1. C4 Photosynthesis
AP Biology
Unit 4

2. Review: C3 Photosynthesis
Regular photosynthesis = CO2 trapped into a 3-c molecule by Rubisco
This 3C enters calvin cycle to produce glucose …. eventually

3. C4 Photosynthesis
Some plants used slightly modified photosynthesis (C4 Photosynthesis)
C4 Photosynthesis = adaptation that evolved due to environment.
Dry/Arid and Protein “issues”

4. Rubisco ALL plants have it and use. Rubisco reacts with CO2
also react with O2 – 2 competing reactions.
Carboxylase Reaction
Good for glucose
Oxygenase Reaction
No good
Uses ATP to burn carbon compound
No sugar

5. Photorespiration Rubisco reacts with O2 instead of CO2 Occurs when:
High O2 concentrations
High heat (closed stomata)
Photorespiration cuts sugar by 25%

6. Why high heat? Hot temp = closed stomata
conserve H2O
Photosynthesis continues  CO2 lvl drops and O2 increase
photorespiration occurs

7. C4 Photosynthesis Certain plants limit photorespiration C4 Pathway*
CAM Pathway*
Both convert CO2 into a 4c molecule
 C4 Photosynthesis

8. Leaf Anatomy C3 plants all photosynthesis occurs in mesophyll cells.
Bundle sheath cells
(no photosynthesis
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9. C4 Pathway
Photosynthesis occurs in mesophyll and the bundle sheath cells.
Light reactions in mesophyll
Calvin Cycle in Bundle sheath
Affinity of PEP Carboxylase for CO2 is much higher than its affinity for O2.
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10. C4 Pathway
CO2 fixed into 4c molecule by enzyme (PEP Carboxylase) in mesophyll
Rubico not present to make mistake

11. C4 Pathway 4c molecule “smuggles” CO2 into the bundle sheath cell
CO2 released
Enters Calvin Cycle
C3 Pathway

12. How? Bundle sheath (BS) cells far from surface
less O2 access
PEP Carboxylase has no affinity for O2
Allows plant to concentrate CO2 in the bundle sheath (where Rubisco is)

13. CAM Pathway Fix CO2 at night Calvin Cycle happens during day
Stomates open
store as 4c molecule
Calvin Cycle happens during day
Stomates closed
prevent H2O loss

14. How?
Collects CO2 at night so that it can be more concentrated during the day
Plant can still do the calvin cycle during the day without losing water

15. Summary of C4 Photosynthesis
C4 Pathway
Separates by space
CAM Pathway
Separates reactions by time

16. Comparison Light intensity is directly related to temp
C4 Plants (CAM and C4) do more photosynthesis at high temp