AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
How carbon dioxide decreased: Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis.
Early Earth's atmosphere contained much more carbon dioxide than today.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Early Earth's atmosphere contained much more carbon dioxide than today. Photosynthesis in algae and plants removed carbon dioxide and stored the carbon in biomass. In photosynthesis, carbon dioxide and water react using light energy to produce glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The glucose is used for growth, so carbon becomes locked in plant and algal tissue. Over millions of years, this process gradually lowered the percentage of carbon dioxide in the atmosphere while raising oxygen. For example, marine algae photosynthesise in surface waters, forming biomass that may sink and become buried. The statement links a biological process to a global atmospheric change, so answers should name photosynthesis, identify algae and plants as the organisms, and explain that carbon is removed from the air and incorporated into biomass.
What earns the marks
- Photosynthesis uses carbon dioxide from the atmosphere as a reactant.
- Algae and plants are photosynthetic organisms that absorb carbon dioxide.
- The carbon from carbon dioxide becomes part of glucose and then biomass.
- Oxygen is released as a by-product, contributing to the rise in atmospheric oxygen.
- Over geological time, repeated photosynthesis by vast numbers of algae and plants reduced the percentage of carbon dioxide in the atmosphere.
- The carbon removed is stored in living tissue and can later be buried, so it is not immediately returned to the air.
Common mistakes and how to put them right
Saying plants and algae 'breathe in' carbon dioxide.
Put it right by stating that carbon dioxide is a reactant used in photosynthesis, not taken in by breathing.
Writing that photosynthesis produces carbon dioxide.
Put it right by stating that photosynthesis removes carbon dioxide and produces oxygen.
Confusing the role of algae with that of animals.
Put it right by noting that algae are photosynthetic organisms, whereas animals do not photosynthesise.
Exam tips
- Name the process as photosynthesis and state the word equation or balanced symbol equation to show carbon dioxide is used.
- Link the removal of carbon dioxide to the storage of carbon in biomass rather than saying the gas simply disappears.
- Use comparative language such as 'decreased the percentage' rather than 'removed all carbon dioxide', because other processes also affected the atmosphere.
Try the lesson
The start of the How carbon dioxide decreased: Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis. lesson, made from checked teaching notes for this specification point.
Have a go first
Which process in algae and plants removed carbon dioxide from the early atmosphere?
Listen
Early Earth's atmosphere contained much more carbon dioxide than today. Photosynthesis in algae and plants removed carbon dioxide and stored the carbon in biomass. In photosynthesis, carbon dioxide and water react using light energy to produce glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The glucose is used for growth, so carbon becomes locked in plant and algal tissue. Over millions of years, this process gradually lowered the percentage of carbon dioxide in the atmosphere while raising oxygen. For example, marine algae photosynthesise in surface waters, forming biomass that may sink and become buried. The statement links a biological process to a global atmospheric change, so answers should name photosynthesis, identify algae and plants as the organisms, and explain that carbon is removed from the air and incorporated into biomass. A strong answer earns marks for these points. Photosynthesis uses carbon dioxide from the atmosphere as a reactant. Algae and plants are photosynthetic organisms that absorb carbon dioxide. The carbon from carbon dioxide becomes part of glucose and then biomass. Oxygen is released as a by-product, contributing to the rise in atmospheric oxygen. A common slip: Saying plants and algae 'breathe in' carbon dioxide. Instead, stating that carbon dioxide is a reactant used in photosynthesis, not taken in by breathing. Exam tip: Name the process as photosynthesis and state the word equation or balanced symbol equation to show carbon dioxide is used.
Key words
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Photosynthesis
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Biomass and carbon storage
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Atmospheric evolution
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See every definition
- Photosynthesis
- A process in which carbon dioxide and water react using light energy to produce glucose and oxygen.
- Biomass and carbon storage
- The carbon from carbon dioxide becomes part of glucose and then biomass, stored in living tissue and able to be buried later.
- Atmospheric evolution
- Over geological time, repeated photosynthesis by vast numbers of algae and plants reduced the percentage of carbon dioxide in the atmosphere.
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