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C4 Cycle
M. D. Hatch and C. R. Slack elucidated the alternative photosynthetic path way for fixing the CO2, called as C4 cycle. The fist stable compound formed in this pathway is a 4 carbon oxaloacetate, hence the name C4 cycle. This pathway is also called as Hatch and Slack pathway in the honor of the scientists who elucidated the pathway. The plants which exhibit this cycle are known as C4 plants. The common example of C4 plants are tropical grasses, sugar cane, maize cynodon etc.
C4 plants leaves exhibit a special kind of anatomy known as Kranz Anatomy. The chloroplasts are dimorphic in nature. In kranz anatomy, the leaves have two types of cells- the mesophyll cells and the bundle sheath cells. The bundle sheath cells are single layered and surrounds the vascular bundles. These bundle sheath cells contain large chloroplasts which lack grana but contain starch. On the other hand mesophyll cells contain small chloroplasts, having garana but lack starch. The chloroplasts of mesophyll cell lack the calvin cycle enzymatic machinery. The bundle sheath cells are bigger and look like a ring or wreath. Kranz in German means wreath and hence it is called Kranz anatomy.
The C4 cycle involves two carboxylation reactions, one taking place in chloroplasts of mesophyll cells carried out by PEP (phosphoenylpyruvate) carboxylase and another in chloroplasts of bundle sheath cells.
Stages of C4 Cycle
C4 cycle consists of four stages as follows
M. D. Hatch and C. R. Slack elucidated the alternative photosynthetic path way for fixing the CO2, called as C4 cycle. The fist stable compound formed in this pathway is a 4 carbon oxaloacetate, hence the name C4 cycle. This pathway is also called as Hatch and Slack pathway in the honor of the scientists who elucidated the pathway. The plants which exhibit this cycle are known as C4 plants. The common example of C4 plants are tropical grasses, sugar cane, maize cynodon etc.
C4 plants leaves exhibit a special kind of anatomy known as Kranz Anatomy. The chloroplasts are dimorphic in nature. In kranz anatomy, the leaves have two types of cells- the mesophyll cells and the bundle sheath cells. The bundle sheath cells are single layered and surrounds the vascular bundles. These bundle sheath cells contain large chloroplasts which lack grana but contain starch. On the other hand mesophyll cells contain small chloroplasts, having garana but lack starch. The chloroplasts of mesophyll cell lack the calvin cycle enzymatic machinery. The bundle sheath cells are bigger and look like a ring or wreath. Kranz in German means wreath and hence it is called Kranz anatomy.
The C4 cycle involves two carboxylation reactions, one taking place in chloroplasts of mesophyll cells carried out by PEP (phosphoenylpyruvate) carboxylase and another in chloroplasts of bundle sheath cells.
Stages of C4 Cycle
C4 cycle consists of four stages as follows
- Carboxylation of the phosphoenylpyruvate in the mesophyll cells to form C4 acid
- Transport of the C4 acid to the bundle sheath cells
- Decarboxylation of the C4 acids within the bundle sheath cells and generation of CO2, which is then reduced to carbohydrate via the Calvin cycle
- Transport of the C3 acid (pyruvate or alanine) that is formed by the decarboxylation step back to the mesophyll cell and regeneration of the CO2 acceptor phosphoenolpyruvate Diffusion gradients along the numerous plasmodesmata drives the shuttling of metabolites between mesophyll and bundle sheath cells, where as concentration gradients and the specialized translocators in the chloroplast envelope facilitate the transport within the cell. The cycle thus effectively shuttles CO2 from the atmosphere into the bundle sheath cells. This transport process results in the much higher concentration of CO2 in the bundle sheath cells than would occur in equilibrium with the external atmosphere. This elevated concentration of CO2 at the carboxylation site of rubisco results in suppression of the oxygenation of ribulose-1,5-bisphosphate and hence of photorespiration.
CO2 concentrating process consumes two ATP equivalents (2 “high-energy” bonds) per CO2 molecule transported. Thus the total energy requirement for fixing CO2 by the combined C4 and Calvin cycles is five ATP plus two NADPH per CO2 fixed. Though there is advantage with the C4 plants, because of this higher energy demand, C4 plants photosynthesizing under nonphotorespiratory conditions (high CO2 and low O2) require more quanta of light per CO2 than C3 leaves do.
Reactions of C4 pathway
Carboxylation
It happens in the chloroplasts of mesophyll cells. Phosphoenolpyruvate, a 3 carbon compound picks up CO2 and forms 4carbon oxaloacetate in the presence of water. This reaction is catalyzed by the enzyme, phosphoenol pyruvate carboxylase.
Phosphoenolpyruvate (PEP) carboxylase
Phosphoenolpyruvate + H2O + CO2 –? oxaloacetate + H3PO4
Breakdown
Oxaloacetate breaks down readily into 4 carbon malate and aspartate in the presence of the enzyme, transaminase and malate dehydrogenase. These compounds diffuse into bundle sheath cells from mesophyll cells.
Malatedehydrogenase
NADP:malate dehydrogenase
Oxaloacetate + NADPH + H+ ? malate + NADP+
Transaminase
Oxaloacetate + glutamate ? aspartate + a-ketoglutarate
Fig. 1. The C4 Pathway
Decarboxylation
In the sheath cells, malate and aspartate undergo decarboxylation and yield free CO2 and 3 carbon pyruvate. The CO2 is used in Calvin’s Cycle in the sheath cell.
Malate ? CO2 +pyruvate
The second Carboxylation occurs in the chloroplast of bundle sheath cells. The CO2 is accepted by 5 carbon compound ribulose bisphosphate in the presence of the enzyme, carboxy dismutase and ultimately yields 3 phosphoglyceric acid. Some of the 3 phosphoglyceric acid is utilized in the formation of sugars and the rest is used for the regeneration of ribulose bisphosphate.
Regeneration of Phosphoenolpyruvate
The pyruvate molecule is transferred to chloroplasts of mesophyll cells where, it is phosphorylated to regenerate phosphoenol pyruvate in the presence of ATP. This reaction is catalyzed by pyruvate phosphokinase leading to the regeneration of phosphoenol pyruvate .
pyruvate phosphokinase
Pyruvate + Pi + ATP ? phosphoenolpyruvate + AMP + PPi
CRASSULACEAN ACID METABOLISM (CAM)
An alternative mechanism of concentrating CO2 at the site of rubisco is found in crassulacean acid metabolism. CAM is the characteristic feature of the family Crassulaceae (Crassula, Kalanchoe, Sedum). Apart from Crassulaceae it is found in numerous angiosperm families. Cacti and euphorbias are CAM plants, as well as pineapple, vanilla, and agave. The CAM mechanism enables plants to improve water use efficiency. CAM plants are the most efficient users of water followed by C4 and C3 plants. Hence CAM plants have a competitive advantage in dry atmospheres. CAM pathway shares lot of similarities with the C4 pathway. Just like C4 plants, formation of the C4 acids is both temporally and spatially separated in CAM plants.
PEP carboxylase in cytosol captures the CO2 in the night and the malate that forms from oxaloacetate product is stored in the vacuole. During the day, the stored malate is transported to the chloroplast and decarboxylated by NADP-malic enzyme, the released CO2 is fixed by the Calvin Cycle.
CAM involves two steps
Reactions of C4 pathway
Carboxylation
It happens in the chloroplasts of mesophyll cells. Phosphoenolpyruvate, a 3 carbon compound picks up CO2 and forms 4carbon oxaloacetate in the presence of water. This reaction is catalyzed by the enzyme, phosphoenol pyruvate carboxylase.
Phosphoenolpyruvate (PEP) carboxylase
Phosphoenolpyruvate + H2O + CO2 –? oxaloacetate + H3PO4
Breakdown
Oxaloacetate breaks down readily into 4 carbon malate and aspartate in the presence of the enzyme, transaminase and malate dehydrogenase. These compounds diffuse into bundle sheath cells from mesophyll cells.
Malatedehydrogenase
NADP:malate dehydrogenase
Oxaloacetate + NADPH + H+ ? malate + NADP+
Transaminase
Oxaloacetate + glutamate ? aspartate + a-ketoglutarate
Fig. 1. The C4 Pathway
Decarboxylation
In the sheath cells, malate and aspartate undergo decarboxylation and yield free CO2 and 3 carbon pyruvate. The CO2 is used in Calvin’s Cycle in the sheath cell.
Malate ? CO2 +pyruvate
The second Carboxylation occurs in the chloroplast of bundle sheath cells. The CO2 is accepted by 5 carbon compound ribulose bisphosphate in the presence of the enzyme, carboxy dismutase and ultimately yields 3 phosphoglyceric acid. Some of the 3 phosphoglyceric acid is utilized in the formation of sugars and the rest is used for the regeneration of ribulose bisphosphate.
Regeneration of Phosphoenolpyruvate
The pyruvate molecule is transferred to chloroplasts of mesophyll cells where, it is phosphorylated to regenerate phosphoenol pyruvate in the presence of ATP. This reaction is catalyzed by pyruvate phosphokinase leading to the regeneration of phosphoenol pyruvate .
pyruvate phosphokinase
Pyruvate + Pi + ATP ? phosphoenolpyruvate + AMP + PPi
CRASSULACEAN ACID METABOLISM (CAM)
An alternative mechanism of concentrating CO2 at the site of rubisco is found in crassulacean acid metabolism. CAM is the characteristic feature of the family Crassulaceae (Crassula, Kalanchoe, Sedum). Apart from Crassulaceae it is found in numerous angiosperm families. Cacti and euphorbias are CAM plants, as well as pineapple, vanilla, and agave. The CAM mechanism enables plants to improve water use efficiency. CAM plants are the most efficient users of water followed by C4 and C3 plants. Hence CAM plants have a competitive advantage in dry atmospheres. CAM pathway shares lot of similarities with the C4 pathway. Just like C4 plants, formation of the C4 acids is both temporally and spatially separated in CAM plants.
PEP carboxylase in cytosol captures the CO2 in the night and the malate that forms from oxaloacetate product is stored in the vacuole. During the day, the stored malate is transported to the chloroplast and decarboxylated by NADP-malic enzyme, the released CO2 is fixed by the Calvin Cycle.
CAM involves two steps
- Acidification
- Deacidification
Acidification
In darkness, the stored carbohydrates are converted into phophoenol pyruvic acid by the process of Glycolysis. The stomata in CAM plants open in dark and allow the free diffusion of CO2 from the atmosphere into the leaf. Now, the phosphoenolpyruvic acid is carboxylated by the enzyme phosphoenol pyruvic acid carboxylase leading to the formation of oxaloacetate which is later reduced to malate. The malate accumulates and is stored in the large vacuoles that are a typical, anatomic feature of the leaf cells of CAM plants. The accumulation of substantial amounts of malic acid, equivalent to the amount of CO2 assimilated at night, has long been recognized as a nocturnal acidification of the leaf.
PEP carboxylase
Phosphoenol Pyruvate + CO2 + H2O ? Oxaloacetic acid + H3PO4
Malic dehydrogenase
Oxaloacetic acid + NADPH2 ? Malic acid + NADP+
Fig.2. Crassulacean acid metabolism (CAM).
Temporal separation of CO2 uptake from photosynthetic reactions: CO2 uptake and fixation take place at night, and decarboxylation and refixation of the internally released CO2 occur during the day. The adaptive advantage of CAM is the reduction of water loss by transpiration, achieved by the stomatal opening during the night.
Deacidification
During day time, when the stomata are closed, the malic acid is decarboxylated to produce pyruvic acid and evolve carbon dioxide in the presence of the malic enzyme. The removal of the malic acid results in the decrease in the acidity of the cell. This is called deacidification. One molecule of NADP+ is reduced in this reaction.
Malic enzyme
Malic acid + NADP+ ? Pyruvic acid + NADPH2 + CO2
The pyruvic acid may be oxidized to CO2 through the pathway of Kreb’s cycle or it may be reconverted to phosphoenol pyruvic acid and synthesize sugar by C3 cycle. The CO2 released by deacidification of malic acid is accepted by ribulose bisphosphate and is fixed to carbohydrate by C3 cycle.
CAM is a most significant pathway in succulent plants. The stomata are closed during day time to avoid transpirational loss of water. As the stomata are closed, CO2 cannot enter into the leaves from the atmosphere. However, they can carry out photosynthesis during the day time with the help of CO2 released from organic acids. During night time, organic acids are synthesized in plenty with the help of CO2 released in respiration and the CO2 entering from the atmosphere through the open stomata. Thus, the CO2 in dark acts as survival value to these plants.
In C4 plants the carboxylase is “switched on,” or active, during the day but where as in CAM plants it is active during the night. In both C4 and CAM plants, PEP carboxylase is inhibited by malate and activated by glucose-6-phosphate.
In darkness, the stored carbohydrates are converted into phophoenol pyruvic acid by the process of Glycolysis. The stomata in CAM plants open in dark and allow the free diffusion of CO2 from the atmosphere into the leaf. Now, the phosphoenolpyruvic acid is carboxylated by the enzyme phosphoenol pyruvic acid carboxylase leading to the formation of oxaloacetate which is later reduced to malate. The malate accumulates and is stored in the large vacuoles that are a typical, anatomic feature of the leaf cells of CAM plants. The accumulation of substantial amounts of malic acid, equivalent to the amount of CO2 assimilated at night, has long been recognized as a nocturnal acidification of the leaf.
PEP carboxylase
Phosphoenol Pyruvate + CO2 + H2O ? Oxaloacetic acid + H3PO4
Malic dehydrogenase
Oxaloacetic acid + NADPH2 ? Malic acid + NADP+
Fig.2. Crassulacean acid metabolism (CAM).
Temporal separation of CO2 uptake from photosynthetic reactions: CO2 uptake and fixation take place at night, and decarboxylation and refixation of the internally released CO2 occur during the day. The adaptive advantage of CAM is the reduction of water loss by transpiration, achieved by the stomatal opening during the night.
Deacidification
During day time, when the stomata are closed, the malic acid is decarboxylated to produce pyruvic acid and evolve carbon dioxide in the presence of the malic enzyme. The removal of the malic acid results in the decrease in the acidity of the cell. This is called deacidification. One molecule of NADP+ is reduced in this reaction.
Malic enzyme
Malic acid + NADP+ ? Pyruvic acid + NADPH2 + CO2
The pyruvic acid may be oxidized to CO2 through the pathway of Kreb’s cycle or it may be reconverted to phosphoenol pyruvic acid and synthesize sugar by C3 cycle. The CO2 released by deacidification of malic acid is accepted by ribulose bisphosphate and is fixed to carbohydrate by C3 cycle.
CAM is a most significant pathway in succulent plants. The stomata are closed during day time to avoid transpirational loss of water. As the stomata are closed, CO2 cannot enter into the leaves from the atmosphere. However, they can carry out photosynthesis during the day time with the help of CO2 released from organic acids. During night time, organic acids are synthesized in plenty with the help of CO2 released in respiration and the CO2 entering from the atmosphere through the open stomata. Thus, the CO2 in dark acts as survival value to these plants.
In C4 plants the carboxylase is “switched on,” or active, during the day but where as in CAM plants it is active during the night. In both C4 and CAM plants, PEP carboxylase is inhibited by malate and activated by glucose-6-phosphate.
Published date : 30 May 2014 07:08PM
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