Photosynthesis
C3, C4, CAM plants
Definition
• C3 plants: all of carbon fixation and photosynthesis happens in
mesophyll cells just on the surface of the leaf.
• C4 plants: carbon fixation and photosynthesis split between the
mesophyll cells and bundle sheath cells.
• CAM (Crassulacean acid metabolism)plants: photosynthesis takes
place in the mesophyll cells, but carbon fixation (and opening of
stomata) takes place at night and the Calvin cycle happens during the
day.
C3 Plants
• Called C3 because the CO2 is first incorporated into a 3-carbon
compound.
• Stomata are open during the day.
• RUBISCO, the enzyme involved in photosynthesis, is also the enzyme
involved in the uptake of CO2.
• Photosynthesis takes place throughout the leaf.
• Most plants are C3.
C4 Plants
• Called C4 because the CO2 is first incorporated into a 4-carbon
compound.
• Stomata are open during the day.
• Uses PEP Carboxylase for the enzyme involved in the uptake of CO2.
This enzyme allows CO2 to be taken into the plant very quickly, and then
it "delivers" the CO2 directly to RUBISCO for photsynthesis.
• Photosynthesis takes place in inner cells (requires special anatomy called
Kranz Anatomy)
• C4 plants include several thousand species in at least 19 plant families.
Example: fourwing saltbush pictured here, corn, and many of our
summer annual plants.
C4 Pathway
C4 cycle
CAM Plants
• Called CAM after the plant family in which it was first found
(Crassulaceae) and because the CO2 is stored in the form of an acid
before use in photosynthesis.
• Stomata open at night.
• CAM plants include many succulents such as cactuses and agaves and
also some orchids and bromeliads
Cactus
Orchids
Bromeliads
Agaves
CAM Pathway
Differences
C3
CO2 acceptor is 5c RUBP
C4
CO2 acceptor is 3c PEP
CO2 fixing enzyme is RUBP carboxylase
CO2 fixing enzyme is PEP carboxylase
First product of photosynthesis is PGA
(3c)
First product of photosynthesis is
oxaloacetic acid (4c)
CO2 fixation occurs once only
CO2 fixation twice – Kranz anatomy