9.1.3Chemistry of the Atmosphere
How Oxygen Increased — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 4 of 11
Part 1 of 5Source of the oxygen
Algae first
- Algae first About 2.7 billion years ago, simple algae and photosynthetic bacteria started to live in the oceans.
- They used sunlight to make food, releasing oxygen as a waste product.
Plants later
- Plants later As land plants spread, they added much more oxygen to the air.
Slow rise
- Slow rise Oxygen built up over about two billion years until it reached roughly 20%, the level seen today.
Part 2 of 5Photosynthesis
| Form | Equation |
|---|---|
| Word | carbon dioxide + water → glucose + oxygen (in light) |
| Symbol | 6CO2 + 6H2O → C6H12O6 + 6O2 |
| Energy | Absorbed from sunlight, so the reaction is endothermic |
| Where | In chloroplasts, which contain the green pigment chlorophyll |
Part 3 of 5Chain of change
1Photosynthetic organisms appear
- Photosynthetic organisms appear They absorb carbon dioxide and release oxygen.
2Oxygen in the oceans
- Oxygen in the oceans At first it reacted with dissolved iron and other substances, so little reached the air.
3Oxygen enters the air
- Oxygen enters the air Once those reactions were used up, free oxygen accumulated in the atmosphere.
4Life diversifies
- Life diversifies Animals and other organisms that need oxygen could evolve.
Part 4 of 5Wider effects
Results of rising oxygen
Less carbon dioxide
- Less carbon dioxide Photosynthesis removed carbon dioxide from the atmosphere at the same time.
Ozone layer
- Ozone layer Some oxygen formed ozone, which absorbs harmful ultraviolet radiation and protected life on land.
Aerobic life
- Aerobic life Organisms using oxygen for respiration released far more energy from food.
Part 5 of 5Balancing the photosynthesis equation
1
Write the formulae: CO2 + H2O → C6H12O6 + O2.
2
Balance carbon first: glucose contains six carbon atoms, so six CO2 are needed.
3
Balance hydrogen next: glucose has twelve hydrogen atoms, so six H2O are needed.
4
Count oxygen on the left: 12 + 6 = 18 atoms; glucose uses 6, leaving 12 for 6O2.
5
The balanced equation is 6CO2 + 6H2O → C6H12O6 + 6O2.
Key terms
| Term | Meaning |
|---|---|
| Photosynthesis | The process by which green organisms make glucose and oxygen from carbon dioxide and water using light. |
| Algae | Simple photosynthetic organisms that live mostly in water. |
| Glucose | A sugar, C6H12O6, made in photosynthesis. |
| Ozone | A form of oxygen, O3, which absorbs ultraviolet light. |
| Endothermic | Absorbing energy from the surroundings. |
Avoid these mistakes
1
Writing that plants take in oxygen during photosynthesis; they take in carbon dioxide.
2
Leaving out the light energy needed on the equation.
3
Saying oxygen appeared suddenly; it increased gradually over billions of years.
4
Forgetting that algae produced oxygen before land plants did.
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