Outer Membrane of The Cell – Cell wall
The cell wall is exclusively present in plant cells including bacteria.
It is present external to the plasma membrane and is formed of dead substances (mainly cellulose) secreted by the protoplasm of the cell.
1. It is a rigid, freely permeable, non-living part of the cell.
2. Plasmolysis –
Living cells are generally turgid due to the presence of water. When cells are immersed in hypertonic solution, water moves out from the cell by osmosis, shrinkage of protoplasm takes place with visible separation of plasma membrane from the cell walls. This is called plasmolysis.
Wilting due to a loss of water. Peel off the lower epidermis of a Rheo discolour leaf. Cut a small piece and put it on a slide. Add a drop of distilled water. Put a cover slip over the epidermis and examine the cells under a microscope. The cells become turgid. Slowly replace the distilled water with concentrated sucrose solution. The cells become plasmolysed. Plasmolysis can be reversible.
Turgid ↔ Plasmolysed
Now put some Rheo leaves in boiling water for a few minutes. This kills the cells. When the above process is repeated with these leaves, we find that plasmolysis does not occur. This concludes that plasmolysis / osmosis does not occur in dead cells.
The cell wall serves a variety of purposes including –
- It maintains cell shape. It is analogous to an external skeleton for every cell.
- It provides support and mechanical strength. That allows plants to get tall, hold out thin leaves to obtain light.
- It prevents the cell membrane from bursting in a hypo-tonic medium (i.e., resists water pressure). In hypo-tonic solution, the cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell and so withstand external media without bursting.
Read Next –
Cell – Nucleus