07 November 2020

Low Photosynthetic Efficiency? Replacing the agricultural paradigm with the evolutionary paradigm

Dr Matt Johnson: Why is Photosynthesis so inefficient?

I was searching for authors complaining about the low efficiency of photosynthesis. This is a good example:

"Why is Photosynthesis so inefficient? Only 5% of the sunlight that hits this field ends up in these grains. Given that plants have a head start of about a billion years, why haven't they already evolved photosynthesis to be more efficient? ...(Plants have different priorities than us!)" (Dr Matt Johnson)

Dr Matt Johnson asks the question Why is Photosynthesis so hopelessly inefficient? while sitting in a field of grain! Not in a tropical rainforest. Clearly, the question arises in an agricultural context. The human point of view. We need to produce more food for the growing world population and end hunger. Higher photosynthetic efficiency means more food for us humans. Plants don't meet human needs.

Wild strawberry Fragaria vesca
wikipedia Ivar Leidus - Own work, CC BY-SA 3.0,
 

cultivated strawberry (source)
 

When we think of strawberries we automatically think of the big strawberries in the supermarket. But we tend to forget that our big, delicious strawberries started long ago as small wild strawberries. See pictures above. In fact, all our modern cultivated fruit and vegetable species were once small. But, we want bigger tomatoes, bananas, potatoes and grains. This is the standard agricultural paradigm nearly everybody is unconsciously using when we see percentages of photosynthetic efficiency which are closer to 0% than 100%.

This agricultural point of view can also be found in a supposedly neutral wikipedia article about the efficiency of photosynthesis:

"Many plants lose much of the remaining energy on growing roots."

Lose energy? How can a plant exist without roots? Something is seriously wrong here. This is a myopic agricultural view of plant anatomy and physiology. The wikipedia article is apparently written from an anthropocentric point of view: roots of most crop plants are not eaten and thus a waste! Why plants need roots follows directly from the definition of photosynthesis (see below).

Although the Britannica gives a neutral definition of photosynthesis, and there is some attention to 'the needs of the whole plant', the agricultural context is already present in the introduction ('agricultural revolutions').

The Life of a Leaf
The Life of a Leaf
But we can also find the idea of low photosynthetic efficiency outside the agricultural context. Biologist Steven Vogel writes:

"Photosynthesis consumes no more than perhaps 5 percent of that energy [= after rejecting almost all the near-infrared portion of sunlight], an amount we'd dismiss as negligible were it not for life's total dependence on it. That other 95 percent makes trouble. Odd idea, admittedly –light as bad for leaves." The Life of a Leaf, 447/990.

This is remarkable and unexpected, because Vogel wrote a insightful book about plants The Life of a Leaf  in which he exhaustively lists all physical constraints of leaves and the adaptations to deal with them [1]. I will return to him.

Karo Michaelian

Physicist Karo Michaelian observes that photon dissipation into heat accounts for 99,9% of the free energy in sunlight and only 0,1% is used for photo-synthesis:

"This represents an extremely poor efficiency for a photosynthetic system that has had the opportunity to evolve for at least 3,500 million years considering that humans have developed systems capable of converting up to 40% of the free energy in sunlight into usable electrical energy within only 40 years of technological innovation." [2], [19]

Many claims in one sentence! I will debunk all of them. 

An authoritative article in Science states that:

"Solar energy conversion efficiencies for crop plants in both temperate and tropical zones typically do not exceed 1%" [18]. Please note the agricultural context: crop plants [25].

When all the reported photosynthesis efficiencies seem to be low, that is for me a reason to dig deeper. Because it is somewhat surprising.


A number between 0 and 100

For a start, such a simple thing as expressing photosynthetic efficiency as a percentage from 0 to 100 can be quite misleading. It suggests that 100% is the maximum efficiency. Wrong. The calculated theoretical maximum energy efficiency of photosynthesis is 26 percent [7]. So, we should not express the efficiency as a fraction of a physical impossible maximum of 100%, but of the maximum that is set by the laws of physics and biochemistry. Those laws cannot be broken. They set the absolute upper limit. If we use the upper limit, the 26% is the maximum and in a sense represents 100%. So, the 5% efficiency is in fact 20% (5/26). If we use the value 11% in wikipedia [8], than photosynthesis achieves 42% of the maximum. So, it makes more sense to express the efficiency relative to what is physically possible. Karo Michaelian uses 0,1% efficiency which is 1/1000. Indeed, that is negligible! (Hmmm, my income is negligible compared to that of a billionaire.)


Why are solar panels so inefficient?

wikipedia solar panels

We must first debunk the 'solar-panels-are-more-efficient-than-photosynthesis' claim before we can make any progress. The following statements can be found on a website of a company that produces solar panels (a company that hardly can be accused of being pessimistic about performance): 

"Most typical silicon solar cells have a maximum efficiency of around 15 percent. (...) excess energy will be released as heat. This is one of the reasons that solar cells have such a low efficiency; they only need a very specific amount of energy in order to work. (...). 23 percent of the energy from the sun has a wavelength too long to be useful to solar panels. (...) Other wavelengths have some excess energy. In fact, another 33 percent of the sun's energy is excess energy that is also unusable for silicon solar cells. Therefore, this leaves only 44 percent of the sun's energy available to silicon solar cells. More of this energy is lost due to reflection and other processes in the cell itself. Hence, while the theoretical maximum efficiency may be higher, the real efficiency of silicon cells is usually around 15 percent." [16]. [my emphasis]

That means that 85% of the solar energy is wasted! Karo Michaelian (2016) quotes the highest efficiency for solar panels he could find: 40% efficiency. For photosynthesis he chose the lowest value he could find: 0.1%. That is a biased and abstract description. Real-life conditions of solar panels must be included in the calculation such as seasons, weather, the accumulation of dust, grime, and pollen and the problem of shade. No surprise, solar panels and photosynthesis have some efficiency problems in common caused by the laws of physics.

Finally, if engineers are able to produce solar panels with 40% efficiency in 40 years [19], why are engineers unable to do the same for photosynthetic efficiency? Why didn't engineers improve photosynthesis efficiency to 40%? If photosynthesis is so poor, wouldn't it be easy to improve? Be assured: scientists are trying very hard! And the stakes are high. I will elaborate on this later.

 

The difference between solar panels and photosynthesis

As we have seen, in only one sense solar panels can be compared with photosynthesis, but in all other respects the comparison breaks down. By simply looking at the formula of photo-synthesis we see that it is a chemical synthesis:

Photosynthesis (wikipedia)

Photo-synthesis is a chemical synthesis of sugar with input carbon-dioxide, water and light. Solar panels fail in all these respects:

  • solar panels have zero percent efficiency for sugar production
  • solar panels have zero percent efficiency for water splitting
  • solar panels have zero percent efficiency for oxygen production
  • solar panels have zero percent efficiency for energy storage
Solar panels fail completely in these respects simply because solar panels are not designed to produce sugar or oxygen or split water. Solar panels only require light (the only limiting factor) and only produce electricity. Plants store the energy  in chemical bonds (sugar).

Water is often plenty available, but not everywhere on earth. Certainly not in areas such as deserts. Typically, deserts would be great for solar panels, but not for plants precisely because of the lack of water. Water can be a limiting factor. And if water is available it has to be transported to the leaves. This definitively shows the difference between solar panels and plants. Further, water uptake requires roots. Roots have to be grown.

CO2 is –despite climate warming– present in very low concentrations in the atmosphere: 415 parts per million, that is just 0.04% [15]. A plant cannot actively grab CO2 from the air. Animals have active breathing and are only interested in oxygen which is present in the atmosphere in a comfortable concentration of 21%. That is more than 500 times higher than carbon-dioxide! Plants don't have active breathing [3]. CO2 ends up inside plant cells mainly by simple diffusion [4]. That CO2 is really a limiting factor is shown when CO2 is experimentally increased: plant growth is stimulated [5]. So, both the concentration of carbon dioxide and the passive diffusion are limiting factors for the rate of photosynthesis. Natural selection can do a lot, but not increasing atmospheric CO2 concentration!

CO2 concentration
figure 1 (source).

Law of Limiting Factors

The Law of limiting factors states that the rate of a physiological process will be limited by the factor which is in shortest supply. Any change in the level of a limiting factor will affect the rate of reaction. (source). In photosynthesis light, temperature and CO2 are limiting factors alone and in combination. Above we have seen the CO2 effect. Here is the temperature effect:

figure 2 (source).

Here is the combination of CO2 and temperature:

 

figure 3 (source).

 

Below the effect of light is shown (figure 4).
 


CO2 / O2 balance is crucial for photosynthesis

 
Above the law of the limiting factors there is an evolutionary legacy. Earth history and biochemistry show that the relative levels of oxygen and carbon-dioxide in earth’s atmosphere are important. Plants require a minimum level of carbon-dioxide and at the same time not too much oxygen. This is because the enzyme involved in photosynthesis (rubisco) doesn't work when carbon-dioxide levels are below 40-65ppm at 21% oxygen level. At 1000ppm carbon-dioxide photosynthesis works very efficiently. At the same time at 2-5% oxygen photosynthesis works well, while at the current level of 21% oxygen there is 25% inhibition of photosynthesis. The oxygen/carbon-dioxide balance has changed during the lifetime of the planet and that has its effect on photosynthesis. The core photosynthesis enzyme rubisco has been adapted to the past [21]
 

Too much sunlight!

Steven Vogel (quoted above) already noted that too much sunlight is bad for plants. Direct full sunlight is needlessly bright. He measured leaf temperature on a hot day: 55°C (130°F)! That is clearly outside the range photosynthesis works best: 20° – 35° C (68° – 95°F). Plants manage to survive those harsh conditions. Why aren't they dead?

figure 4 (source).

Rates of photosynthesis in bright sunlight sometimes exceed the needs of the plants, resulting in the formation of excess sugars and starch. When this happens, the regulatory mechanisms of the plant slow down the process of photosynthesis, allowing more absorbed sunlight to go unused [14].

Although light is required for photosynthesis, too much light can be harmful. To protect the photosynthetic apparatus from oxidative damage, photosynthetic systems possess antioxidant systems that scavenge reactive oxygen species, as well as mechanisms that regulate photosynthesis to minimize their production [20].

At high light the absorption of light energy exceeds a plant's capacity for CO2 fixation [20].

Carotenoids are part of a cycle that renders excess energy beyond the level of light saturation harmless, effectively serving as “lightning rods” in the process [14].

These effects explain why photosynthesis rate has a ceiling.


Plants in natural conditions

“People often think that nature is very efficient. It is, but only in natural conditions. The crops we grow were not born for agriculture; we took them from the forests and placed them in fields. If they had completely adapted to these new conditions they would perform much better." ( wur.nl )

According to photosynthesis researcher Vincent P. Gutschick the components of photosynthesis in wild plants have been nearly optimized by natural selection [10]. That explains why breeding for photosynthesis has enabled few discrete gains in yield. Breeding for higher yield has trade-offs such as lower water-use efficiency.

The authors of an article in Science conclude: "the main evolutionary pressure on photosynthetic organisms is that they survive, not that they have the optimum thermodynamic efficiency!" [17]. Very important.

Recently, researchers claim to have eliminated 3 bottlenecks in photosynthesis in tobacco, thereby increasing yield significantly. They are trying to apply the findings to food crops as cassava, cowpea, maize, soybean and rice [11]. The question arises: if this is good for the plant species, why has natural selection not removed these bottlenecks? The answer can only be found when we find out what the evolutionary reasons are. One example is a wasteful process called photorespiration. It's long been thought that more than 30% of the energy produced during photosynthesis is wasted in the process. Now, a new study suggests that photorespiration wastes little energy and instead enhances nitrate assimilation. The researchers propose that something else is going on that shows plants aren't so stupid [9]. Again, digging deeper is rewarding.


Evolution and adaptation

A leaf is not a simple 'solar panel'. A leaf has many functions: "Assuring access to light, providing mechanical support, coping with heat, deploying from a bud, dealing with wind, getting atmospheric carbon dioxide into the cells, extracting water from soil and raising it upward, deterring herbivores" [22].

Pinus
All these functions often require compromises. Furthermore, evolution designed different life-cycle strategies: annual, biennial, perennial. Each of these lifestyles imply different compromises such as fast or slow growing, making many small leaves or few big leaves, producing seed once in the growing season, or every year; deciduous trees (shed leaves in the autumn), conifers (evergreen). For example, deciduous trees show that trees make choices, they shed their leaves and they don't photosynthesise the whole year. Why? Think about that. Clearly, they don't maximize photosynthesis. Otherwise they wouldn't shed their leaves. Apparently, climatological circumstances make it unprofitable to have leaves the whole year.
Something similar holds for conifers. They have long, thin
'leaves' (needles). That is a very small surface compared to 'normal' leaves. Think about it: why do they do that?

 

Summary of main points

  • In an agricultural context photosynthetic efficiency is meaningful and important because it is all about food production
  • In an evolutionary context photosynthesis efficiency is only one of several factors that contribute to survival and reproductive success.
  • Reporting photosynthesis efficiency is meaningless without a precise description of how it is measured: plant species, wild or domesticated, light spectrum considered (Photosynthetically Active Radiation or all wavelengths), how photon flux is measured, which end-product measured [12], water-use efficiency, CO2 levels, minerals, temperature, which 'losses' are included, in the lab or in the field, duration.
  • Limiting factors are: light, CO2, water,  temperature, nutrients [14]. Evolution cannot improve the physical environment!
  • Whenever a feature of a plant in nature looks wasteful and inefficient, we should study the species to find out why [9].

 

Final thoughts

Is photosynthetic efficiency low? First, use your common sense. Whatever the efficiency, it is a fact that plants are able to grow, survive and multiply. That is a remarkable fact. Photosynthesis has existed for some 3.5 billion years. Plants are the evolutionary success story. They made the earth habitable for animals including humans. Plants produce oxygen. Without atmospheric oxygen animals would not exist on earth [24]. The earth is the only known planet with life. Apparently photosynthesis is powerful enough for the existence of 8 million species on earth [23]. We should not ignore the obvious.

Is photosynthetic efficiency low? It is really too simplistic to conclude that 0.1% is low just because the percentage is low. Period. In this blog I introduced the chemical formula of photosynthesis. Many things follow from that formula. Important: carbon-dioxide. To know what the effect of carbon-dioxide is, one has to know what the history of carbon-dioxide and relative CO2/O2 concentrations were in the history of the earth. Those concentrations formed the selection pressures of the core enzyme rubisco. Further, one has to know what the biochemical sensitivity of rubisco is for different concentrations in order to understand the current efficiency of that molecule. Finally, one has to find out if there exists selection pressure at all for higher efficiency of photosynthesis in the wild. That's why it is wrong to say that "0,1% is an extremely poor efficiency for a photosynthetic system that has had the opportunity to evolve for at least 3,500 million year".

Is photosynthetic efficiency low? A better question would be: how are plants adapted to their natural environment? Plants are not adapted to human needs. From an evolutionary point of view agriculture is unnatural. Wild plants have been taken from their natural environment and transported to a different continent with a different climate. Plants are modified in order to redirect their resources to human goals. We want different parts of the plant: the root, stem, leaves, seeds, fruits, and modify plants accordingly. Plants did not evolve to meet human needs.

 

Acknowledgments

I would like to thank Kasper van Gelderen en Thijs Pons.


Notes

  1. Steven Vogel The Life of a Leaf. A second quote about leaves: "These relatively inefficient solar panels provide all the energy a tree can invest in growth, reproduction, and dispersal - the three central concerns of every organism that has ever lived." (505/506)
  2. Karo Michaelian (2016) Thermodynamic Dissipation Theory of the Origin and Evolution of Life, p.312) paperback (see a previous blog). Karo is right about one thing: "Photosynthesis was well-established on the earth at least 3.5 thousand million years ago." (source). In a next blog I will refute Dissipation theory with evolutionary biology.
  3. CO2 is not always passively transported. C4 plants use a biochemical pump to concentrate CO2 at the locations within the leaf where the RUBISCO enzyme mediates incorporation of CO2.
  4. Andrew Bocarsly of Princeton University: "We've been studying CO2 chemistry for a long time, more than 100 years, and there's very little evidence that we could do what a leaf does."
  5. Effects of Carbon Dioxide on Photosynthesis, Plant Growth, and Other Processes
  6. see wikipedia article Measuring ancient-Earth carbon dioxide concentration.
  7. Energy efficiency of photosynthesis, Britannica (free article)
  8. The theoretical maximum efficiency of solar energy conversion is approximately 11% (wikipedia).
  9. Shedding light on the energy-efficiency of photosynthesisSciencedaily, 2018. "a new study suggests that photorespiration wastes little energy and instead enhances nitrate assimilation,".
  10. Vincent P. Gutschick (1997) Photosynthesis, Growth Rate, and Biomass Allocation, Ecology in Agriculture, 1997 
  11. Third breakthrough demonstrates photosynthetic hacks can boost yield, conserve water. August 10, 2020
  12. End-product: "Only 5% of the sunlight that hits this field ends up in these grains"! (Dr Matt Johnson). "if only agricultural products (e.g., seeds, fruits, and tubers, rather than total biomass) are considered as the end product of the energy-conversion process of photosynthesis, the efficiency falls even further." (source)
  13. C4 plants: "The C4 metabolic pathway is a valuable recent evolutionary innovation in plants, involving a complex set of adaptive changes to physiology and gene expression patterns." (wikipedia).
  14. Britannica. "Several minerals are required for healthy plant growth and for maximum rates of photosynthesis. Nitrogen, sulfate, phosphate, iron, magnesium, calcium, and potassium are required in substantial amounts".
  15. Industrial carbon capture from the air is costly because CO2 has a low concentration in the air and lots of water are required! Just like photosynthesis!
  16. The Average Photovoltaic System Efficiency, Sciencing, April 25, 2017
  17. quoted in this blog Is photosynthesis highly efficient? 2012
  18. Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement, Science,  13 May 2011. (a pdf can be downloaded here)
  19. On page 223 of his book he writes "in only 25 years" for the solar panels and "after more than 3.8 Ga years of biological evolution" for photosynthesis, making the difference still bigger! Exaggeration!
  20. Photosynthesis, R.C. Leegood, in Encyclopedia of Biological Chemistry (Second Edition), 2013.
  21. Evolution on Planet Earth, page 21.
  22. Steven Vogel The Life of a Leaf., 28/990 (ebook)
  23. How many species on Earth? About 8.7 million, new estimate says. Science daily 2011.
  24. "Animals depend on oxygen because they need it to release energy from sugars, fats and proteins. In strict chemical terms, cellular respiration reverses the reaction at the core of photosynthesis. Sugar and oxygen react with each other to make water and carbon dioxide, releasing a lot of energy." Paul Nurse (2020) What is Life? 22 Nov 2020
  25. Nobel prize winner Paul Nurse (2020): "If we could re-engineer plants to carry out photosynthesis even more efficiently than they do..." in: What is Life? 306/360 24 Nov 2020


Previous blogs about photosynthesis