plz give me short description on c4 cycle

Dear Student,

• Occurs in plants like maize, sugarcane – plants adapted to dry tropical regions
• The leaves of C₄ plants have Kranz anatomy. They have special large cells around their vascular bundles called bundle sheath cells. These cells form several layers around the vascular bundles and have large number of chloroplasts, thick walls impermeable to gaseous exchange, and lack intercellular spaces.
• First CO₂ fixation product is a 4-carbon compound called oxaloacetic acid, but C₃ cycle is used as the main biosynthetic phase.
  • Mesophyll cells: Occur in the C₄ cycle
  • Bundle Sheath Cells: Occur in the C₃ ­cycle
• C₄ plants can tolerate high temperature and high light intensity, show greater productivity of biomass, and lack photorespiration.

• Primary CO₂ acceptor: Phosphoenol pyruvate (PEP) – a 3-carbon molecule
• PEPcase (PEP Carboxylase) fixes CO₂ in the mesophyll cells. It forms the 4-carbon compound oxaloacetic acid (OAA), and then other 4-carbon compounds like malic acid and aspartic acid.
• These compounds are transported to the bundle sheath cells. There, C₄ acid breaks down to form C₃ acid and CO₂ (released only in the bundle sheath cells, and enters the C₃ cycle).
• C₃ acid, so formed, is again transported to the mesophyll cells and regenerated back into PEP.
• C₃ cycle cannot directly occur in the mesophyll cells of C₄ plants because of the lack of the enzyme RuBisCO in these cells
• RuBisCO is found in abundance in the bundle sheath cells of C₄ plants. They lack PEPcase.

Cheers!!