How much green skin would we need to photosynthesise?

If we could photosynthesise, Mark asks, how much green skin would we need to make enough energy to live our normal lives?
22 August 2017





If we could genetically engineer cells to make our own energy, like plants do using chloroplasts, how much extra skin surface area would we need in order to function with similar energy levels as today's humans?


Georgia - Plants manage to get their energy from sunshine. Tiny factories in their cells called chloroplasts convert it through a process called photosynthesis, and these chloroplasts are what give plants their green colour.

But say we gave ourselves a green makeover to use the same trick, how big would our skin need to be? Well, you guys have been busy crunching the numbers on the forum:

R.D. suggests at ballpark figures you’d need about a third of a ballpark.

And Board Chemist has reasoned you’d need roughly an oak tree’s leaves worth of area.

While on Twitter, people were dealing with the bigger questions:

Diane is concerned we’d need big flat heads.

And Parallel Fibres was imagining power hungry humans looking like extremely flabby sharpeis.

Well here to shed some light on the answer is Christopher Mason, Associate Professor at Whale Cornell Medicine.

Christopher - Before I get into the calculations, we’ll have to make some assumptions. The first being that human skin cells would be capable of making and supporting chloroplasts, would also have to assume that there isn’t any immune reaction that rejects the chloroplasts. And that melanin, the pigment that gives skin our colour, doesn’t interfere with chloroplast function.

Georgia - Ignoring these caveats then, how energy efficient would our new leafy skin be?

Christopher - Even if these edited humans and plants performed photosynthesis at the same levels, the process would still not be 100% efficient. No chemical reaction every really is. Let’s argue that we would only maybe convert 75% of the Sun’s energy but plants don’t capture photons perfectly either. Current estimates are about 5% of efficiency.

Georgia - This means that plants can only perform the already inefficient photosynthesis on 5% of the light they’re exposed to.

Christopher - We’ll assume that our new skin cells would act about the same. Also, on average, each human has about 1.7 sq metres of skin and only maybe half of it would be exposed to the Sun say if you’re laying on your stomach and getting the rays that way. On a good sunny day sunlight energy levels are about 300 watts per sq metres, which is enough to power a normal light bulb for about three hours. So, if you put all those figures together, it looks like we’d only be capable of collecting about 34 kilojoules of energy per hour, and you’d need 10 million joules per day to survive as a human. So if we wanted to function at our normal energy levels we would need 290 hours of maximum sunlight to collect enough energy to just get through one day.

However, if by some stretch of the imagination we could expand our skin 300 times to about the size of two tennis courts, we would only need to sit in the sun for about one hour.

Georgia - And, if you live in the UK, sadly even one hour of sunshine is asking a bit much. It’s not easy being green.

Thank you Christopher Mason for showing us the light.

Next week, we’re hanging Norm’s question out to dry:

If water is a solid below zero degrees celsius, a gas above 100 degrees celsius, why then does my washing dry when the air temperature is below 100 degrees?


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