I searched previous discussions on photosynthetic animals and there seems to be consensus on photosynthesis not being able to provide sufficient energy for animals. How bad is this deficit?
How much energy does does photosyntheses provide in calories per surface unit per time unit? How does that compare to our body surface and caloric needs per time unit?
Imagine a human covered in the richest photosynthetic layer that plants have in use. Imagine it covering the entire body and it all being exposed under tropical sun with a white wall on the shaded side to reflect the most sunlight possible. Could that human get enough energy to just sit there doing nothing?
Not responsive to the question, but John Varley has a couple of short stories (“Gotta Sing, Gotta Dance” and “Equinoctial”) that deal with humans being paired with semi-sentient plants to achieve what the OP is asking. They do, however, live in orbit around Saturn.
It seems people have 2m[sup]2[/sup] of surface area (cite)
At 1300 W/m[sup]2[/sup] of solar flux in space we can drop that and say we get 1 kW/m[sup]2[/sup] at the earth’s surface. Best plant level photosynthisis seems to be 2-3% (cite).
So 2m[sup]2[/sup]1000W/m[sup]2[/sup].03 –> 60W
Over the course of a day (assuming 24hr illumination) is 5184kJ or ~1240Cal, which seems high but I’m sure someone will straighten me out soon.
I think you just have a lot of overly generous assumptions. First, power density is about one sixth as much as your figure – this cite says 170 W/m^2. Second, a person will only be able to expose part of their skin to direct sunlight, even lying down it’ll be less than half. And third, you’ll only have solar energy for about half the day, though I think that’s included in the average solar radiation figure. Divide your figure by (2*6=24), and our figurative human can only get 100 calories/day.
Another way to think of it – how small can you make a garden to feed a single person? You certainly need a lot more than a 2 m^2 planter.
I guess I was slow, but here’s my physics-based calcs:
I saw an estimate of average solar flux (reaching the gorund) to be 164 Watts/m^2 (over 24 hours). Wiki says photosynthesis efficiency maxes at around 8% (net biomass growth, so that does account for the cost of growing and maintaining the chlorophyll), though anything that actually moves around could never reach, we’ll go with that for the sake of argument. That gives net of 13 Watts/m^2. Since 1 W is 1 joule/second, over 24 hours, at 4 joules per (small) calorie and 1000 small calories per human Calorie, that means 13 W24hours*3600 seconds/hour / 4 joules/calorie /1000 calories/Calorie, we get 283 Calories/square meter/day.
So generously assuming a large flat human at 1 square meter when lying down (only the upper half gets sunlight), and doing nothing but lying flat in the sun all day, they’d get 283 Calories per day.
Typical human Calorie use for a low-activity lifestyle seems to be 2000 Calories per day or so.
So, using incredibly generous assumptions for the photosynthesizing, it’s still only 10% of what they need, and that’s without moving. More realistic assumptions would probably get down to 1% of what a human needs or less.
As **lazybratchse ** says, the other way to figure this is agriculturally-based by finding out how many calories per year we can grow per square meter of crops. This (Holt Online Learning) (not good but the best I could do quickly) says 2000 Cal/m^2/year. Which it’s easy to see is about 1/365 = 0.3% of human needs.
I spent a long shower one morning musing about what the impact would be if we could give humans functional chloroplasts. It wouldn’t be enough to power us completely, of course, but it could help reduce our need for food. I took it quite far before I ran out of hot water.
Thanks for all the answers so far. It seems that in the best of cases we are talking about 283 calories a day. Not even enough to just lie there under the sun. It might help a surfer or a hiker go a little bit longer carrying one less snack bar, but not really something of practical use to anyone.
I specially liked the insight of thinking it on terms of how much arable surface it takes to feed one person.
There is an animal out there that directly uses photosynthesis. It’s a sea slug that harvests chloroplasts from the algae it eats. Once it grows to a certain size (with enough stolen chloroplasts), it can survive on sunlight alone for the rest of their lives (about a year).
The posts that are doing crop analogies are forgetting to take trophic multiplying into account: the crops we eat consume some of that energy for themselves. If we take the canonical 10% as the amount of energy that can be transferred from one trophic level to another (it’s sometimes slightly higher, usually less), we’d have to assume that we could photosynthesize the equivalent of 20 square meters of cropland, not 2.
I was about to add another caveat that you’d have to take into account the fact that it could be higher or lower than that figure because some of the cropland area might not be used, while some of it might be used more efficiently due to having more area exposed over several meters worth of crop, but when you combine the two you can assume that any given light beam will hit a dense crop before it hits the ground…doh! :smack:
But I’d done the calculations myself, awhile ago, and got in the area of the higher calculations in this thread, i.e. around 300-700 kCal/day (the 700 figure was assuming efficiency greater than 3% for photosynthesis, which I was apparently mistaken on?). Still not enough to live on, though.
Isn’t 700 kCal/day nearing the range of fakirs and the similar? A search for “700 calories per day” gives tons of results on people saying “DON’T” as the body goes into starvation mode and eats itself, so to speak. Could a person trained for life to live on less survive on that amount?
Humans are photosynthetic already - it’s just that the product is vitamin D, not sugar.
If we were designing photosynthetic humans from scratch (assuming we had the technology and understanding to do so), I bet we could make an improvement on the 2 square metre garden figures, simply because we could sidestep losses associated with the processes of digesting the garden - and synthesise ATP directly, or something like that.
Even then, we’re not out-engineering evolution, probably. Photosynthesis as a process is very inefficient. As it stands there is no reason to think it could ever power something like a human. Speculation that we could get it to 50% efficiency from 3% is pure sci-fi. Or scy-fy.
Breakdown of glucose converts 56% of the potential energy into heat. (That may sound bad, but it’s actually pretty close to the theoretical maximum.) Thus, even if you could convert light energy directly to ATP, you’d only double the effective energy obtained from photosynthesis. We’re still only talking about 500 calories or so.
Also I bet we could save a bunch of calories cutting out warm bloodedness, plus our melanin can be used to synthesis energy from radioactive sources, instead of just converting it to waste heat, as it does now. Some fungus does this.
The biggest problem I think is humans can not live by calories alone. How would we metabolize our ‘fertilizer’, such as vitamins and minerals? I’d recon you need a lot of them. Normally they come in our food.
More important than vitamins and trace minerals, we’d need a hefty source of protein, since we can’t synthesize our own amino acids.
Of course, if genetic engineering gets to the point of photosynthetic skin, I’m sure we can add little things like protein synthesis and nitrogen fixing.
