Oxygen leaves the leaf
The gas that every animal on Earth depends on is, from the plant’s point of view, what is left over.
The by-product
Oxygen, O2, is produced as a by-product in photosynthesis, and it travels to the atmosphere by diffusion through the stomata.
Out through the same pores the carbon dioxide came in
Nothing about a stoma decides which way a gas moves. Carbon dioxide is being used up inside the leaf, so it diffuses inwards; oxygen is being produced inside the leaf, so it diffuses outwards. Two gases crossing in opposite directions through the same hole, each following its own concentration.
Grade 10 made this point and it is worth keeping
The oxygen we breathe is not something plants make for us — it is what is left over when the plant makes its own food. The same chapter made the sharper version of the point: in a plant, oxygen is the waste of photosynthesis and the requirement of respiration, and carbon dioxide is the reverse. Waste is not a property of a substance. It is a status relative to the process that produced it.
Assignment 2.2
Find out and report
Investigate the various adaptations plants show for absorbing solar energy efficiently during photosynthesis, and prepare a report on them.
Somewhere to start: think about why leaves are flat and thin rather than thick, why they are held out sideways from the stem rather than hanging, why the palisade cells stand upright and sit near the upper surface, and why the chloroplasts inside them move — which you watched them do in activity 2.1.