Alternative Pathways & Photosynthesis Ms. Day AP Biology Chapter 10 (Part 4) Alternative Pathways & Photosynthesis Ms. Day AP Biology

In small groups, decide on an answer: What is the first STABLE product after RUBISCO fixes CO2 into an organic molecule? 1,3-PGA RuBP 3-PGA G3P Why do you think the Calvin cycle is also known as the C3 pathway?

All plants carry out photosynthesis by… Fixing CO2 to a RuBP using RUBISCO RuBP+CO2 breaks down IMMEDIATELY  3-PGA (STABLE!!!) 3-PGA + ATP  1,3-PGA  reduced to G3P by NADPH This is called the Calvin cycle Also known as C3 pathway.

C3 pathway

Alternative mechanisms of carbon fixation Plants evolved ~475 mya Discuss in your lab group… As they moved from water to land, what do you hypothesize was their BIGGEST problem? So…different carbon fixation pathways have evolved in hot, arid (dry) climates

Chemicals in plants that lose water Cells that absorb O2 In small groups, decide on an answer: VOCAB RECALL… What are stomata? Chemicals in plants that lose water Cells that absorb O2 Holes in leaves A gel-like fluid in chloroplasts

STOMATA Less evaporation from sun! Stoma = singular Leaf “pores” or holes usually on UNDERSIDE of leaf…why would they be located here? Less evaporation from sun!

In small groups, decide on an answer: What usually diffuses into stomata during photosynthesis? Water O2 G3P CO2

In small groups, decide on an answer: What usually diffuses OUT OF stomata during photosynthesis? Water O2 G3P CO2

BUT… Stomata Allows O2 out (from photolysis) Allows CO2 in for photosynthesis Why do you need CO2? BUT… Allows H2O out through transpiration (“plant sweating”)

In small groups, decide on an answer: RECALL…Why is transpiration important in plants? It allows water to move into the soil It helps feed the plant It allows water to move to the leaves from the soil It helps move sugar (food) to the roots

On hot, dry days, plants close their stomata to conserve H2O. In small groups, discuss a possible outcome of this scenario: On hot, dry days, plants close their stomata to conserve H2O. What sequence of events might happen to this plant when stomata are closed and why?

Conserves water but limits access to CO2  reduces photosynthesis Causing O2 to build up WHY??? http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::640::480::/sites/dl/free/007251 0846/58644/0021.swf::Stomata

C3 Plants (uses “regular” Calvin cycle) CO2 is attached to RuBP (by Rubisco) Occurs in MESOPHYLL cells of leaf Called C3 plants Ex: Rice, Wheat , Soy But…on hot days, stomata close  reduces sugar production O2 builds up

ribulose bisphosphate carboxylase oxygenase RUBISCO ribulose bisphosphate carboxylase oxygenase In small groups, discuss a possible meaning for the following words Carboxylase? Oxygenase?

RUBISCO BUT… Can bind CO2  carbon fixation Also can bind O2 IF…concentrations of O2 are higher than CO2 in stroma

In your small groups… Using prior knowledge of the light reactions, discuss why there would be a build up of O2 in the stroma?

So…why is binding O2 a problem? Light Reactions gives off O2 Higher temps more sunlight  more light reactions More light reactions  more O2 Therefore, high light intensities and high temperatures favor the second reaction (oxygenase action) of Rubisco

Photorespiration: An Evolutionary Relic? O2 substitutes for CO2 (competitive inhibitor) in active site of Rubisco Rubisco adds O2 in Calvin cycle NOT CO2  product made and splits into 2-C  can’t make G3P  It USES ATP to make 2C sugar BUT…..  NO USABLE SUGAR produced  Photosynthetic rate is reduced 

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

In your small groups… Look at the cross sections of the leaves below In your small groups… Look at the cross sections of the leaves below. How are they different?

C4 Plants Minimize photorespiration Fixes CO2 in MESOPHYLL CELL makes a 3-C PEP molecule  uses PEP carboxylase (enzyme) A new “carbon fixing” enzyme!!! 3-C PEP turned into 4-C intermediates (an organic acid) ACIDS are then stored in plant for later  STORED in bundle sheath cell

What happens next? Bundle sheath cells  deep in leaf tissue (little O2 build up from light rxns) Rubisco can bind CO2 better So… Organic acids  turn back into CO2 in bundle sheath cells Enter C3 Calvin cycle in Bundle Sheath cells  makes glucose

In your small groups… Discuss one advantage that a C4 plant would have over a C3 plant in Florida or Southern California.

Differences compared to a C3 plant: C4 leaf anatomy and the C4 pathway 4C acids Plasmodesma Differences compared to a C3 plant: 2 types of cells: bundle sheath and mesophyll 2 carbon fixation enzymes: PEP carboxylase and RUBISCO CO2 reduced twice

CAM Plants think “AM” has to do with time of day Special 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 INSTEAD…Open stomata at night, fixing CO2 into organic acids first (at night) Ex: pineapple and cacti CAM = crassulacean acid metabolism

CAM Plants During night  stomata open CO2 joins 3-C PEP  4-C organic acids made STORED in central vacoule In morning (light) stomata closed Accumulated acids leaves vacuole Acids broken down to CO2 CO2 released to Calvin cycle (C3) cycle  glucose is made

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

OVERALL TUTORIAL/VIDEO https://www.youtube.com/watch?v=Dq38MpYOb8w https://www.youtube.com/watch?v=g78utcLQrJ4