C3, C4 and CAM Plants PDF

C3,C4 AND CAM PLANTS Carbon Dioxide Fixation: C4 Plants In 1966, Hatch and Slack presented evidence that another pathway for CO2 fixation exists. 1st product from the mesophyll is a 4C compound Pathway incorporates CO2 using phosphoenolpyruvate (PEP) carboxylase enzyme ATP produced...

C3,C4 AND CAM PLANTS Carbon Dioxide Fixation: C4 Plants In 1966, Hatch and Slack presented evidence that another pathway for CO2 fixation exists. 1st product from the mesophyll is a 4C compound Pathway incorporates CO2 using phosphoenolpyruvate (PEP) carboxylase enzyme ATP produced in photophosphorylation is used to convert pyruvate to PEP Carbon Dioxide Fixation: C4 Plants (cont.) The PEP (3C) is carboxylated to three 4-carbon acids (oxaloacetate, malate and aspartate) These acids are converted in the vascular sheath cells in to pyruvate During conversion to pyruvate, a carbon released used either by addition to RuBP or by addition to a 2-carbon molecule to become 3-PGA by RuBP carboxylase After 3-PGA is produced, the Calvin cycle is operative again. Physiological Differences between C3 and C4 C4 able to maintain high concentration of CO2 in bundle- sheath cell thus, avoid wasteful photorespiration. A comparison of significant features of C3 and C4 C3 C4 Photorespiration yes no CO2 compensation (µl CO2 l-1) 20-100 0-5 Temperature optimum (ºC) Photosynthesis 20-25 30-45 Rubisco 20-25 - PEPcase - 30-35 Quantum yield as a function of declining steady temperature Transpiration Ratio 500-1000 200-350 Light saturation(µmole 400-500 Does not photons) saturate  Anatomical differences of C3 and C4 Species C4 species have chloroplast in the vascular sheath cells, C3 species do not. Chloroplast in mesophyll of C3 and C4 are structurally similar but no starch is produced in C4 plants (just 4C compounds) Chloroplast in vascular sheath cells of C4 species are larger and have less developed grana than in mesophyll cell chloroplasts since Calvin Cycle is operative, they store starch)  Crassulacean Acid Metabolism (CAM) Plants Occurs predominately in succulent plants Adapted to arid conditions where low transpiration is a survival necessity Stomata are closed during the day and open at night to absorb CO2 Domestic examples of Cam plants: -Pineapple, cactus Fix CO2 into 4-C acids with PEP carboxylase only at night when stomata are open and energy required comes from glycolysis Photosynthesis mechanism in C4 Plants Photosynthesis mechanism in CAM Plants  Comparison Between C4 and CAM The first product when CO2 is fixed is the same (the c-4 carbon) Low photorespiration occurs in C4 and CAM pathways, although the exact mechanisms are different Different time of the day to fix CO2 CAM at night, C4 during daytime Fixation of CO2 and Calvin Cycle occur in different cells in C4. However, in CAM both processes occur in the same cell (single cell)  C4 Plants C4 plants minimize the cost of photorespiration - By incorporating CO2 into four carbon compounds in mesophyll cells - Phosphoenolpyruvate, pyruvate, malate, oxaloacetate  These four carbon compounds - Are exported to bundle sheath cells, where they release CO2 used in the Calvin Cycle During the day, the stomata close - And the CO2 released from the organic acids for use in the Calvin Cycle  CAM plants Open their stomata at night, incorporating CO2 into organic acids  Photosynthetic Characteristic In 3 Major Plant Group Characteristics C3 C4 CAM Leaf Anatomy Mesophyll with Mesophyll with Mesophyll with no distinct distinct bundle distinct bundle bundle sheath sheath sheath Carboxylating Rubisco PEPcase & PEPcase & enzyme Rubisco Rubisco (separation in (separation in time) space) Photorespiration High (1/3 of total Very low or Very low photosynthesis) absent