Biological Diffusion and Gas Exchange Mechanisms

Exchange of Gases in the Lungs

Alveoli are tiny sacs that always have a very large surface area which is kept moist; this is important for the most effective diffusion of gases. The job of your breathing system is to move air in and out of your lungs. It brings in oxygen-rich air and removes air containing waste CO2.

Exchange in the Gut

The food you eat is broken down in your gut and it forms simple sugars such as glucose and amino acids... but these products of digestion are of no use if they stay in your gut.

Absorption in the Small Intestine

The products of digestion must get into your bloodstream to make the molecules of the food available to your body cells. Only when the molecules are dissolved in water can diffusion take place, and then the digested food molecules are small enough to pass freely through the walls of the small intestine into your blood vessels. The lining of the small intestine is folded into thousands of tiny finger-like projections known as villi; these increase the uptake of digested food by diffusion.

Gills of Fish

Fish need gills to take the oxygen out of the water by diffusion. They are thin, so there is only a short distance for the gases to diffuse across, but gills can't work out of the water.

Tadpoles and Frogs

Young tadpoles have frilly external gills to take oxygen by diffusion, but when they grow, they turn into frogs and they spend time on the ground, but they can still breathe in water; frogs make the exchange of gases through the skin.

Respiratory Systems in Insects

To supply their needs, they have an internal respiratory system which supplies oxygen directly to their cells and removes CO2. Most of the gas exchange takes place in the tracheoles; they are very moist, and air is pumped in and out of them by the insect.

Exchange in Plants

Plants need CO2 and they get it by diffusion through their leaves. The flattened shape of the leaves increases the surface area for diffusion, but they don't always need CO2 because when it is dark, they are not photosynthesising because light is a limiting factor for photosynthesis.

Water Loss in Leaves

Water loss occurs through stomata; they are open to allow CO2 in and to lose water vapour (transpiration). They can be opened or closed by guard cells.

The Transpiration Stream

The transpiration stream is the constant movement of water molecules through the xylem from the roots to the leaves.