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!