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C4 Cycle of Photosynthesis

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Green plants, synthesize the food they need and all other organisms depend on them for their needs. Green plants carry out photosynthesis, a physico-chemical process by which they use light energy to drive the synthesis of organic compounds. Photosynthesis may be defined as the synthesis of carbohydrates by the green organs of a plant in the presence of sunlight from CO2 and H2O. Photosynthesis is a sensitized physiochemical oxidation and reduction mechanism in which water is oxidized and CO2 is reduced. Carbon and oxygen present in the carbohydrates are derived from carbon dioxide and hydrogen from water. Oxygen evolved In photosynthesis comes from water and not from CO2. Oxygen present in product water comes from CO2 and hydrogens of it comes from substrate water. About 170 billion tonnes of dry matter in terms of carbon is produced by this process, of this 55 billion tonnes are formed in the oceans annually. Photosynthesis is the only natural process by which oxygen is liberated into the atmosphere. Chlorophyll (green pigment of the leaf), light, and CO2 are required for photosynthesis to occur. 

Why do Plants Evolve into C4 Cycle?

Because the  C4 plants are more proficient than C3 because of their high pace of photosynthesis and decreased pace of photorespiration. The fundamental catalyst of carbon obsession (Calvin cycle) is RuBisCO, for example, ribulose bisphosphate carboxylase oxygenase. It has a partiality for both CO2 and O2. At the point when carbon dioxide fixation is low, RuBisCO takes up oxygen to perform photorespiration. In C4 plants, photorespiration is profoundly diminished in light of the fact that carbon dioxide fixation is high at the RuBisCO site.

C4 plants have extraordinary leaf life structures called ‘Kranz life systems’ and go through the C4 pathway of photosynthesis. Here CO2 is first acknowledged by phosphoenolpyruvate (PEP) in the mesophyll cells to create 4 carbon natural acids, for example, oxalo acetic corrosive (OAA). OAA is then switched over completely to malic corrosive and shipped to package sheath cells. In the pack sheath cells, CO2 is delivered and it enters the Calvin cycle and is followed up on by RuBisCO. Because of the greater centralization of carbon dioxide in the group sheath cells, photorespiration is limited. 

 C4 cycle/ Pathway

This pathway is called several names like beta-carboxylation pathway or Hatch and Slack pathway. Kortschak, Hartt & Burr (1965) found that 3-PGA is not the initial product of photosynthesis in the case of sugarcane, instead, it was a four-carbon dicarboxylic acid-like malic and aspartic acids. M. D. Hatch and C. R. Slack confirm the results of the above scientists while working on sugarcane plants. Because the first stable compound Is a four-carbon compound, the pathway is termed the C4 pathway, and plants that undergo this pathway are called C4 plants. This pathway occurs in plants of subtropical and tropical regions of the world where the temperature ranges between 30°C to 45°C. About 1500 species of angiosperms belong to 19 families (eg., Maize, sugarcane, sorghum, pearl millet, etc). C4 plants are biologically more evolved than C3 plants. 

Structure of C4 Leaf

The most distinguishable anatomical feature of a C4 leaf is the presence of a bundle sheath surrounding the vascular bundles. This anatomical specialization of a C4 leaf is called Kranz anatomy. While agranal chloroplasts are present in bundle sheath cells, the mesophyll cells of a C4 leaf have grana in their chloroplast (chloroplast dimorphism). The distribution of chloroplast is more in bundle sheath cells. This pathway operates in two photosynthetic cells: mesophyll and bundle sheath, thus exhibiting division of labor. 



Six molecules of carbon dioxide are accepted by six molecules of a primary carbon acceptor called ‘phosphoenol pyruvic acid` (PEP) to form six molecules of oxaloacetic acid (OAA), which is a four-carbon dicarboxylic acid hence this is called C4 pathway. This reaction occurs in the cytosol of the mesophyll cell and is catalyzed by the enzyme PEP carboxylase (PEPcase). OAA, thus formed in the cytosol enters into the chloroplast of the mesophyll cell. Mg+2 is an activator of enzymes RUBISCO and PEP case enzymes. (6)CO2+(6)phosphoenol pyruvic acid +(6)H2O Gives (6)Oxaloacetic acid +(6)H3Po4.


  six molecules of oxaloacetic acid are reduced to six molecules of malic acid, by using the light-generated NADPH. This reaction is catalyzed by the enzyme malic dehydrogenase. 6 Oxaloacetic acid +6 NADPH  gives 6 Malic acid +NADP. I some C4 plants, oxaloacetic acid is converted into aspartic acid by transamination. 

Oxidative Decarboxylation

Six molecules of malic acid formed in the chloroplast of mesophyll cells are transported to the chloroplast of bundle sheath cell and undergo oxidative decarboxylation to form six molecules of pyruvic acid (three-carbon organic acid) and six molecules of CO2. This reaction is catalyzed by the malic enzyme. The oxidation of malic acid during this reaction is associated with the reduction of 6NADPH+ into 6NADPH+H. (6)Malic acid + (6)NADPH + Gives (6) pyruvic acid + (6)NADPH + H+ (6)CO2.This molecule of CO2 generated in the above reaction is utilized in the Calvin cycle (PCR cycle) to synthesize sugars. The bundle sheath may form several layers around the vascular bundles. They are deep-seated and impervious to gaseous exchange due to the presence of Casparian thickenings. As a result, oxygen reaching bundle sheath cells is less but CO2 released is more (by decarboxylation of malic acid). As a result, RUBP undergoes carboxylation only, thereby reducing the chance of photorespiration. 


Six molecules of pyruvic acid produced in bundle sheath cells move to the chloroplast of mesophyll cells. Here it is phosphorylated to six molecules of phosphoenol pyruvate (PEP). The enzyme pyruvate dikinase catalyzes the reaction. Twelve molecules of extra ATP are used to transport 6 CO2 from mesophyll cell into bundle sheath cell. PEP enters the cytosol of the mesophyll cell. During this process, there are two carboxylations ( one each in mesophyll and bundle sheath) and one decarboxylation (bundle sheath cell). To synthesize one molecule of glucose C4 plants utilize 30 ATP and 12 NADPH (energetics of C4 CYCLE).

Crassulacean Acid Metabolism (CAM)

Certain plants, especially succulents which grow under extremely xeric conditions, fix atmospheric CO2 in dark. The process was first observed in the members of the family Crassulaceae (e.g., Bryophyllum, kalanchoe, sedum, etc) hence termed Crassulacean Acid Metabolism (CAM). IT was discovered by Oleary and Rouhani. Similar metabolism has been found to occur in plants like cactus (eg., Opuntia), orchid, portulaca, and pineapple (Bromeliaceae) families. All these plants which carry out Crassulaceaen acid metabolism are referred to as CAM plants. The most characteristic feature of these plants is their stomata (SCOTOACTIVE) remain open at night (in dark) and closed during the day (in light). THUS, CAM is a Kind of adaptation in succulents to carry out photosynthesis without much loss of water. The metabolic pathway of CAM involves acidification, which occurs at night (during the dark), and deacidification (during the daytime) in light. During the night, the organic acid content of CAM plants increases, and the pH of their cell sap decreases whereas, during the day, the organic acid content decreases, and the pH of their cell sap increases. Similarly, the stored food carbohydrates (eg, starch) increase during the daytime and decrease during the night. 

Metabolic Pathway

IN dark (during the night) when the stomata are open, the carbon dioxide combines with phosphoenol acid (PEP) to form oxaloacetic acid (OAA) in the presence of the enzyme PEP carboxylase. Phosphoenol pyruvic acid +CO2 + H2O Gives Oxaloacetic acid +H3PO4. Oxalic acetic acid is subsequently converted to malic acid in presence of the enzyme malic dehydrogenase. The reaction occurs in presence of NADPH. The malic acid produced in dark as a result of acidification is stored in the vacuole. Oxaloacetic acid + NADH Gives Malic acid + NAD. In light (during the day), when the stomata are closed, the malic acid is oxidatively decarboxylated to produce pyruvic acid. This process is termed deacidification. Malic acid + NAD Gives Pyruvic acid + NADH + CO2. CO² released in the above reaction enters into the Calvin cycle to produce starch. CAM pathway mostly resembles the C4 pathway.

 Difference between C3 Cycle and C4 Cycle 

C3 Cycle

C4 Cycle

The first stable product of the carbon pathway is the C3 compound ( PGA – Phospho glyceric acid)  The first stable product of the carbon pathway is the C4 compound (OAA – Oxaloacetic acid)
This occurs mostly in Temperate plants This occurs only in tropical plants 
Leaves do not show Kranz’s anatomy   Leaves show Kranz’s anatomy
Chloroplast dimorphism is not seen  Chloroplast dimorphism is seen
Photorespiration is very high        Photorespiration is not detectable
In C3 plants, transpiration is more In C4 plants, transpiration is less
Less efficient in utilizing atmospheric CO2 More efficient in utilizing atmosphere CO2
Biomass is produced in less quantity Biomass is produced in high quantity
Example: almost all dicot plants  Example: Maize, sugarcane, Sorghum

Conceptual Questions 

Question 1: Which group of plants exhibits two types of photosynthetic cells? What is the first product of carboxylation?


C4 plants exhibit two types of photosynthetic cells. They are mesophyll cells and bundle sheath cells. During the first carboxylation in mesophyll cells, OAA is formed as a stable product

Question 2: What carboxylation enzyme is present in bundle sheath cells and mesophyll cells?


In mesophyll cells, PEP carboxylase enzyme is present, In bundle sheath cells, RuBisCO is present.

Question 3:  Which is the primary CO2 acceptor in C4 plants?


 The primary CO2 acceptor in C4 plants is the 3-carbon molecule phosphoenol pyruvate (PEP). 

Question 4: Give two examples of C4 plants?


Maize and Sorghum. 

Question 5:  why does the Calvin cycle not occur in mesophyll cells?


Mesophyll cells lack of enzyme RuBisCO thus they cannot carry out the Calvin cycle. 

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Last Updated : 21 Oct, 2022
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