Revision notes for AQA GCSE Chemistry How oxygen increased. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
How oxygen increased
What you'll learn
- How algae and later plants increased the amount of oxygen in Earth’s atmosphere.
- The word and symbol equation for photosynthesis.
- Why oxygen levels rose slowly over a very long time.
- How to explain the link between photosynthesis and the evolution of animals.
The big picture: Earth’s atmosphere changed over time
Earth’s atmosphere has not always had the same composition as it does today. In the early atmosphere, there was very little or no oxygen.
Today, oxygen makes up about 21% of the atmosphere, but that oxygen had to be produced over billions of years.
Definition
Atmosphere
The atmosphere is the layer of gases surrounding the Earth.
The key process that increased oxygen was photosynthesis, carried out first by algae and later by plants.
The organisms that made oxygen
Algae came before plants
The first important oxygen-producing organisms were algae. Some early algae lived in water and began producing oxygen about 2.7 billion years ago.
Definition
Algae
Algae are simple photosynthetic organisms, often found in water. They are not the same as land plants, but they can make food by photosynthesis.
Over time, more complex plants evolved. These plants also photosynthesised, adding more oxygen to the atmosphere.
Common Mistake
Plants were not first
Do not write that land plants produced the first oxygen. The GCSE point is that algae first produced oxygen about 2.7 billion years ago, and plants evolved later.
Photosynthesis: the process that releases oxygen
What photosynthesis does
Photosynthesis is the process where algae and plants use light energy to make glucose from carbon dioxide and water. Oxygen is released as a product.
Definition
Photosynthesis
Photosynthesis is the process by which algae and plants use light energy to convert carbon dioxide and water into glucose and oxygen.
The word equation is:
carbon dioxide + water → glucose + oxygen
In symbols, with light shown above the arrow:
6CO2(g)+6H2O(l)→lightC6H12O6(aq)+6O2(g)6\text{CO}_2\text{(g)} + 6\text{H}_2\text{O(l)} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} + 6\text{O}_2\text{(g)}6CO2(g)+6H2O(l)lightC6H12O6(aq)+6O2(g)The oxygen in the atmosphere came from the product oxygen gas, shown as O2(g)\text{O}_2\text{(g)}O2(g).
Key Idea
Oxygen is a product
Photosynthesis removes carbon dioxide from the surroundings and produces oxygen gas. This is why photosynthesis changed the atmosphere over time.
Reading the equation carefully
The equation tells you several important things:
- Carbon dioxide and water are the reactants because they are used up.
- Glucose and oxygen are the products because they are made.
- Light is needed for the process to happen.
- Oxygen is written as O2\text{O}_2O2 because oxygen gas exists as molecules containing two oxygen atoms.
Tip
Reactants and products
In a chemical equation, reactants are on the left of the arrow and products are on the right. For photosynthesis, oxygen is on the right, so oxygen is made.
Example
Using the photosynthesis equation
A student is asked: “Explain why photosynthesis increased the amount of oxygen in the atmosphere.”
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Identify the process in the question: photosynthesis uses carbon dioxide and water in the presence of light.
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Use the equation to decide what is made: oxygen, O2(g)\text{O}_2\text{(g)}O2(g), appears on the right-hand side, so it is a product.
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Link the product to the atmosphere: because algae and plants released oxygen gas over a very long time, oxygen built up in the atmosphere.
Why oxygen did not rise instantly
Even though algae began producing oxygen about 2.7 billion years ago, the atmosphere did not immediately become oxygen-rich.
At first, much of the oxygen produced would not have stayed in the air straight away. Some oxygen dissolved in the oceans, and some reacted with other substances. Eventually, enough oxygen was produced for it to begin accumulating in the atmosphere.
For GCSE Chemistry, the key idea is the overall trend: photosynthesis produced oxygen, and over time the percentage of oxygen in the atmosphere gradually increased.
Common Mistake
Instant oxygen rise
Avoid saying that oxygen suddenly jumped to today’s level as soon as algae appeared. The increase was gradual and happened over hundreds of millions to billions of years.
The timeline you need to know
1. Early Earth had little or no oxygen
The early atmosphere contained very little oxygen. It had much more carbon dioxide than today.
2. Algae began photosynthesis
About 2.7 billion years ago, algae began producing oxygen by photosynthesis.
3. Oxygen appeared in the atmosphere
Soon after algae started producing oxygen, oxygen began to appear in the atmosphere.
4. Oxygen percentage gradually increased
Over the next billion years, plants evolved and the percentage of oxygen continued to increase.
5. Animals could evolve
Eventually, oxygen reached a level that allowed animals to evolve. This is because animals need oxygen for aerobic respiration, the process that releases energy from glucose using oxygen.
Definition
Aerobic respiration
Aerobic respiration is the process in living organisms where glucose reacts with oxygen to release energy.
You do not need detailed respiration chemistry for this sub-topic, but you should understand the link: more oxygen in the atmosphere made animal life possible.
Key Idea
The cause-and-effect chain
Algae and plants photosynthesised → oxygen was produced → oxygen built up in the atmosphere → oxygen levels became high enough for animals to evolve.
Carbon dioxide and oxygen changed in opposite directions
Photosynthesis uses carbon dioxide and produces oxygen. So, as photosynthesis continued:
- the percentage of carbon dioxide decreased
- the percentage of oxygen increased
This does not mean photosynthesis was the only process affecting the atmosphere, but it is the process you need to connect directly to the rise in oxygen.
Example
Explaining atmospheric gas changes
A graph shows carbon dioxide decreasing and oxygen increasing after algae appeared. Explain why both trends happened.
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Use the reactants in the photosynthesis equation: carbon dioxide is on the left-hand side, so algae and plants used it up during photosynthesis.
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Use the products in the equation: oxygen is on the right-hand side, so algae and plants released oxygen gas.
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Connect repeated photosynthesis to long-term change: because photosynthesis happened for a very long time, carbon dioxide gradually decreased while oxygen gradually increased.
How to describe evidence from aquatic plants
The spec mentions that this topic is a good opportunity to show that aquatic plants produce oxygen in daylight.
An aquatic plant is a plant that lives in water. In daylight, it photosynthesises. You can often see bubbles coming from the plant. These bubbles are oxygen gas.
A simple classroom demonstration might involve placing pondweed in water and shining light on it. If bubbles are produced, that supports the idea that photosynthesis produces a gas. The gas can be tested and shown to be oxygen because oxygen relights a glowing splint.
Definition
Glowing splint test
The glowing splint test is used to test for oxygen gas. Oxygen relights a glowing splint.
Common Mistake
Bubbles are not carbon dioxide
In this demonstration, the bubbles from pondweed in light are oxygen produced by photosynthesis, not carbon dioxide. Carbon dioxide is a reactant in photosynthesis.
What the exam usually wants
For this section, exam questions are often about explanation rather than calculation. You should be able to write a clear sequence:
- Algae appeared and photosynthesised.
- Photosynthesis produced oxygen.
- Oxygen gradually built up in the atmosphere.
- Later, plants evolved and added more oxygen.
- Higher oxygen levels allowed animals to evolve.
If asked for the equation, give either the word equation or the balanced symbol equation, depending on what the question asks for.
Exam technique
In the exam
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If the question asks how oxygen increased, name photosynthesis and mention algae first, then plants later.
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Use the equation to support your explanation: carbon dioxide and water are used, while oxygen is produced.
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Include the timescale if needed: algae first produced oxygen about 2.7 billion years ago, and oxygen increased gradually over a very long time.
Self review
Check yourself
- What process produced the oxygen that built up in Earth’s atmosphere?
- Why is it more accurate to say algae produced the first oxygen, rather than plants?
- How does the photosynthesis equation show that carbon dioxide decreased while oxygen increased?
The composition and evolution of the Earth's atmosphere
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