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Explainer: why can’t humans photosynthesise?

You know what, I think we looked better before. clement127/flickr

Little green men feature heavily in movies about aliens. But why aren’t humans green? And why can’t we photosynthesise like plants? It would, after all, save us a whole lot of bother.

The most familiar green organisms are, of course, plants. Plants are green because their cells are packed with internal organelles – organs within cells – called chloroplasts, which are the centres of photosynthesis. These chloroplasts have a rather interesting evolutionary history, as they were once free-living cyanobacteria, independent of plants.

Cyanobacteria are famous for inventing photosynthesis, a process that harnesses the energy in sunlight to make sugar from water and carbon dioxide. But as any inventor can tell you, if you have a great idea then pretty soon everyone else will want a piece of it.

In an astonishing piece of insight, Lynne Margulis realised that the chloroplasts inside plants were domesticated cyanobacteria that had been captured early in the evolution of the plant lineage. It appears that a unicellular ancestor of the land plant engulfed a cyanobacteria, but rather than digesting it, realised that it was a useful acquisition.

But even more fundamental to the functioning of all higher organisms is a second organelle called the mitochondrion. Margulis realised that this too was once a free-living bacterium – in this case one that could harness the chemical energy locked up in sugary substrates such as glucose. So the cells of plants are really chimeras – a single organism made up of the original host plus two captured bacteria. This theory is known as the endosymbiotic theory.

The joy of chloroplasts

Ownership of a chloroplast brings an enormous and immediate benefit. Animals only have mitochondria, which allow them to oxidise glucose and harness the resulting chemical energy to fuel their metabolism. But they have to find a source of glucose. And that means dedicating a substantial part of their day to locating, subduing and consuming food. Plants on the other hand don’t have to bother. They can simply use their chloroplasts to make their own glucose, which they can then pass to the mitochondria to release chemical energy as and when it is required.

So surely everyone else is missing a trick. If plants can bypass finding glucose, then surely animals could too. In fact, many animals have done exactly this. The chloroplast was just too good an invention and many other organisms managed to beg, borrow or steal a chloroplast, mainly from free-living unicellular algae that already had one. This process is known as secondary endosymbiosis to distinguish it from the primary endosymbiotic event, in which the original plant ancestor engulfed a free-living cyanobacterium.

Why can’t I photosynthesise? R~P~M/flickr

It’s not entirely clear why secondary endosymbiosis appears to have occurred many times, while the primary endosymbiosis occurred only once; although recently scientists have discovered a second example of a primary endosymbiosis in the making. In this case the host is a strange amoeba called Paulinella, which appears to be domesticating a cyanobacteria from the new and hence recreating the ancient event that gave rise to the land plants.

The transfer of the chloroplast around the tree of life by secondary endosymbiosis has given rise to a whole host of ecologically important organisms, most of which are unicellular. These organisms, for example, diatoms, dinoflagellates and euglenids, are unknown to most of us and arise from independent acquisitions of a chloroplast from an alga.

Perhaps most interesting of all for our story, these unicellular photosynthesising organisms have themselves been taken up by multicellular animals. These symbioses have evolved independently many times and the relationship between the host and the photosymbiont can take many forms.

Sea slugs can do it – so why can’t we?

For example, there are incredible green sea slugs that steal chloroplasts from the algae on which they graze. The slugs can’t maintain the chloroplasts in working order for long, so they need a constant supply and there’s still debate about whether the chloroplasts are really essential for the slug’s existence. At the other end of the spectrum, many marine organisms such as corals, giant clams and sea squirts are totally dependent on their symbionts and will perish without them.

Elysia grandifolia eating algae. Sylke Rohrlach/flickr, CC BY-SA

So, if slugs and sea squirts can all benefit from photosynthesis, why can’t we?

The answer lies in considering the energy budget of a large active multicellular animal such as a human being. Every day an adult human requires its own body weight in a molecule called ATP, which stores the chemical energy released from the oxidation of glucose.

We need more skin

To produce roughly 60kg of ATP, a typical adult woman therefore requires around 700g of glucose per day. Given the maximum known rates of photosynthesis in higher plants and assuming that the surface area of an adult woman’s skin is around 1.6 m2, a woman with green skin could produce a highly disappointing 1% of her daily demand for glucose through photosynthesis. So to meet her energy demands, a photosynthesising woman would have to have a lot more skin. Indeed, roughly a tennis court’s worth.

To photosynthesise she’d need enough skin to cover a tennis court. Su--May/flicker, CC BY

We must therefore reluctantly conclude that either aliens look substantially weirder than they are currently portrayed in movies or that the photosynthesis that has potentially evolved on other planets colonised by little green men is seriously more efficient than the one that evolved here.

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