Living things, not geology, added the oxygen
Definition
Photosynthesis
The process in which algae and plants use light energy to convert carbon dioxide and water into glucose, releasing oxygen.
Definition
Algae
A diverse group of mainly aquatic organisms that make glucose by photosynthesis.
- The early atmosphere had little or no oxygen, so the oxygen we breathe was added later.
- It was produced by algae and plants through photosynthesis.
- In photosynthesis, organisms use light energy to turn carbon dioxide and water into glucose, releasing oxygen.
- The word equation is: carbon dioxide + water →\rightarrow→ glucose + oxygen.
- The balanced symbol equation is 6CO2+6H2O→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_26CO2+6H2O→C6H12O6+6O2.
Key Idea
Every oxygen molecule in today's air traces back to photosynthesis: living things built up the oxygen over billions of years.
A slow build-up over billions of years
Definition
Glucose
A simple sugar produced from carbon dioxide and water during photosynthesis.
- Algae first appeared in the oceans about 2.7 billion years ago and began releasing oxygen.
- Over the next billion years or so, plants evolved and spread across the land.
- Together they steadily raised the proportion of oxygen in the atmosphere.
- The increase was gradual, not sudden, taking place across an enormous span of time.
- The glucose made in photosynthesis fed the growth of these organisms and stored carbon in their tissues.
Common Mistake
- Do not say oxygen appeared quickly; it built up gradually over roughly a billion years.
- Photosynthesis adds oxygen and removes carbon dioxide, so it changed the air in two ways at once.
Rising oxygen opened the way for animal life
- As oxygen levels rose, the air could support organisms that respire using oxygen.
- This made it possible for animals to evolve and survive on Earth.
- So the increase in oxygen was a turning point in the history of life.
Exam technique
- If asked for the source of atmospheric oxygen, name photosynthesis by algae and plants, not just 'plants'.
- Learn the balanced equation 6CO2+6H2O→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_26CO2+6H2O→C6H12O6+6O2; a correct symbol equation is often worth a mark on its own.
Self review
- Which process increased the amount of oxygen in the atmosphere?
- Which organisms first released oxygen, and about how long ago?
- Write the balanced symbol equation for photosynthesis.
- Was the rise in oxygen sudden or gradual, and over roughly how long?
- Why did rising oxygen levels matter for the evolution of animals?
