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Oxygen increased because algae, starting about 2.7 billion years ago, and later plants produced oxygen by photosynthesis. Free notes and 9 questions answered.
How Photosynthesis Increased Atmospheric Oxygen is part of The composition and evolution of the Earth's atmosphere in GCSE AQA Higher Chemistry.
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Oxygen increased because algae, starting about 2.7 billion years ago, and later plants produced oxygen by photosynthesis. Free notes and 9 questions answered.
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Use this lesson to stay inside The composition and evolution of the Earth's atmosphere while connecting the detail back to the wider Topic 9: Chemistry of the atmosphere topic.
Read the core explanations from How Photosynthesis Increased Atmospheric Oxygen before testing yourself from memory.
Earth's early atmosphere contained almost no oxygen. Yet today, one in every five molecules you breathe is oxygen. What changed everything?
The answer lies with tiny organisms that appeared 2.7 billion years ago. These early algae developed a revolutionary process that would transform our planet forever.
About 2.7 billion years ago, the first algae evolved the ability to photosynthesise. This wasn't just another biological process - it was a planetary game-changer.
These microscopic organisms began pumping oxygen into the atmosphere as a waste product. At first, the oxygen was absorbed by iron in the oceans, forming rust-coloured bands we can still see in ancient rocks today.
Notice that for every 6 molecules of carbon dioxide used, 6 molecules of oxygen are produced. This 1:1 ratio means that as algae removed CO₂ from the atmosphere, they replaced it with O₂.
Oxygen didn't appear in the atmosphere immediately. For the first few hundred million years, all the oxygen produced by algae was absorbed by iron dissolved in the oceans.
Only when the oceans became saturated with iron oxide did free oxygen begin accumulating in the atmosphere. This process took over a billion years.
Worked example: Timeline of Atmospheric Oxygen
Trace the key stages in Earth's oxygen story from 4 billion years ago to today
Answer: It took 2.1 billion years for oxygen to build up from zero to levels that could support animal life
Why did it take so long for animals to evolve? The answer lies in oxygen levels. Early Earth's atmosphere contained less than 1% oxygen - not nearly enough for complex life.
Animals need oxygen for respiration - the process that releases energy from food. Without sufficient atmospheric oxygen, multicellular organisms couldn't survive.
As plants continued photosynthesising over hundreds of millions of years, oxygen levels gradually increased. When they reached around 15-20% of the atmosphere, complex animals could finally evolve.
We can use the photosynthesis equation to calculate how much oxygen is produced. This helps us understand the massive scale of this planetary transformation.
Worked example: Calculating Oxygen from Photosynthesis
If a forest uses 264g of CO₂ in photosynthesis, how much oxygen is produced? [4 marks]
Answer: 192g of oxygen is produced
Exam questions on this topic often test your understanding of the timeline and the photosynthesis equation. Here's how to tackle them effectively.
For 6-mark questions, structure your answer chronologically: early atmosphere → algae evolution → oxygen absorption by iron → atmospheric accumulation → plant evolution → animal evolution.
Exam precision note: In How Photosynthesis Increased Atmospheric Oxygen - How oxygen increased - The composition and evolution of the Earth's atmosphere, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Photosynthesis gradually increased atmospheric oxygen because early photosynthetic organisms used carbon dioxide and water to make glucose, releasing oxygen as a by-product. At first, much of this oxygen reacted with rocks and dissolved iron, so oxygen only built up in the atmosphere slowly over a very long time.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The balanced equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Plants and algae use light energy to convert carbon dioxide and water into glucose, releasing oxygen as a waste product.
The first algae appeared about 2.7 billion years ago and evolved the ability to photosynthesise. They released oxygen as a waste product, starting the long process that transformed Earth's atmosphere.
Animals need oxygen for respiration, the process that releases energy from food. Without enough atmospheric oxygen, multicellular organisms couldn't survive, which is why animals evolved only after oxygen levels rose.
Oxygen increased because algae, starting about 2.7 billion years ago, and later plants produced oxygen by photosynthesis. At first the oxygen was absorbed by iron in the oceans, but once the iron could absorb no more, free oxygen built up in the atmosphere, which later made animal evolution possible.
Algae began photosynthesising about 2.7 billion years ago, but free oxygen only appeared in the atmosphere about 2.4 billion years ago, once the iron in the oceans could absorb no more. About 600 million years ago, oxygen reached levels enabling animals.
Photosynthesis using 264g of carbon dioxide produces 192g of oxygen. Moles of CO₂ = 264 ÷ 44 = 6 mol; the equation gives 6 mol of O₂ for 6 mol of CO₂, so the mass of oxygen = 6 × 32 = 192g.
Animals need oxygen for respiration, the process that releases energy from food, so complex animals could only evolve once oxygen had built up. When oxygen reached around 15-20% of the atmosphere, complex multicellular life became possible.
The first oxygen made by algae was absorbed by iron dissolved in the oceans, forming rust-coloured bands still seen in ancient rocks today. Only when the oceans became saturated with iron oxide did free oxygen begin to build up in the atmosphere, about 2.4 billion years ago.
Complex animals could only evolve when oxygen reached around 15-20% of the atmosphere, about 600 million years ago. Early Earth's atmosphere contained less than 1% oxygen, not nearly enough for complex life.