Plants, Photosynthesis, and the RbcL Gene
Plants are a fundamental part of the existence of life. They utilize the energy of the sun in conjunction with inorganic compounds to manufacture carbohydrates and create biomass (Freeman, 2008). This biomass forms the basis of the food web as we know it. All heterotrophs depend on the existence of plants either directly or indirectly to provide food (Vitousek et al., 1986). Plants are also necessary for the existence of terrestrial habitats. When plants break apart or die they eventually fall to the ground. This mass of plant parts compiles and is broken down by decomposers, which in turn creates soil. Soil then holds nutrients and water for future generations of plants. Not only do plants make soil, they support it. The root systems of plants keep the soil and the nutrients contained in it from being quickly eroded away. The presence of the plants softens the impact of rainfall as well, another source of erosion. Plants are important moderators of environmental temperatures too. Their existence provides shade, which reduces the temperature beneath them and the relative humidity (Freeman, 2008).
Plants also remove atmospheric carbon from the atmosphere and make it biologically useful. As a by-product of this process, plants create oxygen gas, a molecule vital for many organisms to oxidize glucose to CO₂. This reverse-photosynthesis process (respiration) results in the production of ATP, an energy source required to perform necessary cellular functions. This conversion of CO₂ into O₂ allows the existence of terrestrial animals. Plants also break down organic waste molecules made by heterotrophs such as nitrate and convert them into energy, continuing the carbon cycle. Plants are important to humans specifically not only because they provide a source of food, but also a source of building materials, fuel, fiber, and medicine. All of these things are made possible by the ability of plants to photosynthesize, which is dependent on the rbcL gene (Freeman, 2008).
The rbcL gene is a valuable tool for assessing phylogenetic relationships. This gene is found in the chloroplasts of most photosynthetic organisms. It is an abundant protein in leaf tissue and very well may be the most abundant protein on earth (Freeman 2008). Thus this gene exists as a common factor between photosynthetic organisms and can be contrasted with the rbcL genes of other plants in order to determine genetic similarities and differences. It codes for the large subunit of the protein ribulose-1, 5-biphosphate carboxylase/oxygenase (rubisco) (Geilly, Taberlet, 1994).
Rubisco is an enzyme used to catalyze the first step in carbon fixation: carboxylation. This is achieved by the addition of CO₂ to ribulose biphosphate (RuBP). Atmospheric CO₂ enters the plant through the stomata, which are small pores on the bottom of leaves used for gas exchange, and then reacts with RuBP.These two molecules attach, or fix, allowing carbon to become biologically available. This leads to the production of two molecules of 3-phosphoglycerate. These new molecules are then phosphorylated by ATP and then reduced by NADPH, making them into glyceraldehyde-3-phosphate (G3P). Some of this G3P is used to create glucose and fructose, while the rest of it serves as a substrate for a reaction which results in the regeneration of RuBP (Freeman, 2008).
In addition to catalyzing the reaction between CO₂ and RuBP, rubisco is also responsible for catalyzing the introduction of O₂ to RuBP. This in turn decreases the rate of CO₂ absorption by the plant due to the fact that O₂ and CO₂ compete for the same active sites. The reaction of O₂ with RuBP also results in photorespiration. Photorespiration decreases the overall rate of photosynthesis due to the fact it consumes ATP. It also creates CO₂ as a by-product, essentially undoing carbon fixation. This reaction is a maladaptive trait, successfully reducing the fitness of the organism. It is speculated that this trait evolved during a time when the atmosphere was made up of significantly more CO₂ and less O₂, before the presence of oxygenic photosynthesis (Freeman, 2008). Now that the atmospheric conditions have changed and oxygenic photosynthesis exists, the ability for a photosynthesizing organism to take up O₂ has become maladaptive, but the ability remains. With this in mind, the evolution of organisms could very well affect the ability of scientists to use the rbcL gene as an identification tool due to the fact the gene may change.
Freeman, Scott. Biological Science. San Francisco: Pearson/Benjamin Cummings, 2008. Print.
Gielly, Ludovic, and Pierre Taberlet. "The Use of Chloroplast DNA to Resolve Plant Phylogenies: Noncoding versus RbcL Sequences." Mol Biol Evol 11.5 (1994): 769-77. Print.
Vitousek, Peter M., Paul R. Ehrlich, Anne H. Ehrlich, and Pamela A. Matson. "Human Appropriation of the Products of Photosynthesis." BioScience 36.6 (1986): 368-73. Print.