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UNIT 2: Metabolic Processes Chapter 4: Photosynthesis 4.2 Fixing Carbon Dioxide: The Light-Independent Reactions UNIT 2 Chapter 4: Photosynthesis Section 4.2 4.2 Fixing Carbon Dioxide: The LightIndependent Reactions Plants convert carbon dioxide to carbohydrates by using ATP and NADPH produced in the light-dependent reactions, along with enzymes in the stroma. The initial step involves converting carbon dioxide into organic compounds in a process called CO2 assimilation. This step occurs in the Calvin cycle. The reactions in the Calvin cycle can be grouped into three phases. The end organic molecule is glyceraldehyde-3-phosphate (G3P). UNIT 2 Chapter 4: Photosynthesis The Calvin Cycle: Phase 1 Section 4.2 Calvin Cycle Carbon Dioxide Fixation • CO2 bonds with ribulose-1,5-bisphosphate (RuBP). The resulting six-carbon compound is unstable and breaks down into two 3-phosphoglycerate (PGA) molecules. • The reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase (also named rubisco). • Plants that use this method to fix carbon are called C3 plants, and the pathway is called C3 photosynthesis. UNIT 2 Chapter 4: Photosynthesis Section 4.2 The Calvin Cycle: Phase 2 Reduction • PGA is activated by ATP and then reduced by NADPH and converted to glyceraldehyde-3-phosphate (G3P). In their reduced, higherenergy state, some of the G3P molecules leave the cycle to become glucose, while other G3P molecules remain to replenish RuBP. UNIT 2 Chapter 4: Photosynthesis Section 4.2 The Calvin Cycle: Phase 3 Regenerating RuBP • • ATP energy is used to break and re-form the bonds to make RuBP from G3P. The Calvin cycle must be completed six times in order to synthesize one molecule of glucose. Of the 12 G3P molecules that are produced in six cycles, 10 are used to make RuBP and 2 are used to make one glucose molecule. UNIT 2 Chapter 4: Photosynthesis Section 4.2 The Calvin Cycle The net equation for the Calvin cycle is: 6CO2 + 18 ATP + 12 NADPH + water → 2 G3P + 16 Pi + 18 ADP + 12 NADP+ G3P is the starting substance for many carbohydrates and some plant oils and amino acids. UNIT 2 Chapter 4: Photosynthesis Section 4.2 Adaptation to Photosynthesis Oxygen competes with carbon dioxide for the same active site on rubisco. In a process called photorespiration, oxygen reacts with RuBP, and different products are formed, thus reducing the efficiency of photosynthesis. In controlled lab conditions, photosynthesis is more efficient and the oxygen inside the leaf increases. In nature, photosynthetic efficiency ranges from 0.1 percent to 3 percent. Continued… UNIT 2 Chapter 4: Photosynthesis Section 4.2 Adaptations to Photosynthesis Leaves prevent water loss in hot, dry conditions by closing their stomata. Water is conserved, but carbon dioxide is prevented from entering. Continued… UNIT 2 Chapter 4: Photosynthesis Section 4.2 Adaptations to Photosynthesis C4 and CAM plants live in hot and dry climates. They have evolved mechanisms to reduce photorespiration and improve the efficiency of photosynthesis. C4 plants separate the uptake of CO2 from the Calvin cycle in different types of cells. CAM plants do something similar with a biochemical pathway. In both cases, they increase the ratio of carbon dioxide to oxygen for the Calvin cycle. UNIT 2 Chapter 4: Photosynthesis C4 Plants C4 plants use energy to “pump” carbon dioxide into the bundle-sheath cells, where it becomes concentrated. Included among the C4 plants are food crops such as corn, sorghum, sugarcane (shown here), and millet. Also included are grasses such as crabgrass and Bermuda grass. Section 4.2 UNIT 2 Chapter 4: Photosynthesis Section 4.2 CAM Plants The opening and closing of stomata in CAM plants, such as pineapple and cacti, are opposite from most plants. The stomata are open at night and closed in the daytime. When the carbon dioxide is removed from the fourcarbon compound malate in the daytime, it cannot leave the cell because the stomata are closed. In cool climates, CAM plants are very inefficient, because they use energy to drive the reactions that store carbon dioxide. UNIT 2 Chapter 4: Photosynthesis The Energy Cycle in Plant Cells Both aerobic respiration and photosynthesis occur in plants and are closely related. Together they represent a plant cell’s energy cycle. Section 4.2 UNIT 2 Chapter 4: Photosynthesis Section 4.2 Aerobic Respiration and Photosynthesis UNIT 2 Chapter 4: Photosynthesis Section 4.2 Review Section 4.2