Hatch & Slack Pathway

Most of the C4 plants are monocots (Tropical grasses), which belong toGramineae & Cyperaceae families. C4 plants are adapted to hot and dry environments.

C4 pathway is also called:

  • CO2 Concentrating Mechanism 
  • Co-operative Photosynthesis 
  • Dicarboxylic Acid Cycle (DCA Cycle)

Eg. : C4 plants. Sugarcane, Maize, Sorghum. • Wheat, Rice, and Barley are C3 species.

C4 plants are special because : 

(i) They lack a process called photorespiration so have greaterproductivity of biomass. 

(ii) They tolerate higher temperature 

(iii) They show a response to high light intensities 

(iv) They have a special type of leaf anatomy (Kranz anatomy) 


Kranz Anatomy

Kranz (Wreath) Anatomy- Present in leaves of C4 plants.

The bundle sheath cells may form several layers around the vascular bundles, they are characterized by

i. having a large number of chloroplasts.

ii. thick walls impervious togaseous exchange.

iii. no intercellular spaces. 

License to NCERT


  • C4 plants total of 30 ATP and 12 NADPH2 are utilized for the synthesis of one glucose. 
  • In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the RuBisCO enzyme site. This takes place when the C4 acid (malic or aspartic acid) from the mesophyll cell is broken down in the bundle sheath cells to release CO2 (CO2 pumping), this results in increasing the intracellular concentration of CO2. In turn, this ensures that the RuBisCO functions as a carboxylase minimizing the oxygenase activity. 
  • In addition, in C4 plants, the site of O2 evolution (mesophyll cell) and site of RuBisCO activity (Bundle sheath cell) are different.
  • The evolution of the C4 photosynthetic system is probably one of the strategies for maximizing the availability of CO2 while minimizing water loss. C4 plants are twice as efficient as C3 plants in terms of fixing carbon (making sugar). However, a C4 plant loses only half as much water as a C3 plant for the same amount of CO2 fixed

Photorespiration

  • This is the reaction where RuBP combines with CO2 to form 2 molecules of 3PGA, that are catalyzedby RuBisCO. 
  • RuBisCO is the most abundant enzyme in the world (Do you wonder why?) and is characterized by the fact that its active site can bind to both CO2 and O2 – hence the name.
  • The light-dependent uptake of O2 & release of CO2 in green cells of C3 plants is called Photorespiration. This process creates an important difference between C3 and C4 plants. 
  • It occurs in chloroplast, peroxisome & mitochondria. 
  • RuBisCO is characterized by the fact that its active site can bind to both CO2 and O2 – hence the name. This binding is competitive. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme. (Usually, RuBisCO has a much greater affinity for CO2 than for O2 ). 
  • Conditions for photorespiration – High light intensity (High O2, Low CO2 ) and high temperature.

Mechanism of Photorespiration


  • In the photorespiratory pathway, there is neither synthesis of sugars, nor ATP and NADPH2 rather it results in the release ofCO2 with the utilization of ATP. 
  • The biological function of photorespiration is not known yet. 




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