UNIT 2: Metabolic Processes Chapter 5: Photosynthesis: The Energy of Life pg. 210-240 5.4: Alternative Mechanisms of Carbon Fixation pg. 231 234 Photosynthesis requires reactants; CO 2 and H 2 O, to produce C 6 H 12 O 6, H 2 O and O 2. Atmospheric gases available to photosynthetic organisms are 0.04% CO 2 and 21.0% O 2. Preventing Water Loss Plants require water to perform photosynthesis; they acquire this water from their environment, using their roots to absorb the water from the soil. Water enters the cell creating the cytosol. Carbon dioxide also required for photosynthesis enters the plant through the stomata of the leaf. The water in the cytosol and carbon dioxide supply the chloroplast, where the process of photosynthesis occurs. Environmental factors change which may hinder water, carbon dioxide concentrations in the plant and negatively influence photosynthesis. Plants have special adaptations to limit the impact of these environmental factors. Plants have a waxy water proof layer called a cuticle. This covering prevents water lose by transpiration. This ensures that there is not a rapid loss of water and photosynthesis may continue. The exchange of gases with the atmosphere is controlled by the stomata of the leaves. These stomata can open and close during the day or night. When the stomata are open during the day, gas exchange takes place; carbon dioxide enters the leaf and oxygen leaves entering the atmosphere. At night the stomata close and photosynthesis stops.
Stomata are small pores in the surface of a leaf that can be opened and closed to control the exchange of gases between the atmosphere and the leaf interior. (Stoma singular) Photorespiration: The Problem with Rubisco Photorespiration is the catalysis of O2 instead of CO2 by rubisco into RuBP, which slows the Calvin cycle, consumes ATP, and results in a release of carbon. All metabolic reactions are catalyzed by enzymes. The rate or efficiency is dependent on the availability of reactants, enzymes, temperature, ph, and inhibitors. Rubisco, is the most abundant enzyme on earth, is a very slow working enzyme. The rate of photosynthesis occurs fast, the rate of this reaction is supported by the abundance of rubisco. Rubisco (Ribulose-1,5-bisphosphate carboxylase oxygenase) can also bind with oxygen molecules and catalyzes RuBP with oxygen, but the product is not used by the cell. This molecule must be converted back to useful molecules and release RuBP. The process is long and occurs in the chloroplast, peroxisomes and mitochondria. The final product released is carbon dioxide. This process is called photorespiration. C4 Plants C4 Cycle is an alternative form of carbon fixation that some plants use, particularly in hot weather, to increase the concentration of CO2 available for the Calvin cycle reactions. C 4 plants are plants that are found in hot, dry climates, and have internal leaf structures and mode of carbon fixation that minimizes photorespiration. To prevent photorespiration in these plants, the Calvin cycle is found in special cells called bundle-heat cells, which are connected to mesophyll cells. This structure reduces the exposure of rubisco to oxygen
In the C 4 plant CO 2 combines with phosphoenolpyruvate (PEP), carboxylation reaction facilitated by PEP carboxylase, producing oxaloacetate. The oxaloacetate is then reduced to malate. As the malate passes into the bundle-sheath cell, it loses a CO 2 molecule, decarboxylation reaction, and becomes pyruvate. Pyruvate leaves the bundle-sheath cell returning to the mesophyll cell, where it converted to PEP by ATP, phosphorylation reaction. The CO 2 molecule released into the bundle-sheath cycle is now used in the Calvin cycle (C3 cycle), with rubisco facilitating the reaction. Oxygen is prevented from interacting with Rubisco, preventing photorespiration, because oxygen is unable to reach the bundle-sheath cell. This C 4 cycle is energy consuming since ATP is used to convert pyruvate into PEP. Figure 3: C4 cycle, pg. 232
CAM Plants Crassulacean Acid Metabolism (CAM) is a metabolic pathway, used mostly by succulent plants, in which the Calvin cycle and the C4 cycle are separated in time for better efficiency of CO2 fixation. Plants that run their C4 cycle in the same cells as their Calvin cycle (2 phases), unlike the C 4 plant, that use mesophyll cells and bundle-sheath cells, are called Crassulacean Acid Metabolism or Cam plants. These types of plants live in hot and dry climates during the day and cool at night, such as; cacti and succulent species. These plants open their stomata at night to release oxygen that has built up during photosynthesis throughout the day and take in carbon dioxide. The carbon dioxide enters the C4 cycle right away, where it is fixed into molecules of malate. The malate is stored as malic acid in the cell s vacuoles for use during the day. During daylight, the C 4 cycle stops, the stomata are closed and gas exchange ceases. The surplus malic acid diffuses from the vacuoles into the cytosol and is oxidized into pyruvate and carbon dioxide is released. The pyruvate is recycled back to malate during the night. The carbon dioxide enters the Calvin cycle, as Rubisco carboxylizes it into 3-phosphoglycerate.
Figure 4: C4 cycle and Cam plants, a) Some C4 plants, such as sugarcane, control the location of the C4 and Calvin cycles. b) CAM plants, such as Pineapple, also use a C4 Cycle, carrying out the two cycles in the same cells but at different times. pg. 233