Alternate Mechanisms of Carbon Fixation Also Known As… Why can’t they just leave good enough alone. Let’s cut them all down.

What’s the problem?!?!?!  Why can’t the plants just run the C 3 thing and be happy with that?  Some plants live in areas in which CO 2 is very hard to come by – deserts have fewer organisms and carbon sources so their air may be less than that of a more populated area.  Others may live in areas where it is so hot that opening the stomata during the day may not be possible because water loss would be rapid and substantial during the day.  Both of these problems have been solved by plants that make clever adjustments in how they harvest their CO 2. They use two slightly adjusted mechanisms – they are the C 4 pathway and the CAM pathway.

Why Do We Have C 4 Plants?  The reason for plants becoming a C 4 plant is this – the enzyme that takes in CO 2 at the beginning of the Calvin cycle, called rubisco, actually loves O 2 more than CO 2.  When oxygen gets in the active site for rubisco, a process called photorespiration occurs – it is brought on by hot weather because the stoma close to preserve water and the CO 2 in the leaf is used up and its levels drop.  The C 4 pathway is seen as an evolutionary adaptation that avoids this problem of photorespiration.

The C 4 Pathway  An enzyme, phosphoenolpyruvate carboxylase, catalyzes a reaction that fixes a carbon dioxide molecule to a molecule of PEP – this makes a four-carbon molecule of oxaloacetate. The first product made has four carbons, hence the name C 4 photosynthesis.  These plants can do this because they have two types of photosynthetic cells – bundle sheath cells and mesophyll cells. The two cell types are found close together within the leaf of a C 4 plant. Connection between the two cell types is made through the plasmodesmata.  [PEP + CO 2  Oxaloacetate  Malate]  [Pyruvate + CO 2 (for Calvin)  Pyruvate + ATP  PEP]  Only CO 2 gets into the Calvin Cycle – photorespiration is eliminated and efficiency skyrockets. Corn is an example of a C 4 plant and look at its growth within one season!  This process is endergonic – it consumes energy (ATP).

The C 4 Pathway

The CAM Plants  This is a special pathway developed for water-bearing plants such as cacti and pineapples. They are the opposite of other plants because they open their stomata at night and close them during the day. (It is just way too hot to risk the water loss during the day so stomata stay shut.)  The carbon dioxide is captured by organic acids (much like the C 4 plants) only it is done during the night. Once the day comes, the organic acids release the carbon dioxide into the Calvin cycle. The two steps take place in the same part of the leaf but at different times of the day.  Both the C 4 and CAM pathways are seen as evolutionary adaptations for plants to maximize the production of carbohydrates.

The CAM Plants

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