How oxygen increased — AQA GCSE Combined Science
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How oxygen increased explained
Photosynthesis is the process by which algae and plants use light energy to convert carbon dioxide and water into glucose and oxygen.
Read the full explanation
The balanced symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This process released oxygen into the early atmosphere over billions of years, gradually raising its percentage. The glucose produced is used for respiration, growth and storage, while the oxygen either enters the atmosphere or is used in respiration. In exams, you may be asked to identify reactants and products, balance the equation, or explain how photosynthesis changed the atmosphere. Remember that the oxygen now in the atmosphere is a direct result of this process acting on a global scale over geological time.
Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve.
About 2.7 billion years ago, algae began producing oxygen through photosynthesis. This oxygen initially dissolved in the oceans and then entered the atmosphere. Over the following billion years, plants evolved and photosynthesis continued on a massive scale, gradually raising the atmospheric oxygen concentration. Eventually, oxygen levels became high enough to support the evolution of animals that rely on aerobic respiration. This sequence of events is a key part of the history of Earth's atmosphere. In exams, you may be asked to describe the timeline, explain why oxygen increased slowly, or discuss the consequences for animal life. Remember that the increase was gradual and took billions of years, not a sudden change.
Your focus
- Write the balanced symbol equation for photosynthesis.
- Describe how algae and plants produced oxygen in the early atmosphere.
- Explain the link between photosynthesis and the increase in atmospheric oxygen.
Show all 6 objectives
- Outline the sequence of events that led to oxygen accumulation in the atmosphere.
- Explain why the increase in oxygen was gradual and took billions of years.
- Relate the rise in atmospheric oxygen to the evolution of animals.
How oxygen increased exam tips
Marking Points
- State that algae and plants carry out photosynthesis, using light energy to convert carbon dioxide and water into glucose and oxygen.
- Write the balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Identify the reactants as carbon dioxide and water, and the products as glucose and oxygen.
- Explain that the oxygen produced is released into the atmosphere, contributing to the increase in atmospheric oxygen over time.
- Recognise that photosynthesis also removes carbon dioxide from the atmosphere, which is why carbon dioxide levels decreased as oxygen increased.
- 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 continued to produce oxygen.
- Describe how the percentage of oxygen gradually increased over time.
- Explain that this increase in oxygen eventually enabled animals to evolve, as they require oxygen for aerobic respiration.
Examiner Tips
- 💡Learn the balanced symbol equation for photosynthesis; you may be asked to write it from memory or complete it.
- 💡When explaining how oxygen increased, always link the process to the organisms (algae and plants) and the time scale (billions of years).
- 💡Use the correct chemical formulae and state symbols if required; for example, CO₂(g) and O₂(g).
- 💡Use the specific time frame 'about 2.7 billion years ago' when referring to the first oxygen production by algae.
- 💡When describing the increase in oxygen, emphasise the gradual nature and the role of both algae and plants.
- 💡Link the rise in oxygen to the evolution of animals, explaining that animals need oxygen for respiration.
Common Mistakes
- Writing the equation with incorrect balancing, such as CO₂ + H₂O → C₆H₁₂O₆ + O₂. Correction: ensure six carbon dioxide and six water molecules are used to produce one glucose and six oxygen molecules.
- Confusing photosynthesis with respiration, stating that plants take in oxygen and release carbon dioxide. Correction: photosynthesis takes in carbon dioxide and water and releases oxygen; respiration does the opposite.
- Forgetting that algae, not just land plants, were responsible for the initial oxygen production. Correction: algae first produced oxygen about 2.7 billion years ago, before plants evolved.
- Stating that oxygen appeared suddenly or that animals evolved at the same time as algae. Correction: oxygen increased gradually over billions of years, and animals evolved much later once oxygen levels were sufficient.
- Confusing the order of events, such as saying plants evolved before algae. Correction: algae first produced oxygen about 2.7 billion years ago, and plants evolved later.
- Assuming that oxygen levels rose to the current percentage immediately. Correction: the increase was gradual and took about a billion years or more to reach a level that could support animals.