Photosynthesis in Higher Plants

Photosynthesis is the physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It is the primary source of all food on earth and of the oxygen in the atmosphere. The chapter begins with early experiments: Joseph Priestley's bell jar experiments with a candle, a mouse and a mint plant, Jan Ingenhousz's discovery that sunlight is essential, Julius von Sachs's evidence of glucose being stored as starch, T.W. Engelmann's action spectrum using Cladophora and aerobic bacteria, and Cornelius van Niel's insight that the oxygen released comes from water. You will learn about the chloroplast and the pigments chlorophyll a and b, xanthophylls and carotenoids. You will study the light reactions: photosystems I (P700) and II (P680), the splitting of water, the Z scheme of electron transport, cyclic and non-cyclic photophosphorylation, and ATP synthesis by chemiosmosis. The chapter explains the Calvin cycle, which needs 18 ATP and 12 NADPH for one glucose, the C4 pathway with Kranz anatomy, photorespiration, and Blackman's law of limiting factors.

Detailed Worksheet: Photosynthesis in Higher Plants Section A - Definitions (10 marks) 1. What is an action spectrum? How did Engelmann show it? (2 marks) 2. What is photolysis of water? Where does it occur? (2 marks) 3. What is Kranz anatomy? Give an example of a plant showing it. (2 marks) 4. What is photorespiration? Why is it called wasteful? (2 marks) 5. State Blackman's law of limiting factors. (2 marks) Section B - Calculations and Applications (15 marks) 6. Write the overall balanced equation of photosynthesis using water as the hydrogen donor, showing the origin of the oxygen released. How many molecules of oxygen are released for each molecule of glucose formed? (3 marks) 7. Calculate the number of ATP and NADPH molecules needed in the Calvin cycle for one molecule of CO2 fixed and for one molecule of glucose. How many turns of the cycle are needed to make one glucose? (3 marks) 8. In C4 plants, two extra ATP are needed per CO2 fixed (5 ATP and 2 NADPH per CO2 in total). Calculate the ATP and NADPH needed to make one glucose in a C4 plant. Why is the C4 pathway still advantageous in hot, dry climates? (3 marks) 9. The atmospheric CO2 concentration is about 0.04%. Express this in parts per million. If C3 plants show increased photosynthesis up to about 0.05%, calculate the percentage increase in CO2 concentration needed. Why do greenhouse farmers enrich CO2 for tomatoes? (3 marks) 10. In a light reaction, 2 photons are absorbed for each electron to travel through both photosystems (1 photon at each). Calculate the photons needed to release one O2 molecule (4 electrons) and to make one glucose molecule. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the Z scheme of light reaction. (4 marks) 12. Draw a labelled diagram of the Calvin cycle showing its three stages. (3 marks) 13. Draw a labelled diagram of the transverse section of a C4 leaf showing Kranz anatomy. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the chemiosmotic hypothesis of ATP synthesis in chloroplasts. What three processes create the proton gradient across the thylakoid membrane? (5 marks) 15. Compare the C3 and C4 pathways on the basis of the first CO2 acceptor, the first stable product, the enzymes, the cells involved and the effect of photorespiration. (5 marks) 16. Explain the factors affecting photosynthesis with graphs. Why is light rarely a limiting factor in nature, except for plants in shade? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw diagrams and graphs neatly with labels. Write equations correctly.