OCR GCSE Combined Science: Biology: Cell-level Systems

A gardener notices that a tomato plant grows much faster on a warm, bright day than on a cold, dull one. Inside the plant's leaf cells, photosynthesis is taking place.

Explain how light intensity, temperature and carbon dioxide concentration can each act as a limiting factor on the rate of photosynthesis. Refer in your answer to what happens as each factor increases. (6 marks)

A student uses a light microscope to look at a human cheek cell. In the image, the cell measures 30 mm across. The real (actual) width of the cheek cell is 60 μm.

(Remember: 1 mm = 1000 μm.)

(a) Calculate the magnification of the image. Show your working. (3 marks)

(b) Using the same microscope at a magnification of ×500, the nucleus of another cell appears 5 mm across in the image. Calculate the real width of the nucleus, in micrometres (μm). (1 mark)

Enzymes are biological catalysts. A student investigated how the enzyme amylase breaks down starch at different temperatures. The reaction was fastest at 40 °C. At 10 °C it was very slow, and at 70 °C it had almost stopped.

(a) Explain why the reaction was slower at 10 °C than at 40 °C. (2 marks)

(b) Explain why the reaction had almost stopped at 70 °C. (1 mark)

During hard exercise, muscle cells may not receive enough oxygen and switch to anaerobic respiration.

(a) Name the product formed when anaerobic respiration takes place in human muscle cells. (1 mark)

(b) State one way in which the amount of energy released by anaerobic respiration compares with that released by aerobic respiration. (1 mark)

(c) Write the word equation for aerobic respiration. (1 mark)

A bacterial cell and a human liver cell were compared under a microscope.

Describe two structural differences between a bacterial (prokaryotic) cell and a human liver (eukaryotic) cell. (2 marks)

The genome of an organism is stored in its DNA.

Name the type of molecule that DNA codes for the production of. (1 mark)