In C4 plants, atmospheric carbon dioxide is initially fixed in mesophyll cells before being transferred to bundle sheath cells to minimize photorespiration. What is the correct chronological sequence of biochemical events in the C4 (Hatch-Slack) pathway from initial carbon fixation to its entry into the Calvin cycle?
- 1Carboxylation of phosphoenolpyruvate (PEP) by PEP carboxylase to yield oxaloacetate in mesophyll cells
- 2Reduction of oxaloacetate to malate within the mesophyll chloroplasts
- 3Translocation of malate through plasmodesmata into adjacent bundle sheath cells
- 4Decarboxylation of malate in bundle sheath cells to release carbon dioxide and pyruvate
- 5Fixation of released carbon dioxide by RuBisCO to initiate 3-phosphoglycerate formation in the Calvin cycle
Answer
The correct sequence of the Hatch-Slack (C4) pathway is: (1) Carboxylation of PEP to oxaloacetate in mesophyll cells -> (2) Reduction of oxaloacetate to malate -> (3) Translocation of malate into bundle sheath cells -> (4) Decarboxylation of malate releasing CO2 and pyruvate -> (5) Fixation of released CO2 by RuBisCO in the Calvin cycle.
The correct order follows the spatial and biochemical sequence of the C4 pathway: atmospheric carbon dioxide is first fixed by PEP carboxylase in mesophyll cells to form oxaloacetate, which is reduced to malate. Malate is translocated through plasmodesmata into bundle sheath cells, where it undergoes decarboxylation to yield pyruvate and release carbon dioxide. Finally, RuBisCO fixes this released carbon dioxide in the Calvin cycle.
Step-by-Step Solution
Key Concept
Spatial carbon fixation and intercellular shuttle in C4 photosynthesis (Hatch-Slack pathway)