CCEA A Level Biology 5.2 Photosynthesis Exam Questions

About CCEA A Level Biology 5.2 Photosynthesis

Photosynthesis is section 5.2 of Unit A2 2. CCEA requires the light-dependent stage on the thylakoids, including photoactivation of photosystems II and I, the Z-scheme, non-cyclic photophosphorylation, reduction of NADP and photolysis of water. You also need the light-independent stage in the stroma: fixation of CO2 by rubisco onto ribulose bisphosphate, reduction of glycerate phosphate to triose phosphate and regeneration of RuBP. C4 and CAM plants and cyclic photophosphorylation are not required. You must interpret absorption and action spectra, compensation points and limiting factors. Practicals are paper chromatography of pigments with Rf values and DCPIP as a hydrogen acceptor.

What you need to know

• Sites The light-dependent stage occurs on the thylakoid membranes; the light-independent stage occurs in the stroma.
• Light-dependent stage Light excites electrons in photosystem II. They pass along carriers to photosystem I, releasing energy to make ATP (photophosphorylation). Electrons from photosystem I reduce NADP to reduced NADP. Electrons lost from PSII are replaced from water, which is split into hydrogen ions, electrons and oxygen.
• Light-independent stage Rubisco fixes CO2 onto ribulose bisphosphate (5C), forming two molecules of glycerate phosphate (3C). These are reduced to triose phosphate using ATP and reduced NADP. Five out of six triose phosphates regenerate RuBP; one in six is used to make glucose and other compounds.
• Pigments and spectra An absorption spectrum shows which wavelengths each pigment absorbs; an action spectrum shows the rate of photosynthesis at each wavelength. Peaks are in the blue and red regions.
• Limiting factors Light intensity, CO2 concentration and temperature can each limit the rate. Rate is measured as CO2 uptake or O2 production. Gross photosynthesis = net photosynthesis + respiration. At the compensation point the rates of photosynthesis and respiration are equal.
• Practicals Separate chloroplast pigments by paper chromatography and calculate Rf values. DCPIP changes from blue to colourless when reduced by isolated chloroplasts in light, showing the role of hydrogen acceptors.

Common mistakes in 5.2 Photosynthesis

• Saying oxygen comes from carbon dioxide. Oxygen is released from the photolysis of water.
• Calling the light-independent stage the dark reaction that only happens at night. It happens in light too, as it needs ATP and reduced NADP from the light-dependent stage.
• Saying rubisco combines CO2 with glycerate phosphate. Rubisco fixes CO2 onto ribulose bisphosphate.
• Saying a factor is limiting just because it is low. A factor is limiting if increasing it increases the rate.
• Forgetting that most triose phosphate is recycled. Only 1 in 6 molecules leaves the cycle to make sugars.

Try three 5.2 Photosynthesis exam-style questions