Fig. 1 shows a cell from the inside surface of a child's cheek. A laboratory technician stained the cells with methylene blue before viewing them using a li...

Fig. 1 shows a cell from the inside surface of a child's cheek. A laboratory technician stained the cells with methylene blue before viewing them using a light microscope. The stain made one dark structure easier to see. The cell is an animal cell, so it does not have structures used by a plant to make food. Table 1 gives measurements made from one cell image. The image was displayed on a computer screen, where the cell was much larger than its actual size.

(a) What is structure A? [1]
(b) Describe one feature shown in Fig. 1 that supports the conclusion that this is an animal cell. [1]
(c) Complete the calculation to give the actual width of the cell in mm.
actual width = 60 ÷ 600 [1]
(d) Give the actual width in micrometres, µm. [1]

A science club investigated cells in saltwater fish. Table 1 shows the mean number of mitochondria counted in electron micrographs of cells from two organs. Fig. 1 shows one cell from a gill filament. Fish gills exchange oxygen and carbon dioxide with water. The cells also use energy to move some ions between blood and water, helping to control the concentration of salts in the body. The club used images at the same magnification and counted mitochondria in equal areas of cytoplasm.

(a) What is organelle A? [1]
(b) Describe the difference shown in Table 1. [1]
(c) Explain why gill cells contain many mitochondria. [2]
(d) Give the name of the process that releases energy in mitochondria. [1]