1. Cell membranes are flexible but need a certain amount of control in shape to retain their structure. Name the molecule present in cell membranes that restricts movement of other molecules making up the membrane.
Answer
Cholesterol.
2. Name some ways in which cells are adapted to increase the rate of movement of ions and water soluble molecules.
Answer
By increasing their surface area and the number of channel or carrier proteins present.
3. Name the type of transport protein that is involved only in facilitated diffusion and not active transport.
Answer
Channel proteins.
4. The figure below shows some structures in the phospholipid bilayer. Give the letter(s) of the structure(s) that could be used for facilitated diffusion.
Answer
A and B.
5. Name the properties that molecules must have to pass through D.
Answer
Non-polar, small, lipid soluble.
6. Explain why the cell surface membrane is described as fluid mosaic.
Answer
Fluid: the phospholipids can move around within their layer.
Mosaic: the proteins in the cell membrane form a pattern like a mosaic.
7. The table below shows the concentration of ions in seawater and in the cytoplasm of a single-celled alga that lives in seawater. Explain and prove how potassium ions enter the cells of the alga.
Answer
Potassium ions enter the cells by active transport. To prove this, culture the cells in seawater with no oxygen present (in an atmosphere of nitrogen) and measure the concentration of potassium ions inside the cells. When aerobic respiration does not take place, little or no ATP is produced. Since ATP is required for active transport, the concentration of potassium ions inside the cells will be much lower or remain the same as in the seawater.
8. Give some roles of sodium ions in living organisms.
Answer
Sodium ions are involved in co‑transport (such as the uptake of glucose into epithelial cells of the gut), in generating action potentials, and in maintaining the water potential of the cytoplasm and body fluids.