In a maize leaf, a mesophyll cell fixes 12 carbon dioxide molecules with PEP…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In a maize leaf, a mesophyll cell fixes carbon dioxide molecules with PEP. (a) How many carbon atoms are in one PEP, and how many in one OAA? (b) How many carbon atoms do the OAA hold in all? (c) In which cell is the carbon dioxide set free again, and what 3-carbon compound goes back? (d) What is needed to make PEP from that compound?
Need a reminder? The key ideaShow
C4 plants (maize, sugarcane, sorghum): Kranz anatomy (large bundle sheath cells with many chloroplasts, thick walls, no intercellular spaces, round the veins). In a mesophyll cell, PEP (3C) plus carbon dioxide gives OAA (4C) with PEP carboxylase. OAA becomes malic acid or aspartic acid. It goes to a bundle sheath cell, where carbon dioxide is set free. Pyruvate (3C) goes back to the mesophyll to become PEP with ATP. RuBisCO and the Calvin cycle work in the bundle sheath cells and are absent from the mesophyll cells. C4 plants lack photorespiration, have a high temperature optimum and high productivity.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. PEP and OAA. PEP has carbon atoms. With one carbon atom from carbon dioxide, OAA has carbon atoms.
Step 2. Twelve OAA. carbon atoms.
Step 3. Where it is set free. The 4-carbon acid goes to a bundle sheath cell. There it releases carbon dioxide, and pyruvate (3C) goes back to the mesophyll.
Step 4. Making PEP. Pyruvate becomes PEP again, using ATP.
Check. Twelve carbon dioxide molecules brought in of the carbon atoms, and the other came from PEP, which is .
Answer: (a) PEP: atoms, OAA: atoms; (b) atoms; (c) in the bundle sheath cell, pyruvate; (d) ATP.
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