Converting sunlight into alternative forms of energy could solve the problem of consuming non-renewable fuel resources. Is it possible to increase yields, biomass and get rid of the food crisis in this way?
How does the photosynthesis reaction take place?
The essence of photosynthesis is that the energy of visible light is converted into the energy of chemical bonds of organic substances.
In other words, with the help of the energy of light, the body removes electrons from the molecule and transfers them to carbon dioxide molecules, reducing and converting them into molecules of organic matter, which can then be oxidized again, gaining energy.
The whole system of plant photosynthesis reactions in one scheme: 6СО2 + 6H2O = glucose (С6H12О6) + 6О2.
One of the key steps in this complex and multi-step process is carbon dioxide sequestration. When this happens, carbon dioxide is attached to a compound called ribulose (1,5) bisphosphate, a sugar with two phosphate groups.
And the enzyme directs this reaction – ribulose bisphosphate carboxylase, or rubisco (RuBisCO).
RuBisCO is an enzyme that is an absolutely complex of 16 protein chains at once. Most enzymes catalyze thousands of chemical transformations every second.
However, Rubisco only processes 3 to 10 molecules of carbon dioxide, depending on conditions. Such a low quality of the enzyme can only be compensated for by its quantity: by weight, it accounts for up to 30% of all water-soluble plant proteins, which makes it the most abundant protein on the planet.
Types of photosynthesis
Living organisms have two types of pigments that can act as photosynthetic receivers (antennas).
In the overwhelming majority of organisms, chlorophylls play the role of antennas; less common is the case in which the vitamin A derivative retinal serves as an antenna. In accordance with this, chlorophyll and chlorophyll-free photosynthesis are distinguished.
• Chlorophyll-free photosynthesis
The system of chlorophyll-free photosynthesis is distinguished by a significant simplicity of organization, in connection with which it is assumed evolutionarily to be the primary mechanism for storing the energy of electromagnetic radiation. The efficiency of chlorophyll-free photosynthesis as a mechanism for energy conversion is relatively low (only one H + is transferred per absorbed quantum).
• Chlorophyll photosynthesis
Chlorophyll photosynthesis differs from bacteriorhodopsin in a significantly higher energy storage efficiency. For each effectively absorbed quantum of radiation against the gradient, at least one H + is transferred.
Anoxygenic
Anoxygenic (or anoxic) photosynthesis occurs without oxygen evolution. Purple and green bacteria, as well as heliobacteria, are capable of anoxygenic photosynthesis.
Oxygenic
Oxygenic, or oxygenic photosynthesis is accompanied by the release of oxygen as a by-product. In oxygenic photosynthesis, non-cyclic electronic transport occurs, although under certain physiological conditions, only cyclic electronic transport occurs. An extremely weak electron donor, water, is used as an electron donor in a non-cyclic flow.
Hunger in agriculture
The population of the Earth, despite the second demographic transition, is constantly growing. If we could, at will, increase fertility in proportion to population growth, there would be no big problem.
However, today man has mastered about a third of the land suitable for agriculture. Almost all suitable territories in South Asia, in the Middle East and North America have already been plowed up, and the development of the remaining areas threatens us with erosion.
The place on the planet may simply run out, so we need to find new ways to increase food production. This has already been done before.
The last time this happened was due to the “green revolution” of the 1950s and 1970s. Then the development of new high-yielding varieties of cereals, the introduction of pesticides and advanced irrigation systems made it possible to dramatically – almost twice – increase the yield.
How to speed up photosynthesis
The cornerstone of this problem is rubisco, the enzyme we’ve already talked about.
However, it turned out to be not so easy. Targeted mutagenesis of individual amino acid residues did not lead to any noticeable results.
The method of direct evolution of enzymes was also applied to it: in it, a huge collection of gene variants of Rubisco is created by the method of introducing random mutations. All this variety was applied on E. coli – Escherichia coli. Using this approach, the researchers were able to increase the activity of Rubisco cyanobacteria, which works well in E. coli cells.
But the same method didn’t work with plants. In addition, the enzyme is assembled from parts of two different “manufacturers”: the genes encoding Rubisco chains are found not only in the cell nucleus, but also in the chloroplast genome, which complicates manipulation with them. Researchers have to work with two genomes at once, using different techniques of gene modification.
But the scientists did not give up on this. They came up with a new idea: to increase the amount of rubisco, since the leaves of the plants are literally filled with it. For this, the authors used GMO methods. However, overexpression of Rubisco genes was not enough – something else was needed to assemble the enzyme.
Over the past years, it has become clear that several folding proteins are involved in the assembly of Rubisco – RAF1 and RAF2 (RuBisCO Assembly Factor). These proteins (called chaperones) tend to stabilize the assembled protein chain during assembly, giving it time to fold properly.
This was the problem of previous studies: Rubisco genes really actively synthesized protein “building blocks” of the enzyme, but the lack of chaperones did not allow collecting a sufficient amount of Rubisco from semi-finished polypeptide chains. The number of chaperones also needed to be increased.
Therefore, the authors took these conditions into account, and as a result, the total rubisco content in the leaves of transgenic maize increased by 30%.
Because of this, not all of the additional enzyme was involved in the process of photosynthesis. However, in spite of everything, the final fixation of carbon dioxide still increased by 15%. This has significantly accelerated the growth of GM maize.
As a result of the study, Chinese scientists in 2020 managed to accelerate the photosynthesis of algae and a flower. Scientists have accelerated photosynthesis of the green alga Chlorella pyrenoidosa and the higher plant Arabidopsis thaliana using a light-harvesting polymer. The polymer increased their activity of photosynthetic systems due to electrostatic and hydrophobic binding to the walls of photosynthetic cells.
According to the authors, due to their good ability to absorb green light, solubility in water and biocompatibility, such synthetic polymers are potentially suitable for use in the production of biofuels, as well as the development of energy and ecology.
Output
Currently, photovoltaic cells operating in an aqueous environment are efficient, but clearly imperfect. Artificial photosynthesis is still quite effective as a tool for binding atmospheric carbon and at the same time produces a stable flow of charged particles (protons and electrons).
Thus, photosynthetic cells could be combined with solar panels – for example, already installed on the roofs of private houses in the United States.
The solar battery could give part of the energy it receives for electrolysis. In this case, photovoltaic cells connected to it would participate in binding carbon dioxide and splitting water to obtain hydrogen, which is an environmentally friendly fuel.
The development of catalysts for such processes would allow not to be limited to the reproduction of ordinary photosynthesis, but to synthesize, for example, proteins or enzymes. We have already learned how to scale solar cells, so we could scale photovoltaic cells with them.
Technologies like these could help decompose toxic waste or plastic, yielding hydrogen and energy.
