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Photosynthesis
Photosynthesis

The Theme of Unity and Diversity in Photosynthesis

While there are many similarities in how cells conduct photosynthesis, there are also many differences. In order to find their own niche in the ecosystem, plants have continued to evolve in the hopes of maximizing their ability to perform photosynthesis. The result? A wide variety of "biological masterpieces."

If you look at the photosystems in different organisms today, you will see that they absorb different wavelengths of light. These differences are attributed to changes in the pigments in the different photosystems. Nonetheless, the similarities in the overall architecture and structure of the photosystems also speak to their common evolutionary roots.

The specialization of photosynthesis in different organisms that use particular wavelengths of light is responsible for the diversity in colored organisms we see on our planet. Plants appear green, for instance, because they use primarily red and blue light for photosynthesis. Yellow and green light is not absorbed as well, and is therefore reflected, which results in the pretty green colors of forests and other shrubbery. Yes, we really wanted to use the word shrubbery.

Besides specializing their photosystems, organisms have had to balance the amount of water (H2O) needed for photosynthesis while at the same time letting carbon dioxide (CO2) into the system. Some plants, such as cacti, are called CAM plants, or Crassulacean acid metabolism plants. These plants only open their stomata to collect CO2 at night because it helps to minimize the amount of H2O lost in the hot and dry daytime weather. They only fix CO2 at night. CAM plants primarily store their CO2 in the form of malate, which can be broken down to release CO2 inside the leaf during the day, when it can be acted on by RuBisCo.

Although ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) prefers to use CO2, it can use oxygen (O2) if the concentration of CO2 becomes too low. This process, called photorespiration, is a way for the plant to use extra oxygen while at the same time producing CO2. However, it does not seem to result in any useful energy forms, and the reason for its existence is still debated today. Photorespiration proves to be a real problem for plants living in hot and dry areas. In these climate conditions, plants are often forced to close their stomata to prevent the loss of H2O. The result, however, is that CO2 cannot enter the cells. With CO2 concentrations getting lower and lower, photorespiration becomes a favored process. Some plants, such as corn, a type of C4 plant, have evolved a special procedure to try to decrease the amount of photorespiration that happens inside the plant. In these cases, photosynthesis is actually spatially separated within the leaf.

In C4 plants, RuBisCo is only present in the bundle sheath cells, and carbon fixation only occurs there. These cells are not exposed to the air, which effectively decreases the availability of O2 to RuBisCo. CO2 is pumped into these cells by the mesophyll cells that perform the light reactions. This pumping is an advantage because the cells can conduct photosynthesis with a much lower concentration of CO2. Furthermore, it allows cells to close their stomata during hot and dry conditions because CO2 can be stored and used later. The mesophyll cells capture CO2 by attaching each one to a 3-carbon molecule, making it a 4-carbon molecule. This attachment is also why they are called C4 plants. In C4 plants, not only does photosynthesis occur in different cells, but the chloroplasts within those cells have also become specialized to deal with their specific roles in the photosynthesis. And you thought that the assembly line was invented in the 1900s. Psh. Plants have had it down pat for a few million years before that.

Why don’t all plants use the C4 system? Because, dear Shmooper, it is only advantageous under certain conditions, like those in hot and dry climates, where CO2 is limiting. Unfortunately, it takes a lot of energy to pump CO2 into those bundle sheath cells. C4 plants comprise about 4% of plant species, but represent a higher percentage of our food crops.5

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