Chapter 8: Photosynthesis

Chapter 8: Photosynthesis. NULL.

Carbon fixation – Calvin cycle. To build carbohydrates cells use: Energy = ATP from light-dependent reactions. Cyclic and noncyclic photophosphorylation. Drives endergonic reaction. Reduction potential = NADPH from photosystem I. Source of protons and energetic electrons..

Calvin cycle. Convert inorganic carbon into organic carbon. Named after Melvin Calvin (1911–1997). Also called C 3 photosynthesis First intermediate molecule, phosphoglycerate , has three carbons. Key step is attachment of CO 2 to the 5-carbon sugar, ribulose 1,5-bisphosphate (RuBP) to form 3-phosphoglycerate (PGA). Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco ..

Calvin vs Krebs. Unlike the citric acid cycle, carbon fixation is geared toward producing new compounds, so the nature of the cycles is quite different..

Three phases of Calvin cycle. Carbon fixation RuBP + CO 2  PGA Reduction PGA is reduced to glyceraldehyde 3-phosphate (G3P). Regeneration of RuBP G3P is used to regenerate RuBP. Three turns incorporate enough carbon to produce a new G3P Six turns incorporate enough carbon for 1 glucose.

ATP, NADPH, and carbon dioxide are used in the Calvin cycle to produce energy rich organic compounds, like sugars (see appendix).

Output of Calvin cycle. Glucose is not a direct product of the Calvin cycle G3P is a 3 carbon sugar Used to form sucrose. Major transport sugar in plants. Disaccharide made of fructose and glucose. Used to make starch. In time of extensive photosynthesis Through glucose-1-pohsphate Insoluble glucose polymer. Stored for later use..

Energy cycle. Photosynthesis uses the products of respiration as starting substrates. Respiration uses the products of photosynthesis as starting substrates. Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse. Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria..

Chloroplasts and mitochondria: completing an energy cycle..

Photorespiration. Rubisco has two enzymatic activities: Carboxylation Addition of CO 2 to RuBP. Favored under normal conditions. Photorespiration Oxidation of RuBP by the addition of O 2 . Favored when stoma are closed in hot conditions. Creates low-CO 2 and high-O 2 . CO 2 and O 2 compete for the active site on RuBP.

Photorespiration conditions. Heat absorbed by leaves causes water to evaporate out of stomata. Closing stomata prevents water loss, but leads to photorespiration (see appendix).

Photorespiration. Image result for photorespiration.

Types of photosynthesis. C 3 Plants that fix carbon using only C 3 photosynthesis (the Calvin cycle). C 4 and CAM Add CO 2 to phosphoenolpyruvate (PEP) to form 4 carbon molecule. Use PEP carboxylase. Greater affinity for CO 2 , no oxidase activity. C 4 – spatial solution to photorespiration . CAM – temporal solution to photorespiration..

Comparison of C3 and C4 pathways of carbon fixation.

C 4 plants. Corn, sugarcane, sorghum, and a number of other grasses. Initially fix carbon using PEP carboxylase in mesophyll cells. Produces oxaloacetate that is converted to malate, transported to bundle-sheath cells. Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO 2 . Efficient carbon fixation using CO 2 by rubisco via the Calvin cycle..

C 4 plant carbon fixation. C4 plants use C3 and C4 carbon fixation pathways in different locations to maintain high levels of CO2 preventing photorespiration (see appendix).

C 4 pathway. C4 plants have evolved to minimize photorespiration C 4 pathway, although it overcomes the problems of photorespiration, does have a cost. To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone. Each CO 2 transported costs 2 ATP to convert pyruvate back to PEP. C 4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C 3 pathway alone..

CAM plants CAM=Crassulacean (family of flowering plants with succlent leaves) Acid Metabolism.

CAM plant carbon fixation. CAM plants use C3 and C4 carbon fixation pathways at different times of day to prevent photorespiration (see appendix).

Compare C 4 and CAM. Both use both C 3 and C 4 pathways. C 4 – two pathways occur in different cells. CAM – C 4 pathway at night and the C 3 pathway during the day..

End of Chapter 8 (1 of 2). McGraw Hill Education.

End of Chapter 8 (2 of 2). McGraw Hill Education.