The oxygen on Earth is not inexhaustible, but it circulates in a dynamic equilibrium system. Oxygen on Earth comes mainly from plants and certain bacteria through photosynthesis. These organisms use solar energy to convert water and carbon dioxide into oxygen and glucose. Here are some key points about the oxygen cycle:

1. Photosynthesis: Plants, algae, and certain bacteria release oxygen through photosynthesis. This process is widespread on Earth, especially in forests, grasslands, and marine plankton, providing large amounts of oxygen to the atmosphere.
2. Respiration: All animals (including humans) and some microorganisms consume oxygen and produce carbon dioxide during life activities. This process is called respiration.
3. Oxygen circulation: Oxygen circulates in the atmosphere and water. Oxygen in the atmosphere is constantly consumed by the respiration of plants and animals, while plants in water bodies replenish the oxygen in the atmosphere by releasing oxygen through photosynthesis.
4. Atmospheric stability: The concentration of oxygen in the Earth’s atmosphere has stabilized at about 21%, an equilibrium concentration that the biosphere has maintained for thousands of years, enough to support most life on Earth.
5. Geological History: Oxygen concentrations have varied significantly over the long history of the Earth. Billions of years ago, oxygen concentrations in the Earth’s atmosphere were much lower than they are now, but with the advent of photosynthesis and increased biodiversity, oxygen concentrations have gradually stabilized at current levels.

Although oxygen concentrations on Earth are relatively stable, human activities, such as burning fossil fuels, are altering the gas balance in the atmosphere, leading to the greenhouse effect and climate change. These changes may affect the oxygen cycle and the stability of the biosphere, so we need to take steps to reduce our impact on the environment and protect the ecological balance of the planet.