AQA Biology Unit 3 GCSE

B3.1 - How do dissolved materials get into and out of animals and plants?

• Dissolved substances move by diffusion 

• Substances are sometimes absorbed against a concentration gradient. This requires the use of energy from respiration. The process is called active transport. It enables cells to absorb ions from very dilute solutions. Other substances such as sugar and ions, can also pass through cell membranes.

 • Many organ systems are specialised for exchanging materials.

 • In humans:

 − the surface area of the lungs is increased by the alveoli

 − and that of the small intestine by villi.

 • The lungs are in the upper part of the body (thorax) protected by the ribcage and separated from the lower part of the body (abdomen) by the diaphragm.

 • The breathing system takes air into and out of the body so that oxygen from the air can diffuse into the bloodstream and carbon dioxide can diffuse out of the bloodstream into the air.

 • The alveoli provide a very large, moist surface, richly supplied with blood capillaries so that gases can readily diffuse into and out of the blood.

 • The villi provide a large surface area with an extensive network of capillaries to absorb the products of digestion by diffusion and active transport.

 • In plants:

 – carbon dioxide enters leaf cells by diffusion

 – most of the water and mineral ions are absorbed by root hair cells.

 • The surface area of the roots is increased by root hairs and the surface area of leaves by the flattened shape and internal air spaces.

 • Plants have stomata to obtain carbon dioxide from the atmosphere.

 • Plants lose water vapour from the surface of their leaves. This loss of water vapour is called transpiration. Transpiration is more rapid in hot, dry and windy conditions. Most of the transpiration is through stomata. The size of stomata is controlled by guard cells which surround them. If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.

B3.2 - How are dissolved materials transported around the body?, How does exercise affect the exchanges taking place in the body?

• The heart pumps blood around the body. Blood flows from the heart to the organs through arteries and returns through veins. In the organs, blood flows through capillaries. Substances needed by cells in the body tissues pass out of the blood, and substances produced by the cells pass into the blood through the walls of the capillaries.

• There are two separate circulation systems, one to the lungs and one to all the other organs of the body.

• Blood plasma transports:

− carbon dioxide from the organs to the lungs

− soluble products of digestion from the small intestine to other organs

− urea from the liver to the…