The Light Reactions: Quick Revision

A brief note on The Light Reactions. Read it before a test. Try each question at the end before you open its answer.

Photosystems

• Antenna: hundreds of pigment molecules in a light harvesting complex. Reaction centre: one chlorophyll a molecule.
• PS II: P680. PS I: P700. Named by discovery, PS II works first.
• Water splitting at PS II, in the lumen: $2\,\text{H}_2\text{O} \to 4\,\text{H}^+ + \text{O}_2 + 4\,e^-$.
• Grana: both photosystems. Stroma lamellae: PS I only, no PS II, no NADP reductase.

Electron flow

• Non-cyclic: water, PS II, acceptor, PQ, cytochrome b6-f, PC, PS I, acceptor, NADP+. Gives ATP, NADPH and oxygen. The Z scheme.
• Cyclic: PS I only, the electron returns to PS I. Gives ATP only. In the stroma lamellae, and in far-red light.
• Each electron is lifted twice, so it takes two photons.

Chemiosmosis

• Needs a membrane, a proton pump, a proton gradient and ATP synthase.
• The lumen gains protons: water splitting, the carrier from the stroma. NADP reductase removes protons from the stroma.
• CF0 is the channel. CF1 faces the stroma and makes ATP.
• Each pH unit is a factor of $10$ in the proton concentration.

A worked example

A chloroplast splits $16$ water molecules. How many oxygen molecules, NADPH molecules and photons?

1. Oxygen. Two water give one oxygen, so $16 \div 2 = 8$.
2. Electrons. $16 \times 2 = 32$.
3. NADPH. Two electrons for each, so $32 \div 2 = 16$.
4. Photons. Two for each electron, so $32 \times 2 = 64$.

Practice with new numbers

A program to try

The program counts the products of the light reactions for a number of water molecules.

Mistakes to avoid

• Cyclic flow gives ATP only. No NADPH and no oxygen.
• PS II is P680, PS I is P700.
• The lumen gains protons. NADP reductase takes protons from the stroma.
• CF0 is the channel, CF1 makes ATP.
• Each electron needs two photons in the non-cyclic path.

Check yourself

1. Which photosystem splits water?

2. What does the cyclic path give, and what does it not give?

3. How many photons does one electron need in the non-cyclic path?

4. Name two ways the lumen gets protons.

5. What makes ATP in a thylakoid, and on which side?