C4 Pathway or Hatch-Slack Pathway

M. D. Hatch and C. R. Slack, working on sugarcane, discovered that in some plants the very first stable product of CO2 fixation is not the 3-carbon compound of the ordinary Calvin cycle but a 4-carbon dicarboxylic acid, oxaloacetic acid (OAA) — plants that fix CO2 this way are accordingly called C4 plants, in contrast to C3 plants, whose first CO2-fixation product is the 3-carbon phosphoglyceric acid via the Calvin cycle alone. The C4 pathway (also called the Hatch-Slack pathway) occurs in tropical and sub-tropical grasses and some dicotyledons, prominent examples being sugarcane, maize and sorghum.

C4 plants have a distinctive leaf anatomy called Kranz ('wreath') anatomy: a ring of bundle-sheath cells surrounds each vascular bundle. The bundle-sheath chloroplasts are large, with little or no grana development, while the mesophyll chloroplasts, by contrast, are smaller but have well-developed grana.

In the mesophyll cells, atmospheric CO2 is accepted by the 3-carbon compound phosphoenolpyruvic acid (PEP), catalysed by the enzyme PEP carboxylase (PEPCO), forming the 4-carbon oxaloacetic acid; this is then converted into another 4-carbon compound, malic acid, which is transported into the chloroplasts of the bundle-sheath cells. There, malic acid (4-C) is converted back to pyruvic acid (3-C), releasing CO2 in the process — concentrating CO2 in the bundle-sheath cells to a much higher level than in the surrounding air. Because the bundle-sheath chloroplasts contain the enzymes of the Calvin cycle, this locally high CO2 concentration ensures that RuBP carboxylase (Rubisco) is driven almost entirely toward its carboxylase role in the Calvin cycle rather than toward its wasteful oxygenase role in photorespiration. The sugar produced by the Calvin cycle here is exported into the phloem. …