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

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

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

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 O2 out and CO2 in for photosynthesis
Allows H2O out through transpiration (“plant sweating”)

7. On hot, dry days, plants close their stomata (“pores”)
Conserving water but limiting access to CO2  reduces photosynthesis
Causing oxygen to build up
WHY???
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8. 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.

9. 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

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

11. 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.

12. 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 

13. 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

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. 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

19. 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

20. C4 leaf anatomy and the C4 pathway
Plasmodesma

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, 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

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

24. 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

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