How oxygen increased: Algae and plants produced the oxygen that is now in the atmosphere by photosynthesis, which can be represented by the equation:

The idea

Photosynthesis is the process by which algae and plants use light energy to convert carbon dioxide and water into glucose and oxygen. 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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Writing the equation with incorrect balancing, such as CO₂ + H₂O → C₆H₁₂O₆ + O₂.

  Put it right: 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.

  Put it right: 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.

  Put it right: Algae first produced oxygen about 2.7 billion years ago, before plants evolved.

Exam 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).