Photorespiration
&
Alternative Methods of
Carbon Fixation
Photorespiration
Stomata are pores in
the leaf surface,
through which:



Carbon dioxide enters
the plant
Water evaporates from
the plant
Oxygen leaves the
plant
Photorespiration
In hot, dry climates, plants close their
stomata

Pro:
plant doesn’t lose as much water

Cons:
Plant doesn’t receive carbon dioxide
Plant can’t get rid of oxygen
Photorespiration
Rubisco:


Enzyme that unites CO2 and RuBP in the first
step of the Calvin Cycle
HOWEVER, it can also cause O2 to join RuBP
Higher concentration (O2 or CO2) “wins”
and is joined to RuBP
When O2 is joined to RuBP,
photorespiration occurs
Photorespiration as a Problem
Photorespiration:


Does not produce any ATP
Prevents the synthesis of glucose
Therefore, some plants in hot, dry climates
have developed ways to combat
photorespiration
Alternative Methods of
Carbon Fixation
C3 Pathway
Uses only the Calvin
Cycle
Called C3 because
PGA is a 3-C
compound
Occurs in mesophyll
cells
C4 Pathway
Different location
within the leaf
Mesophyll cells pump
CO2 into bundlesheath cells, which
are impermeable to
CO2
This increases the
concentration of CO2
C4 Pathway, cont’d
Since the
concentration of CO2
is greater than the
concentration of O2,
the Calvin Cycle is
favored over
photorespiration
Examples:


Corn
Sugar cane
C3 vs. C4
Different LOCATIONS within the leaf
CAM pathway
Occurs at a different
time
Stomata open at night
and close during the
day

Pros:
Prevents water loss
Prevents CO2 from
leaving the leaves

Con:
Plant grows slowly
CAM Plants, cont’d…
CO2 is stored
overnight until the
light reactions can
supply ATP and
NADPH to drive the
Calvin Cycle
Examples:


Cacti
Pineapples