Alternate Mechanisms
of Carbon Fixation
Also Known As…
Why can’t they just leave good enough
alone. Let’s cut them all down.
What’s the problem?!?!?!
 Why can’t the plants just run the C3 thing and be happy
with that?
 Some plants live in areas in which CO2 is very hard to
come by – deserts have fewer organisms and carbon
sources so their air may be less than that of a more
populated area.
 Others may live in areas where it is so hot that opening
the stomata during the day may not be possible because
water loss would be rapid and substantial during the day.
 Both of these problems have been solved by plants that
make clever adjustments in how they harvest their CO2.
They use two slightly adjusted mechanisms – they are
the C4 pathway and the CAM pathway.
Why Do We Have C4 Plants?
 The reason for plants becoming a C4 plant is this
– the enzyme that takes in CO2 at the beginning
of the Calvin cycle, called rubisco, actually loves
O2 more than CO2.
 When oxygen gets in the active site for rubisco, a
process called photorespiration occurs – it is
brought on by hot weather because the stoma
close to preserve water and the CO2 in the leaf is
used up and its levels drop.
 The C4 pathway is seen as an evolutionary
adaptation that avoids this problem of
photorespiration.
The C4 Pathway
 An enzyme, phosphoenolpyruvate carboxylase,
catalyzes a reaction that fixes a carbon dioxide molecule
to a molecule of PEP – this makes a four-carbon
molecule of oxaloacetate. The first product made has four
carbons, hence the name C4 photosynthesis.
 These plants can do this because they have two types of
photosynthetic cells – bundle sheath cells and mesophyll
cells. The two cell types are found close together within
the leaf of a C4 plant. Connection between the two cell
types is made through the plasmodesmata.
 [PEP + CO2  Oxaloacetate  Malate] 
[Pyruvate + CO2 (for Calvin)  Pyruvate + ATP  PEP]
 Only CO2 gets into the Calvin Cycle – photorespiration is
eliminated and efficiency skyrockets. Corn is an example
of a C4 plant and look at its growth within one season!
 This process is endergonic – it consumes energy (ATP).
The C4 Pathway
The CAM Plants
 This is a special pathway developed for water-bearing
plants such as cacti and pineapples. They are the
opposite of other plants because they open their stomata
at night and close them during the day. (It is just way too
hot to risk the water loss during the day so stomata stay
shut.)
 The carbon dioxide is captured by organic acids (much
like the C4 plants) only it is done during the night. Once
the day comes, the organic acids release the carbon
dioxide into the Calvin cycle. The two steps take place in
the same part of the leaf but at different times of the day.
 Both the C4 and CAM pathways are seen as evolutionary
adaptations for plants to maximize the production of
carbohydrates.
The CAM Plants
FIN