Calvin Cycle and Photorespiration PDF

Calvin Cycle and Plant Adaptation Chapter 4.2 Calvin Cycle - Learning Goals Location Name the reactants and ALL of the products (PGA, BPG, G3P, RuBP) Follow the C’s Energy molecules involved  Calvin Cycle Reactions: Overview of C3 Photosynthesis A cyclical series of re...

Calvin Cycle and Plant Adaptation Chapter 4.2 Calvin Cycle - Learning Goals Location Name the reactants and ALL of the products (PGA, BPG, G3P, RuBP) Follow the C’s Energy molecules involved  Calvin Cycle Reactions: Overview of C3 Photosynthesis A cyclical series of reactions Utilizes atmospheric carbon dioxide to produce carbohydrates Known as C photosynthesis 3 Involves three stages: Carbon dioxide fixation Carbon dioxide reduction RuBP regeneration Calvin Cycle Reactions I - Carbon Dioxide Fixation CO is attached to 5-carbon RuBP molecule 2 Results in a temporary 6-carbon molecule This splits into two 3-carbon molecules (3PG) Reaction is accelerated by RuBP Carboxylase (Rubisco) CO is now “fixed” because it is part of a carbohydrate 2 4 Calvin Cycle Reactions II - Carbon Dioxide Reduction 3PG is reduced to BPG BPG is then reduced to G3P Utilizes NADPH and some ATP produced in the light reactions 5 Calvin Cycle Reactions III - Regeneration of RuBP RuBP used in CO2 fixation must be replaced Every three turns of Calvin Cycle, Five G3P (a 3-carbon molecule) are used To remake three RuBP (a 5-carbon molecule) 5 X 3 carbon = 3 X 5 carbons 6 The Calvin Cycle recap 7 Importance of the Calvin Cycle G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules G3P The hydrocarbon skeleton of G3P can form Fatty acids and glycerol to make plant oils glucose phosphate fatty acid synthesis amino acid synthesis Glucose phosphate (simple sugar) + Fructose (which with glucose = sucrose) fructose phosphate Starch and cellulose Amino acids sucrose starch cellulose Photorespiration - Learning Goals Be able to explain the problem that C3 plants have with photorespiration (ie. why it happens etc.) Be able to explain the adaptations that C4 and CAM plants have to minimize photorespiration Photorespiration In hot and/or dry conditions Stomata of C3 plants must close to avoid wilting/water loss. But there is a consequence (think gas levels): CO2 decreases and O2 increases Problem: rubisco can not differentiate between CO2 and O2 Photorespiration O2 starts combining with RuBP, leading to the production of CO2 instead of fixation of CO2 This is called photorespiration which reduces efficiency (ie. it is BAD for C3 plants) Can you spot a few reasons why this is ‘sub-optimal’ for the C3 plant? Leaf anatomy C3 vs. C4 plants C3 Plant C4 Plant mesophyll cells bundle sheath vein bundle sheath vein cell stoma cell stoma 12 C4 Photosynthesis – a solution to photorespiration 1. Compartmentalize rubisco with specialized anatomy 2. Use a ‘better’ enzyme to grab sparse CO2 to feed to rubisco CO2 is fixed to PEP (a C3 molecule) resulting in oxaloacetate, a C4 molecule in a “Pre-Calvin” cycle 13 C4 Photosynthesis – a solution to photorespiration In hot & dry climates C4 plants avoid photorespiration Net productivity is about 2-3 times greater than C3 plants In cool, moist environments, C4 plants can’t compete with C3 plants. Why not? Crabgrass is a C4 plant. When do you notice crabgrass in your neighbourhood?  CAM Photosynthesis – not even trying to fix carbon during the day CAM = Crassulacean-Acid Metabolism CAM plants partition carbon fixation by time During the night CAM plants fix CO2 Form C4 molecules (ex. malate etc) which are stored in large vacuoles During daylight NADPH and ATP are available Stomata are closed for water conservation C4 molecules release CO2 to Calvin cycle 15 Climatic Adaptation: Photosynthesis Each method of photosynthesis has comparative advantages and disadvantages which depend on the climate C4 plants most adapted to: High light intensities High temperatures Limited rainfall C3 plants better adapted to Cooler temperatures (below 25°C) High moisture CAM plants are better adapted to extreme aridity Source: New phytologist 16 C3 vs C4 vs CAM recap  Aerobic Respiration and Photosynthesis form an energy cycle The products of one are the reactants of the other