Carbon dioxide fixation is the process in plants and algae by which the atmospheric carbon dioxide is converted into organic carbon compounds, such as carbohydrates, usually by photosynthesis. C3 Plants are those plants which survive solely on C3 fixation and tend to thrive in areas where the intensity of the sunlight is moderate, temperature is moderate, carbon dioxide concentrations are around 200 ppm or higher and plenty of ground water. The C3 plants that originated during the Mesozoic and Paleozoic eras; predate the C4 plants and still represent approximately 95% of Earth’s plant biomass. The C3 plants lose 97% of the water taken up through their roots to transpiration. Examples of C3 plants include rice and barley. C3 plants cannot grow in hot areas because the enzyme Ribulose-1, 5-bisphosphate carboxylase oxygenase (RuBisCO) incorporates more oxygen into the organic substance Ribulose-1, 5-bisphosphate (RuBP) as temperatures increase. This leads to photorespiration, which leads to a net loss of carbon and nitrogen from the plant, which potentially limits its growth. In dry areas, C3 plants shut their stomata to reduce the water loss, but this stops CO2 from entering the leaves and therefore, reduces the concentration of CO2 in the leaves. This lowers the CO2:O2 ratio and therefore also increases photorespiration. C4 and CAM plants have adaptations that allow them to survive in hot and dry areas where they can out-compete C3 plants.
The Calvin Cycle
It was discovered by Melvin Calvin and Andy Benson, Berkley. Plants, which use only the Calvin cycle for fixing the carbon dioxide from the air, are known as C3 plants. About 85% of plant species are C3 plants. They include the cereal grains: wheat, rice, barley, oats, peanuts, cotton, sugar beet, tobacco, spinach, soybeans and most trees are C3 plants. Most lawn grasses such, as rye and fescue are C3 plants. The cycle uses ATP as an energy source and consumes NADPH2 as reducing power for adding high-energy electrons to make the sugar. Net equation: The sum of reactions in the Calvin cycle is the following 3 CO2 + 6 NADPH + 5 H2O + 9 ATP → Glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi There are three steps of the cycle
Step 1 Carboxylation CO2 is incorporated into a five-carbon sugar named ribulose bisphosphate (RuBP). The enzyme, which catalyzes this first step, is RuBP carboxylase oxygenase (RuBisCO). It is the most abundant protein in chloroplasts.
Step 2 Reduction
ATP and NADPH2 from the light reactions are used to convert 3-phosphoglycerate to glyceraldehyde 3-phosphate, the three-carbon carbohydrate precursor to glucose and other sugars.
Step 3 Regeneration More ATP is used to convert glyceraldehyde 3-phosphate back to RuBP, the acceptor for CO2, thereby completing the cycle. For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P). For each G3P synthesized, the cycle spends nine molecules of ATP and six molecules of NADPH2. The light reactions sustain the Calvin cycle by regenerating the ATP and NADPH2.
Till 1965 it was believed that Calvin cycle was the only pathway of CO2 fixation in photosynthesis. In 1957, Kortschak and co-workers reported synthesis of a 4-C organic acid as the first stable product of photosynthesis in sugarcane. In 1967, two Australian scientists Hatch and Slack thoroughly investigated the complete pathway in the plants where the first stable product of CO2 fixation was a 4-C compound. This pathway was known as C4 cycle. It is also known as Hatch Slack pathway. The plants, which exhibit this cycle, are known as C4 plants. The common example of C4 plants is tropical grasses, sugar cane, maize etc. The anatomy of C4 leaves is known as Kranz anatomy. In this case, the leaves have two types of cells, the mesophyll cells and the bundle sheath cells. The bundle sheath cells are single layered and surround the vesicular bundles. They contain few large chloroplasts and lack grana. On the other hand, the mesophyll cells contain large number of normal chloroplasts. They lack enzymes of Calvin cycle and do not contain starch.
The steps involved in the C4 pathway are
i. In the mesophyll cells, the C4 cycle occurs, the primary acceptor of CO2 is a 3-C compound phosphoenol pyruvic acid. It combines with CO2 in presence of the enzyme phosphoenol pyruvate carboxylase (PEPCO) to form a 4-C compound oxaloacetic acid. It is the first stable product of C4 pathway.
ii. Oxaloacetic acid is then reduced to malic acid using NADPH produced during light reaction. The reaction is catalysed by the enzyme malic-dehydrogenase.
iii. Sometimes the oxaloacetic acid is converted to aspartic acid by a transamination reaction. However aspartic acid has no role in the cycle.
iv. The malic acid formed in mesophyll cell is transported to bundle sheath cells where they are decarboxylated in the presence of specific malic enzyme to produce pyruvic acid.
v. The CO2 so liberated by decarboxylation of malic acid is accepted by ribulose 1, 5 bisphophate and enters the Calvin cycle.
vi. The pyruvic acid formed in the bundle sheath cells are transported back to mesophyll cells where they are phosporylated in the presence of ATP produced in light reaction to form phosphoenol pyruvic acid in presence of enzyme pyruvate phosphate dikinase. Thus the phosphoenol pyruvic acid is regenerated which can take part again in the cycle.
Crassulacean Acid Metabolism (CAM)
The CAM pathway (Crassulacean Acid Metabolism) is utilized by cacti, other succulents and members of the crassulaceae. The CAM pathway uses up more energy that result in stunted growth. CAM plants open their stomata at night to take in carbon dioxide and close them (reducing water loss) in the day. The CAM plants represent a metabolic strategy adapted to extremely hot and dry environments. The acidity arises from the opening of their stomata at night to take in CO2 and fix it into malic acid for storage in the large vacuoles of their photosynthetic cells. In the heat of the day, the stomata close tightly to conserve water and the malic acid is decarboxylated to release the CO2 for fixing by the Calvin cycle. PEP is used for the initial short-term carbon fixation as in the C4 plants, but the entire chain of reactions occur in the same cell rather than handing off to a separate cell as with the C4 plants. In the CAM strategy, the processes are separated temporally, the initial CO2 fixation at night and the malic acid to Calvin cycle part taking place during the day. Stomata opens only at night when the temperature is low and the relative humidity higher, the CAM plants use much less water than either C3 plants or C4 plants. Some varieties convert to C3 plants at the end of the day when their acid stores are depleted if they have adequate water and even at other times when water is abundant.