Diffusion occurs down a concentration gradient - this is where molecules move from an area of high concentration to an area of low concentration. It occurs across membranes, between the outside and inside of cells.

Visking tubing can be used to model diffusion in the body. Visking tubing is a selectively permeable membrane. It selects which molecules can pass through as it has pores of a certain size. Molecules that are too big are unable to pass through the pores. Molecules that are small enough can pass freely in and out of the membrane.

Starch is a large molecule and is unable to pass through the pores in the membranes of the small intestine. The enzyme amylase breaks down the starch into maltose, then a second enzyme maltase breaks the starch into small molecules of glucose. These molecules are then able to pass through the pores in the small intestine.

We can use visking tubing to demonstrate this. The starch is unable to pass through the visking tubing, but if this is broken down into glucose using enzymes, the glucose will diffuse from the high concentration in the visking tubing out into the beaker of water. The water can be tested at regular intervals using Benedict's reagent. As the glucose diffuses into the water the test will go from negative to positive.

The rate of diffusion

The rate of diffusion can be affected by several factors:

FactorHow the factor affects the rate of diffusion
The concentration gradientThe greater the difference in concentration, the quicker the rate of diffusion
The temperatureThe higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly
The surface area of the cell membrane separating the different regionsThe greater the surface area, the faster the rate of diffusion

Diffusion, surface area and volume

For a single-celled organism like an amoeba, substances diffuse into and out of the cell across its surface. Once inside, because of the amoeba's size, substances will need to diffuse 1μm or less to where they are needed, for instance for respiration.

Simple organisms therefore take in substances over their body surface. Their needs are determined by their volume. As organisms increase in size, their surface area does not increase at the same rate as their volume.

It is therefore important that multicellular organisms have specialised gas exchange surfaces and a transport system to take gases from the respiratory surface to all other body cells. In mammals the lungs have a large surface area to volume ratio, and the circulatory system collect oxygen and delivers carbon to this system for gas exchange. This maintains the concentration gradient at the gas exchange surface and also delivers the oxygen to all body cells.