Kranz Anatomy
What is Kranz Anatomy? 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.
Thus, in C4 plants the RUBISCO (RuBP carboxylase-oxygenase) activity is compartmentalized in the bundle sheath chloroplast as a result of differential gene expression. This shows that there is a division of labour between two different types of photosynthetic cells (mesophyll cells and bundle sheath cells). In order to perform this modified CO2 fixation through C4 cycle, the C4 plants possess structurally and functionally different chloroplasts in their mesophyll and bundle sheath cells.
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The present post describes the similarities and differences between the chloroplasts of mesophyll cells and bundle sheath cells in C4 plants.
Similarities between Mesophyll and Bundle Sheath Cells & Chloroplasts
Ø Both mesophyll cells and bundles sheath cells are living cells with plenty of cytoplasm and prominent nucleus.
Ø Chloroplasts in both mesophyll and bundle sheath are double membrane bound organelles.
Ø In both cells, the chloroplasts are autonomous organelles contain its own genetic materials.
Ø Chloroplasts of both cells contain a fluid matrix called stroma.
Ø Both possess the thylakoid membrane system.
Ø Both contain the green pigments- chlorophylls.
Difference between Mesophyll and Bundle Sheath Cells & Chloroplasts
Sl. No. Mesophyll Cells & Chloroplasts Bundle Sheath Cells & Chloroplasts 1 Mesophyll chloroplasts are comparatively smaller. Bundle sheath chloroplasts are comparatively larger. 2 Mesophyll cells are thin walled and gas permeable cells. Bundle sheath cells are comparatively thick-walled and gas impermeable cells. 3 Vacuoles in the mesophyll cells are comparatively larger. Bundle sheath cells possess smaller sized vacuoles. 4 Mesophyll cells possess randomly arranged chloroplasts. Bundle sheath has centrifugally arranged chloroplasts. 5 Chloroplasts of mesophyll cells possess stacked thylakoid membranes. Chloroplasts of bundle sheath cells possess unstacked thylakoid membranes. 6 Mesophyll chloroplasts possess well developed and large grana. Bundle sheath chloroplasts do not contain grana (if present very few and ill-developed). 7 In mesophyll cells, the light dependent reaction is favoured. In bundle sheath cells, the light independent reactions are favoured. 8 Large amount of ATPs and reduced coenzymes are generated in the mesophyll cells. Very little or no ATPs or reduced coenzymes are generated in the bundle sheath cells. 9 Plenty of oxygen is evolved in the chloroplasts of mesophyll cells. In bundle sheath chloroplasts, the evolution of oxygen is literally absent due to the absence of light reaction. 10 No CO2 fixation takes place in the chloroplasts of mesophyll cells. Initial CO2 fixation takes place in the cytoplasm of mesophyll cells by the enzyme PEP carboxylase. CO2 fixation takes place in the bundle sheath cells. The CO2 released by the OAA is accepted by RuBP and enter into C3 cycle. 11 RUBISCO is literally absent in the chloroplasts of mesophyll cells. Higher concentration of RUBISCO occurs in the bundle sheath cells. 12 Starch grains are absent in the chloroplasts of mesophyll cells. Starch grains are abundantly present in the chloroplasts of bundle sheath cells.
