What is Photosynthesis?

by Govindjee and Rajni Govindjee

Photosynthesis converts massive amount of Sunlight into electrical and then chemical energy. The input is carbon dioxide (CO2), water (H2O), minerals and light, and the output is carbohydrates (food) that we need for our nourishment, and oxygen that we need to breathe [Ref. 1] This oxygenic photosynthesis occurs in higher plants (e.g., rice, maize, wheat, mosses, ferns, forest trees, shrubs, etc); in green, red, brown and yellow algae, and even blue-green cyanobacteria. There are photosynthetic bacteria (e.g., purple and green bacteria; and heliobacteria) that can produce carbohydrate (food), but no oxygen. They are called anoxygenic photosynthesizers. Instead of the all-abundant water, they use H2S or even organic matter. Oxygenic photosynthesizers use the green pigment Chlorophyll a, located in protein complexes in photosynthetic membranes, to run the photochemistry of the process, whereas the anoxygenic photosynthesizers use Bacteriochlorophyll instead. The set of photosynthetic reactions are arbitrarily divided into (1) the light phase (that produces the reducing power and ATP, the energy currency of life); and (2) the dark phase (where the products of light phase are used to convert CO2 to carbohydrates).

Photosynthesis is the most important biological process on Earth. It serves as the World's largest solar battery. The primary reactions have close to 100% quantum efficiency (i.e., one quantum of light leads toone electron transfer); and under most ideal conditions, the overall energy efficiency can reach 35%. Due to losses at all steps in biochemistry, one has been able to get only about 1 to 2% energy efficiency in most crop plants. Sugarcane is an exception as it can have almost 8% efficiency. However, many plants in Nature often have only 0.1 % energy efficiency. Due to massive vegetation, the total productivity is very high indeed. (Deforestation is a bad deal for all of us because it would add to the already increasing CO2 in the atmosphere and its attendant consequences, such as global warming.) The photosynthesis of the past is what had stored the Sun's energy that ultimately produced coal; natural gas; and the petroleum (called petrol in India and gas in USA). Photosynthesis also provides us with the fiber, the clothing, and indirectly all the building materials including our Macs and PCs. In the villages in India, firewood, used for cooking and heating, also owes its existence to Photosynthesis. We cannot leave out the dried "cowdung" (gobar) from the scene. The cow that produced it clearly ate hay that was the dried form of what photosynthesis had produced for her. Thus, we depend upon the process for our existence in more ways than is often considered. Perhaps, the Earth is the only hospitable planet for our lives. In short, our Sun God (Suraj Devta) has given us this life through Photosynthesis.

The major problem is that increasing population pressures may cause havoc in our Society if the future Photosynthesis cannot support it. Thus, it is essential to understand the intricacies of the process and exploit it to our benefit. We need to learn how to improve crop productivity; how to go after sustainable agriculture; and how to invent means such that plant biotechnology becomes our friend, not our enemy; and how to mold plants by genetic engineering to provide us with cheap vaccines and medicines. Finally, the impact of global climate change on Photosynthesis and of Photosynthesis on global climate change needs to be understood [Refs. 2 and 3]. Thus, it is necessary to train scientists who will exploit the molecular and cellular aspects of photosynthesis, and also those who will go after integrating the information at a systems level. Both must go on hand in hand in order for the future to be bright for our grand children and great grand children.

Applications. Several photosynthesis-based (and, some distant) opportunities for the human race are highly promising.

[1] As mentioned earlier, the primary reaction of photosynthesis is highly efficient. Thus, attempts are being made to produce artificial systems to just do that and to produce chemical energy in artificial systems (e.g., in membrane vesicles, the liposomes). A research group at Arizona State University has succeeded in producing ATP (the energy currency of life) in such systems [Ref. 4].

[2] Since water is available in huge quantities on our Earth, and since hydrogen is a clean fuel, another effort is being made at Golden (Colorado) and Berkeley (California) to use the green alga Chlamydomonas reinhardtii to trick it in converting water into oxygen and hydrogen. The problem is that hydrogen production machinery is sensitive to oxygen. Thus, researchers are attempting to separate in time the two processes. We await results of such research.

[3] Genetic Engineering is another powerful approach that is being used to produce plants that are, for example, resistant to frost; insects; drought and pathogens (disease causing organisms), etc. A specific example is the development of a cotton variety that would be resistant to caterpillars that are eating the leaves and destroying the crops. The best hope for the developing countries is, of course, the increased yield of plants under marginal lands (such as in dry and saline soils).

[4] Another exciting approach, also by genetic engineering, is to construct plants that have added nutritional values (such as plants that make lots of Vitamin E; crop plants that are rich in specific proteins; rice containing iron in the form of ferritin; and canola plants that produce palm oil). Such engineering approaches [Ref. 5] can increase quality and quantity of food to meet the needs of the increasing World population.

[5] A highly exciting new application is constructing plants that produce medicines (plants have been doing it since they came to be on our Earth, but now we can direct them to make what we need and in quantities we need). In particular, efforts at Boyce Thompson Institute aim to produce vaccines in bananas that will revolutionize their delivery to children in developing countries [Ref. 6]. It will be affordable and would increase the life expectancy and health of millions. What a delightful thought! .

Sun shines each day and does not charge us any money for the light it gives us. The light falling on Earth is very clean and will be there for a very long time as long as the Sun lasts. In addition to the current applications, mentioned above, Photosynthesis-based technology could also include (1) using the concept of efficient "energy capture" to our artificial systems just as plants have been doing: thousands of chlorophyll a molecules (antenna) serving one center where the process occurs efficiently; (2) the use of compounds, produced by plants, in triggering reactions that kill cancer cells; and (3) the use of photosynthetic organisms in cleaning of aqueous surface environments (lakes, etc). An excellent example is in the use of cyanobacteria that literally eat up the nitrates from ground water and clear it for us. The opportunities that photosynthesis-based technology provides us are enormous. The success, however, requires a concerted effort on the part of biophysicists, biochemists, molecular biologists, plant physiologists, microbiologists, geneticists, agronomists, physicists, chemists, bio-technologists and engineers to come together and ask what they can do for the World, not what the World can do for them.


[1] Hall, D.O. and K.K. Rao (1999) Photosynthesis. 6th Edition,Cambridge University Press, Cambridge, UK. (ISBN 0-521-64497 6, paperback), 214 pages

[2] Walker, D. (1992) Energy, Plants and Man. Oxygraphics, Sheffield, U.K. (ISBN 1 870232 05 4, paperback), 277 pages.

[3] Falkowski, P. G. and J.A. Raven (1997) Aquatic Photosynthesis. Blackwell, Oxford, UK (ISBN 0-86542-387-3, paperback), 375 pages.

[4] Steinberg-Yfrach, G., J.-L. Rigaud, E.N. Durantini, A.L. Moore, and T. Moore (1998) Light-driven production of ATP catalysed by FoF1-ATP Synthase in an artificial membrane. Nature 392: 479-482.

[5] Shintani, D. and DellaPenna, D. (1998) Elevating the Vitamin E content of plants through metabolic engineering. Science 282: 2098-2100.

[6] Langridge, W.H. (2000) Edible Vaccines. Scientific American 283: 66-71.

For further reading, see the following four selected URLs:

[1] http://www.life.uiuc.edu/govindjee contains sites for a brief presentation on the basics of photosynthesis; a full description of the "Photosynthesis Process"; a program "Photosynthesis and Time"; an article on "Photosynthesis and the World Wide Web", and a chapter on "Milestones in Photosynthesis", among other items.

[2] http://esg-www.mit.edu:8001/esgbio/ps/psdir.html has a good hypertext on Photosynthesis at MIT's experimental study group; it covers both the "light" and the "dark" reactions of photosynthesis.

[3] http://www.biotech-resource.com is a great portal to biotech resources.

[4] http://photoscience.la.asu.edu/photosyn/ is the most visited site; it has several basic articles; list of books at all levels on photosynthesis including the Series "Advances in Photosynthesis". In particular, an excellent source of information on "Genetic Engineering and Society" can be found at this site. For a direct access, go to http://photoscience.la.asu.edu/photosyn/courses/BIO_343/default.html