Describe why diffusion is important in plants and animals

Diffusion is the movement of gases across a partially permeable membrane from a high concentration to a low concentration and is important to allow the movement of substances across cell membranes. For example, it is important to animals to allow the movement of oxgyen from the blood capillaries to tissues across cell membranes, as oxygen is required for aerobic respiration which is a key metabolic process. Diffusion is also important in the absorption of small molecules and ions in the digestive system and nutrients into the epithelial cells lining the gastrointestinal tract to allow extraction of energy. Additionally, water is absorbed by osmosis, a type of diffusion, in the large intestine of animals which is important for water retention and hyrdation of the body. 

In plants, diffusion of gases such as CO2 and O2 is required for photosynthesis, where CO2 diffuses into cells and O2 diffuses out of cells (as a waste product of photosynthesis). Diffusion of water is also important in plants at the roots, as this is how plants gain water, a key substrate for photosynthesis.