Photosynthesis in Higher Plants · Topic 1

The Light Reactions

Photosynthesis is how a green plant turns sunlight into chemical energy. It runs in two connected stages. This chapter covers the first — the light reactions — which need light and take place in the thylakoid membranes of the chloroplast.

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light + 12 H₂O + 12 NADP⁺ + 18 ADP + 18 P_i ⟶ 12 NADPH + 18 ATP + 6 O₂

Key idea: The whole job of the light reactions is to trap light energy and store it as two molecules — ATP and NADPH. Together these are the assimilatory power, later spent by the Calvin cycle to build sugar from CO₂.

Where it happens

Inside every chloroplast are stacks of flattened sacs called thylakoids. Their membranes hold all the machinery of the light reactions.

Thylakoid membrane — site of the light reactions.
Thylakoid lumen — the inner space, where protons gather and water is split.
Stroma — the surrounding fluid, where the Calvin cycle later runs.

The pigments that catch light

Light is captured by photosynthetic pigments — coloured molecules that absorb specific wavelengths.

Chlorophyll a — the chief pigment and the reaction centre; the only pigment that actually converts light energy into chemical energy.
Accessory pigments — chlorophyll b and carotenoids. They absorb other wavelengths, widen the usable range of light, pass that energy to chlorophyll a, and protect it from damage.

Why leaves are green: chlorophyll absorbs strongly in the blue and red bands but reflects green — which is why leaves look green and why photosynthesis peaks in blue and red light.

Two photosystems

The pigments are organised into two light-harvesting units, each named for the wavelength its reaction-centre chlorophyll absorbs best.

Photosystem II

Reaction centre: P680 · absorbs 680 nm

Acts first in the chain, though numbered second. It is the photosystem that splits water.

Photosystem I

Reaction centre: P700 · absorbs 700 nm

Acts second. It re-energises the electron and passes it on to make NADPH.

When a pigment absorbs a photon, its reaction-centre chlorophyll becomes excited and ejects a high-energy electron. What happens to that electron is the heart of the light reactions.

The Z-scheme: following the electron

Because the electron rises and falls in energy as it moves from PS II to PS I, the pathway drawn out looks like a sideways letter Z — hence the Z-scheme.

Figure 2.1 — Electrons flow from water → PS II → electron transport chain → PS I → NADPH. Each photosystem lifts the electron with a burst of light; each downhill run releases energy used to make ATP. This one-way path is non-cyclic photophosphorylation.

Step by step:

Light excites PS II (P680); it releases a high-energy electron.
The electron travels downhill through the electron transport chain — releasing energy that drives the synthesis of ATP.
Light excites PS I (P700), lifting the electron again.
The electron is finally passed to NADP⁺, reducing it to NADPH.

Splitting water — the oxygen we breathe

When PS II gives up its electron, it needs a replacement. That electron comes from water. PS II drives the splitting of water — photolysis — on the inner (lumen) side of the membrane:

2 H₂O ⟶ 4 H⁺ + 4 e⁻ + O₂

This one reaction does three things: it replaces PS II's lost electrons, it releases protons (H⁺) into the lumen that help make ATP, and it sets free the oxygen — the O₂ every animal breathes is a by-product of this step.

The products, and a second route

The pathway above — water to NADPH in one direction — is non-cyclic photophosphorylation, and it makes both ATP and NADPH.

Sometimes the plant needs extra ATP but not NADPH. Then only PS I works and the electron simply cycles back to PS I instead of going to NADP⁺. This is cyclic photophosphorylation, and it makes only ATP (no NADPH, no O₂, since water is not split).

Chemiosmosis — how the ATP is made: splitting water and pumping electrons crowds protons into the thylakoid lumen. This gradient drives H⁺ back out through the enzyme ATP synthase, and that flow powers the assembly of ATP.

Watch & listen

Reinforce the whole topic with these curated resources:

▶ Video: Photosynthesis (Amoeba Sisters) ▶ Video: The light-dependent reactions (Khan Academy) 🔊 Audio: Photosynthesis (BBC In Our Time)