gas exchange

Process by which green plants and some bacteria manufacture carbohydrates from water and atmospheric carbon dioxide, using the energy of sunlight. Photosynthesis depends on the ability of chlorophyll molecules within plant cells to trap the energy of light, in order to split water molecules, giving off oxygen as a by-product. The hydrogen of the water molecules is then used to reduce carbon dioxide to simple carbohydrates.

Gas exchange in the alveolus. Blood enters the lungs low in oxygen and high in carbon dioxide. The carbon dioxide diffuses from the blood to the alveolus, where carbon dioxide levels are low. Oxygen diffuses from the alveolus, where oxygen levels are high, into the blood where the level is low, so blood leaves the lungs high in oxygen and low in carbon dioxide.

Movement of gases between an organism and the atmosphere, principally oxygen and carbon dioxide. All aerobic organisms (most animals and plants) take in oxygen in order to burn food and manufacture ATP (adenosine triphosphate, which provides the energy for all cell reactions). The resultant oxidation reactions release carbon dioxide as a waste product to be passed out into the environment. Green plants also absorb carbon dioxide during photosynthesis, and release oxygen as a waste product.

It is a characteristic of living organisms that they exchange materials with the environment. As well as the exchange of oxygen and carbon dioxide during aerobic respiration, nutrients must be taken up from the environment to feed the organism. In humans, these include carbohydrates, proteins, minerals (see mineral salts), and vitamins. Water is another important substance that has to be absorbed. However, in plants, only carbon dioxide, water, and minerals are required. The carbon dioxide is used in photosynthesis, which then releases oxygen as a waste product. All oxygen gas in the air on Earth is believed to have been produced by photosynthesis.

Specialized respiratory surfaces have evolved during evolution to make gas exchange more efficient. In humans and other tetrapods (four-limbed vertebrates), gas exchange occurs in the lungs, aided by the breathing movements of the ribs. Many adult amphibia and terrestrial invertebrates can absorb oxygen directly through the skin. The bodies of insects and some spiders contain a system of air-filled tubes known as tracheae. Fish have gills as their main respiratory surface. In plants, gas exchange generally takes place via the stomata and the air-filled spaces between the cells in the interior of the leaf.