OCR A-Level Biology: Photosynthesis and Respiration

The light-dependent stage of photosynthesis takes place on the thylakoid membranes of the chloroplast. It produces ATP, reduced NADP and oxygen.

Describe and explain how the events of non-cyclic photophosphorylation lead to the production of ATP and reduced NADP. In your answer you should refer to photolysis, the electron transport chain and chemiosmosis.

(6 marks)

A student used a respirometer to measure gas exchange in a sample of germinating sunflower seeds kept at a constant 20 °C. By running the apparatus once with potassium hydroxide (to absorb CO₂) and once without it, they found the volume of oxygen consumed and the volume of carbon dioxide produced over a fixed period.

Gas	Volume exchanged in 10 minutes / cm³
Oxygen consumed	1.60
Carbon dioxide produced	1.12

The respiratory quotient (RQ) is given by:

$RQ = \frac{CO_2 \text{ produced}}{O_2 \text{ consumed}}$RQ=O2​ consumedCO2​ produced​

(a) Calculate the RQ for this sample. Show your working and give your answer to 2 decimal places. (2 marks)

(b) Using your value, identify the main respiratory substrate being used and explain why this substrate gives the RQ you calculated. (4 marks)

A researcher measured the rate of photosynthesis of pondweed (Elodea) as the volume of oxygen released per hour, while changing the light intensity. The whole experiment was carried out twice: once at a low CO₂ concentration (0.04%) and once at a raised CO₂ concentration (0.10%). Temperature was held constant at 25 °C throughout.

Light intensity / a.u.	Rate at 0.04% CO₂ / cm³ O₂ h⁻¹	Rate at 0.10% CO₂ / cm³ O₂ h⁻¹
1	8	8
2	15	16
4	24	31
6	25	44
8	25	52

(a) State and explain which factor is limiting the rate at a light intensity of 2 a.u. in both sets of results. (2 marks)

(b) At a light intensity of 8 a.u., the rate at 0.10% CO₂ is much higher than at 0.04% CO₂. Use the data to explain what this shows about the factor limiting the rate at high light intensity. (3 marks)

Some herbicides work by interfering with photosynthesis. DCMU is a chemical that blocks the electron transport chain of the thylakoid membrane, preventing electrons from passing along the chain after they leave photosystem II.

A leaf is treated with DCMU and then given bright light and plenty of CO₂.

Suggest and explain the effect of DCMU on:

(a) the production of ATP and reduced NADP in the light-dependent stage; (3 marks)

(b) the levels of GP and RuBP in the Calvin cycle shortly afterwards. (2 marks)

Cyanide is a respiratory poison. It binds to the final carrier in the electron transport chain of the inner mitochondrial membrane and prevents electrons from being passed to oxygen.

A cell respiring aerobically is exposed to cyanide.

Explain why exposure to cyanide causes the cell's production of ATP to fall to a very low level. (4 marks)

During vigorous exercise, a muscle cell may not receive enough oxygen for aerobic respiration. Under these conditions pyruvate produced by glycolysis is converted to lactate.

Explain why this conversion of pyruvate to lactate allows glycolysis to continue when there is no oxygen. (3 marks)