8.22 - Photosynthesis and rising oxygen levels

Photosynthesis changes atmospheric gases

The early atmosphere contained little or no oxygen gas and much more carbon dioxide than today. Early photosynthetic organisms helped change this. The specification calls them “primitive plants”; the earliest important oxygen producers included cyanobacteria, which are bacteria rather than modern land plants.

Photosynthesis uses light energy to make glucose from carbon dioxide and water, releasing oxygen:

carbon dioxide + water → glucose + oxygen

Carbon dioxide is a reactant; oxygen is a product. This is not simply one CO₂ molecule turning into O₂: water is also needed and carbon becomes part of glucose and other organic material.

Production is not the same as accumulation

As photosynthetic organisms grew and spread, the total amount of photosynthesis could increase, using more carbon dioxide and releasing more oxygen.

At first, much of the oxygen reacted with dissolved substances in oceans and with rocks. Oxygen production therefore did not immediately create an oxygen-rich atmosphere. Oxygen accumulates only when its supply exceeds its removal. Over long periods, changes in these sources and sinks allowed atmospheric oxygen to rise.