No chemical process is more important to life on Earth than photosynthesis —the series of chemical reactions that allow plants to harvest sunlight and create carbohydrate molecules. Without photosynthesis, not only would there be no plants, the planet could not sustain life of any kind. In plants, photosynthesis occurs in the thykaloid membrane system of chloroplasts. Many of the enzymes that allow photosynthesis to occur are transmembrane proteins embedded in the thykaloid membranes. What then is the chemistry involved?
The most basic summary of the photosynthesis process can be shown with a net chemical equation
6CO 2 (g) + 6 H 2 O(l) + hν → C 6 H 12 O 6 ( s ) + 6O 2 ( g )
The symbol hν is used to depict the energy input from light (in the case of most plants, sunlight). This chemical equation, however, is a dramatic simplification of the very complicated series of chemical reactions that photo-synthesis involves. It also implies that the only product is glucose , C 6 H 12 O 6 ( s ), which is also a simplification.
Still, take a moment to look at this chemical equation. If one were to guess where the various atoms in the reactants end up when products are produced, it would be reasonable to suggest that the oxygen atoms in the O 2 ( g ) were those originally associated with carbon dioxide. Most scientists believed this to be true until the 1930s when experiments by American biologist Cornelius van Niel suggested that oxygen- hydrogen bonds in water must be broken in photosynthesis. Further research confirmed his hypothesis and ultimately revealed that many reactions are involved in photosynthesis.
There are two major components of photosynthesis: the light cycle and the dark cycle. As implied by these names, the reactions in the light cycle require energy input from sunlight (or some artificial light source) to take place. The reactions in the dark cycle do not have to take place in the dark, but they can progress when sunlight is not present.
The critical step of the light cycle is the absorption of electromagnet radiation by a pigment molecule. The most famous pigment is chlorophyll , but other molecules, such as β- carotene, also absorb light (see Figure 1). Together, these pigment molecules form a type of light harvesting antennae that is more efficient at interacting with sunlight than would be possible with
the pigments acting alone. When the light is absorbed, electrons in the pigment molecule are excited to high energy states. A series of enzymes called electron transport systems help channel the energy present in these electrons into reactions that store it in chemical bonds.
For example, one major chemical reaction that results from the absorbed light energy (and excited electrons) involves water and nicotinamide adenine dinucleotide phosphate (NADP + ). The net reaction is shown by the chemical equation
2 NADP + + 2 H 2 O → NADPH + O 2 + 2H +
This is an example of an oxidation –reduction reaction, and it shows that the light cycle is the stage of photosynthesis when water breaks up. The amount of energy required to make this reaction proceed is greater than what can be provided by a single photon of visible light. Therefore, there must be at least two ways that plants harvest light energy in photosynthesis. These two systems are referred to as photosystem I (PSI) and photosystem II (PSII), although the numbers associated with these names do not imply which one happens "first."
At the same time that NADPH is being produced, the combination of the photo systems also produces a concentration gradient of protons. Enzymes in the cell use this proton gradient to produce ATP from ADP. Thus, the light cycle produces two "high energy" molecules: NADPH and ATP.
With the high energy products provided by the light cycle, plants then use reactions that do not require light to actually produce carbohydrates. The initial steps in the dark cycle are collectively called the Calvin cycle, named after American chemist Melvin Calvin who along with his coworkers determined the nature of these reactions during the late 1940s and early 1950s.
The Calvin cycle essentially has two stages. In the first part of the cycle, several enzymes act in concert to produce a molecule called glyceraldehyde-3-phosphate (GAP). (See Figure 2). Note in the illustration that this molecule has three carbon atoms. Each of these carbon atoms comes originally from carbon dioxide molecules—so photosynthesis completes the amazing task of manufacturing carbohydrates out of air (the source of the carbon dioxide). This stage of the Calvin cycle is sometimes called carbon fixing. In order to carry out this synthesis of GAP, the Calvin cycle consumes some of the NADPH and ATP that was produced during the light cycle.
The carbon dioxide needed for this step enters through pores in the photosynthetic leaf (called stromata). Plants close these pores during hot, dry times of the day (to prevent water loss) so the details of carbon fixing vary for plants from different climates. In hot climates, where stomata are closed for a higher percentage of time, the trapping of carbon dioxide has to be more efficient than in cooler climates. This biochemical difference in photosynthesis helps explain why plants from one climate do not grow as well in warmer (or cooler) places.
The second stage of the cycle builds even larger carbohydrate molecules. With more than half a dozen enzyme-catalyzed reactions in this portion of the dark cycle, five-and six-carbon carbohydrates are produced. The five-carbon molecules continue in the cycle to help produce additional GAP, thus perpetuating the cyclic process.
Photosynthesis is central to all life on the planet and has been for many thousands of years. As a result, there are numerous variations in the way it occurs in different cells. The efficient collection of carbon dioxide mentioned earlier is one example of variation in photosynthesis. Other differences occur when the process takes place in bacteria rather than plants. Nonetheless, the description provided here outlines the basic concepts that would be noted in all photosynthesis. These differences pose the research questions that continue to challenge scientists today.
Foyer, Christine H. (1984). Photosynthesis. New York: Wiley.
Govindjee, and Coleman, W. J. (1990). "How Plants Make Oxygen." Scientific American 262:50–59.
Wong, Kate (2000). "Photosynthesis's Purple Roots." Scientific American. Available from http://www.sciam.com .