OxfordAQA International A Level Biology 3.3.2 Photosynthesis Exam Questions

About OxfordAQA International A Level Biology 3.3.2 Photosynthesis

Photosynthesis is a major Unit 3 topic. You need to describe the light-dependent reaction on the thylakoid membranes, including the excitation of electrons in chlorophyll, the electron transfer chain, photolysis of water and the production of ATP and reduced NADP by chemiosmosis. You also need to describe the light-independent reaction in the stroma, where rubisco catalyses the reaction of carbon dioxide with ribulose bisphosphate. Finally you need to explain how carbon dioxide concentration, light intensity and temperature act as limiting factors. Required practical 7 investigates a limiting factor on the rate of photosynthesis. It links closely to respiration and energy transfer.

What you need to know

• Light-dependent reaction Light is absorbed by chlorophyll, exciting electrons that leave the chlorophyll. The electrons pass along an electron transfer chain, and the energy released moves protons across the thylakoid membrane.
• Chemiosmosis Protons diffuse back through ATP synthase, which produces ATP from ADP and inorganic phosphate. Electrons and protons reduce NADP to form reduced NADP.
• Photolysis Light splits water into protons, electrons and oxygen. The electrons replace those lost by chlorophyll, and the oxygen is a by-product.
• Light-independent reaction In the stroma, rubisco catalyses the reaction of carbon dioxide with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). ATP and reduced NADP reduce GP to triose phosphate (TP).
• Calvin cycle Some TP is used to regenerate RuBP, which uses ATP. The rest is converted into useful organic substances such as glucose, amino acids and lipids.
• Limiting factors The rate of photosynthesis is limited by whichever of light intensity, carbon dioxide concentration or temperature is in shortest supply. Growers raise these factors in glasshouses to increase yield, as long as the extra cost is less than the value of the extra yield.

Common mistakes in 3.3.2 Photosynthesis

• Saying oxygen comes from carbon dioxide. Oxygen comes from the photolysis of water.
• Saying the light-independent reaction happens in the dark. It does not need light directly but needs ATP and reduced NADP from the light-dependent reaction, so it stops soon in the dark.
• Writing that rubisco produces triose phosphate directly. Rubisco produces GP, which is then reduced to TP.
• Mixing up where each stage happens. The light-dependent reaction is on the thylakoid membranes; the light-independent reaction is in the stroma.

Try three 3.3.2 Photosynthesis exam-style questions