Biol 414 Exam 1 Essay Practice

Overall, the process of photosynthesis involves using water and light in order to produce sugars and oxygen. Photosynthesis is broken down into two sets of reactions, the light reaction and the dark reaction (also known as the Calvin Cycle). During the light reaction, H2O serves as an electron donor, which is made a better donor using ligth, and NADPH and ATP is produced. Druing the Calvin Cycle, Carbon dioxide is “fixed” from the atomosphere and used to make sugars. The enzyme involved in this process is called Rubisco (ribulose 1,5 biphosphate carboxylase oxygenase). As seen in the name, rubisoc can bind both CO2 (as it does in phososynthesis) and oxygen. When rubisco binds oxygen instead of CO2, glycolate is pruduced. This product must go thorugh an alternate pathway in order to be converted into a metabolite that can be used in the Calvin Cycle. This process, called photorespiration, uses extra energy. Since Rubisco’s affinity for oxugen incerases as temperatures incerease, plants that inhabit hot, dry climates have come up with adaptations in order to reduce energetic costs of photorespiration.

C3 plants, or the ancestral plant photosynthesis type, do not have any adaptations to avoid phototrespiration. For these plants, both a parts of photosynthesis take place within the same cell, in chloroplasts, which takes in both atmospheric oxygen anc carbon dioxide. Since these plants do not have any adaptations to reduce phtotrespiration, they are best suited to moderate climiates taht have sufficent rainfall and high levels of atmospheric CO2. Thier leaf are often larger, broader, and have more stomata in order to take up air and CO2. Suffiencet rainfall is necessary to blance water lost through transpriation from the stomata. For these reasosn, C3 plants are fouond in moderate climates.

C4 plants are plants that have adapted photosynthesis in oder to be better adaptaed to high, dry temperatures wieh low atmosphitc CO2 by reducing energy lost to photorespiration and water lost to transpiration. In C4 platns, photosynthesis is separated spartially between two cells. The light reacitons take place in mesophyll cells. These cells also take up CO2 and react it to form PEP via the enzyme PEP carboxylase. This forms a four carbon compunrf (gives the C4 name). This id then transported to bundle sheath cells that are located deep in the plant and the site of teh calvin cycle. Here, the CO2 is taken form the PEP comound and used in the calvin cycle. With the bundle sheath cell being deep within the plant, the concentration of oxygen is reeuce. Additioanlly, the plant can create a CO2 concntration gradient with high conentraiton with in the bundle cell to increase Rubisoc’s affinity for CO2. These differences help C4 plants reduce phtotorespiration. C4 plants have smaller, thinner leaves with fewer stomatat inorder to reduce the amount of water lost to transpiration. For these reasons, C4 plnats are better adapted to hot, dry conditions.