The C₄ carbon pump.

In a Kranz leaf, PEPcase in the mesophyll fixes CO2 onto PEP as OAA, a C4 acid crosses into the bundle sheath, releases CO2 beside RuBisCO, and the C3 remnant returns to remake PEP.

Place both enzymes so CO2 at RuBisCO is at least 5× the air level, then name the primary acceptor and first stable product.

RuBisCO in C4 mesophyll; C4 plants skip the Calvin cycle; acceptor and first product confused.

• C₄ plants are adapted to dry tropical regions. Their first CO₂ fixation product is the 4-carbon OAA, but they still use the Calvin cycle as their main biosynthetic route.
• Compared with C₃ plants they have a special leaf anatomy, tolerate higher temperatures, respond to high light intensity, lack photorespiration and have greater biomass productivity. Maize and sorghum are examples.
• Kranz anatomy: around the vascular bundles lies a wreath-like layer of large bundle sheath cells (Kranz means wreath). These cells can be in several layers; they have many chloroplasts, no intercellular spaces, and thick walls that gases cannot pass through.
• In mesophyll cells, the primary CO₂ acceptor is phosphoenol pyruvate (PEP), a 3-carbon molecule, and the enzyme is PEP carboxylase (PEPcase). Mesophyll cells lack RuBisCO.
• The OAA formed in mesophyll is converted to other 4-carbon acids such as malic acid or aspartic acid, which move to the bundle sheath cells.
• Inside bundle sheath cells the C₄ acids break down, releasing CO₂ plus a 3-carbon molecule, which goes back to the mesophyll to be converted into PEP again, so the cycle is completed.
• The released CO₂ enters the Calvin cycle in the bundle sheath cells, which are rich in RuBisCO but lack PEPcase.
• This pathway is also known as the Hatch and Slack pathway, and in effect it pumps CO₂ to where RuBisCO is, keeping CO₂ concentration high there.

Teaching this idea? Put the simulation on your own page or LMS with this code.