Photosynthesis · Paper 2 — Free Biology SL Practice Questions

A student investigated the rate of photosynthesis in pondweed (Elodea) by counting the number of oxygen bubbles produced per minute at different light intensities. The results are shown below. Light intensity (arbitrary units) — 0 — 20 — 40 — 60 — 80 — 100 Oxygen bubbles per minute — 0 — 8 — 15 — 20 — 22 — 23

In the light-dependent reactions of photosynthesis, electrons pass along an electron transport chain embedded in the thylakoid membrane. This generates a proton gradient across the membrane, which drives ATP synthesis. During active photosynthesis: - pH of the thylakoid lumen $\approx 4.5$ - pH of the stroma $\approx 8.0$

Isolated chloroplasts from spinach (Spinacia oleracea) were illuminated with saturating white light in the presence of $^{14}\text{C}$-labelled carbon dioxide ($^{14}\text{CO}_2$). The light was then switched off, and the concentrations of $^{14}\text{C}$-labelled 3-phosphoglycerate (3-PGA) and ribulose-1,5-bisphosphate (RuBP) were measured over the following 120 seconds. Time after light off — $[\text{3-PGA}]\ /\ \mu\text{mol}\,\text{mg}^{-1}$ chlorophyll — $[\text{RuBP}]\ /\ \mu\text{mol}\,\text{mg}^{-1}$ chlorophyll $0\,\text{s}$ — $12.0$ — $4.0$ $30\,\text{s}$ — $15.0$ — $1.5$ $120\,\text{s}$ — $18.0$ — $0.5$