The C4 pathway and photorespiration

C4 plants, like maize and sorghum, are adapted to hot, dry tropical regions. Their leaves have a special Kranz anatomy (Kranz means wreath): the veins are surrounded by a ring of large bundle sheath cells with many chloroplasts, thick walls that gases cannot pass through, and no spaces between the cells.

The Hatch and Slack pathway works in two places:

• In the mesophyll cells, a 3-carbon acceptor, PEP (phosphoenol pyruvate), takes up CO₂ with the help of the enzyme PEPcase, forming the 4-carbon OAA. This becomes malic or aspartic acid, which moves into the bundle sheath. Mesophyll cells lack RuBisCO.
• In the bundle sheath cells, the 4-carbon acid releases its CO₂. The 3-carbon leftover goes back to the mesophyll to remake PEP. Here the CO₂ enters the Calvin cycle. Bundle sheath cells have RuBisCO but no PEPcase. (In C3 plants, the Calvin cycle runs in the mesophyll.)

Why go to all this trouble? Because RuBisCO can grab oxygen as well as CO₂, and the two compete for its active site. In C3 plants, some RuBP binds O₂ instead, giving one PGA and one phosphoglycolate. This is photorespiration: it makes no sugar, no ATP and no NADPH, actually uses up ATP, and releases CO₂. It is wasteful.

C4 plants pump CO₂ into the bundle sheath, so the CO₂ concentration around RuBisCO is high and it rarely grabs oxygen. So C4 plants have no photorespiration, are more productive, and tolerate higher temperatures.