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The Endodermis is one of the most important adaptations of terrestrial plants. In the absence of an endodermis plants would not be able to regulate water uptake by the roots and maintain the water balance of the plant. Bryophytes lack an endodermis and they are unable to maintain large aerial systems. Plants began to develop large shoots only after they developed this adaptation. I will try to illustrate how the endodermis regulates solute movement and consequently regulates water movement at an elementary level.
Keep in mind that molecules move from areas of high concentration to areas of low concentration. This includes water. Solute molecules like Sucrose, Na & Cl reduce the water concentration. We usually think of diluting solutions by adding water, however, dissolving 50 grams of sucrose in 100 ml of water would "dilute" the water molecules. If this solution was placed in one side of a U-Tube which was divided into two by a semipermeable membrane (permeable to water but impermeable to sucrose) & Pure water was placed in the other half, what would happen??? The sucrose molecules can't go anywhere, but the water molecules can cross the membrane.
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Sucrose solution is in the left half of the tube. Pure water is in the right half. The membrane will allow water molecules to pass through but sugar is blocked. There is a greater concentration of water molecules on the right side of the membrane. Thus, there will be a net movement of water molecules from the right to the left side of the membrane. |
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The movement of water molecules from an area of high concentration to one of low concentration results in a volume increase in the left side of the tube. Thus the solution level rises. |
What would happen if there were no membrane?
The Sucrose and water molecules would become evenly distributed in the solution.
The endodermis separates the Cortex from the Stele. If the endodermal cell walls were purely composed of Cellulose & there were a lot of Sucrose molecules in the cortex, but none in the stele, what would happen to the sucrose molecules.
Sucrose molecules are represented as red balls below. These are a thousand fold bigger than actual sucrose molecules, and can easily fit between cellulose fibrils in the cell wall.
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Sugar molecules are abundant in the cortex (outside) and absent in the stele (inside). |
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The sucrose molecules have moved through the apoplast (Cell Walls & Spaces) into the Stele. Bye! Bye! |
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