C4 Photosynthesis
AP Biology
Unit 4
Review: C3 Photosynthesis
• During “regular”
photosynthesis, CO2 is
trapped into a 3-carbon
compound by Rubisco
 C3 Photosynthesis
• This 3 carbon compound
then goes through the
calvin cycle to produce
glucose (eventually)
C4 Photosynthesis
• Certain plants go through a slightly
modified photosynthesis process (C4
Photosynthesis)
• C4 Photosynthesis is an adaptation that
evolved due to the environment these plants
are in.
Rubisco
• ALL plants have it and use it for the Calvin
Cycle.
• Rubisco usually reacts with CO2, but it can
also react with O2 – 2 competing reactions.
Competing Reactions
• Rubisco can react with CO2 (Carboxylase
Reaction) – good for glucose output 
CH2OPO32C O
H C OH
H C OH
CH2OPO32Ribulose 1,5-bisphosphate
CO2
O-
O
C
H C OH
CH2OPO323-Phosphoglycerate
O-
O
C
H C OH
CH2OPO323-Phosphoglycerate
Competing Reactions
• Rubisco can also react with O2 (Oxygenase
Reaction)
– not good for glucose output
– Even though CO2 is eventually regenerated, it
wastes time and energy (occupies Rubisco)
CH2OPO32C O
H C OH
H C OH
CH2OPO32Ribulose 1,5-bisphosphate
O2
O-
O
CH2OPO32-
C
H C OH
CH2OPO323-Phosphoglycerate
C
O
O-
Phosphogly colate
Photorespiration
• When Rubisco reacts with O2 instead of
CO2
• Occurs under the following conditions:
– High O2 concentrations
– High heat
• Photorespiration is estimated to reduce
photosynthetic efficiency by 25%
Why high heat?
• When it is hot, plants close their stomata to
conserve water
• They continue to do photosynthesis  use
up CO2 and produce O2  creates high O2
concentrations inside the plant 
photorespiration occurs
C4 Photosynthesis
• Certain plants have developed ways to limit
the amount of photorespiration
– C4 Pathway*
– CAM Pathway*
* Both convert CO2 into a 4 carbon intermediate
 C4 Photosynthesis
Leaf Anatomy
• In C3 plants (those that do C3
photosynthesis), all photosynthesis
processes occur in the mesophyll cells.
Mesophyll cells
Bundle sheath cells
(no photosynthesis
Image taken without permission from http://bcs.whfreeman.com/thelifewire|
C4 Pathway
• In C4 pathway plants
photosynthesis occurs in
both the mesophyll and
the bundle sheath cells.
– Light reactions in
mesophyll
– Calvin Cycle in Bundle
sheath
Image taken without permission from http://bcs.whfreeman.com/thelifewire|
C4 Pathway
• CO2 is fixed into a 4carbon intermediate
first
• Has an extra enzyme–
PEP Carboxylase that
initially traps CO2
instead of Rubisco–
makes a 4 carbon
intermediate
C4 Pathway
• The 4 carbon
intermediate “smuggles”
CO2 into the bundle
sheath cell
• The bundle sheath cell is
not very permeable to
CO2
• CO2 is released from the
4C molecule  goes
through the Calvin Cycle
C3 Pathway
How does the C4 Pathway limit
photorespiration?
• Bundle sheath cells are far from the
surface– less O2 access
• PEP Carboxylase doesn’t have an affinity
for O2  allows plant to collect a lot of CO2
and concentrate it in the bundle sheath cells
(where Rubisco is)
CAM Pathway
• Fix CO2 at night and
store as a 4 carbon
molecule
• Keep stomates closed
during day to prevent
water loss
• Same general process
as C4 Pathway
• Has the same leaf
anatomy as C3 plants
How does the CAM Pathway
limit photorespiration?
• 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
Summary of C4 Photosynthesis
• C4 Pathway
– Separates by
space (different
locations)
• CAM Pathway
– Separates
reactions by time
(night versus day)
• Light intensity is
directly related to
temperature
• C4 Plants (CAM
and C4 Pathway)
are able to do more
photosynthesis at
high temperatures
Net Photosynthesis (mg CO 2/dm2/hr)
Comparison
Light Intensity (cal/cm 2/min)