Plants take in carbon dioxide and expel oxygen through photosynthesis, while animals breathe in oxygen and expel the carbon dioxide. Known as the oxygen cycle, this mirrored use of carbon dioxide and oxygen does have a few exceptions.
During the daytime, plants absorb water from their roots and carbon dioxide through tiny pores, the stomata, on the leaves. The heat of the sun triggers photosynthesis in the green parts of the plant that contain chlorophyll. One product of photosynthesis is sugar, which the plant uses for food immediately or stores in the roots or other tissues. Oxygen is also produced, which is released back into the atmosphere. Aquatic plants release oxygen into the water.
At night, a plant's breathing process is reversed. The stomata pull oxygen from the air and release carbon dioxide. Without the energy from the sun, plants cannot photosynthesize food. They need the oxygen to break down stored carbohydrates and maintain their metabolism.
Animals need oxygen, night and day. Land animals have lungs that breathe in air and extract the oxygen. Sea mammals also breathe in this fashion. Fish and other animals that live entirely underwater usually have gills. Water washes over the gills and oxygen is extracted.
As of 2015, a creature of the genus Spinoloricus, which lives in sediments at the bottom of the Mediterranean Sea, is the only multicelled animal discovered that survives without oxygen.