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Photosynthesis and Chloroplasts

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Photosynthesis and Chloroplasts

Answer every question. Use biological terms where appropriate and show your working for calculations.

Photosynthesis: key ideas

Photosynthesis converts light energy into chemical energy stored in organic molecules. In plants, it takes place mainly in chloroplasts. The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Light-dependent reactions in the thylakoid membranes produce ATP and reduced NADP; oxygen is released when water is split. In the stroma, the Calvin cycle uses carbon dioxide, ATP and reduced NADP to make triose phosphate (TP). Some TP contributes to carbohydrate production, while the rest helps regenerate RuBP.

Core knowledge and processes

1.Which chloroplast compartment contains the enzymes and substrates for the Calvin cycle?

Thylakoid space
Stroma
Intermembrane space
Outer membrane

2.Complete the overall photosynthesis equation: 6CO₂ + 6H₂O → ________ + 6O₂. Name the two products of the light-dependent reactions that are used directly in the Calvin cycle.

3.Match each term to its best description.

Chlorophyll
ATP synthase
Rubisco
RuBP
Thylakoid space
Reduced NADP

Uses ion flow across a membrane to make ATP
Catalyses the joining of carbon dioxide to RuBP
Carries reducing power for reactions in the stroma
Absorbs light energy in photosystems
Five-carbon molecule that accepts carbon dioxide
Compartment where hydrogen ions accumulate

4.For each process, state whether it belongs to the light-dependent reactions or the Calvin cycle, and give its main location in a chloroplast: (a) water is split and oxygen is released; (b) carbon dioxide is fixed to RuBP; (c) ATP is formed as hydrogen ions flow through ATP synthase; (d) TP is used to regenerate RuBP.

5.Describe how the thylakoid membrane and its compartments enable ATP and reduced NADP to be made during the light-dependent reactions. Include electron transport, hydrogen ions, ATP synthase and chemiosmosis.

Draw and label a chloroplast. Include the outer and inner membranes, stroma, a granum, one thylakoid and its space, a stroma lamella, chloroplast DNA and a ribosome. Add arrows to show where the light-dependent reactions and Calvin cycle occur.

Investigating photosynthesis

A class estimates photosynthesis rate by counting oxygen bubbles released by pondweed. They keep temperature, carbon dioxide concentration and the length of pondweed constant.

Light intensity (arbitrary units)

Oxygen bubbles per minute

6.Using the results table, describe how the measured photosynthesis rate changes as light intensity rises from 8 to 64 units. Identify the intensity range where the results first show a plateau.

7.Calculate the percentage increase in bubble rate between light intensities of 8 and 32 units. Show your calculation.

8.Suggest one reason the bubble rate might stop increasing between 32 and 64 units, even though the lamp is brighter. Give one limitation of using bubble counts to estimate photosynthesis rate.

9.Choose two factors other than light intensity that influence photosynthesis. For each, explain how a low level can restrict the rate and how a very high level can also reduce the rate or fail to increase it further.

10.A leafy plant is kept in complete darkness for several days. Explain why a leaf is likely to contain little newly made starch. Distinguish between the immediate effect on the light-dependent reactions and the later effect on the Calvin cycle and carbohydrate storage.

11.A greenhouse operator increases light intensity and carbon dioxide concentration, but the air becomes excessively warm. Explain how these changes could affect the overall photosynthesis rate. Link the light-dependent reactions to the Calvin cycle, and describe how the limiting factor may shift as conditions change.