Study Of Plant Cells -plasmolysis

Experiment : Plasmolysis Study of Plant cells in Hypertonic Solution Objective To observe the cells of an onion peel in hypertonic solution. Apparatus and material required An onion, glass slide, coverslip, forceps, needle, brush, blade, blotting paper, dropper, concentrated salt or sugar solution and a compound microscope. Theory When a plant cell is placed in a s solution that has a higher solute concentration(hypertonic compared to the cell sap) water is lost from the cell by osmosis (exosmosis). As the concentration of water molecules in the cell is higher than the outer hypertonic solution, net movement of water molecules is from their region of higher concentration to their region of lower concentration across the selectively permeable plasma membrane. This process is called exosmosis. If exosmosis continues in a plant cell, cytoplasm along with the nucleus and vacuole shrinks to a small irregular mass due to loss of water. Eventually it pulls away form the cell wall. The space between, the cell wall and cytoplasmic mass is filled with the salt or sugar solution due to the permeability of the cell wall. This phenomenon is plasmolysis and the cell is said to be plasmolysed. Plasmolysis is a reversible phenomenon. When the plasmolysed cell is placed in water, which is hypotonic compared to cell sap, the cell gains water due to endosmosis. As a result. The cell membrane, cytoplasm and vacuole regain their normal position. This phenomenon is called deplasmolysis. Procedure 1. 2. 3. 4. 5. 6. 7. 8.

Remove the outermost peel of the onion. Cut out a small part from an inner fleshy scale leaf using the blade. Pull out the transparent peel of the scale leaf using the forceps. Using the brush place the piece of onion peel on a clean glass slide. Put a drop of the concentrated salt or sugar Solution on the peel with the help of the dropper. Put a clean coverslip by tilting it over the needle. In this way you can prevent air bubbles form getting trapped under the coverslip. Remove excessive solution with the blotting paper. Observe the mounted onion peel immediately under the low and high magnification of the compound microscope. Observe it again under the microscope after some time. Note the shrunken cytoplasm of the cells.

Observations After putting the drop of the hypertonic solution on the cells of the onion peel, their cell membranes withdraw from the cell walls; the enclosed cytoplasm nucleus and vacuole lose water and shrink to one side of the cell. The cell walls remain at their original positions. The space between the cell wall and the shrunken cytoplasm is filled with the salt or sugar solution.

Precautions 1. 2. 3. 4. 5. 6.

Use a fresh onion peel. Avoid folding the peel. Do not let the peel dry out. Use a clean, dry glass slide and coverslip Avoid trapping air bubbles under the coverslip. Remove excessive solution from the slide with the blotting paper. These pictures are only for reference. Do not draw them in your Practical Files