With the aid of respiration (metabolism) it is possible for life forms to convert external biomass (food) and oxygen into carbon dioxide and water, and obtain energy in the process. Carbon dioxide and water are released, so to say 'exhaled'. Photosynthesis and respiration form a closed cycle.
Green plants and algae are capable of completely closing this cycle. In daylight hours they conduct photosynthesis with sunlight as the energy supplier and build up the body's own biomass. Oxygen is a by-product of this process, which is given off to the environment. At night green plants and algae switch over their metabolism and use their energy reserves and in the process they consume oxygen.
All other life forms can only effect a portion of the carbon cycle. They lack the green pigments that are necessary to conduct photosynthesis. They consume energy-rich food and convert it into the body's own biomass. This process is referred to as respiration. In the respiration process oxygen is consumed and carbon dioxide is given off.