron is passed to NADP, which together with a proton forms reduced NADP.
The products passed to the Calvin cycle are ATP and reduced NADP.
Overall electron route: water → PSII → chain → PSI → NADP.
Photolysis: splitting water
When PSII passes its excited electron away, it is left oxidised and cannot function again until that electron is replaced. The replacement comes from water.
Photolysis of water
2H2O → 4H+ + 4e− + O2 (driven by light, in the thylakoid space)
Water splitting produces three things and the plant uses all three differently:
Electrons replace those lost from chlorophyll a in PSII, so the photosystem can work again.
Protons (H+) accumulate inside the thylakoid, contributing to the gradient that makes ATP, and are later used to reduce NADP.
Oxygen is left over. It is a waste product of photosynthesis, and it diffuses out through the stomata.
This is the fact to protect above all others in this topic. The oxygen in the atmosphere came from water, split to keep photosystem II supplied with electrons. It is a by-product of an electron-replacement problem. Nothing in photosynthesis is trying to make oxygen.
The path of the electron
Because the electron is lifted to a high energy level, falls, and is lifted again, the diagram of this pathway is traditionally drawn as a Z shape.
Each fall down the chain releases energy. Each rise costs a photon. The electron ends higher than it started, and the difference is what the plant has banked.
Step by step
Light strikes PSII. Energy funnels to the reaction centre and an electron in chlorophyll a is excited to a higher energy level and passed to an electron acceptor.
Water is split. Photolysis supplies replacement electrons to PSII, releases protons into the thylakoid space, and releases oxygen as waste.
The electron falls down the chain. It passes between carriers in the thylakoid membrane, releasing energy at each transfer.
That energy pumps protons from the stroma into the thylakoid space, building a gradient used to make ATP.
The electron reaches PSI, now at a low energy level, and replaces the electron PSI has lost.
Light strikes PSI. The electron is excited again to an even higher energy level.
NADP is reduced. The electron, along with a proton, reduces NADP to reduced NADP, which carries it to the Calvin cycle.
🧠
Two lifts, one destination
Light is a lift, not an escalator: it takes the electron up in one jump. Two lifts are needed because one photon cannot raise an electron all the way from water to NADP.
The products, and where they go
Product
Where it comes from
What happens to it
ATP
Energy released as electrons fall down the chain
Passed to the Calvin cycle in the stroma
Reduced NADP
An electron from PSI plus a proton
Passed to the Calvin cycle to reduce GP
Oxygen
Photolysis of water
Waste — diffuses out of the leaf, or is used in the plant’s own respiration
Protons
Photolysis, and pumping across the membrane
Build the gradient for ATP synthesis, then reduce NADP
Compare it with respiration and it becomes familiar. An electron transport chain in a folded membrane, protons pumped across, a gradient used to make ATP, and a final acceptor at the end. Respiration’s final acceptor is oxygen; photosynthesis’s is NADP. Respiration gets its electrons from glucose; photosynthesis gets them from water, using light.
Worked examples
WE 1
Explain the role of water
Explain the role of water in the light-dependent reactions. (4 marks)
Point 1: the process
Water is split by photolysis, using light energy, in the thylakoid space.
Point 2: electrons
This provides electrons that replace those lost from chlorophyll a in photosystem II, so PSII can continue to function.
Point 3: protons
It releases protons, which contribute to the gradient used to make ATP and are used to reduce NADP.
Point 4: oxygen
Oxygen is produced as a waste product and diffuses out of the leaf.
2H₂O gives 4H+ + 4e⁻ + O₂ — and all three are accounted forgive all three products; answers that only mention oxygen score one mark
WE 2
Trace the electron
Describe the path taken by an electron from water to reduced NADP. (4 marks)
Step 1
Photolysis of water releases an electron, which passes to photosystem II to replace one lost from chlorophyll a.
Step 2
Light excites the electron at PSII to a higher energy level and it is passed to an electron acceptor.
Step 3
It travels along the electron transport chain of carriers, losing energy at each transfer, which is used to make ATP.
Step 4
It arrives at photosystem I, is excited again by light, and is passed with a proton to NADP, forming reduced NADP.
water → PSII → chain → PSI → NADPthe arrow chain at the end is worth writing out — it shows the sequence at a glance
WE 3
Predict the effect of an inhibitor
A herbicide blocks the electron acceptor of photosystem II. Predict its effect on oxygen production and on the Calvin cycle. (3 marks)
Point 1: oxygen
Electrons cannot leave PSII, so the chlorophyll is not oxidised and there is no demand for replacement electrons. Photolysis stops and oxygen production ceases.
Point 2: the chain
No electrons enter the electron transport chain, so no protons are pumped and ATP synthesis stops. NADP is not reduced.
Point 3: the Calvin cycle
Without ATP and reduced NADP, GP cannot be reduced and the cycle stops, so no sugars are made.
Block PSII and both stages fail, with no oxygen releasedmany herbicides really do work this way, which is why the plant dies rather than just stops growing
💡 Exam tips
Use the correct terms: photolysis for splitting water, photoactivation for exciting the electron.
State that oxygen comes from water and is a waste product.
Remember PSII acts before PSI.
Say reduced NADP (or NADPH), never reduced NAD.
Link energy released along the chain to proton pumping, not directly to ATP.
Name both products passed on: ATP and reduced NADP.
⚠ Common mistakes
Saying oxygen comes from carbon dioxide. It comes from the photolysis of water.
Saying light directly makes ATP. Light excites electrons; the falling electrons drive proton pumping, which makes ATP.
Writing NAD instead of NADP.
Saying photolysis needs an enzyme called photolysase. It is driven by light energy at PSII.
Getting the photosystem order wrong. Water feeds PSII, not PSI.
Saying the electron returns to water. In the non-cyclic route it ends on NADP.
Up next: Photophosphorylation. This page said energy from the falling electrons is “used to make ATP”. The next one explains exactly how — and introduces a second route the electron can take.
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