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C4 Pathway

C4 plants: Previously it was thought that the formation of 3-phosphoglycerate was always the initial step in carbon dioxide fixation during photosynthesis in all plants.

However, soon it was discovered that in plants such as sugar cane, the initial products were not 3-phosphoglycerates; instead they were 4-carbon acids. Subsequently it was shown that numerous plants follow this pattern. These plants, known as C4 plants perform normal Calvin-Benson cycle. In addition to this, they add a stop that acts as a 'CO2 pump' to increase the rate of photosynthesis even at low levels of CO2 within their leaves.

In C4 photosynthesis, mesophyll cells in the leaf pick up carbon dioxide which combines with phosphoenol pyruvate in the presence of the enzyme phosphoenol pyruvate carboxylase to form a four-carbon carbon compound(oxaloacetic acid). These four-carbon compounds diffuse into adjacent bundle sheath cells, where the four carbon compounds undergo decarboxylation (removal of CO2). The enzyme ribulose-biphosphate carboxylase (RuBP carboxylase) picks up this CO2 and the usual Calvin-Benson cycle of C3 photosynthesis takes place.

The C4 system of photosynthesis, in effect, 'pumps' CO2 from a region where its concentration is low (the intercellular spaces within the leaf) to be where it is relatively more abundant (the bundle sheath layer). The c (PEP-carboxylase) in the mesophyll chloroplasts can, by its great affinity, take up CO2 when RuBP Carboxylase could not. The four carbon acids are then transported to the bundle sheath, allowing the release of sufficient carbon dioxide for photosynthesis. A comparison of photosynthesis in C3 and C4 plants is given in the table.


In the chloroplast, the enzyme ribulose biphosphate carboxylase, may catalyse a reaction between ribulose-biphosphate and oxygen. Further reactions lead to the formation of the reduced two carbon compound glycolate. Glycolate leaves the chloroplast and diffuses into the microbody where it is oxidised. Later reactions lead to the formation of CO2which diffuses out of the cell. The rate of the overall process is roughly proportional to the light intensity. Because these reactions are dependent on light and because of the uptake of oxygen and release of carbon dioxide, this process is called photorespiration.

Table - Comparison of photosynthesis in C3 and C4 plants


C3 Plants

C4 Plants

Perform Calvin-Benson cycle



Primary carbon dioxide acceptor

Ribulose biphosphate (Ribulose 1,5-diphosphate) RuBP

Phosphoenol pyruvate (PEP)

CO2 fixing enzyme First product of CO2 fixation

RuBP carboxylase 3-phosphoglycerate

PEP carboxylase Oxaloacetate

Affinity of carboxylase to CO2



Leaf anatomy photosynthetic cells


Mesophyll + bundle Sheath

Classes of chloroplasts






The role of photorespiration in the life of the plant is unknown. However, it has been found that C4 plants appear to indulge in little photorespiration in comparison to C3 plants. The reduction in photorespiration in C4 plants may be an adaptation to maximize the efficiency of their photosynthesis.


Halobacteria live in salty environments where the salt concentration is much higher than in the oceans. These bacteria exhibit a form of photosynthesis in which there is no involvement of chlorophyll. Instead a protein called bactetiorhodopsin located in the plasma membranes of the halobacteria, serve as light-absorbing material. This protein is about 4.5 nm in length and is organised into seven helical regions perpendicular to the plane of the plasma membrane. When light is absorbed by bacteriorhodopsin, protons are transported through the membrane. This pumping of protons out of the cell results in ATP synthesis by a similar mechanism that operates in chloroplasts and mitochondria. The ATP thus formed, serves as the immediate source of energy for the metabolism of the bacteria. Under anaerobic conditions, the halobacteria synthesise a substance called retainal which is a carotenoid pigment. Retinol combines with protein to form bacteriorhodopsin. It is interesting to note that the carotenoid pigment, retinol is found in the vertebrate eye where it plays a key role in vision.

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