What is Kranz Anatomy?
The word Kranz means “wreath” or “ring”. Kranz anatomy is a specialized structure in C4 Plants where the mesophyll cells are clustered around the bundle-sheath cells in a ring-like fashion. The number of chloroplasts in the bundle-sheath cells is more than that in the mesophyll cells. This is found in C4 grasses such as maize and a few dicots. The Kranz anatomy is developed in three different steps:
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Initiation of procambium
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Bundle sheath and mesophyll cell specification
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Chloroplast development and integration of the C4 cycle
Also read: Photosynthesis in Higher Plants
Kranz Anatomy in C4 Plants
The light-dependent reactions and the Calvin cycle are separated in the C4 plants. The Calvin cycle occurs in the bundle-sheath cells and the light-dependent reactions occur in the mesophyll cells.
The atmospheric oxygen is fixed first to form 4-carbon compound oxaloacetate in the mesophyll cells, catalyzed by PEP carboxylase.
Oxaloacetate is converted to malate which is transported to the bundle-sheath cells.
Malate dissociates in the bundle-sheath cells to release carbon dioxide.
Rubisco fixes the carbon dioxide and converts it into sugars.
Carbon dioxide is constantly pumped into the bundle sheath cells by the mesophyll cells, the carbon dioxide concentration around Rubisco is always higher. This reduces photorespiration.
In the majority of plants, carbon dioxide is fixed into a 3 carbon compound by the action of Ribulose bisphosphate carboxylase oxygenase (Rubisco). Rubisco can also catalyze a reaction with oxygen giving a wasteful process known as photorespiration. To overcome this, the C4 pathway fixes atmospheric carbon dioxide using the enzyme phosphoenolpyruvate carboxylase. Carbon dioxide is then released for refixation by Rubisco.
In C4 grasses such as maize, the mesophyll cells surround the bundle sheath cells, and the bundle sheath cells surround the veins.
Differences Between Mesophyll Cells and Bundle-Sheath Cells
|
Mesophyll Cells |
Bundle-Sheath cells |
|
Many well-developed and large grana are present. |
Grana are very small and poorly developed or might be absent. |
| RuBP carboxylase is absent. No C3 cycle occurs. | RuBP carboxylase is present in high concentration. C3 cycle occurs. |
|
High activity of photosystem II. |
Low activity of photosystem II. |
|
No starch grains present. |
A lot of large starch grains present. |
| Key enzymes for starch synthesis are absent. |
Key enzymes for starch synthesis are present. |
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Frequently Asked Questions
Define Kranz anatomy?
Kranz anatomy is a special structure in the leaves of C4 plants where the tissue equivalent to spongy mesophyll cells is clustered in a ring around the leaf veins outside the bundle sheath cells.
Which plants exhibit Kranz anatomy?
Kranz anatomy is exhibited by C4 plants. They consist of two photosynthetic cell types. These include bundle sheath cells that surround the vascular bundles and the mesophyll cells that surround the bundle sheath cells.
What is the function of vascular bundles?
Vascular bundles are a part of the transport system in vascular plants. The vascular tissues exist in two forms, xylem and phloem, which facilitate the transport of water and minerals. Both these tissues are present in vascular bundles.
Do C3 plants have Kranz anatomy?
No C3 plants do not have Kranz anatomy. In these plants, the bundle sheath cells do contain chloroplast and carbon dioxide fixation occurs only once.
What is the difference between C3 and C4 plants?
The main differences between the C3 and C4 plants are that the bundle sheath cells of C3 plants do not contain chloroplast whereas the bundle sheath cells of C4 plants do. Carbon dioxide fixation in C3 plants takes place only once, whereas that in C4 plants takes twice.
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