C₄ plants like maize, sugarcane; pearl millet, etc. have evolved a wonderful mechanism to avoid photorespiration.

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

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 into 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 carboxylation and suppressing photorespiration.

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.