Why can’t we photosynthesize?

We can’t photosynthesize because we don’t have chloroplasts, and we wouldn’t get enough food out of photosynthesis to make it worthwhile, anyway.

The simple answer to why we can’t photosynthesize is that we don’t have any of the biological equipment necessary for the process. However, things become more complex when we consider that there are certain animals, including at least one vertebrate, that can photosynthesize.

In plants, photosynthesis takes place in special units inside the cell called plastids. Plastids containing chlorophyll, the green pigment that captures light for photosynthesis, are called chloroplasts. Humans can’t make plastids, as we don’t have the genes for this process.

It turns out that plants originally didn’t have the genes for it either, so 1.5 billion years ago, they found some bacteria that did have the genes, and invited them over—forever. Plastids were originally photosynthetic bacteria called cyanobacteria, which took up residence inside other cells in a symbiotic (mutually beneficial) relationship. The cyanobacteria benefited from having a safe home with strong cell walls and a constant supply of water; their new hosts benefited from the sugars produced by the cyanobacteria. It was the beginning of a beautiful friendship, and now, 1.5 billion years later, the algae and plants that resulted from this union have conquered the world.

Living on borrowed plastids

If plants are playing host to someone else’s chlorophyll, why can’t we? There are many species of sea slug that eat algae, suck out their plastids, and give them new homes in their skin. They are able to keep the plastids alive for up to 10 months before they must consume more algae to replace them. Sea slugs that do this can live for up to 9 months without eating, relying on photosynthesis to feed them instead.

Even more intriguing, biologists have recently discovered that the spotted salamander (Ambystoma maculatum) can also play host to photosynthesizing guests inside its own cells. This is an exciting discovery because vertebrates (animals with backbones, such as salamanders and humans) have immune systems that are supposed to stop foreign cells from living inside their bodies. If salamanders can overcome this barrier, is it possible that humans can? With a bit of genetic tinkering, we might be able to engineer humans with plastids in their skin cells, so that they can photosynthesize and enjoy a supply of free sugar whenever the sun shines.

All the same, even if we could do this, it probably wouldn’t be worth it. Humans don’t have that much skin to photosynthesize with, and photosynthesis is not a particularly efficient process. According to one analysis, a photosynthesizing human lying naked in the sun at midday for an hour would produce just 15 calories of energy, equivalent to about a sixth of an apple. Humans need around 2,400 calories per day, so in order to survive by photosynthesis alone, a “green” person would have to sunbathe for 150 hours a day!