BBC Future has an interesting round-up of animals that can photosynthsize like plants, but concludes:
Even if the symbionts took, even if the controlling genes were successfully added, would this make a difference to us? Probably not. Photosynthesis is a useless ability without some way of exposing yourself to as much of the Sun’s energy as possible. That requires a large surface area, relative to their volume. Plants achieve that with large, horizontal, light-capturing surfaces – leaves. Elysia, the sea slug, being flat and green, looks like a living leaf. It’s also translucent, so light can pass through its tissues to the chloroplasts within.
Humans, on the other hand, are pretty much opaque columns. Even if our skin was riddled with working chloroplasts, they would only manufacture a fraction of the nutrients we need to survive. “Animals need a lot of energy, and moving at all doesn’t really jive well with photosynthesis,” says Agapakis. “If you imagine a person who had to get all of their energy from the sun, they’d have to be very still. Then, they’d need a high surface area, with leafy protrusions. At that point, the person’s a tree.”
Full Story: BBC: Will we ever… photosynthesise like plants?