What is Life Efficiency?

What is Life Efficiency? We now have a scientific overview of how an ecosystem works. Green plants share out the space available to the ecosystem among themselves and on professional lines. Each kind of plant has a separate niche, specializing in living on good soil or bad, being early in the season or late, being big or little. And these green plants trap some of the energy of the sun to make fuel. Some of this fuel they use, some are taken by animals, and much goes to rot.

The fuel taken by the animals at the bottom of the Estonian pyramid is mostly burned up by the herbivores themselves, but a portion is taken by their predators, and so on for one or two more links up the food chains. At each level in the pyramid, there are many species of animals, the numbers of each being set by its chosen profession or niche. All the animals and plants use much of their fuel to make as many babies as possible, and many of these babies are used as fuel by other animals.

Every animal and plant in this ecosystem has an appointed place defined both by its level in the pyramid and by its niche. All these living things are tied together in a great web of eating and being eaten, and an ecosystem is a complex community of energy consumers, all straining to get the most and do their best with it. The result of all these individual efforts is the self-perpetuating mechanism of nature at which we wonder.

But how good is that mechanism really? It certainly works, and it undoubtedly is long-lasting, but is it efficient? This question has more than academic interests because the future of our human population depends on the fuel-gathering efficiencies of ecosystems. So, we ask whether the plants and animals of wild ecosystems are efficient converters of energy and whether the agricultural ecosystems on which we depend are better or worse than the wild ones.

Once we know the answers to these questions, we want to know what sets the limits to efficiency and whether we can do anything to improve upon whatever it is. We first look at the plants, because they perform the most important task of subverting the sun to make fuel and ask how efficient they are as factories of fuel.

The plants that now exist must be “fit” plants, they must be able to leave more offspring than have plants that might have been, which in turn means that they must be able to win more food than could have been which means that they must be more efficient at trapping the sun than were the might-have-been. Thus, a Darwinian ecologist expects all plants to be superbly efficient.

We see that the green receptors and transducers of energy that we call “leaves” are indeed stacked up on the face of the earth in the formidable array. So far so good. But we expect the chemistry and thermodynamics of those green transducers to be as efficient as the leaves are abundant.

We hear engineers talk about the efficiency of automobiles or steam engines, by which they mean how much of the energy supplied as fuel is converted to useful work. They often talk of efficiencies of 20 or 30 percent. With these thoughts, we turn to the practical measurements of what plants and animals can really do.

The efficiency of plants was first determined by a fine piece of armchair scholarship. It was done by Nelson Transeau in an office of an old building of The Ohio State University in Columbus when he was seeking material for a presidential address to the local academy of sciences.

The plant on which this scholar mused was the humble com plant, so suitable for armchair scholarship because anything measurable about com can be found out from the library. No one had thought before how to measure its efficiency, but they had measured everything an ingenious man might need to calculate it.

A crop of com begins with the bare, plowed ground, a place of zero production, zero efficiencies. The corn then grows, zealously defended by the farmer from browsing animals and pests, until maturity. During the intervening weeks, the com plants have been receiving sunlight and converting it first to sugar, then to all the other ingredients of the plant’s structure.

Every calorie these com plants trapped had one of two possible fates: either it was burned by the plant itself to do the work of growing and living or it was still there at harvest time, dormant as potential energy in that standing crop. Com plants have been weighed often enough, and an agricultural handbook readily gives average figures for the yield of grain, leaves, stems, roots, everything.

Also known is how many calories are in a gram of grain, leaves, roots, and the rest; just as the number of calories in a gram of sugar or ice cream is known. So, one can add up the calories in a field of corn. Finding out how many calories the plants have burned during their lives is trickier, but, as we shall see, this can be discovered too.

Transeau mused about an acre of land in the state of Illinois, a good place to begin because someone had measured how many calories came onto the land of that state from the sun on a typical summer’s day. A nice crop of good corn growing on that acre would constitute a population of ten thousand plants. These grew from germination to harvest, as it happened, in exactly one hundred days.

Now it was necessary only to go to the handbooks to find out how much poundage was represented by ten thousand well-grown com plants. Transeau did this, then did a little calculation to convert all the cellulose, protein, and other chemicals they represented back into the sugar from which they had originally been made. In his mind’s eye, Transeau saw not a field of ten thousand yellowing, rustling plants but a beautiful pile of glistening white sugar. The sugar weighed 6,678 kilograms.

Now Transeau needed only to know how much sugar these ten thousand plants had burned in their hundred days of life, and his own notebooks gave him this figure. Transeau had pioneered the measurement of breathing in plants, and by the time of that presidential address of 1926 he had all the figures he needed. These had come from com plants that Transeau had grown in glass chambers to which he could control the air supply.

He measured the carbon dioxide going into the chambers and the carbon dioxide coming out. In total darkness his experimental plants would respire as an animal does, burning sugar to give them calories for work, disposing of the combustion gases into the air.

The excess carbon dioxide coming out of the glass chambers was thus a measure of the combustion, a measure of sugar burned. Transeau’s notebooks told him how much sugar typical com plants of varying ages would bum in a day.

It was simple now to work out how much sugar would have been burned by ten thousand plants in one hundred days, and soon Transeau could see a second glistening white pile beside the first, a pile of sugar the plants had first made and then burned. This second pile weighed 2,045 kilograms, so the two piles combined weighed 8,723 kilograms.

This was all the sugar made by the cornfield that summer. Now the end was in sight. 8,723 kilograms of the sugar glucose represents 33,000,000 calories, but the man who had measured the sun streaming onto Illinois had found that one acre in a hundred days of summer received 2,043,000,000 calories, more than fifty times as much.

If you put one of these figures over the other and multiply by a hundred you get Transeau’s result, which was that corn plants, on prime land in Illinois, where they were given every care and attention, were only 1.6 percent efficient.

And so, to our amazement we find, not the 20 or 30 percent efficiency of a steam engine, not some super efficiency suggested by ideas of survival of the fittest or the marvelous workings of nature, but a miserable 1.6. Could the scholar in his armchair have got his sums wrong? People have made all Transeau’s suggested measurements on real crops, not only com but other high-yielding plants such as sugar beets, and they have come up with the same general answer: about 2 percent.

They also measured the rates of sugar production in photosynthesis more directly, by monitoring the flow of raw materials and waste products to and from the plants, and numerous studies have confirmed the estimates from crops. Our rich productive crops on rich productive soil are only 2 percent efficient.

Perhaps there is something wrong with agriculture. Perhaps it is only planted, and grown in unnatural conditions that are so abysmally inefficient. But there is no escape this way either. It is harder to measure the efficiency of wild plants than of crops, but it can be done.

You cannot harvest a field of wild plants all the same age, as you can with com plants, but it has proved to be not beyond the wit of computer-minded man to make samples and calculate the potential wild crop. We now know that wild plants do about as well as tame plants. A very rough figure of 2 percent describes the efficiency of them all when they grow in very favorable circumstances.

Most wild plants achieve nothing like the 2 percent of agriculture because they do not have it so well. So, it is ours to reason why. What curious circumstance prevents 98 percent of the sun’s energy from getting into the living things staked out to wait for it in such an eager array?

What we know of these things has been told to us by laboratory people. A plant is grown in a glass chamber, with rigid controls on all the conditions of its life so that it is comfortable and not disturbed; like a baby in an incubator. The breathing of the plant is monitored by measuring the gases it takes and gives to its chamber.

When it is busy converting energy by making sugar from carbon dioxide and water, it releases the oxygen that sensors can detect; when it is respiring in the dark it releases carbon dioxide. You can do wet chemistry on samples; you can make a plant use a radioisotope of carbon and then measure activities, or you can wire the container to the fine expensive electronics of a modem analyst’s laboratory.

But, whatever way the measurements are taken, one can infer the rate at which the laboratory plant makes the sugar “glucose,” and hence the rate at which it fixes energy. Using a water plant, such as a tiny green alga, makes things easier because the water simplifies the chemistry. Then you shine lights of known intensity into its glass incubator, recording precisely what it does.

The first startling discovery is that half the kinds of light shone on the plant have no apparent effect on its chemistry. Half the total energy of sunlight is in the red end of the spectrum; what we call infrared light. We cannot see this light, but it floods down on us as warm rays, of low intensity it is true, but together add up to half the energy getting to us from the sun. If red lamps are shone on the plant in its water bath, the chemistry of the water does not change. Plants cannot trap the energy of the far-red wavelengths any more than we can see them. Plants use only “visible” light.

We have obviously found one of the reasons for the inefficiency of plants, but we give a Darwinian biologist a curious question to answer while we are at it. Why should plants be made like people’s eyes so that they only make use of “visible” light? Plants must operate according to the rules of our Darwinian game, striving to wrest the largest possible number of calories from their surroundings so that they can turn them into babies.

They have been refined by natural selection to do this for a few thousand million years and should be very good at it. And yet they seem incapable of using half the energy pouring down on them. When this discovery was first made, an ingenious idea was put forward to explain it.

Plants, it was noted, had all first evolved in the sea, and the red light does not penetrate very far through water but is rapidly absorbed. Any skin diver knows that everything looks blue down below the surface. A plant growing in an underwater place never has the redder rays shining on it and must do all its work of living with the bluer half of the spectrum.

So, it was argued, that the ancestors of all plants evolved to be able to use only the energetic rays that penetrate water, essentially the visible light. Plants, however, have now lived on the land for several hundred million years, and it is very difficult for a biologist to believe that in all that time they could not adapt to this new brighter world with its red light. Fortunately for our peace of mind, modem physical chemists have come up with a better explanation.

The process of fixing energy (what we call “photo-synthesis,) involves violent disturbance to electrons as they spin in their orbits around atoms, and it takes a fierce pulse of energy to do this. The radiations of visible light are intense enough to fix energy, but the radiations of the red are not. Life, not for the only time, bows before the harsh reality of physical laws and does what it can with only half of the energy coming from the sun.

The red light can warm plants and does; it also evaporates water from them; helping drive the plants’ circulation systems, but that is all. Since the laws of physics let plants use only half the sunlight, we ought to amend our efficiency calculation accordingly. We double the calculated efficiencies of wild vegetation and crop plants alike; bringing them up from a miserable 2 percent to nearly as miserable 4 percent. Steam engines and automobiles still manage 20 percent or better, and the greater part of our question about the inefficiency of plants remains.

The next enlightenment to come from laboratory science is that the efficiency of plants depends on the strength of the light. If one shines a very dim light into the laboratory bottles containing the plants, say the light of the dawning or twilight, the plants do amazingly well.

If one calculates the efficiency with which they are using the meager resource of light, one may well find that they are doing as well as 20 percent efficient or even more. This does not compare so unfavorably with steam engines and automobiles, particularly when one reflects that a plant must do its own maintenance as it works, whereas steam engines are made and looked after by others.

So, we learn that in dim light the efficiency of plants compares quite favorably with the efficiency of man-made machines. They are not very productive in dim light, of course, because the total energy available is so slight. Twenty percent of very little is still very little, and dim light means poor production of sugar.

But plants in dim light yet use what energy is available to them with tolerable efficiency. Why then do they not maintain this high efficiency when light is abundant and the potential riches in sugar to be won are very large? If brighter and brighter lights are shone into the plant incubators, the rate of sugar production goes up. This we would expect.

But the efficiency progressively falls until it levels off, not at 2 or 4 percent, but at about 8 percent. It is still at about 8 percent when the very highest rates of photosynthesis, of making sugar, are reached. Eight percent of an optimum amount of light gives the highest flow of energy into living things that the bottled plants can be made to achieve.

If the plants are given still more light, both their efficiency and the rate of production fall, and a time comes when production ceases altogether. That too fierce a light should stop the plant from working completely is not surprising. Presumably, the plant is being cooked. It is the low efficiency with which light of optimum brightness is used for which we must find an explanation.

At this stage in the research, our original problem has been compounded rather than solved. We began by asking why crops and vegetation were so inefficient at handling the sunlight with which Providence provided them, and we have not got an answer yet.

What we have done is to show that plants are much more efficient at handling dim light than they are at handling the noonday sun and that algal cultures in laboratory incubators may be twice as efficient in bright sunlight as a field crop (8 percent, as opposed to 2 percent or 4 percent, depending on the wavelengths supplied).

Why are all plants comparatively inefficient in bright light? And why are all plants more efficient in dim light? Why are algal cultures in laboratory incubators twice as efficient as wild vegetation? The last question is the easiest, and we will take it first.

An algologist once taunted his colleagues, and tempted the public, with the figures from laboratory experiments with algae. See! These plants are 8 percent efficient—far, far better than the com and the other plants we eat! It is foolish to grow inefficient crops when we could all fatten on green algal scum instead! This theme recurs in newspaper articles about the world food crisis.

It is a myth that is probably as impossible to eradicate as the myth that Tyranosaurus rex was a ferociously active predator. But myth it is. Algae are not more productive than other plants. The catch about algal culturing is that it is the culturing that leads to higher average efficiencies, not the algae. Any actively growing plant that can be introduced to one of the small laboratory cultures will do as well as the algae.

A whole seedling can be put in a small laboratory container, made comfortable, and it will convert the energy of light to the energy of glucose with an efficiency of 8 percent or so, depending on the wavelengths supplied. A piece cut out of a leaf can be made to do the same on its own in the nutrient solution, away from its parent plant. When conditions are the same, algae are no more and no less efficient than the crop plants with which they were so favorably compared.

We now know that any healthy young plant, com included, which grows in a well-watered field with enough fertilizer, does as well as the algae (or any other plants) in the incubators. Its efficiency is that same rough 8 percent of the laboratory cultures. But the special thing about the plant in the field is that it grows old. When it is old it feels its age and does not work very well. So, the average efficiency over its lifetime has to be much less than the 8 percent efficiency of its youth.

At the start of Transeau’s hundred days, his Illinois acre was bare of plants and there was no production. At the end of a hundred days, there were ten thousand senile individuals who were not doing very much. Somewhere in the intervening time, the field was nicely covered with fresh green leaves turning in their 8 percent, but the average for the whole hundred days had to include the beginning and the end, which brought the average efficiency down to 2 percent. Wild vegetation in temperate latitudes faces the same harsh reality: a spring without leaves,  autumn with pretty colors but diminishing green.

The great deception concerning algal culture came very largely from the accidental circumstance that it was convenient for plant physiologists to use fresh-water algae in their experiments. Such cultures are not good ways of producing food (even if we wanted to eat green scum) because culturing requires massive amounts of work and energy compared with conventional crop husbandry.

If these inputs of the energy were fed into the efficiency equation, we would find that the calculated efficiency was drastically lowered. Algae are no more efficient than any other kind of plant. The answer to our third question is that crops and wild vegetation are less efficient “overall” than cultures or growing seedlings because of the physical vicissitudes of life, bare ground in spring, old age before the winter, shortage of water and nutrients, of the debilitating presence of neighbors.

Now we must solve the mystery of the dim-light efficiency and the failure of even the favored young to do better than 8 percent. We can find a plausible answer to both questions by pondering the supply of raw materials a plant uses in the essential chemistry of photosynthesis. Plants make sugar out of carbon dioxide and water.

When water is in short supply the plants grow miserably, as we all know. But when water is abundant it is available to plants in virtually unlimited amounts. The other raw material, carbon dioxide, however, is always scarce, even though it is always present.

Carbon dioxide is a rare gas. It is present in the atmosphere at an average concentration of about 0.03 percent by volume, a quite tiny proportion. And carbon dioxide is the essential raw material out of which plants must make sugar. Plant leaves are thin and pierced with multitudes of tiny breathing holes (stomates) for they must suck in carbon dioxide from as many directions as possible if they are to keep their sugar factories going.

Even so the rate at which they can soak up the precious gas is strictly limited. It seems reasonable to suggest that it is this shortage of raw material that sets a limit to the sugar-producing powers of plants growing on even the most favorable sites. Plants are inefficient as machines for converting sunlight because they face a shortage of raw materials.

When a plant is grown in dim light, its energy factories cannot work very fast, this being the simple consequence of lack of their light ‘‘fuel.” In dim light, they have carbon dioxide to spare, and only considerations of thermodynamics and plant chemistry inhibit the rate of photosynthesis. The plants in this case turn out to be highly efficient.

But as such plants are given lighter, their demand on the carbon dioxide supply quickly grows, until they soon are using it as fast as it can be extracted from the air. At this moment plants are working as fast as their factories can be made to run. They are then about 8 percent efficient. If they are given more fuel still, as by shining the noonday sun on them, they can only waste the surplus, degrading it to heat, pouring it away.

We can test the hypothesis that carbon dioxide limits the productivity of plants by pumping a little extra into our plant incubators and seeing what happens. If we do this, the rate of sugar production goes up and the efficiency of energy conversion in bright light is slightly increased.

If we give the plants too much carbon dioxide, we suffocate them; but this need not disturb us. Plants have evolved in a world in which carbon dioxide is scarce, and their chemistry has adapted accordingly. Yet the dependence of sugar production on the carbon dioxide supply is clearly shown by these experiments.

It is well to insert a small word of caution about the generality of this result. The logic that so scarce a commodity such as carbon dioxide ought to limit the rate of production is sound, and the experimental data are convincing demonstrations that we are on the right line. But some of the consequences of a shortage of carbon dioxide are very complex and may impose second-order restrictions on photosynthesis.

Plants must “pump” large volumes of gas as they extract their carbon and this pumping may introduce its own restraints. Flooding the plant tissues with oxygen in the flux of air will have its own consequences for reactions dependent on chemical oxidations and reductions. Opening the stomates must result in the escape of water.

And so on. All operations that boost production in the plant factories must involve their own constraints and we can expect many fresh limits to appear as plants evolve to make the most of the carbon dioxide supply in different circumstances.

These possibilities are reflected in many modem debates about alternative “pathways” of chemical synthesis in plants. But with this bit of mealy-mouthing, we can yet say that plants are generally inefficient as converters of energy because carbon dioxide is a rare gas in the terrestrial atmosphere.

This finding is of great significance to practical people for it means that there is a very narrowly defined limit to the possibilities for growing human foodstuffs. Our ultimate yields are set by the carbon dioxide in our air, and there is nothing we can do to push plants to do better. Our so-called high-yielding strains of wheat and the rest are in fact no more efficient than the wild plants they replace, whatever the gentlemen of the green revolution may claim.

All that the agriculturalists have done is to make plants put more of their total capital of sugar into parts that people like to eat. High-yielding wheat makes more grain at the expense of stalk, roots, and the energy to defend itself against pests and weeds. The finest efforts of science have not made any plant one jot more efficient than those nature made.

To a biologist brooding on the great conundrums of life, the inefficiencies of plants have a different message. The fuel supply for all life is restricted to some small fraction of what comes from the sun. A theoretical upper limit is about 8 percent, but this will be reached only for very short periods in very small places. All plants face youth and senescence, and virtually all face the changing seasons.

All suffer at times from a shortage of water or nutrients; none works at full efficiency for long. When we think of the average condition of life on earth, we think of deserts, mountainsides, and polar ice caps, as well as fertile flood plains. The average productivity of the earth must be very low, certainly much lower than that of 1.6 percent of Transeau’s cornfield. Probably only some fraction of 1 percent of the solar energy striking the earth gets into living things as fuel for plants and food for animals.

When we try to explain the numbers and kinds of plants and animals, we must remember this great restriction in the fuel supply. Plant-eating animals, for instance, can get only a small portion of the sugar made by the plants on which they feed.

This is hard to measure, but practical people generally accept an upper estimate of about 10 percent. We may think, therefore, that on good pasture land, herbivores get 10 percent of 2 percent of the sun. A tiger hunting those herbivores might theory get 10 percent of 2 percent of the sun. And so, on up the food chains.

We come then to the proposition that the numbers of the different kinds of plants and animals on earth are set by the amount of carbon dioxide in our air. Carbon dioxide sets the rate of plant production and is hence the ultimate arbiter of the food supply of all animals. If our earth had been forged with more carbon dioxide at its surface, the plants would have delivered more food and the opportunities for animals would have been greater.

We might even have had tiger-hunting dragons then, and the ferocious tyranosaur would have been less mythical. But the chemistry of the earth’s surface keeps the concentration of carbon dioxide low, by mechanisms quite out of reach of plants and animals. And so, the answer to many general questions about the numbers of animals as well as to the inefficiency of plants becomes, “Because it is very little carbon dioxide in our air.”

Reference: Paul Colinvaux

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