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

REVIEW…light and dark reactions DARK RXNS http://www.science.smith.edu/departments/Biology/Bio2 31/calvin.html LIGHT AND DARK RXNS http://www.campbellbiology.com/ http://www.wwnorton.com/college/biology/discoverbio3/ full/content/index/animations.asp http://bcs.whfreeman.com/thelifewire/content/chp08/08 02001.html

CO2 3-PGA ATP ADP RUBISCO ADP ATP 1,3 PGA RuBP NADPH G3P NADP+ G3P G3P GLUCOSE

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

Remember Stomata Leaf “pores” usually on UNDERSIDE of leaf…why? Allows O2 out and CO2 in for photosynthesis Allows H2O out through transpiration (“plant sweating”)

On hot, dry days, plants close their stomata (“pores”) Conserving water but limiting access to CO2  reduces photosynthesis Causing oxygen to build up WHY??? http://academic.kellogg.edu/herbrandsonc/bio111/animations/002 1.swf

All plants carry out photosynthesis by… adding carbon dioxide (CO2) to a RuBP Catalyzed by the enzyme RUBISCO RuBP+CO2 breaks down IMMEDIATELY  3-PGA 3-PGA + ATP  1,3-PGA  reduced to G3P by NADPH The process is called the Calvin cycle and the pathway is called the C3 pathway.

C3 Plants (“regular” calvin cycle) Plants add CO2 in carbon fixation to RuBP (by Rubisco) Occurs in MESOPHYLL cells of leaf Called C3 plants b/c 1st product of carbon fixation is a 3 carbon compound Ex: Rice, Wheat , Soy But…on hot days, stomata close  reduces sugar productionO2 builds up Ex: droughts little water but HOT  little sugar production

RUBISCO ribulose bisphosphate carboxylase oxygenase (RUBISCO) Can bind CO2 Also has affinity for/can bind O2 Depends on concentrations of CO2 and O2

So…why is this a problem? Light Reactions gives off O2 More O2 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.

Photorespiration: An Evolutionary Relic? In photorespiration O2 substitutes for CO2 (competitive inhibitor) in active site of enzyme Rubisco Rubisco adds O2 to Calvin cycle NOT CO2  product made and splits into 2-C  can’t make G3P  Occurs in light (photo) and uses O2 and releases CO2 (respiration) NO ATP is made  it USES ATP  NO SUGAR produced  The photosynthetic rate is reduced 

Why photorespirate? Ancient atmosphere = little O2 Current atmosphere = lots of O2 Rubisco has an affinity to bind to O2 and CO2 So…now it is inevitable that Rubisco will bind some O2 b/c there is so much O2 in air

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

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

C4 Plants C4 plants minimize the cost of photorespiration Takes CO2 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

ACIDS = 4-C oxaloacetic acid (OAA) then Malate (4C) exported to bundle sheath cells Malate  CO2 +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 O2) Rubisco can bind CO2 better

C4 leaf anatomy and the C4 pathway Plasmodesma

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, C4 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

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 CO2 into organic acids first (at night) CAM = crassulacean acid metabolism

CAM Plants During night  stomata open CO2 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  CO2 released Calvin cycle (C3) cycle is used to produce sugar using energy made in light rxns

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

NOTE: PGAL = G3P http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/interactivemedia/activities/load.html?10&F