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Articles by Brian Jackson

Photosynthesis and You

16 December 2013

I heard the claim recently that once science has the ability to produce a "photosynthesis pill," human beings' problems with obesity and malnutrition are at an end. After all, if food's no longer a problem, most of society's ills kind of evaporate, and we can finally have that commune the vegans have been promising us for decades.

I've decided to test this theory with a little back-of-the-napkin mathematics inspired by Randall Monroe's brilliant article, Green Cows. So let's try this out:

At ground level, average solar irradiance (incoming power from our sun) is around 1361 watts per square meter.

While most plants have about a 2% efficiency, some sugar canes have a much greater efficiency in the 8-10% range. According to the textbook Photosynthesis by Hall and Rao, the maximum photosynthesis rate happens around 100 watts per square meter, which we have covered.

Furthermore, a human body has around two square meters of surface area, all put together. Let's assume for the sake of this experiment that I'm willing (for the sake of world peace and prosperity, remember) to be naked as a jaybird and surrounded by reflectors so every part of me can catch the sunlight. I suppose my photosynthesis pill had better prevent sunburn too, but that's another story.

So far, that gives me 200 watts while I'm lit up. Or, to be more pragmatic, that's 0.0478 (dietary) Calories per second.

Now, let's go ahead and assume I live in Jacksonia, where every day is the summer solstice and I get 15 hours of sun a day. That's 54,000 seconds of brilliant sunlight where I can sit and meditate on how much better this is than eating all those icky carbs and red meats.

Over the 15 hours, that gives me 2581 Calories. Decent! That's enough to feed me for a day, plus a little extra so maybe I can play non-competitive shareball with the other nude perma-sunbathers if I don't have to work too hard to do it.

But wait, what about my trip to Dayton to visit my family? There's no way I can match the efficiency of a sugar cane in this climate; my efficiency is going to be closer to a normal plant. Let's say I'm still more efficient than the average crop. Say, maybe 4% efficiency? That's better than wheatgrass can do, and better than the 2% that crops in Ohio average. Anyway, I've still got 56 watts per square meter to work with.

Wait, did I say December? I'll need to pack some shorts and a T-shirt, then. That's only 9.5 hours of sunlight, and I'm covering 60% of my skin now. That's 65.3 watts, or 0.0166 Calories per second. Over the course of a day, now I can generate 567 Calories provided I'm outdoors, aligned towards the sun, and I can somehow avoid shivering, which eats up some of that energy.

So I can produce about three pop-tarts' worth of calories. Two and a half, if they're Smore's. I love Smore's.

But I'm still wearing shorts and a T-shirt, producing twice as much energy as plants that survive in Ohio can produce, and I still have to avoid breaking a sweat, because then I'll need water and salts. Oh, and I'd better not walk, because then I'll need calcium to keep from developing osteoporosis or stress fractures. And I'd better not move at all, because I need to conserve protein.

This is so much better than eating.

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