Humans are about 100W animals, and sunlight is an average of a 250W source. The efficiencies of plants at photosynthesis is somewhere about 1 to 5 %, let’s take 1 %. So we’d need about 40 square meters of a leaf-organ. Lol.

But, let’s imagine a magical future where these proteins and chemical pathways and eneryg losses are mere distractions to our intent and this 1% efficiency means nothing, and we can push it to its theoretical best of something like 11-12%. That’s not too bad… 3-4 square meters.

And you never need to eat.

I wonder if we can design a trailer-car which is basically your food/organics kit.

@Noumenon : If you move, you’ve kind of spent all the energy you stayed still to harness and was barely enough to supply life-maintenance functions…

Animals are not all obliged to move energetically. A sloth could get enough power from the sun for daily life, if it could bear the exposure. Sea stars (starfish, echinoderms) living in shallow water could afford to live entirely on sunlight. It’s possible, even likely, that echinoderm lineages have taken up photosynthesis many times, gone sessile, and then died out before we had a chance to see them. It would be hard but not necessarily impossible to tell from fossils that it had happened.

On a planet where nothing survives without migrating, there might be no real distinction between plants and animals. The plants would move, and the animals would bask, and each might eat the other.

Where nothing happens very fast, there’s no need for a brain to be centralized, or for it to do nothing but think. A slow brain might better be distributed over your entire skin surface, along with photosynthetic jiggery, and be powered directly by it; or double as a circulatory system or dyspeptic liver. If it doesn’t have to be very fast, you don’t need to have so much of it, or have it so densely interconnected as ours are. A slow brain, no matter how clever, might not seem to be a brain at all if you aren’t looking for it.

Anybody looking for non-human intelligent life had better not assume it lives just as fast as we do. It might be right here and we just don’t notice it.

I’m trying to promote the idea that true photosynthesis ends with the production of ATP and NADPH [Carbon Dioxide Fixation in the Dark Ocean] [The Photosynthesis Song and a Pet Peeve]

In photosynthetic bacteria and algae the ATP and NADPH molecules are used for all kinds of things including synthesis of proteins, lipids, and nucleic acids. This is also true in plants although in flowering plants the energy is often stored temporarily as sucrose or starch.

Furthermore, the fixation of carbon using the Calvin cycle is not restricted to photosynthetic species. The two processes, photosynthesis and carbon fixation, are truly uncoupled. Our bias stems (no pun intended) from the fact that the original studies of photosynthesis were done in flowering plants where there’s a direct correlation between oxygen release (photosynthesis) and carbon dioxide uptake (Calvin Cycle) because so much of the chemical energy produced by photosynthesis is devoted to synthesis of sucrose and starch.

You don’t see such a direct correlation in algae and bacteria but photosynthesis in those organisms was only looked at much later.