Photorespiration and
Plant Diversity
On hot dry days…
► Plants
close stomata to prevent water loss
► This
means less CO2 available for
photosynthesis
► O2
builds up in mesophyll and competes
with CO2 for active site of rubisco
► Some
intermediates of Calvin degraded
into CO2 and H20 – photorespiration
 Occurs in presence of light
 Requires O2
 Results in CO2 and H20 (same as cellular
respiration), but no ATP produced
► Photorespiration
efficiency
reduces photosynthetic
C4 and CAM plants have
adaptations to reduce
photorespiration
Review of Leaf Cross-Sections:
C3 Leaf
C4 Leaf
C4 Plants
► First
fix CO2 into 4-C oxaloacetate
 Mesophyll cells
 Prior to Calvin Cycle
 Uses enzyme PEP carboxylase – high affinity
for CO2
► Oxaloacetate,
through intermediates,
delivers CO2 to Calvin Cycle
 In bundle sheath cells
 Requires input of energy to recycle PEP
Pros & Cons
► In
high light areas, photorespiration is
negliglible in C4 plants
 Photosynthetic yield is worth expenditure of 30
ATPs
► In
lower light/temperature, C3 plants are
favored
CAM Plants
► Found
► Only
► Use
in plants in xeric conditions
open stomata at night
PEP carboxylase to fix CO2
► During
day, CO2 is removed from malate
 CO2 then enters Calvin
C4 vs. CAM plants
► Both
use PEP carboxylase and form
oxaloacetate
► In
C4 plants – C4 and C3 pathways occur in
different locations
► In
CAM plants, CAM and C3 pathways occur
at different times