Normal Photosynthesis: Or, How to Make
“Nutritive organic molecules from inorganic sources”
By John V. Richardson Jr.
Let’s start with the simplified biochemistry formula for the normal metabolic pathway (i.e., the series of chemical reactions which produce sugar, or stored energy, from sunlight, etc.) within plant cells:
CO2 + H2O + E = CH2O + O2
On the left hand side (LHS) of the equation, the earth’s atmosphere, which is composed primarily of nitrogen and oxygen, provides the primary source of CO2 (but only about .038% of the atmosphere) and H2O (as rainfall or water vapor or humidity, up to 4% of the atmosphere) while the sun provides the solar radiation or radiant energy (E) which reaches the earth’s surface. On the right hand side (RHS) of the equation, the reactions produces CH2O, the hydrates of carbon (aka sugar) and O2, which is important in cellular respiration (the opposite of photosynthesis, but more on that latter).
At the first level of our southwest desert ecology pyramid, the primary producers are land-based plants (which can be said to be autotrophic because they are self-sustaining or self-nourishing from inorganic materials, which is a good thing since they are rooted in their habitat and can’t go to McDonald’s at will). Their stomata (or pores in the leaves and stems) take in the CO2 and their roots take in H2O. This intake is carried out by chlorophyll, which is embedded in the light-gathering cells called chloroplasts, inside the plant, which processes the LHS of the equation, storing glucose and releasing O2. The specific type of photosynthesis (literally, “gathering of light”) may be characterized as C3, C4, or CAM, depending upon their particular adaptation or water use efficiency (WUE).
The first type is called C3 because the CO2 is taken into a three-carbon compound using an enzyme named ribulose-1,5-biphosphate carboxylase/oxygenase (or, RuBisCO, for short) which is found in the plant’s leaves and is usually active only during the day. In a C3 plant, the stomata are open during the day and photosynthesis takes place in the plant’s leaves. The C3 photosynthesis is advantageous in a more temperate climate. However, one disadvantage of this process is that trapped oxygen (due to closed stomata) is more attractive to the RuBisCO enzyme than CO2 and, thus, photo respiration can occur instead of sugar building. As a result, the desert plant stops growing, especially if not enough water is available during long, hot, bright sunny conditions.
Most desert plants are C3; examples, nonetheless, include most of the winter annuals (which bloom in the “cool, wet season, and when water is available and conservation is not required” (Sowell, p. 35) and the creosote bush (Larrea tridentata; formerly Larrea Divaricata) which has been described as “the most widespread, conspicuous, and successful” according to Jaeger (1941, #294). Finally, C3 plants are more efficient than C4 and CAM plants “under cool and moist conditions and under normal light” like the desert’s winter (Fiero, 2006).
The second type is called C4 because the CO2 is taken into a four-carbon compound first for carbon fixation. Like C3 plants, their stomata are also open during the day, but the photosynthesis takes place in their inner cells. This time the uptake of CO2 is handled by the phosphoenolpyruvate carboxylase (or just PEP carboxylase for short) enzyme rather than RuBisCO as in C3 plants for photosynthesis, so that photorespiration is less likely to occur. The primary advantage of C4 is that it works well in intense light conditions and higher temperatures during the desert summers. Although this C4 process is less efficient, it is also better than the risk of cellular respiration in an unusually hot desert ecosystem. The best examples of C4 desert plants are most of the summer annuals and many native grasses.
The third and final type is called CAM, which is short for crassulacean acid metabolism (and is named after the succulent plant family in which the process was discovered). Unlike C3 and C4 plants, these open their stomata at night when there is no sunlight, the temperature is lower, and there are slower or no winds. CO2 is stored as an acid. The stored acid from overnight is released during the day to the RuBisCO enzyme for photosynthesis; interestingly, though, if water is abundant, then these desert plants can open their stomata during the daytime as well and process the CO2 just like a C3 plant does (a situation which is known as being facultative, able to adapt to different conditions; the C3 and C4 plants are obligate). The primary adaptive advantage is better water storage and WUE than the C3 desert plants. Notably, such plants can also “idle” their processing by closing their stomata during the day and nighttime and thus survive under extremely harsh arid conditions. Examples of such xerophytic CAM plants include many succulents such as the cactus family and agaves, plus the famous resurrection plant which can recover literally within hours after rainfall.
In summary, and risking over simplification, if you see a desert plant in the winter months, it is likely to be C3; during the summer, it is more likely to be a C4 plant; but, if you see a cactus (CAM process) plant, then you know its habitat is likely to be extremely harsh.
PRINT AND ONLINE SOURCES OF REFERENCE:
3. Brad Fiero, “Types of Photosynthesis,” Pima Community College (Arizona) (1 November 2006; original 2001) at http://wc.pima.edu/~bfiero/tucsonecology/plants/plants_photosynthesis.htm (accessed 14 April 2008).
4. Fort Hays State University (Kansas), Department of Biological Sciences, “Desert Plants” at http://www.fhsu.edu/biology/Eberle/DesertSW/DesertPlants.htm (accessed 15 April 2008).
5. Edmund C. Jaeger, Desert Wild Flowers (Palo Alto: Stanford University Press, 1941).
7. John Sowell, Desert Ecology: An Introduction to Life in the Arid Southwest (2001).
 Usually a protein, serving as a “catalyst,” which speeds up a chemical process; as an aside, the term catalyst was coined by the Swedish chemist Jons Jakob Berzelius (1779-1848).
 The Opuntia bigelovii, aka Teddy bear cholla, can survive ambient temperatures up to 138 degrees, according to Bowers, 1993, p. 5.