C3, C4, and CAM Plants

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C3, C4 and CAM Plants
Introduction
Plants can be classified as c3, c4 or cam plants with respect to how they combat photorespiration. The failure of certain plants to adapt towards eliminating photorespiration means that the plant may not survive certain conditions. On the other hand, plants that have mechanisms to reduce photorespiration may require higher ATP (Adenosine triphosphate) energy for the processes to take place. These classifications are therefore important in the determination of crop adaptability to different environments and their food yields for economic productivity.
C3 plants
C3 plants are those that do not have any photosynthetic mechanism to reduce photorespiration. Standard mechanism of carbon fixation by rubisco enzymes of the Calvin cycle occurs in the mesophyll cell i.e. spongy tissue in the middle of the leaf. Most species of the earth including all trees, rice, wheat, and soybeans are in the C3 category. C3 crops are best abundant to cool wet environments because of their inability to counter photorespiration and consequent higher water requirements.
C4 plants
In the C4 category of plants, photorespiration effect is reduced through separation of the Calvin cycle and the light-dependent reactions. About 3% of all vascular plants which include sugarcane, crabgrass and corn are C4 plants. C4 plants do well in hotter sunny environments rather than cooler conditions. This is because the benefit of reduced photorespiration is mostly realized in hotter conditions.

Under hot conditions, the ATP cost of moving CO2 from the mesophyll cell to the bundle sheath cell is high. The water consumption of a C4 type of plants is relatively low because they are metabolism mechanisms are efficient in water utilization.
CAM plants
Plants that apply the crassulacean acid metabolism pathway to reduce counter the photorespiration phenomenon are known as CAM plants. Crassulacean acid metabolism path was discovered in the Crassulaceae family of plants by scientist and hence the name. CAM plant is adapted to very hot and dry environment. Examples of CAM plants are pineapple and cactus. The C4 pathway has evolved over twenty times to adapt to the hot and dry weather.
Photorespiration and Its Influence on C3, C4 and CAM Plants
Photorespiration refers to the phenomenon that occurs when the carbon is fixing rubisco enzyme of the Calvin cycle, grabs oxygen instead of carbon dioxide which is important in plant growth. Rubisco’s affinity to O2 and CO2 is highly influenced by temperatures. Its tendency to bind to CO2 molecules is higher than about eighty times that for O2, however, rubisco’s affinity to oxygen molecules rises as temperatures increase. This means that hot and dry environments are prone to photorespiration and thus the need for plants to have special features to mitigate the problem.
Photorespiration affects the plant through loss of energy and displacement of carbon. It starts in the chloroplast when oxygen molecule is attached to RuBP in the oxygenase by rubisco enzyme. This forms two molecules: a two-carbon compound and a three-carbon compound which is a normal Calvin cycle intermediate. The two-carbon compound (phosphoglycolate) does not take part in the cycle and hence it’s lost. Through reactions that involve transfer across different cell organelles, plants have to put some phosphoglycolate to recover some of the lost carbon. A quarter of phosphoglycolate that enters the cycle is lost as CO2. C3 plants as discussed above have no special features to counter photorespiration unlike for C4 and CAM plants (Troughton & Card 189).
In C4 plants, the Calvin cycle is made to occur in special cells around the leaf veins called the bundle-sheath cells while light-dependent reactions take place in the mesophyll cells. When C4 photosynthesis takes place, atmospheric carbon dioxide is fixed to form oxaloacetate which is a simple 4-carbon organic acid. This first step of CO2 fixation takes place in the mesophyll cells by the PEP Carboxylate enzyme that has no tendency to bind O2. This is preceded by the conversion of oxaloacetate to malate which is then transported into the bundle-sheath cell where it’s broken. A CO2 molecule is released and then fixed by rubisco, and the Calvin cycle converts it to sugar.
In CAM plants, separation of light dependent reactions and Calvin cycle are done in time rather than in spatial space by opening the stomata at night and closing it by the day. When stomata are open, CO2 diffuses into leaves, and it’s fixed by PEP carboxylase enzyme into oxaloacetate then converted into an acid which is stored till the next day in vacuoles. Photosynthesis occurs during the day without having to open the stomata.
Similarities and differences between C3, C4 and CAM plants
All this-these crops are similar in the way carbon dioxide is converted into carbohydrates. The formula used in the different photosynthetic processes is the basic formula where light energy is utilized in the conversion of carbon dioxide and water into oxygen and sugar as follows:
CO2 + H2O C6H12O6 + O2
Another similarity for C3, C4, and CAM plants is that all use the Calvin cycle to process carbon dioxide into sugar.
The major difference between C3, C4 and CAM plants is the way they respond to photorespiration. C3 plants are not adapted to deal with the problem while CAM plants have the most efficient mechanism, and the C4 plants are intermediate (Sternberg, Michael, & Henry Ajie 2477). This makes them possess different advantages and disadvantages over each other, and the plants are made to suit different environments i.e. C3 plants for cold, wet environments, and CAM plants are adapted to Very hot and dry environments. These differences in mechanisms can be enumerated as follows:
For the C3 type of plants, CO2 formed into three (3) carbon compounds while CO2 in C4 plants is first converted into four (4) carbon compounds before conversion into sugar.
photosynthesis takes place in most of the leaf tissues in C3 plants while it only takes place within special cells that are found in the veins of the leafs in a special structure known as Kranz anatomy.
The leaf pores are open for gas exchange in daytime allowing water loss in C3 and C4 plants while CAM plants have their leaf pores closed during the day to prevent water loss.
C4 plants are excellently adapted to minimum CO2 levels than C3 plants due to the presence of PEP carboxylase which is a special molecule that enables plants to grab CO2 more efficiently and directly deliver it to the photosynthesis process (Kanai & Edwards 487).
While single CO2 fixation takes place in C3 plants, C4 and CAN plants exhibit double fixation of CO2.
Different leaf anatomies are associated with the three i.e. normal leaf anatomy for C3 plants, Kranz anatomy for C4 plants and xeromorphic leafs for CAN plants.
C3 plants have the highest CO2 compensation point of 30-70ppm while C4 and CAM plants follow with CO2 compensation points of 10ppm and 5ppm respectively.
The primary enzyme utilized in fixation of CO2 fixation in C3 plants is rubisco while PEP carboxylase is the first enzyme involved in C4 and CAM plants.
For C3 plants, the first stable product of CO2 fixation is 3-PGA (3-Phosphoglyceric acid) while OAA (Oxalo acetic acid) is the first stable product produced in C4 and CAM processes.

Work Cited
Troughton, John H., and K. A. Card. “Temperature effects on the carbon-isotope ratio of C3,
C4 and crassulacean-acid-metabolism (CAM) plants.” Planta 123.2 (1975): 185-190.
Sternberg, Leonel, Michael J. Deniro, and Henry Ajie. “Stable hydrogen isotope ratios of
saponifiable lipids and cellulose nitrate from CAM, C3 and C4 plants.”
Phytochemistry 23.11 (1984): 2475-2477.
Kanai, R., and G. E. Edwards. “Purification of enzymatically isolated mesophyll protoplasts
from C3, C4, and Crassulacean acid metabolism plants using an aqueous dextran-
polyethylene glycol two-phase system.” Plant Physiology 52.5 (1973): 484-490.