Atmospheric oxygen is needed by all terrestrial and aquatic plants and animals for respiration. Twenty-one percent of Earth’s atmosphere has oxygen and this is rapidly cycled between plants, animals, and the atmosphere through photosynthesis and respiration.
During photosynthesis, atmospheric carbon dioxide is converted into glucose inside the leaves of plants. It is a by-product of chemical change and is delivered by the plants back into the atmosphere. The oxygen can also be delivered through the root system, giving oxygen to the soil. Submerged aquatic plants and phytoplankton deliver oxygen produced in course of photosynthesis into the water. Both terrestrial and aquatic plants provide oxygen for respiration by other plants and animals.
Respiration is performed by both plants and animals. During respiration, organic carbon compounds are broke down by molecular oxygen. Food which is made by plants in course of photosynthesis contains organic carbon compounds. This food is directly or indirectly consumed by animals and humans for their growth and development. Plants conjointly use the food made by them during photosynthesis for their own growth. During aerobic respiration (respiration that takes place in the presence of oxygen), the food is burnt and energy is released. The energy is used by the body for various works. However, when all of the oxygen in water or saturated soil has been consumed, some microbes undergo anaerobic respiration (respiration that does not require oxygen) and other compounds for oxygen, like iron, manganese, nitrate, and sulfate, are used for respiration by these microbes. Anaerobic respiration is common in wetland soils,anaerobic respiration is common which is frequently flooded and have lower oxygen concentrations than drier soils. When oxygen re-enters the soil or water, aerobic respiration system again begins.
The oxygen throughout photosynthesis comes from broken water molecules. In photosynthesis, the plant absorbs water and carbon dioxide. After the absorption, the water molecules are disassembled and converted into sugar and oxygen. The water molecules are also split into oxygen and hydrogen and the latter (hydrogen) is joined to the carbon dioxide to create sugars. Excess oxygen gathered after all these are delivered into the atmosphere throughout respiration.
In another regard, plants absorb carbonic gas (CO2) and water (H2O) in course of photosynthesis. The Chlorophyll in the leaves absorb sunlight and the water and carbon dioxide are used for making sugar. This sugar is stored for energy. Excess sugar is left behind in the root to work as starch and the excess water and carbon dioxide will evaporate.