Question

why do plant cells have an additional layer surrounding the cell membrane....
what is the layer known as.​

Answer 1

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The cell wall is the additional protective covering outside the cell membrane which makes the cell rigid. It provides mechanical support and turgidity. It is made up of cellulose in plants, peptidoglycan or murein in bacteria, and chitin in fungus. It is a dead and permeable layer which allows all type of solutes and solvent. It gives shape to the plant cell.

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Basically, there are two important reasons for the presence of cell wall in plants which are :-

• Plants are stationary living organisms. They can't move from one place to another. Thus, they have to suffer from storms, cyclones, etc at the same place. And for this, they need extra mechanical support and the cell walls in each cells provides this strength to the cell.
• The plant's cell wall helps in the prevention of cell bursting due to osmotic pressure during endosmosis. The balance the osmotic pressure by applying pressure in the opposite directions. Hence, in this way also, they provide turgidity to the cell. They give shape to the cell.

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The cell wall is made up of middle lamella, primary wall and secondary wall. The middle lamela is the cementing player and is common to two consecutive cells (aside to each other). It is mainly composed of calcium and magnesium pectate.

The primary wall is the first formed elastic wall and is thinner with respect to secondary wall. It is formed of cellulose microfibrils and pectin. The secondary wall is thicker than primary wall. They occur inner to primary wall in mature and non-dividing cells only.

⏺ Refer to the attachment....

In the bacteria cell, an extra protective layer may present outside the cell known as Glycocalyx. In some bacteria, it is in the form of loose sheath called slime layer, other have thick and tough covering called capsule.

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