We’d still need to eat
Photosynthesis is only 3 to 6 per cent efficient. If you stood naked outdoors all day, you would generate less than 240 calories – about three chocolate digestive biscuits’ worth. If you weren’t eating anything to supplement this, photosynthesis wouldn’t even be enough to offset the heat your naked body would lose. You’d die of hypothermia before you starved.
We’d still need oxygen
Plants generate all of the oxygen they need as a by-product of their own photosynthesis. But since you’d need to eat extra food to power your more active metabolism, you’d also need to breathe extra oxygen in order to convert all of that food into energy. Photosynthesis would let you lower your breathing rate by 10 per cent at most. Photosynthetic astronauts would still need oxygen tanks.
We might not even be green
Most plants need light to synthesise chlorophyll. If humans used the same mechanism, our skin would only be green where it was exposed to the Sun. Office workers and many people in northern latitudes probably wouldn’t get enough sunshine to tint their skin more than pale yellow, except on their face and hands. Those working rolling shifts might cycle between green and white.
More thought experiments:
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