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

2. 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?

3. 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.

4. C3 pathway

5. 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

6. 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

7. 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!

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

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

10. 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”)

11. 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

12. 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?

13. Conserves water but limits access to CO2  reduces photosynthesis
Causing O2 to build up
WHY???
0846/58644/0021.swf::Stomata

14. 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

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

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

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

18. 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

19. 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 

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

21. 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

22. 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?

24. 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

25. 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

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

27. 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

28. 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

29. 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

31. NOTE: PGAL = G3P

33. OVERALL TUTORIAL/VIDEO