| MadSci Network: Botany |
The following information was provided by Dr. Bill Rizzo, plant physiologist and coworker.
The evolution of different carbon fixation and anatomical pathways is a response to multiple environmental stressors such as low carbon dioxide concentrations, high oxygen concentrations, water scarcity, and high light and temperature conditions. The first discovered photosynthetic pathway was the C3 (Calvin - Benson; “dark” reaction) pathway wherein carbon dioxide is fixed by an enzyme called ribulose 1-5 bi-phosphate (RUBISCO) producing a 3-carbon product (3-phosphoglyceraldehyde, or 3-PGA), hence the name for the cycle.
In the C4 (or Hatch-Slack) pathway, carbon dioxide is initially fixed by the action of a different enzyme, phosphoenolpyruvate carboxylase (PEPCASE) into a 4-carbon product; usually oxaloacetate, but aspartate and malate are products in some species.
PEPCASE is much more efficient than RUBISCO in fixing carbon dioxide. RUBISCO will bind readily to oxygen, producing end-products like glycolate that must be metabolized. This process is called photorespiration, and it uses up many of the products of the light reactions of photosynthesis, constituting a metabolic drain on the plant.
Now anatomy enters the picture. In C3 plants, carbon dioxide is fixed in cells just below openings (stomates) in the leaf surface. In many environments, particularly hot, well-lit environments, this area of the plant leaf experiences low concentrations of carbon dioxide and high concentrations of oxygen; conditions not favorable to efficient operation of RUBISCO. In C4 plants, the cell geometry is arranged in a bundle-arrangement called Kranz anatomy, wherein intial C4 fixation takes place by PEPCASE in cells in the CO2-poor/O2 rich area (in mesophyll cells). The fixation product is then transported to cells in the interior of the plant leaf (bundle-sheath cells), and the CO2 is removed (decarboxylated). The decarboxylation produces a carbon dioxide-rich environment away from the oxygen-rich leaf surface, where the newly released carbon dioxide can be efficiently re-fixed by RUBISCO, where it enters the normal C3 pathway.
Because C4 plants use CO2 more efficiently, they are better competitors in CO2 poor environments (obviously), but they also need open stomates (for CO2 uptake) much less than C3 plants, which reduces water loss - an important consideration in arid environments. C4 plants also waste less photosynthetic material on photorespiration, and have higher light requirements than C3 plants. In addition, they often have higher ratios of carbon to nitrogen in their tissues than C3 plants, which make them poorer forage for grazing animals.
CAM (Crassulacean Acid Metabolism) metabolism is an extreme adaptation to very arid (desert) environments, but is otherwise similar to C4 metabolism. In CAM plants, water loss through stomates is so high that they can seldom “afford” to open their stomates for CO2 uptake during the heat of the day. These plants, including cacti and euphorbs, open their stomates at night for CO2 uptake. This CO2 is also fixed by forming C4 acids like oxaloacetate. These C4 acids are then decarboxylated during the daytime when the stomates are closed, and the CO2 is fixed by RUBISCO.
Finally, and confusingly, both algae and seagrasses have physiological and anatomical adaptations that essentially allow them to function 'in-between' normal C3 and C4 plants.