B3.1 Movement of molecules in and out of cells

B3.1 Movement of molecules in and out of cells

We need to understand how biological and environmental systems operate when they are working well in order to be able to intervene when things go wrong. Modern development and technological research allow us to do so.

B3.1 Movement of molecules in and out of cells

The cells, tissues and organs in plants and animals are adapted to take up and get rid of dissolved substances. Different conditions can affect the rate of transfer. Sometimes energy is needed for transfer to take place.

Candidates should use their skills, knowledge and understanding to:

• evaluate the development and use of artificial aids to breathing, including the use of artificial ventilators
• evaluate the claims of manufacturers about sports drinks
• analyse and evaluate the conditions that affect water loss in plants.

B3.1.1 Dissolved substances

a) Dissolved substances move by diffusion and by active transport.

b) Water often moves across boundaries by osmosis. Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules.

Additional guidance:

Use of the terms turgor and plasmolysis is not required.

c) Differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.

d) Most soft drinks contain water, sugar and ions.

e) Sports drinks contain sugars to replace the sugar used in energy release during the activity. They also contain water and ions to replace the water and ions lost during sweating.

f) If water and ions are not replaced, the ion / water balance of the body is disturbed and the cells do not work as efficiently.

g) Substances are sometimes absorbed against a concentration gradient. This requires the use of energy from respiration. The process is called active transport. Active transport enables cells to absorb ions from very dilute solutions.

h) Many organ systems are specialised for exchanging materials. The effectiveness of an exchange surface is increased by:

• having a large surface area
• being thin, to provide…