Edexcel GCSE Combined Science: Biology: Key Concepts in Biology

A biologist examines two cells under a microscope: a bacterial cell (a prokaryote) and a single liver cell from a mouse (a eukaryote).

Compare the structures of the bacterial cell and the liver cell. In your answer you should describe both similarities and differences, and refer to named sub-cellular structures. (6 marks)

A student observes a single cheek cell using a light microscope. In the image, the cell measures 30 mm across. The student is told that the real (actual) width of the 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 magnification, the nucleus of the cell appears 6 mm wide in the image. Calculate the real width of the nucleus, in micrometres (μm). (1 mark)

The diagram description below refers to the enzyme amylase, which breaks down starch into sugars.

(a) Enzymes are described as biological catalysts. State what is meant by the term catalyst. (1 mark)

(b) Amylase will break down starch but it will not break down protein. Explain why amylase is specific to starch. (2 marks)

In a core practical, a student investigated how temperature affects the activity of the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. The student measured the volume of oxygen gas produced in 2 minutes at each temperature. The results are shown below.

Temperature / °C	Volume of oxygen produced in 2 minutes / cm³
10	8
20	16
30	28
40	41
50	12
60	0

(a) Describe how the rate of reaction changes as the temperature increases from 10 °C to 40 °C. (1 mark)

(b) Explain why almost no oxygen is produced at 60 °C. (2 marks)

Oxygen moves from the air in the alveoli (air sacs) of the lungs into the blood by diffusion.

Explain why oxygen diffuses from the alveoli into the blood. (2 marks)

Osmosis is a special type of diffusion involving water.

Define the term osmosis. (1 mark)