Exchanging Substances

Exchange surfaces allow materials to pass in and out of organisms, helping them maintain all aspects of their metabolism.
They have several adaptations to increase their effectiveness including: being thin, to maintain a short diffusion distance; having a large surface area, for efficient absorption; and (in animals) having a good blood supply to maintain a steep concentration gradient, and in (plants) are able to control the loss and gain of water by having stomata.
The effectiveness of an exchange surface can also be increased by being ventilated - movement of the environment (for example air) aids the maintenance of the concentration gradient.

Diffusion is the spreading out of particles from an area of high concentration to an area of low concentration.
It is a passive process, meaning it happens naturally and requires no energy.
Factors that can increase the rate of diffusion include: a high concentration gradient, high temperature and a large surface area.

Osmosis is a special type of diffusion which concerns water molecules. It is the movement of water molecules from an area of higher water concentration to an area of lower water concentration through a partially permeable membrane.

Active transport is the movement of substances from an area of low concentration to an area of high concentration - against the concentration gradient.
This process requires energy from respiration, and allows cells to absorb ions from very dilute solutions.

Organisms that have a larger surface area to volume ratio have a more efficient exchange of materials.
Cells and tissues are organized in such a way to increase this ratio.

In the lungs, gas exchange occurs in the alveoli - they have an enormous surface area, very thin walls and a rich blood supply to maximise the exchange of oxygen and carbon dioxide.
Diseases such as emphysema can damage the alveoli, reducing the surface area for gas exchange and leading to difficulties with breathing.

In leaves, the main gas exchange surface is the spongy mesophyll covered by a thin layer of cells called the epidermis.
Stomata, tiny pores largely found on the underside of the leaf, allow for the movement of gases into and out of the leaf.
The size of the stomata pores is controlled by guard cells in response to environmental conditions.