C3, C4 and CAM plants
Alternative Methods of Carbon Fixation
 Rubisco – the double agent!
 Normally rubisco adds CO2 to RuBP but when O2 is
very plentiful it adds it to RuBP
 This is called Photorespiration – it occurs in light!
 Produces phosphoglycolate (a 2C compound) & one
PGA

Ie. It removes PGA molecules from the Calvin cycleDecrease
CO2 fixation and less sugar formed
 Reduces the efficiency of photosynthesis
Rubisco - The Double Agent!
HELPFUL!
ANNOYING!!
3
Photorespiration
 Oxygen acts as a competitive inhibitor.
 Rubisco is an evolutionary remnant (from a time in
earth’s history when O2 was not as prevalent
 Some plants have adapted strategies to work
around this!
Introduction
 C3, C4 and CAM
photosynthesis are the
three types of
photosynthesis in green
plants.
 C3 photosynthesis is the
photosynthesis we have
learned about in class.
 C4 and CAM
photosynthesis are
adaptations to arid
conditions.
C3 Photosynthesis
 C3 is so called because
the first compound
made when carbon
dioxide is fixed from the
atmosphere has three
carbons (PGA).
 Stomata are open during
the day.
 RuBisco is the enzyme
involved in carbon
fixation.
C3 Photosynthesis
 Photosynthesis takes place
throughout the leaf.
 Most plants are C3 because
it requires fewer enzymes
and less specialized
machinery than C4 and
CAM photosynthesis.
 It is the most efficient form
of photosynthesis under
normal light intensity,
lower temperature and
normal moisture.
C4 Photosynthesis
 Called C4 because
carbon dioxide is fixed
into a four carbon
compound in the
mesophyll cells
 Stomata open during the
day.
 Dark Reactions take
place in inner cells called
Kranz Anatomy –
Bundle-sheath cells.
C4 Photosynthesis
 Uses the enzyme PEP




carboxylase to fix carbon dioxide
into a four carbon compound
called Oxaloacetate (OAA)
Converted to 4C malate which
then crosses the membrane into
the bundle-sheath cells.
Malate is decarboxylated
resulting in pyruvate.
CO2 produced goes to the Calvin
Cycle – much like in C3 plants.
Pyruvate crosses the membrane
back into mesophyll cells and
converted into PEP.
C4 Photosynthesis
 This works faster than C3
photosynthesis under
high light intensity and
high temperature
because it delivers CO2
directly to RuBisCO
thereby maximizing
carbohydrate formation
and preventing product
loss due to
photorespiration.
C4 Photosynthesis
 It is also more efficient in
terms of water use because
PEP carboxylase brings in
CO2 faster and therefore
does not need to keep the
stomata open for as long,
thereby minimizing water
loss.
 C4 photosynthesis is
common in plants that
grow mainly during the
intense heat of summer in
North America (i.e. Corn).
CAM Photosynthesis
 Crassulacean acid
metabolism (CAM)
photosynthesis is
another adaptation for
plants that are in arid
conditions.
 Stomata are closed
during the day and open
at night to reduce water
loss through
transpiration.
CAM Photosynthesis
 These plants fix carbon dioxide
during the night, going to a
similar process as C4 pathway
until creates malate
 Malate is stored in a vacuole
until daytime.
 During the day, once the light
dependent reactions have
created enough energy to fuel
them, the malate transfers out of
the vacuole and is
decarboxylated to produce
carbon dioxide
 The CO2 is ready then to be
fixed by RuBisCO and enter the
Calvin cycle to create
carbohydrate.
CAM Photosynthesis
 CAM plants often have thick,
reduced leaves with a low SA to
V ratio; thick cuticle; and
stomata sunken into pits.
 Some store water in vacuoles
(succulent plants).
 CAM plants can also be
recognized as plants whose
leaves have an increasing sour
taste during the night yet
become sweeter-tasting during
the day. This is due to malic acid
stored in the vacuoles of the
plants' cells during the night and
then used up during the day.
 That is why we let pineapple
ripen before eating! 
CAM Photosynthesis
 CAM plants can “CAM-idle”
which allows them to keep
their stomata closed at all
times during extremely arid
conditions and therefore any
oxygen they give off in
photosynthesis is
immediately used in cellular
respiration and vice versa.
 This allows the plant to
survive dry periods and
recover very quickly when
water returns.
CAM vs. C4 Photosynthesis
Similarities
 Both are a response to arid
conditions.
 Both use water more
efficiently than C3 plants.
 Both minimize the amount of
photorespiration by proving
carbon dioxide directly to
RuBisCO.
Differences
 CAM plants provide CO2
temporally (stockpile it at night
and provide it during the day)
whereas C4 plants provide CO2
spatially (take it from the outer
cells and provide it to the inner
Kranz Anatomy).
 C4 plants require special Krantz
Anatomy.
 C4 plants have stomata open
during the day while CAM
plants have stomata open at
night.