1. Photosynthesis
Alternative methods of carbon
fixation in hot, arid climates

2. You may want to set up your
notes like this
C3 Plants
C4 Plants
CAM Plants

3. Plant evolutionary history
 Life began in water
 The first plants were aquatic
 Since plants evolved to living out of water,
they have had a hard time dealing with
dehydration

4. Plant adaptations
Most adaptations involve some sort of
trade off
For plants, the trade off with
photosynthesis is that they lose water to
the environment through transpiration

5. A little plant anatomy
 The openings on leaves are called stomata
 This is where CO2 and O2 can enter and exit
the plant
 Water is also lost through the stomata
 Plants often close the stomata on very hot
days, but CO2 intake is cut off

6. C3 plants
“3” comes from
the 3-carbon
molecule produ
ced after CO
2
is fixed
Most plants
Fix carbon by
attaching CO2 to RuBP

7. C3 Plants
On hot days, they close their stomata
part way
Produce little sugar then

8. C3 plants
As the CO2 that the plant does have
gets used up, rubisco will fix O2 and
send it into the Calvin cycle instead of
CO2

9. Peroxisomes & mitochondria split the
new compound and release CO2
This is called photorespiration
Occurs in the
light
Consumes oxygen and
releases CO2

10. Photorespiration
uses ATP but
makes no ATP
Photorespiration
makes no sugars
Photorespiration
decreases
photosynthesis
output

11. So why would any
plant do this…
Photorespiration may
be an evolutionary
leftover
In early atmosphere,
with little O2, it didn’t
matter if Rubisco had an
affinity for O2
Today, with so much O2
in the atmosphere, it is
inevitable that some O2
will be fixed instead of

12. C3 Crop plants
Rice, wheat, soybeans
May lose as much as 50% of the
Carbon fixed in the Calvin Cycle
to photorespiration

13. 3 Things
What are 3 things you know for sure
about C3 plants?

14. C4 plants
C4 plants have
adaptations that
allow them to
minimize the
effects of
photorespiration
Many are
tropical grasses

15. C4 plants
“4” comes fro
m the 4-carb
on
compound th
at is formed
when CO
is fixed in C
2
4 plants
Have an alternate
means of fixing
carbon
C4 plants fix carbon
out in the
cytoplasm before it
enters the Calvin
Cycle

16. C4 plant adaptations
 1st step is binding CO2 to PEP
(phosphoenolpyruvate) by the enzyme PEP
carboxylase to make a 4-C compound:
oxaloacetate
PEP has a high affinity for CO2 and none for
O2

17.  2. The 4-carbon compound enters
photosynthetic cells
 CO2 is released from PEP and the Calvin
cycle continues as normal
 This adaptation allows C4 plants to keep a
high concentration of CO2 in the
photosynthetic cells, preventing Rubisco from
binding to O2 instead of CO2

18. C4 plants thrive in hot
climates where
stomata will be
closed often
Some important C4
plants are sugarcane,
corn, tropical grasses

19. 3 Things
What are 3 things you know for sure
about C4 plants?

20. CAM plants
Another alternate method of carbon
fixation
Common in succulents like cacti

21. CAM plants
CAM plants keep stomata closed during
the day to minimize water loss
Only open stomata at night

22. Closed stomata during the day
means no CO2 entering the
plant during the day

23. CAM plants
 When stomata are
open at night, CAM
plants take in CO2 and
fix it by incorporating
it into organic
molecules
 This is called
Cr a
crassulacean acid namssulaceae is the
e of the family
of
metabolism
plants that inc
lud
succulents
e

24. CAM plants
 Cells of CAM plants
store the organic
molecules that CO2 was
fixed to in their
vacuoles until morning
 Light reactions then
produce ATP and
NADPH to drive the
Calvin Cycle
 The CO2 in the organic
molecules is released
for use in the Calvin
Cycle

25. CAM plants
Important CAM plants include
pineapple and agave

26. 3 Things
What are 3 things you know for sure
about CAM plants?

27. Comparison of C3, C4, and CAM plants
C3 plants
Most plants
C4 plants
Tropical grasses
like corn,
sugarcane
CAM plants
Succulents,
pineapple, agave
Fix carbon in Calvin Fix carbon in
cycle - attach CO2
cytoplasm - attach
CO2 to PEP
to RuBP
Fix carbon at night
only, fix it to
organic molecules
Enzyme - Rubisco
Enzyme – PEP-ase
Enzyme – PEP-ase
Most energy
efficient method
1/2 way between
these two
Best water
conservation
Loses water
through
photorespiration
Loses less water
Loses least water