AQA Biology Unit 3 (B3) - Chapter 1: Exchange of Materials (HT)

• How active transport in cells differs from diffusion and osmosis
• The lungs and the small intestines have very large surface areas for the exchange of substances
• The lungs have millions of air sacs called alveoli
• Carbon dioxide and oxygen are exchanged in the lungs
• The blood absorbs the products of digestion from the small intestines
• Thousands of villi increase the surface area of the small intestines so that absorption takes place
• Plants exchange carbon dioxide and oxygen through the surface of the leaves
• Plants take water and minerals up through the roots
• Root hairs greatly increase surface area of roots
• Water is lost by plants through transpiration
• Temperature, humidity and wind speed all affect the rate of transpiration

Substances get in and out of cells via diffusion, osmosis and active transport

Osmosis and diffusion involve movement of substances from a higher concentration to a lower concentration.

A cell may need to take up a substance against the concentration gradient. This involves energy and is called 'active transport'.

Active transport is the only one of the three methods that requires energy. It is only worth the cell using energy to do this if the substance is really needed.

The lungs are in the thorax and are protected by the rib cage.

The lungs exchange carbon dioxide (a waste product of respiration) and oxygen (needed for aerobic respiration) with the atmosphere.

They have a large surface area, provided by the millions of tiny air sacs (alveoli) which is moist and thin so diffusion takes place quickly.

Oxygen diffuses into the many capillaries surrounding the alveoli and carbon dioxide diffuses back out into the lungs.

The surfaces of the lungs are moist as diffusion takes place much quicker through a wet surface (i.e. in water).

Digested food is absorbed by the capillaries alongside the small intestine (gut).

The small intestine has a rich blood supply. The surface area of the gut is greatly increased by the finger-like projections into the small intestine called villi (singular: villus).

Absorption by the blood is by diffusion (when there is a lower concentration of that molecule in the blood) and active transport, where movement is against the concentration gradient.

Fish exchange oxygen through their gills.

Frogs exchange oxygen through their skin.

Insects exchange oxygen through holes in their sides (called spiracles) leading to a series of tubes.

All living organisms need to exchange gases. They need oxygen for respiration and to remove carbon dioxide. These organisms have a number of features in common:

• They have a large surface area
• They are moist
• The gases are transported away quickly to maintain a high concentration gradient
• The membranes which the gases diffuse through are thin.

Plants exchange gases through their leaves. They take up water and minerals through their roots. Leaves and roots are adapted for efficient exchange of materials.

Gases diffuse in and out of leaves through tiny holes called stomata.

• Oxygen  is needed for respiration and is a waste product of photosynthesis
• Carbon dioxide is needed for photosynthesis and is a waste product of respiration. The movement of gases depends upon which process is taking place the quickest.

Leaves are flat and very thin so the gases do not need to diffuse very far. There are also internal air spaces.

Water and mineral ions are taken up by the roots which have thousands of tiny projections called root hairs to increase their surface area.

Water evaporates from the leaves. This can be a problem.

Plants take up gases through the stomata.

They take up water and can lose water through the stomata.

If plants lose too much water, they will wilt.

Water passes from the roots to the leaves. It evaporates from the leaves in a process known as transpiration.

Stomata (usually found on the underside of the leaf) allows gases to be exchanged but also water to be lost.

Guard cells control the size of the stomata. They can close the stomata to prevent water loss. More water is lost on hot, windy, dry days.

Why? Because on hot days, there is more energy to cause evaporation. On dry days, the air can hold more water. On windy days, any build up of humidity (water vapour) around the plant is blown away.