Differences between Mesophyll and Bundle Sheath Cells and Chloroplasts in C4 Plants
(Mesophyll vs Bundle Sheath Chloroplasts: A Comparison Table)
The C4 cycle or Hatch and Slack pathway of dark reaction of photosynthesis are characterized by two structurally and functionally different chloroplasts in their leaves. The leaves of C4 plants such as maize possess the classical Kranz anatomy. In Kranz anatomy, each vascular bundle is surrounded by a ring of bundle sheath cells, followed by one or more concentric layers of mesophyll cells. Bundle sheath cells have thick cell walls and contain centrifugally arranged chloroplasts with large starch granules and unstacked thylakoid membranes, whereas the mesophyll cells contain randomly arranged chloroplasts with stacked thylakoids and little or no starch grains.
The mesophyll chloroplasts in C4 plants are highly specialized to do the light dependent reactions of photosynthesis whereas the bundle sheath cells are specialized to perform the light independent reactions. In C4 cycle, the atmospheric CO2 is first accepted by PEP in the cytoplasm of the mesophyll cells and converted to OAA with the help of the enzyme PEP carboxylase. OAA is then transported from the mesophyll cells to the bundle sheath cells. In the bundle sheath cells, OAA releases molecular CO2 and which is accepted by the regular RuBP to run the Calvin cycle or C3 cycle for the synthesis of carbohydrate precursors.