U3 AOS2 Topic 2: Comparison of C3, C4 and CAM

Rubisco is an enzyme which is used to catalyze carbon fixation during light independent stage i.e. calvin cycle of photosynthesis. It helps to fix inorganic CO2 into an organic glucose molecule. It binds with CO2 to carry out carbon fixation but sometimes it binds to oxygen molecule to carry out another useless reaction called as photorespiration as it has affinity to bind with both CO2 and O2.

                                        

Scientists have categorized the plants into three groups on the basis of their ability to avoid photorespiration which are C3, C4 and CAM plants.

                                                        

C3 plants:

85% of plants on earth are C3 plants. These plants do not have any mechanism to avoid photorespiration. In these plants photosynthesis can take place only in one type of cell i.e. mesophyll cells where after light dependent stage, CO2 is fixed by rubisco enzyme in calvin cycle. The first molecule formed after carbon fixation is a 3 carbon compound called as 3-PGA therefore, these plants are called as C3 plants.

C4 plants:

In C4 plants light dependent stage is similar to that in C3 plants but the difference lies in light independent stage which occurs in two different cell types i.e. mesophyll cells and bundle sheath cells. The initial reaction of carbon fixation occurs in mesophyll cell by a different enzyme PEP carboxylase which unlike rubisco has no affinity for O2 and fixes CO2 into a 4 carbon compound oxaloacetate which then converts into malate. The remaining reaction occurs in bundle sheath cell where malate is broken down into CO2 which can enter calvin cycle.  The first molecule formed after carbon fixation is a 4 carbon compound therefore these plants are called as C4 plants.

CAM plants:

In these plants only one type of cell called mesophyll cells are involved but the difference lies in time. Stomata are open during night time instead of daytime so that more CO2 is available for PEP carboxylase enzyme to carry out carbon fixation. The first molecule i.e. oxaloacetate is converted into malate which is stored in vacuole of mesophyll cells until daytime when stomata are closed. During daytime malate is transported out of vacuole and is broken down into CO2 which can enter calvin cycle in same fashion as happens in C3 and C4 plants.

	C3	C4	CAM
Limits photorespiration	No	Yes	Yes
Cell types involved	Mesphyll cells	Mesophyll and bundle sheath cells	Mesophyll cells
Stomata open	Day	Day	Night
Advantages	Doesn’t consume extra energy	Minimizes photorespiration	Minimizes photorespiration
Best adapted to	Cool environment	Hot environment	Very hot and dry environment