What If Humans Could Photosynthesize?

How cool would it be if you could photosynthesize? No more cooking or grocery shopping. If you got hungry, all you’d need is some
tasty sunlight! It sounds awesome, but it turns out that it’s
not so easy being green. Photosynthesis allows plants to take sunlight
and carbon dioxide and turn it into sugar, which they use for food. Oh and it also makes oxygen, which is good
if you like breathing and all. So it sounds simple enough. You sit in the sun and BAM you have a food
supply. Let’s get into the mechanics of photosynthesis. Chloroplasts are cute little organelles in
plant cells that make photosynthesis happen. They contain the molecule chlorophyll, the
stuff that makes plants green. More importantly, the chlorophyll captures
the sun’s energy. Plants are green because chlorophyll is especially
good at absorbing blue and red light, while green and yellow light gets reflected back
to your eye, which is why they, you know, look green. If we had to photosynthesize, we’d need
chloroplasts. So we’d be green. Pretty awesome. So you got your chlorophyll and you’re soaking
up some rays. But it’s not a very efficient process. Estimates vary, but plants only convert about
5 to 9% of the total available energy from the sun’s rays. Since photosynthesis is pretty inefficient
at converting sunlight into energy, we would need to absorb a LOT of light. Plants evolved broad, flat leaves to increase
their surface area and maximize the amount of sun hitting their chloroplasts. So we’d need leaves. Great. We’ll be able to get plenty of sunlight,
but now we run into another issue. Our skin. That thick, oily layer of dead skin cells
covering our entire body would need some adjusting. It prevents carbon dioxide from getting to
the cells in our body. We do exhale CO2 as a waste product, but that
CO2 is mainly produced around the brain and in muscles. To photosynthesize, we’d need carbon dioxide
near the surface of our sk0in. And we’d need a lot more of it than we produce,
so our skin needs to be more porous–like 100 times more porous! That would allow plenty of CO2 in for our
human photosynthetic needs. Carbon dioxide is crucial in photosynthesis. We won’t get too deep into it, but here’s
roughly how it works: Chlorophyll absorbs sunlight. The sun’s energy splits water molecules,
knocking electrons into motion through the chloroplast. This kicks off a series of reactions that
creates molecules that store energy, the most famous one being ATP. The split water molecules also release the
oxygen that we humans need to breathe. Good work plants! Now the plant pulls in carbon dioxide from
the air. Those energy molecules help put carbon dioxide
and the hydrogens from water together in the Calvin Cycle, making sugars like glucose,
the sweet, sweet sugar that feeds plants. But these sugars wouldn’t give us enough
energy to get up and move around. Trees use a lot less energy than us. A typical tree consumes around 200 calories
a day. A typical human consumes around 2000 calories
a day, since we need more fuel for our big needy human brains and for walking around. So we probably wouldn’t make enough energy
from photosynthesis to power our usual high energy human activities. Unless you count watching Netflix. So for us to photosynthesize, we’d be green,
need leaves, have weird porous skin to let CO2 in and we’d have to lie out in the sun
and not move much. Okay that last part doesn’t sound too bad. We’d still have to eat food to get the proteins
the human body requires, but we’d be good on the sugars and fats. Overall photosynthesis doesn’t seem to be
the way to go for humans. But it does work for some sea slugs and giant
clams. These guys eat photosynthesizing algae or
bacteria and use their chloroplasts to make sugars for them to use. So don’t humans do that? Well, scientists think it’s just not worth
it. The energy we would spend to feed and manage
photosynthetic symbionts is better spent foraging and taking care of our other biological needs. So just get the salad. And marvel at the chemistry that keeps life
going. Would you want to photosynthesis? Let us know in the comments. And check out some more plant filled science
while you’re here. Thanks for watching!


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