Mimicking the energy efficient C4 photosynthesis could be a solution, but we are not there yet
Waste handling is a necessary evil. Not every bit of what we buy from shops is useful. Take fresh food for example. We separate waste matter such as vegetable skins from the useful. Some waste materials are biodegradable that we distinguish from other waste and put in compost bins. The absolute waste then goes into the council bins. All this takes our energy and time that we could better use in cooking. We don’t realise it that much in the hustle and bustle of everyday life. But our efficiency of doing useful things is reduced because of such wasteful activities.
Plants have a similar problem. Their cells have a factory to capture carbon dioxide and cook it using sunlight to make useful sugars. The factory is called chloroplast and the process is photosynthesis. The raw materials capturing machine inside the factory sometimes picks up oxygen molecules instead of carbon dioxide from the atmosphere. Obviously oxygen cannot be sent further down the carbon dioxide workflow to manufacture sugars. Yet it must deal with these unwanted molecules. Upon detection, oxygen molecules are therefore fed into a different workflow (called photorespiration) where they travel through other factories — perixosome and mitochondrion — to be converted into something useful, much like how we indirectly use the compostable waste. Needless to say, this waste-handling process consumes a lot of ATP — the cell’s energy unit — and other resources.
It might not have hurt much during the time when carbon dioxide concentration in the atmosphere was high, some 30 odd million years ago. Well, the enzyme machinery — Rubisco — that captures and directs carbon dioxide along the photosynthesis pathway was invented long long ago, perhaps 3.5 billion years ago!! Back then there wasn’t any oxygen in the atmosphere or in ocean waters. So there wasn’t any need for worrying about such accidental intruders. Oxygen buildup in the atmosphere, which started around 2.4 billion years ago, would have necessitated the photorespiration pathway early enough. Plants were forced to handle the oxygen intruders to continue functioning. Around 30 million years ago, carbon dioxide level in the atmosphere began to drop. The genetic machinery of evolution picked up this environmental change. Nature’s efforts at innovation led to a modified workflow of photosynthesis. Genetic changes added a new machinery ahead of Rubisco which tends to concentrate carbon dioxide thereby reducing chances of oxygen molecules being fed to Rubisco. It is like somebody is pre-processing the fresh food and supplying us with only the useful parts that we can cook straightaway.
The new process is called C4 photosynthesis as opposed to the old C3 photosynthesis. C3 photosynthesis is still the dominant mechanism in operation in the world today. Yet C4 photosynthesis is gathering momentum, particularly where temperatures are higher. It is popular among grasses — notably maize, sorghum and sugarcane. Last 8 million years have seen massive land areas transformed into C4 grasslands, e.g. the Savannahs of Africa. Some scientists even attribute our upright position to adaptation to tall C4 grasses of Africa.
With improved efficiency of photosynthesis, these relatively few early adopter plants now account for as much as 23% of carbon fixation on the planet. One wonders then wouldn’t it be helpful if a major consumer food grain, like rice (also a grass) which is still on C3, is forced to adopt C4 photosynthesis. That would certainly solve food problem for a human population fast approaching 10 billions. All we need is understand the genetics behind the C4 machinery and incorporate the appropriate gene into the species. It isn’t that easy though. The C4 machinery that is prefixed to the C3 machinery is slightly different in each of the C4 species because they all seem to have evolved this feature independently. This means that the genes that make the machinery, even though similar, are different. True to the saying “nature does not reinvent the wheel”, this machinery is hardly new. It is present in most plants and used for different purposes. C4 plants just use the same old machinery for the specific purpose of forcing carbon dioxide into Rubisco. Each C4 plant recruits the machinery in a slightly different way to give the same effect. We still do not know fully how this all is controlled. Therein lies the difficulty in making use of this technology of nature for the benefit of mankind. Someday maybe. We need to continue to focus on understanding how gene expression is regulated in this particular case.