Gaseous exchange (e.g. breathing) as the name suggests involve the release/ transfer of gases. Aerobic respiration requires a supply of oxygen and a constant release of carbon dioxide. Gaseous exchange provides these conditions by the trapping of oxygen from the surroundings and the release of carbon dioxide to the surroundings. At no point in time should respiration and gaseous exchange be confused.
Gaseous exchange mechanisms
We know that all living things must metabolize energy rich organic molecules. In most cases this is done through aerobic respiration. To sustain life each organism must have a supply of oxygen gas and an ability to release the carbon dioxide. Also anaerobic organisms must be able to acquire carbon dioxide. As a result each organism must have a mechanism of gaseous exchange. This mechanism varies depending on the size and nature of the organism.
Single celled organisms such as bacteria are in constant contact with their environment. As such, they are able to conduct gaseous exchange through their cell membranes via diffusion.
Gaseous exchange is also very important in plants. Plants, though complex, rely on a simple diffusion mechanism to exchange gases with its environment. In the presence of sunlight plants photosynthesize using carbon dioxide to make simple sugars. Plants also respire aerobically to produce carbon dioxide, however, during the day most of this carbon dioxide is used in photosynthesis. At night when the rate of photosynthesis eases there will be an excess of carbon dioxide produced. Therefore, plants must have a way to gain carbon dioxide and release oxygen in the day, and release carbon dioxide and gain oxygen at nights.
Plants do this by way of diffusion through pores in the leaves call the stomata.
Animals exchange gases through diffusion across membranes as well however, in most cases such as mammals, the mechanism is a lot more complex and involves the circulatory system.
Less complex animals such as earthworms can survive with diffusion across the skin into their tiny blood vessels. In more complex animals such as warm blooded mammals a constant supply of oxygen is needed to keep up with the large demand. This is accomplished by breathing. During the process of breathing carbon dioxide diffuses out and is exhaled; while inhaled air that is rich in oxygen diffuses across the moist cells of the alveoli.