The Difference between C-3, C-4, and CAM Plants!
During the Calvin Cycle, CO2 enters and with the help of the enzyme rubisco, is fixed into 3-PGA, a three carbon molecule.
This is not very efficient because rubisco accidentally binds with O2 as well as CO2, which makes absolutely useless products through a process called photorespiration. Photorespiration makes NO ATP and NO sugar whatsoever. Hurrah. In fact, the products of photorespiration must now be broken down by peroxisomes, resulting in a waste of time and energy.
What do certain plants do to avoid this?
This is a plant modification for dry environments, in which plants such as corn and sugar cane must adapt to hot and sunny environments while being as productive as possible.
C-4 Plants exhibit Kranz anatomy, which means that C-4 leaves have bundle-sheath cells that lie under mesophyll cells, deep, deep down in the leaf so that O2 cannot interfere with the Calvin Cycle.
How does this happen?
Well, the Hatch-Slack pathway is a biochemical pathway that describes a C-4 Plant’s process of sequestering CO2 deep into the bundle sheath cells, avoiding photorespiration.
First, CO2 enters the mesophyll of the leaf from the stomata and combines with a 3-carbon molecule, PEP, to form the 4-carbon oxaloacetate (and hence, the name C-4. Whoaaa. I know right?). The enzyme PEP carboxylase does NOTbind with O2, so this is far more efficient for the plant.
The oxaloacetate is turned into malic acid, which pumps through the plasmodesmata (plant cell junctions) into adjacent bundle sheath cells.
Here, deep in the leaf, CO2 is finally released and incorporated into PGAL by the Calvin cycle. This results in a highly efficient process for C-4 plants, and also minimizes the length of time stomata must remain open (which means less water transpired!)
These nifty plants (such as cacti) carry out a funky form of photosynthesis called crassulacean acid metabolism, which is another adaption to dry conditions. They differ from C-3 plants because they keep their stomata closed in the day and open at night. CAM plants store CO2 in organic compounds (malic acid) at night, and then undergo Calvin Cycle during the day. Weird, but effective.
I hope this helped a little! Good luck to everyone taking AP Bio tomorrow <3
– Renee from APstudying
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