Question

Why do plant cells have an additional layer surrounding the cell membrane? What is this layer known as? What is it made up of?

Answer 1

Hint: Plants require protection from osmotic stress and mechanical pressure. These functions are provided by this additional layer.

Complete answer: A major difference between plant and animal cells is the presence of a cell wall. It surrounds the cell membrane (also called the plasma membrane) of plant cells. The cell wall provides tensile strength and protection against mechanical and osmotic stress. It helps cells to develop turgor pressure.

Turgor pressure – It is the pressure of the cell contents against the cell wall.
1. Plant cells have a lot of molecules dissolved in the cytoplasm.
2. The high concentrations of these molecules in the cytoplasm cause the moving of water into the cell under normal conditions. This makes the central vacuole of the cell swell and presses against the cell wall. In the presence of healthy water supply, the turgor pressure keeps a plant from wilting. In drought conditions, plants may wilt but the cell walls help to maintain the structural integrity of the stem, leaves, and all other structures.
3. These are those conditions where the cells have lost most of their water, the vacuole is shrinking and has lost its turgidity.
4. Plant cell walls are primarily made of cellulose.
5. Cellulose is the most abundant macromolecule on Earth.
6. Its fibers are long, linear polymers of glucose molecules.
7. The cellulose fibers aggregate into bundles, generally, 40 fibers are present in a bundle.
These bundles are called microfibrils. Microfibrils are embedded in a hydrated network of many different polysaccharides.
8. The cell wall does not lose its shape and stays in its place.

Note: In plants, cell wall allows them to have strong, upright structures and vacuoles help in the regulation of water movement and storage of other metabolites.