In this post, I will be touching on the oxygen cycle.

The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs: the atmosphere (air), the biosphere (living things), and the lithosphere (Earth's crust). The main driving factor of the oxygen cycle is photosynthesis, which is responsible for the modern Earth's atmosphere and life.

Just as water moves from the sky to the earth and back in the hydrologic cycle, oxygen is also cycled through the environment. Plants mark the beginning of the oxygen cycle. Plants are able to use the energy of sunlight to convert carbon dioxide and water into carbohydrates and oxygen in a process called photosynthesis. This means that plants "breathe" in carbon dioxide and "breathe" out oxygen. Animals form the other half of the oxygen cycle. We breathe in oxygen which we use to break carbohydrates down into energy in a process called respiration.

The photosynthesis takes place in the green plants, whereas the aerobic respiration takes place in all living organisms. The oxygen cycle is determined by the aerobic respiration of glucose, which consumes oxygen and produces carbon dioxide and water, and by the photosynthesis, which consumes carbon dioxide and water to produce organic matter (sugars) and molecular oxygen.

Carbon dioxide produced during respiration is breathed out by animals into the air, so oxygen is created in plants and used up by animals, but the oxygen cycle is not actually quite that simple. Plants must break carbohydrates down into energy just as animals do. During the day, plants hold onto a bit of the oxygen which they produced in photosynthesis and use that oxygen to break down carbohydrates. But in order to maintain their metabolism and continue respiration at night, the plants must absorb oxygen from the air and give off carbon dioxide just as animals do. Even though plants produce approximately ten times as much oxygen during the day as they consume at night, the night-time consumption of oxygen by plants can create low oxygen conditions in some water habitats.

In conclusion, in the day, photosynthesis produces oxygen which is taken in by humans and animals for respiration and carbon dioxide is given out during repiration and is taken in for photosynthesis. In this way, the oxygen and carbon dioxide can be "reused" and thus forming a cycle.