Introduction:
The plasma membrane is a thin layer that surrounds the cell and separates its internal environment from the external environment. It is composed of lipids, proteins, and carbohydrates. One of the most important properties of the plasma membrane is its semi-permeability.

What is a semi-permeable membrane?
A semi-permeable membrane is a type of membrane that allows certain molecules or substances to pass through while blocking others. In the case of the plasma membrane, it allows some molecules to enter or exit the cell while keeping others out.

How does the plasma membrane work?
The plasma membrane has a lipid bilayer that is composed of two layers of phospholipids. The hydrophobic tails of the phospholipids face each other, forming the interior of the membrane, while the hydrophilic heads face the external and internal environment, making the surface of the membrane.

Membrane proteins:
The membrane proteins are embedded in the lipid bilayer. They play an important role in the semi-permeability of the plasma membrane. Some of the proteins act as channels or transporters, which allow specific molecules to pass through the membrane.

Transport mechanisms:
There are two main mechanisms by which molecules can pass through the plasma membrane: passive transport and active transport.

Passive transport:
Passive transport does not require energy. It occurs when molecules move from an area of high concentration to an area of low concentration, down the concentration gradient. This process can occur through diffusion, osmosis, or facilitated diffusion.

Active transport:
Active transport requires energy in the form of ATP. It occurs when molecules move from an area of low concentration to an area of high concentration, against the concentration gradient. This process can occur through primary active transport or secondary active transport.

Conclusion:
In summary, the plasma membrane is called a semi-permeable membrane because it allows some molecules to pass through while blocking others. The selective permeability of the membrane is due to its composition of lipids and proteins, as well as the transport mechanisms that occur across the membrane.

Δ Plasma Membrane is called a semi permeable membrane because it allows or permits the entry and exist of some materials in and out of the cell. Δ It also prevents movements of some other materials. Therefore, it is also called a selectively permeable membrane. √ HOPE IT HELPS YOU