How oxygen increased — AQA GCSE Chemistry
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How oxygen increased explained
This statement explains how oxygen levels in the atmosphere increased due to photosynthesis by algae and plants.
Read the full explanation
You need to know that algae first produced oxygen about 2.7 billion years ago, and that oxygen soon appeared in the atmosphere. Over the next billion years, plants evolved and the percentage of oxygen gradually increased until it was high enough for animals to evolve. The process of photosynthesis can be represented by the equation: carbon dioxide + water → glucose + oxygen, or 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. You should be able to describe this sequence and explain the link between photosynthesis and rising oxygen levels. This is assessed by your ability to recall the timeline and explain the cause of oxygen increase.
Your focus
- Describe the role of algae and plants in producing oxygen through photosynthesis.
- Recall the word and balanced symbol equations for photosynthesis.
- Explain how the increase in atmospheric oxygen enabled animals to evolve.
How oxygen increased exam tips
Marking Points
- State that algae first produced oxygen about 2.7 billion years ago.
- Describe how oxygen appeared in the atmosphere soon after algae began photosynthesis.
- Explain that over the next billion years plants evolved and gradually increased the percentage of oxygen.
- Recall the word equation for photosynthesis: carbon dioxide + water → glucose + oxygen.
- Recall the balanced symbol equation for photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Explain that the increase in oxygen enabled animals to evolve.
Examiner Tips
- 💡Learn the timeline: algae (2.7 billion years ago), then oxygen appearance, then plants evolving, then gradual increase in oxygen.
- 💡Practice writing both the word and balanced symbol equations for photosynthesis, ensuring correct formulas and state symbols if required.
- 💡When explaining how oxygen increased, always link it to photosynthesis by algae and plants.
Common Mistakes
- Confusing the order of events, such as stating that plants evolved before algae. Correction: algae first produced oxygen about 2.7 billion years ago, and plants evolved later.
- Writing the photosynthesis equation incorrectly, such as omitting water or glucose. Correction: ensure the equation includes carbon dioxide, water, glucose, and oxygen, and is balanced.
- Thinking that oxygen levels increased suddenly rather than gradually. Correction: emphasise that the percentage of oxygen gradually increased over about a billion years.