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Comment by matt4077 | Hacker News Reader

This needs the additional information that photosynthesis is incredibly inefficient. It's <5% IIRC, so we already have solar panels almost a magnitude better than what nature did.

(RuBisCO as the protein at the center of the process is also quite strange: it's huge and slow. As in 'this ain't funny any more, start working' slow with about a reaction per second.)

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The comparison of photosynthesis vs energy generated by solar panels isn't very good, because it neglects the whole biology of the organism, which is optimized for things other than maximum photosynthetic output. For example, leaves are often targets of herbivory, so a plant might want to make a tradeoff [1] of less efficient photosynthesis for better herbivory defense. Or, it might be too costly to do photosynthesis, which requires the input of carbon dioxide, water, and light. In a very dry & hot environment, to get enough carbon dioxide into the leaf, the leaf will lose water by having its stomata open. Google "leaf economics spectrum" for a quick tutorial (the concept isn't 100% correct but it's a good starting point). Compare the leaves of tropical plants (say a banana palm) to arid plants (mesquite tree).

[1] I use this a teleological shorthand for "the selective environment has weeded out species that happen to fall on the wrong side of the tradeoff."

This is not quite right.

Solar panels create an electric current, not fuel. This is a much easier task.

When you read that photosynthesis has 1-2% efficiency, that's because they're measuring the efficiency of converting light into sugar.

Exactly. I suppose the energy density of sugar is pretty high. The non-nuclear fuel with the highest volumetric energy density is jet fuel[1] at 37.4 MJ/L. I wonder how the most efficient natural compound (adenosine triphosphate / sugar / etc) compares...

[1] https://en.wikipedia.org/wiki/Energy_density#Energy_densitie...

It's not the energy density of a fuel that matters so much as its net energy content and conversion efficiency.

You'll find lipids compare favourably to jet fuel (kerosene), which is distantly related to plant-generated lipids in the first place.

ATP + H20 -> ADP + P produces only 30kJ/mol. A mole of ATP weighs ~500g. You likely get a lot more energy from burning it (the Wikipedia doesn't say), as you do with jet fuel, but that reaction is too hard to reverse to be useful for organisms.

A mole of glucose produces 2800kJ/mol when you burn it. A mole weighs ~180g.

Using http://lmgtfy.com/?q=jet+fuel+density to get a range of 775.0-840.0 g/L for jet fuel, that gives us 37.4MJ/807.5g (taking the average of the jet fuel's mass) against 2.8MJ/180g for glucose.

Scaling the denominator of each to 1kg, that's ~46.3MJ/kg for jet fuel and ~15.6MJ/kg for glucose.

Given how biochemically cheap glucose is to produce compared to jet fuel, I'm surprised it's only off the state-of-the-art by a factor of three.

I don't understand. Photosynthesis has been hailed as one of the most efficient processes by nature.

http://www.scientificamerican.com/article/when-it-comes-to-p...

That particular stage in the hugely complex reaction pathway is hailed as efficient. The whole thing, considerably less so.

I believe the trick is that they harvest "95 percent of it from the light they absorb", emphasis added. It's sort of like saying an efficient solar system only loses 5% of the energy absorbed by the solar panels.

Photosynthesis may be quite efficient internally, but it's not very good at capturing all of the available power. It really is 3-6%, where even cheap solar panels can achieve 15-20% or higher these days.

Photosynthesis is a complex process. That article only covers one step of many.

Efficiencies are ~1-3% in most plants, with 5% a possible high. Algae can reach 10% without various artificial stimulation. Given gro-lites and other factors (which tend to defeat the purpose), much higher per-hectare yields have been achieved.

As others note, plants offer many, er, plantly services. They're (usually) self-supporting, have disease, insect, and pest deterrance capabilities, self-transport water and minerals, and arrange for their own replication.

In many cases, a seed in a hole, or even on bare ground, is all the infrastructure you need to start manufacturing a new plant. Constructed infrastructure tends to have higher investment requirements.

Does anyone know what the evolutionary bottlenecks/tradeoffs are for more efficient photosynthesis?

It is a very very old protein, stuck in local maximum more than anything else.

> stuck in local maximum more than anything else

What's the argument that this is true, rather than it being better than more efficient proteins on some other metric?

Plants in arid environments have evolved a more efficient carbon fixation process -- https://en.wikipedia.org/wiki/C4_carbon_fixation -- in response to decreased water in the environment.

Despite this, C3 plants still dominate the planet, so somehow selection pressure hasn't shifted strongly towards C4 plants in non-arid environments.

According to the wikipedia page, C4 is more efficient in water usage, but less efficient than C3 in energy production.

https://en.wikipedia.org/wiki/C4_carbon_fixation

Oh makes sense. I somehow read over that sentence.

Selection pressure may well have shifted towards C4 in the future if we hadn't come along and been kind enough to dig up some fossilized carbon and put it back into circulation.

I suspect there might well be an evolutionarily balanced tradeoff between "construction cost" and "lifetime energy output" of a plant's photosynthesis machinery - as well as a likely overabundance of available solar energy in terms of what a plant actually can use. (Why build a potentially more complex and fragile photosynthesis pathway that's 50% more efficient, if you can instead just grow twice as much leaf surface area?)

But there are places where plants compete fiercely for small amounts of sunlight, such as in forests (especially tropical rainforests). Doesn't your theory suggest they'd use more efficient systems there?

https://en.wikipedia.org/wiki/C4_carbon_fixation

Example of such an evolutionary step. C4 vs C3 photosynthesis.

C4 is more efficient in water usage, but less efficient than C3 in energy production

Don't forget plants get the energy back from making a leaf in a few weeks. Solar panels take considerably longer. Generally plants are optimized to compete with others in their niche and as such it's exponential growth that's most important not total energy capture.

Is it actually tropical rainforests, or temperate, that limit sunlight most? Old-growth PNW forests often have large patches of western hemlocks that produce so much shade that no other tree can grow under it, not even conifers - and unless there's a fire to clear some space out, it can stay that way for centuries.

I don't know but my guess is to have more efficient collection/processing may require bigger leaves which may be too heavy or catch more wind damaging branches.

Or maybe trees are just lazy by nature?

But this does use solar panels.

> The device uses solar electricity from a photovoltaic panel to power the chemistry that splits water into oxygen and hydrogen. Microbes within the system then feed on the hydrogen and convert carbon dioxide in the air into alcohol that can be burned as fuel

Only the hydrogen + co2 -> alcohol part uses biological components.

but solar panels don't grow out of dirt and replicate themselves.

Yet.

And the first thing that comes to mind is a gray goo scenario.

Life is a gray goo scenario.

Pink goo? Red goo?

Red goo? My circulatory system uses a copper based hemoglobin analogue, you insensitive clod :-)

(arthropod blood)

https://en.wikipedia.org/wiki/Hemocyanin

Green goo and pink goo are the accepted terms, at least according to @cstross.

> but solar panels don't grow out of dirt

Neither do leaves, they grow out of the air ;)

https://www.youtube.com/watch?v=N1pIYI5JQLE

Photosynthesis is between 0.1% and 2% efficient.

Seems that increasing the efficiency of photosynthesis could lead to another agricultural revolution.

Do wonder what knock on effects that may have.

Would not surprise me if it makes such plants more susceptible to diseases.

In fact, increasing the efficiency of photosynthesis is the goal of one such project: c4 rice [1].

[1] - https://c4rice.com/the-science/photosynthetic-pathways/

I remember my biology teacher's description of photosynthesis: "If you have a river of gold running through your backyard, you don't care how much of it you splash all over the place when you carry the buckets full back to your house."

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