1. ADAPTIONS TO TEMPERATURE AND DROUGHT
KEY DIFFERENCE: HOW CO2 IS CAPTURED C3 C4
CAM
WHEAT
POTATO
RICE
CORN
SUGERCANE
PINEAPPLE
CACTUS
VAST MAJORITY OF PLANTS ≈ 85 % of species)
≈7 % of species, common in deserts
≈3 % of species, most rare
THRIVE IN MODERATE TEMPERATURES, PLENTIFUL GROUNDWATER, CO2 > 200 ppm
THRIVE IN HIGH
TEMPERATURES, DROUGHT
THRIVE IN HIGH
TEMPERATURES, DROUGHT
ADVANTAGE:
ENERGY EFFICIENT
CHALLENGES:
WATER LOSS IN HOT, DRY CONDITIONS
ADVANTAGE:
MORE DROUGHT RESISTANT
CHALLENGES: LESS ENERGY EFFICIENT
ADVANTAGE:
EXTREMELY DROUGHT RESISTANT
CHALLENGES:
VERY ENERGY INEFFICIENT

2. KEY DIFFERENCES IN HOW CO2 IS CAPTURED
C4: SEPARATION IN SPACE;
CAPTURE CO2 IN MESOPHYLL CELL; CALVIN CYCLE IN SEPARATE CELL
CAM: SEPARATION IN TIME
CAPTURE CO2 AT NIGHT; CAN USE CO2 DURING THE DAY
ENZYME USED
FOR CO2 CAPTURE:
RUBISCO
PEPCO
PEPCO

3. TRANSPIRATION – process of evaporation of water from surface of leaves; helps cool plant
Stoma
Note: As CO2 is taken in through stoma (opening on surface of leaf), O2 and H2O exit

4. The addition of CO2 to RuBP to form 3-PGA is catalyzed by the enzyme Rubisco
-Very inefficient enzyme
Slow rate means that many copies of enzyme are required in each chloroplast

5. PHOTORESPIRATION: Rubisco can also catalyze the addition of O2 to PGA with disastrous consequences for plants
Rubisco evolved at time in earth’s history in which atmospheric oxygen concentration was very low
No evolutionary pressure to exclude O2

6. O2 will outcompete CO2 for active site;
PHOTORESPIRATION: O2 CAN OUTCOMPETE CO2 FOR RUBISCO ACTIVE SITE UNDER CERTAIN CONDITIONS
Normal Temperature, [CO2] ≥ [O2]
If [O2] >> [CO2] ,
O2 will outcompete CO2 for active site;
O2 affinity increases at higher Temp
Rubisco normally binds CO2 more tightly than O2

7. PHOTORESPIRATION O2 is added to RuBP, siphons C out of Calvin cycle
H2O escapes from stomata on hot, dry days
Hot, dry conditions → Stomata CLOSE to reduce H2O loss.
Stomata closed →[CO2] , [O2]
O2 outcompetes CO2 to bind to Rubisco
O2 is added to RuBP, siphons C out of Calvin cycle
Link to Boyer photorespiration animation

8. PHOTORESPIRATION O2 is added to RuBP, siphons C out of Calvin cycle
H2O escapes from stomata on hot, dry days
Hot, dry conditions → Stomata CLOSE to reduce H2O loss.
Stomata closed →[CO2] , [O2]
O2 outcompetes CO2 to bind to Rubisco
O2 is added to RuBP, siphons C out of Calvin cycle
Link to Boyer photorespiration animation

9. O2 will outcompete CO2 for active site;
PHOTORESPIRATION: O2 CAN OUTCOMPETE CO2 FOR RUBISCO ACTIVE SITE UNDER CERTAIN CONDITIONS
Normal Temperature, [CO2] ≥ [O2]
If [O2] >> [CO2] ,
O2 will outcompete CO2 for active site;
O2 affinity increases at higher Temp
Rubisco normally binds CO2 more tightly than O2

10. Impact of Photorespiration on Calvin cycle
Transfer of O2 ultimately causes loss of carbon, instead of gain of carbon in Calvin cycle

11. PEPCO HAS VIRTUALLY NO AFFINITY FOR O2
C4 AND CAM PLANTS USE THE ENZYME PHOSPHOENOLPYRUVATE CARBOXYLASE (PEPCO) TO CAPTURE CARBON DIOXIDE TO FORM A 4 CARBON CARBOXYLIC ACID
3C + 1C → 4C
REACTION CATALYZED BY PEPCO
PEPCO HAS VIRTUALLY NO AFFINITY FOR O2
5C + 1C → 6C → 3C + 3C
O2 CAN COMPETE WITH CO2; PHOTORESPIRATION

12. Fig
C4 plants have a different anatomy than C3 plants and capture CO2 initially not in 3-carbon PGA but rather in 4 –carbon oxaloacetate
C4 leaf anatomy
The C4 pathway
Mesophyll
cell
Mesophyll cell
CO2
Photosynthetic
cells of C4
plant leaf
PEP carboxylase
Bundle-
sheath
cell
Oxaloacetate (4C)
PEP (3C)
Vein
(vascular tissue)
ADP
Malate (4C)
ATP
Pyruvate (3C)
Bundle-
sheath
cell
Stoma
CO2
Video link excellent
Calvin
Cycle
Note that Calvin cycle takes place in a completely separate cell, the Bundle –sheath cell, than CO2 capture.
Sugar
Vascular
tissue
Video Link to C4

13. Comparing efficiency of CO2 capture in C3 by RUBSICO vs C4 Plants by PEPCO as a function of [CO2]
C4 plants capture CO2 with greater efficiency than C3 at low [CO2];
PEPCO BINDS CO2 MORE TIGHTLY THAN RUBISCO AT LOW [CO2]
C3 plants are more efficient at higher [CO2] due to C4 plants reaching saturation at lower [CO2]

14. HOW DOES TEMPERATURE IMPACT RATE OF PHOTOSYNTHESIS IN C4 VS C3 PLANTS?
C4 IS MUCH MORE EFFICIENT AT HIGHER TEMPERATURE; C3 IS MORE EFFICIENT AT LOWER TEMPERATURES

15. What do you notice about CO2 capture (yellow) and water loss (transpiration) rates (blue) at different times of the day?

16. What do you notice about CO2 capture (yellow) and water loss (transpiration) rates (blue) at different times of the day?
PEPCO
RUBISCO
ENZYME PEPCO CATALYZES CAPTURE OF CO2 AS A FOUR CARBON ORGANIC ACID; CO2 DELIVERED TO RUBISCO BY FROM 4 C ACID

17. Figure 10.19 C4 and CAM photosynthesis compared
Table Link

21. Link to Smith Calvin cycle animation
Link to University of Arizona Biology Project

22. Mechanisms of Carbon Fixation
KEY DIFFERENCE- HOW CO2 IS CAPTURED

24. FEATURE C3 C4
CAM

27. NOTE HOW CO2 IS CAPTURED AS AN ORGANIC ACID, OXALOACETATE IN REACTION CATALYZED BY ENZYME PEP

28. Link to Boyer photorespiration animation

29. CAM plants capture CO2 as 4-carbon organic acids, oxaloacetate at night and release CO2 to Calvin cycle during the day

30. PHOTORESPIRATION: Rubisco can also catalyze the addition of O2 to PGA with disastrous consequences for plants
Rubisco evolved at time in earth’s history in which atmospheric oxygen concentration was very low
No evolutionary pressure to exclude O2
Rubisco binds CO2 more tightly than O2, but if [O2] is much higher than [CO2] , O2 will out compete CO2 for active site; also higher temperatures increase relative affinity for O2.
Transfer of O2 ultimately causes loss of carbon, instead of gain of carbon in Calvin cycle

31. Photorespiration: An evolutionary relic?
Photosynthesis first evolved in the absence of oxygen
Oxygen is not detected (as rust in iron-rich rocks) until ~ 2.7 bay.
In most plants, initial fixation of CO2, via the enzyme RUBISCO, forms a three-carbon compound
In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
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