How Oxygen Increased (AQA GCSE Combined Science: Trilogy: Chemistry): Revision Note

Why did oxygen levels increase?

• Primitive plants and algae began photosynthesising which used up carbon dioxide from the atmosphere and released oxygen

• Algae first evolved around 2.7 billion years ago and during the next billion years or so small green plants began to appear

• As more and more plants began to appear the levels of oxygen began to increase which allowed for more complex life forms to evolve

• The balanced symbol equation for photosynthesis is:

• Over billions of years, photosynthesis caused the amount of oxygen in the atmosphere to increase and the amount of carbon dioxide to decrease

• This trend continued until around 200 million years ago when the composition of the atmosphere reached similar characteristics as today