Edexcel A-Level Biology: Energy for Biological Processes

The final stage of aerobic respiration takes place on the inner mitochondrial membrane and is responsible for synthesising most of the ATP produced from one molecule of glucose.

Describe and explain how the electron transport chain and chemiosmosis bring about the synthesis of ATP during oxidative phosphorylation. In your answer you should refer to the role of reduced coenzymes, the proton gradient, ATP synthase and the part played by oxygen.

(6 marks)

A student used a respirometer to investigate the respiratory substrate being used by germinating seeds of three different species. For each species they measured the volume of carbon dioxide produced and the volume of oxygen consumed over the same period at 20 °C. The respiratory quotient (RQ) is given by:

$RQ = \frac{\text{volume of CO}_2 \text{ produced}}{\text{volume of O}_2 \text{ consumed}}$RQ=volume of O2​ consumedvolume of CO2​ produced​

Species	Volume of CO₂ produced / cm³	Volume of O₂ consumed / cm³
P	14.0	14.0
Q	10.5	15.0
R	18.0	18.0

(a) Calculate the RQ for species Q. Show your working. (2 marks)

(b) Typical RQ values are about 1.0 for carbohydrate, 0.7 for lipid and 0.9 for protein. Using your answer to (a) and the data in the table, identify the main respiratory substrate being used by species Q, and explain what the RQ values for species P and R indicate about their substrate. (4 marks)

A student investigated the effect of light intensity on the rate of photosynthesis of pondweed by counting the volume of oxygen collected in a capillary tube over 5 minutes at each light intensity. The carbon dioxide concentration and temperature were kept constant at 25 °C. The results are shown below.

Light intensity / arbitrary units	Volume of O₂ collected in 5 min / mm³
10	12
20	24
30	36
40	45
50	45

(a) Calculate the rate of oxygen production, in mm³ min⁻¹, at a light intensity of 30 arbitrary units. Show your working. (2 marks)

(b) With reference to limiting factors, explain the shape of these results between 10 and 50 arbitrary units. (3 marks)

A species of diving turtle can stay submerged in cold, oxygen-poor mud at the bottom of a pond for many hours during winter. Researchers found that while it is submerged its muscle cells switch almost entirely to anaerobic respiration, the concentration of lactate in its blood rises steadily, and the animal becomes very sluggish. When the turtle surfaces and breathes again, its lactate concentration falls back to normal over several hours.

Suggest and explain why the turtle becomes very sluggish while submerged, and why it must eventually return to the surface to breathe. (5 marks)

A chemical called compound Z makes the inner mitochondrial membrane freely permeable to protons (H⁺), so that protons can leak straight back into the matrix without passing through ATP synthase. When mammalian cells are treated with compound Z, the rate at which they consume oxygen rises sharply, but the cells make much less ATP and their temperature increases.

Explain why treating cells with compound Z reduces ATP synthesis but increases their oxygen consumption and temperature. (4 marks)

In the light-independent reactions (Calvin cycle) of photosynthesis, carbon dioxide is fixed in the stroma of the chloroplast.

Describe the carbon-fixation step of the Calvin cycle. In your answer, name the enzyme involved, the carbon dioxide acceptor, and the first stable product that is formed. (3 marks)