It is the movement of substances in and out of cells from a lower concentration to a higher concentration.
So it is going against the concentration gradient.
Unlike Osmosis and Diffusion, it requires energy.
Example - the movement of mineral ions in soil (low concentration) to plant (higher concentration).
Exchange of gases in the lungs.
Lungs are in the thorax and are protected by the rib cage.
They exchange CO2 and O2 with the atmosphere.
O2 is needed for aerobic respiration. CO2 is a waste product of respiration.
The lungs have a very largesurface area due to the millions of alveoli (air sacs). They also have a thin and moist surface area. This all helps the diffusion of gases.
O2 diffuses into the capillaries (blood) surrounding the alveoli and CO2 diffuses back into the lungs to be released into the atmosphere.
Exchange in the gut.
Digested food is absorbed but the blood in capillaries alongside the small intestine.
Absorption of food by the blood is by Diffusion and Active Transport.
The small intestine has a rich blood supply.
The surface of the small intestine is covered in villi which greatly increases the surface area and make diffusion possible.
Villi are small finger-like projections which have a rich blood supply (which produces a steep concentration gradient), large surface area and a very thin wall (only one cell thick) which all helps diffusion.
Exchange of materials in other organisms.
All living organsims need to exchange O2 and CO2.
They have certain features in common:
- large surface area
- moist surface
- gases are transported away quickly to maintain a high concentration gradient
- thin membranes which gases diffuse from
Fish exchange O2 through gills.
Frogs exchange O2 through their skin.
Insects oxygen through holes in their sides.
Exchange in plants.
Plants exchange gases through their leaves which are flat and thin and have a lot of internal air spaces.
They take up water through their roots which have root hairs to increase surface area.
Water evaporates through leaves.
They respire all the time:
- during the day oxygen is produced by photosynthesis, so diffusion into leaves is mainly CO2.
- at night oxygen diffuses through leaves, as only respiration can take place.
Gases diffuse though tiny holes in leaves called stomata:
- O2 is needed for respiration and is a waste product of photosynthesis.
- CO2 is needed for photsynthesisancd is a waste product of respiration.
The movement of thses gases depend on which process is taking place most quickly.
Plants can take up and lose water through stomata which are on the underside of the leaves.
Stomata allow gases to be exchanged and water to be lost.
If they lose too much water they wilt.
They lose water through their leaves through evaporation, this is called transpiration.
Guard cells ensure that the right amount of water is lost by adjusting the size of the stomata.
More water is lost:
- on hot days as there is more energy
- on dry days as the air can hold more water
- on windy days as any build up of humidity around the plant is blown away