Question 1

What are the products of the light reaction?

Accepted Answer

The light reaction produces ATP, NADPH and oxygen (O₂). It does NOT produce glucose — glucose is made later in the dark reaction (Calvin cycle) using the ATP and NADPH supplied by the light reaction.

Question 2

Where does photolysis of water occur?

Accepted Answer

Photolysis occurs on the inner side of the thylakoid membrane (the lumen side), associated with PSII. It requires Mn²⁺, Ca²⁺ and Cl⁻ ions, and follows the equation 2H₂O → 4H⁺ + O₂ + 4e⁻.

Question 3

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

Accepted Answer

Non-cyclic photophosphorylation involves both PSII and PSI, starts from water, and yields ATP + NADPH + O₂. Cyclic photophosphorylation involves only PSI (in the stroma lamellae, which lack PSII and NADP reductase); the electron returns to P700 and the only product is ATP — no NADPH and no O₂.

Question 4

Why is it called the Z-scheme?

Accepted Answer

When the electron carriers are plotted against their redox potential (energy), the path of the electron rises sharply at each photosystem (photon absorption) and falls through the carriers between them. The resulting shape resembles the letter Z. The vertical axis represents redox potential, not physical position.

Question 5

Where do protons accumulate in the chloroplast?

Accepted Answer

Protons (H⁺) accumulate in the thylakoid lumen — the space inside the thylakoid. They come from photolysis of water and from the PQ/cytochrome b₆f shuttle, creating the proton gradient that drives ATP synthase (CF₀–CF₁) via chemiosmosis.

Light Reaction 3D — Follow the Electron Through the Thylakoid

Practice

Learned it? Now prove it under the clock.