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/Bio231/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/chp08/0802001.html

CO2 3-PGA

RUBISCO

ATP

ADP

1,3 PGA

NADPH

NADP+

G3P

G3P

G3P

GLUCOSE

RuBP

ATP

ADP

Alternative mechanisms of carbon fixation

Plants evolved ~475 myaBig 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/0021.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 compoundEx: Rice, Wheat , Soy

But…on hot days, stomata close reduces sugar productionO2 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 photorespirationO2 substitutes for CO2 (competitive

inhibitor) in active site of enzyme RubiscoRubisco 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 PlantsUsually found in high daytime temps and intense sunlightEx: Corn, Sugarcane, crab grass

Have different leaf anatomy than C3 plantC3= uses mesophyll cellsC4= 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 cellsMalate 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

PlasmodesmaPlasmodesma

C4 Plants Characteristicswell adapted to (found in) habitats

with:high daytime temperaturesintense sunlightlow water conditions (drought)

Advantage = photorespiration not as much of a problemEx: crabgrass, corn (maize),

sugarcaneToday, 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 Plantsthink “AM” has to do with time of day

Type of C4 plantIn CAM plants, C3/C4 pathways NOT

separate in leaf anatomy but by TIMENO 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 openCO2 joins 3-C PEP 4-C

oxaloacetic acid (OAA) 4-C malate in central vacoule

In morning (light) stomata closedAccumulated malate leaves

vacoule broken down CO2 released Calvin cycle (C3) cycle is used to produce sugar using energy made in light rxns

CAM Plants CharacteristicsUsually found in high daytime tempscool night temps intense sunlightLow 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