Answer:
Every cell is surrounded by a cell membrane which is semi-permeable in nature. The need for cells to be surrounded by a semi-permeable membrane is to control the passage of material coming in and going out of the cell.
This membrane helps the cell to maintain a homeostatic condition inside the cell by allowing selective molecules to cross across the cell. The homeostatic condition is necessary for the proper functioning of the cell and cell survival.
Cell membrane is made up of lipid bilayer which contains membrane proteins embedded into it. Some proteins are freely movable in the cell membranes due to the fluid nature of the membrane.
These proteins are necessary for selective transport of substances across the membrane. These are also called transporter proteins.
All cells need a plasma membrane that acts as a selectively permeable barrier in order to regulate the passage of molecules and ions in and out of the cell.
Small molecules (eg, oxygen and carbon dioxide) can diffuse across the plasma membrane in favor of a concentration gradient.
Moreover, higher molecules (e.g., glucose) move through the plasma membrane by membrane proteins called carriers.
There are carriers that function in favor of a concentration gradient (facilitated diffusion) and carriers that function against a concentration gradient (active transport).
In conclusion, all cells need a plasma membrane that acts as a selectively permeable barrier in order to regulate the passage of molecules and ions in and out of the cell.
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