Cell Wall – Definition, Functions and Difference Between Cell Wall and Plasma Membrane

One of the most pressing Biology Topics of our time is the conservation of endangered species and habitats.

What is Cell Wall and Explain the Process of Plasmolysis in Plant Cells

In plant cells, there occurs a rigid cell wall that lies outside the plasma membrane. The cell wall is non-living and freely permeable and is secreted by the cell itself for the protection of its plasma membrane and cytoplasm. It determines the shape of a plant cell and prevents desiccation of cells. It is made up of a fibrous polysaccharide (carbohydrate) called cellulose. The plant cell wall, thus, consists of tiny cellulose fibers called microfibrils, glued together by a mixture of polysaccharides. Each microfibril is made up of thousands of cellulose molecules bound together by pectins and hemicellulose.

As you may recall from your observation of cells of onion peel, the presence of a cell wall makes plant cells clearly visible as distinct units when viewed under a microscope.

Functions of a Cell Wall

The cell wall of plants performs the following functions:

• It permits the plant cell to become turgid. As water enters the vacuole through osmosis, the plant cell expands. The cell wall has to be strong enough to resist this expansion and so enables the cell to become turgid.
• It provides mechanical strength to support the cell. The cellulose microfibrils are very strong. The strength may be further increased by the addition of lignin in tissues such as the xylem. In cells such as collenchyma, extra cellulose is added to the cell wall to increase mechanical support.
• It is freely permeable to water and substances in solution.
• It has narrow pores, called pits, through which fine strands of cytoplasm, called plasmodesmata, are able to pass. These intercellular connections allow the exchange of materials between living cell contents.
• The cell walls of adjacent cells are glued together by the middle lamella. Middle lamella is a jelly-like substance made up of calcium and magnesium pectate.

The Process of Plasmolysis

When a living plant cell loses water through osmosis, there is a shrinkage or contraction of the protoplasm away from the cell wall. This phenomenon is called plasmolysis. Thus, if a living plant cell is immersed in a concentrated sugar solution, the concentration of water molecules inside the cell will be higher than outside. As a result, water will move by osmosis from the higher water potential inside the cell to the lower water potential outside. The cell contents will shrink away from the cell wall and it will be plasmolyzed.

Mount a complete Rheo leaf in water on a slide and examine the cells of the leaf under the high power of a microscope. Note the location of small green granules which represent the chloroplasts having chlorophyll pigment. Put a few drops of a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute, so that process of osmosis may occur and water may come out from the leaf cells. Again observe the leaf under the microscope. You will observe that the cell contents are separated from the cell wall, i.e., the space between the cell wall and chloroplast-containing cell content (protoplasm) is widened. These changes can be explained by the fact that the outer medium is hypertonic, so water moves out of leaf cells to cause plasmolysis.

Plasmolysis. A-A turgid or normal plant cell of Rheo; B-D – Successive stages in the shrinkage of cell content (protoplasm) from the cell wall.

Now place some Rheo leaves in boiling water for a few minutes. This kills the cells of a leaf. Then mount one boiled leaf on a slide and observe it under the microscope. Put a few drops of a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute and observe it again. In this experiment, you will observe that plasmolysis does not occur. This means that the process of osmosis does not occur in boiled plant (leaf) cells. In other words, it is proved by this experiment that the property of selective permeability exists only in the living plasma membrane. Thus, only living cells possess selectively permeable plasma membranes and they are, therefore, able to absorb water by osmosis.

Differences between Cell Wall and Plasma Membrane

Cell Wall	Plasma Membrane
1. It occurs in plant cells.	1. It is found in both plant and animal cells.
2. It lies outside of the cells.	2. It lies on the outside of animal cells and the inner cell wall in plant cells.
3. It is nonliving and quite thick in plant cells.	3. It is living and quite thin.
4. It is rigid.	4. It is flexible.
5. It is generally permeable.	5. It is selectively permeable.
6. It is formed of cellulose, hemicellulose, and pectin.	6. It is formed of lipids and proteins and a small number of small carbohydrates (i.e., oligosaccharides).
7. Its major function is to provide protection and strength to the cell.	7. Its major function is to hold cellular contents and control the passage of materials in and out of the cell.