How Oxygen Increased

Section 1 of 7Source 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.

Section 2 of 7Photosynthesis

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

Section 3 of 7Chain 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.

Section 4 of 7Wider 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.

Section 5 of 7Balancing 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.

Section 6 of 7Key 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.

Section 7 of 7Exam technique

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.