Keeping the air in balance
Two processes add carbon dioxide to the air and one takes it out again. That is the whole balance, and one side of it is this chapter.
Putting carbon dioxide in
Respiration and combustion. Every living thing adds to it around the clock, and every fire and engine adds to it as well.
Taking carbon dioxide out
Photosynthesis. The work of removing from the environment the carbon dioxide gas added to it by processes such as respiration and combustion is done by photosynthesis.
What balance means here
Photosynthesis contributes to keeping the composition of oxygen and carbon dioxide in the atmosphere balanced. Grade 10 gave you the figure: carbon dioxide is about 0.03 per cent of the air. That number has stayed small not because little is produced but because roughly as much is removed as is added — and it changes when that stops being true.
The carbon cycle
The same carbon atoms move round and round: from the air into a plant as carbon dioxide, into an animal as food, and back into the air through respiration, decay or burning. Photosynthesis is one of the principal processes that keeps that cycle going — it is the step that brings carbon out of the air and into living things.
Photosynthesis and respiration are each other run backwards
One takes carbon dioxide and water and makes glucose and oxygen; the other takes glucose and oxygen and gives back carbon dioxide and water. Grade 10 named them anabolic and catabolic. Between them they account for the whole of the cycle above, and each one’s waste is the other’s raw material.
In, out, and round again
Photosynthesis removes the carbon dioxide that respiration and combustion add, helps keep atmospheric O2 and CO2 balanced, and is a principal process maintaining the carbon cycle.