CIE A Level Biology Topic 13: Photosynthesis Exam Questions

About CIE A Level Biology Topic 13: Photosynthesis

Photosynthesis in the Cambridge 9700 syllabus explains how light energy is transferred to chemical energy in chloroplasts. You need to relate chloroplast structure to function, describe the roles of chlorophyll a, chlorophyll b, carotene and xanthophyll, and separate pigments by chromatography using Rf values. The light-dependent reactions on the thylakoids include photoactivation, photolysis of water, cyclic and non-cyclic photophosphorylation, and production of ATP and reduced NADP. The Calvin cycle in the stroma fixes carbon dioxide using rubisco. You also need to explain limiting factors and interpret data from investigations, often using redox indicators or aquatic plants.

What you need to know

• Chloroplast structure Thylakoid membranes stacked into grana hold pigments and electron carriers; the stroma contains enzymes of the Calvin cycle, starch grains, DNA and 70S ribosomes.
• Pigments Chlorophyll a and b and the accessory pigments carotene and xanthophyll absorb different wavelengths. Absorption and action spectra show which wavelengths are absorbed and used. Rf = distance moved by pigment / distance moved by solvent front.
• Light-dependent reactions Light excites electrons in photosystems. In non-cyclic photophosphorylation, photosystems I and II are used, water is split (photolysis) and ATP and reduced NADP are made. Cyclic photophosphorylation uses photosystem I only and makes only ATP.
• Calvin cycle Rubisco fixes CO2 with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). ATP and reduced NADP reduce GP to triose phosphate, some of which regenerates RuBP.
• Limiting factors Light intensity, carbon dioxide concentration and temperature can each limit the rate. The factor in shortest supply limits the rate.
• Investigations Use redox indicators such as DCPIP with isolated chloroplasts, or count bubbles or measure gas volume from aquatic plants at different light intensities.

Common mistakes in Topic 13: Photosynthesis

• Saying the light-dependent reactions produce glucose; they produce ATP, reduced NADP and oxygen.
• Writing that oxygen comes from carbon dioxide; it comes from the photolysis of water.
• Confusing the Calvin cycle products; GP is reduced to triose phosphate, not directly to glucose.
• Calling the light-independent stage the dark reaction and saying it only happens in the dark; it needs the products of the light-dependent stage.
• Calculating Rf from the origin to the top of the paper; measure to the solvent front.

Try three Topic 13: Photosynthesis exam-style questions