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Chapter 10 (Part 4) Alternative Pathways & Photosynthesis (Plant Evolution) Ms. Gaynor AP Biology.

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Presentation on theme: "Chapter 10 (Part 4) Alternative Pathways & Photosynthesis (Plant Evolution) Ms. Gaynor AP Biology."— Presentation transcript:

1 Chapter 10 (Part 4) Alternative Pathways & Photosynthesis (Plant Evolution) Ms. Gaynor AP Biology

2 REVIEW…light and dark reactions DARK RXNS 31/calvin.html 31/calvin.html LIGHT AND DARK RXNS full/content/index/animations.asp full/content/index/animations.asp html html



5 Alternative mechanisms of carbon fixation Plants evolved ~475 mya Big problem for terrestrial plants = dehydration So…different carbon fixation pathways have evolved in hot, arid (dry) climates

6 Remember Stomata Leaf pores usually on UNDERSIDE of leaf…why? Allows O 2 out and CO 2 in for photosynthesis Allows H 2 O out through transpiration (plant sweating)

7 On hot, dry days, plants close their stomata (pores) Conserving water but limiting access to CO 2 reduces photosynthesis Causing oxygen to build up WHY??? 1.swf 1.swf

8 All plants carry out photosynthesis by… adding carbon dioxide (CO 2 ) to a RuBP Catalyzed by the enzyme RUBISCO RuBP+CO 2 breaks down IMMEDIATELY 3-PGA 3-PGA + ATP 1,3-PGA reduced to G 3 P by NADPH The process is called the Calvin cycle and the pathway is called the C3 pathway.

9 C 3 Plants (regular calvin cycle) Plants add CO 2 in carbon fixation to RuBP (by Rubisco) Occurs in MESOPHYLL cells of leaf Called C 3 plants b/c 1 st product of carbon fixation is a 3 carbon compound Ex: Rice, Wheat, Soy But…on hot days, stomata close reduces sugar production O 2 builds up Ex: droughts little water but HOT little sugar production

10 RUBISCO r ib u lose bis phosphate c arboxylase o xygenase (RUBISCO) Can bind CO 2 Also has affinity for/can bind O 2 Depends on concentrations of CO 2 and O 2

11 So…why is this a problem? Light Reactions gives off O 2 More O 2 dissolves in cytoplasm of cell at higher temperatures b/c more light rxns occuring Therefore, high light intensities and high temperatures (above ~ 30°C) favor the second reaction (oxygenase action) of Rubisco.

12 Photorespiration: An Evolutionary Relic? In photorespiration O 2 substitutes for CO 2 ( competitive inhibitor ) in active site of enzyme Rubisco Rubisco adds O 2 to Calvin cycle NOT CO 2 product made and splits into 2-C cant make G3P Occurs in light ( photo ) and uses O 2 and releases CO 2 ( respiration ) NO ATP is made it USES ATP NO SUGAR produced The photosynthetic rate is reduced

13 Why photorespirate? Ancient atmosphere = little O 2 Current atmosphere = lots of O 2 Rubisco has an affinity to bind to O 2 and CO 2 So…now it is inevitable that Rubisco will bind some O 2 b/c there is so much O 2 in air

14 Alternative mechanisms of carbon fixation Photosynthetic adaptations to MINIMIZE photorespiration and OPTIMIZE Calvin Cycle 2 Types of plants have adaptated: C4 Plants CAM Plants

15 C4 Plants Usually found in high daytime temps and intense sunlight Ex: Corn, Sugarcane, crab grass Have different leaf anatomy than C3 plant C3= uses mesophyll cells C4= uses mesophyll AND bundle sheath cells



18 C 4 Plants C 4 plants minimize the cost of photorespiration Takes CO 2 into MESOPHYLL CELL turns it to 3-C PEP molecule using PEP carboxylase (enzyme) 3-C PEP turned into 4-C intermediates (different acids) ACIDS are then stored in plant for later moves into Bundle Sheath cell

19 ACIDS = 4-C oxaloacetic acid (OAA) then Malate (4C) exported to bundle sheath cells Malate CO 2 +pyruvic acid in normal Calvin cycle (C3 cycle) Pyruvic acid moves back to mesophyll cell reforms 3-C PEP Bundle sheath cells deep in leaf tissue (little O 2 ) Rubisco can bind CO 2 better

20 C 4 leaf anatomy and the C 4 pathwayPlasmodesma


22 C4 Plants Characteristics well adapted to (found in) habitats with: high daytime temperatures intense sunlight low water conditions (drought) Advantage = photorespiration not as much of a problem Ex: crabgrass, corn (maize), sugarcane Today, C 4 plants represent about 5% of Earth's plant biomass and 1% of its known plant species. Despite this scarcity, they account for around 30% of terrestrial carbon fixation

23 CAM Plants think AM has to do with time of day Type of C4 plant In CAM plants, C3/C4 pathways NOT separate in leaf anatomy but by TIME NO BUNDLE SHEATH CELLS…only mesophyll cells like C3 plants Open their stomata at night, incorporating CO 2 into organic acids first (at night) CAM = c rassulacean a cid m etabolism

24 CAM Plants During night stomata open CO 2 joins 3-C PEP 4-C oxaloacetic acid (OAA) 4-C malate in central vacoule In morning (light) stomata closed Accumulated malate leaves vacoule broken down CO 2 released Calvin cycle (C3) cycle is used to produce sugar using energy made in light rxns


26 CAM Plants Characteristics Usually found in high daytime temps cool night temps intense sunlight Low soil moisture Advantage = photosynthesis can occur during day when stomata are closed Ex: pineapple and cacti

27 NOTE: PGAL = G3P eractivemedia/activities/load.html?10&F


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