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OCR GCSE Chemistry A · J248
OCR J248 · C6.3b Check the specification (PDF) (opens in a new tab)
The Earth's early atmosphere is thought to have contained little or no oxygen. Today's air is very different: roughly one-fifth of it is oxygen. That change took place gradually over billions of years, rather than when the atmosphere first formed.
The main source of the increase was photosynthesis. Early algae in water used light energy to make glucose from carbon dioxide and water. Oxygen was released as a product:
Algae were producing oxygen about 2.7 billion years ago. As photosynthesis continued, oxygen gradually built up in the atmosphere. Plants evolved later and also photosynthesised, adding more oxygen. At the same time, photosynthesis took carbon dioxide out of the air. Over a very long period, these changes helped produce the oxygen-rich atmosphere in which animals could evolve.
Algae, followed by plants, released oxygen through photosynthesis; oxygen built up gradually over billions of years.
Describe a sequence: little oxygen in the early atmosphere; algae photosynthesised; plants appeared later; oxygen gradually built up.
Do not say that volcanoes produced the oxygen-rich atmosphere. Photosynthesis was the main source of the increase.
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Last updated
Photosynthesis
The process by which algae and plants use light energy to make glucose from carbon dioxide and water, releasing oxygen.
Oxygen
A gas made of molecules containing two oxygen atoms; it is released during photosynthesis and makes up about one-fifth of today's atmosphere.
Put your knowledge into practice — try past paper questions for Chemistry A
Photosynthesis
The process by which algae and plants use light energy to make glucose from carbon dioxide and water, releasing oxygen.
Oxygen
A gas made of molecules containing two oxygen atoms; it is released during photosynthesis and makes up about one-fifth of today's atmosphere.