The Four - Carbon Pathway

C₄ pathway

C₄ plants like maize, sugarcane; pearl millet, etc. have evolved a wonderful mechanism to avoid photorespiration, which is considered a wasteful process.

In these plants three or 2 types of photosynthetic cells namely mesophyll cells and bundle sheath cells. The bundle sheath cells are arranged in a wreath like manner, thus calling them as kranz anatomy (kranz : wreath).

One more speciality of C₄ plants is that they contain dimorphic chloroplasts, that is chloroplasts in bundle sheath cells are agranal. The presence of two types of cells allows the occurrence of light reactions and carbon reactions separately.

In C₄ plants, light reactions occur in the mesophyll cells, whereas CO₂ assimilation occurs in the bundle sheath cells. This type of separation does not allow O₂ released in mesophyll cells to escape in to the bundle sheath cells. This prevents the oxygenation of RuBP, which is present in the bundle sheath cells.

Transverse section of maize leaf showing the arrangement of mesophyll and bundle-sheath cells. The C₄ pathway takes place in the mesophyll cells, and the C₃ pathway (Calvin cycle) operates in the bundle-sheath cells. Both types of cells contain chloroplasts.

To further avoid photorespiration, C₄ plants have evolved a CO₂ concentrating mechanism called the C₄ pathway. The main objective of C₄ pathway is to build up a high concentration of CO₂ in the vicinity of Rubisco, thus favouring carboxytition and suppressing photorespiration.

Many plants like maize and sugar cane, are far more efficient at taking up CO₂ than C₃ plants. The CO₂ acceptor in C₄ plants is phosphoenolpyruvate (PEP). PEP reacts with CO₂ to form oxaloacetic acid which is reduced by NADPH to form malic acid. The malic acid then reacts with RUBP to form pyruvic acid and PGA. The pyruvic acid is then phosphorylated by ATP to regenerate PEP while PGA is converted to triose phosphate as far as C₃ plants. These reactions are called the Hatch - Slack pathway.

These plants, however first fix carbon dioxide in the four - carbon compound oxaloacetic acid. C₄ plants have two rings of cells surrounding the vascular bundles. The outer mesophyll cells fix CO₂ using PEP and pass malic acid to the inner bundle sheath cells. Here CO₂ is released for acceptance by RUBP and the eventual formation of triose phosphate. The photosynthetic efficiency of C₄ plants is due to their ability to fix CO₂ in environmental conditions where CO₂ is the limiting factor. The C₄ photosynthetic pathway is more efficient than the C₃ pathway due to the absence of photorespiration in C₄ plants.

Differences between C₃ and C₄ Plants

C3	C4
CO2 acceptor is 5c RUBP	CO2 acceptor is 3c PEP
CO2 fixing enzyme is RUBP carboxylase	CO2 fixing enzyme is PEP carboxylase
First product of photosynthesis is PGA (3c)	First product of photosynthesis is oxaloacetic acid (4c)
CO2 fixation occurs once only	CO2 fixation twice - Kranz anatomy

 

 

Crassulacean acid metabolism (CAM)

CAM refers to a mechanism of photosynthesis that occurs only in succulents and other plants that grow in dry conditions.

a) In CAM plants, CO₂ is taken up by the leaves, which are present on green stems through the stomata. But the stomata remain open only during the night. During the day it remains closed to conserve moisture.

b) The CO₂ taken is fixed in the same way as in C₄ plants, to from malic acid which is stored in the vacuole.

c) This malic acid formed during the night is used as a source of CO₂ during the day for photosynthesis to proceed via the C₃ pathway.

Thus CAM is a kind of adaptation in certain plants (grown in dry conditions) to carry out photosynthesis without much loss of water, which is otherwise unavoidable in C₃ and C₄ photosynthetic mechanisms.

CAM pathway showing carbon dioxide uptake through open stomata during night and its utilisation for the formation of malic acid which is stored in the vacuole. During day, the malic acid is decarboxylated to release carbon dioxide which is re-fixed to produce starch inside choloroplast via C₃ calving cycle.