Where the oxygen comes from
The gas this chapter has treated as a leftover is, at the scale of a planet, the most consequential thing photosynthesis produces.
The principal source
Photosynthesis is the principal process by which the oxygen needed for the survival of aerobic organisms, and for the combustion of substances, is released.
Two things depend on it entirely
Aerobic organisms
Every organism that respires aerobically — which is nearly all of them, plants included — needs oxygen to release energy from its food. Grade 10 was clear that every living cell respires, plant cells among them.
Combustion
Nothing burns without oxygen. Every fire, every engine and every furnace is running on gas that came out of a leaf.
The plant does both sides of this
A plant releases oxygen in photosynthesis and consumes oxygen in respiration, and it does both at once during daylight. Over a day it releases far more than it uses, which is why there is any left over for the rest of us. A plant is not an oxygen machine; it is an organism that happens to have a large surplus.
Principal does not mean only
The word in the definition is principal. Oxygen enters the atmosphere by other routes too, in far smaller quantities. What makes photosynthesis the principal one is scale, not exclusivity — and most of that scale is in the oceans, in organisms that are not plants at all.