Atmosphere Contents

1. Structure And Composition

Oxygen is essential for animal life in the oceans and on land

The main reservoir of free oxygen (oxygen which is not chemically combined with other elements and is therefore available for respiration) is the atmosphere. The oceans and freshwater (rivers and lakes) also contain dissolved oxygen, which is released by aquatic plants through photosynthesis or absorbed from the air at the surface. Oxygen is produced by photosynthesis where carbon dioxide combines with water in the presence of sunlight to produce plant matter (carbon), releasing oxygen to the atmosphere. Oxygen circulates between the atmosphere (as free oxygen and within carbon dioxide), the biosphere and the hydrosphere, but the proportions in each of these components of the oxygen cycle is stable over long periods of time. This is because the quantity of oxygen in the atmosphere is so large that fluctuations in the amount used for respiration and fuel burning or released by photosynthesis have very little impact on it.

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Why is oxygen important?