The Atmosphere

History (continued).

The first green plants appeared on the Earth about 2 billion years ago
(in the sea) and about 0·5 billion years ago on land.

Green plants use photosynthesis to convert
carbon dioxide from the air into glucose (sugar) plus oxygen.

water  +  carbon dioxide       glucose      +    oxygen.
6H₂O_(l)  +    6CO_2(g)           C₆H₁₂O_6(aq)  +    6O_2(g)

The green catalyst used by the plant is chlorophyll,
this is what makes plants look green.
The energy for the reaction is provided by sunlight.

Oxygen is given off by the reaction,
and many of the earlier bacteria on the planet were killed by it.
Oxygen was a pollutant gas to some life at that time!

Over millions of years, photosynthesis reduced the amount
of carbon dioxide in the atmosphere, and replaced it with oxygen.
Carbon dioxide became increasingly contained in fossil fuels
and carbonate rocks (chalk and limestone - see the carbon cycle).

Continued on the next page.

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