Plasma membrane is known as selectively permeable membrane. it is semi -permeable. it only allows the entry of selected material.

Semi-permeability of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell and separates its internal environment from the external environment. It is composed of a phospholipid bilayer with embedded proteins, and it plays a crucial role in maintaining homeostasis by controlling the movement of substances in and out of the cell.

Definition of Semi-permeability

Semi-permeability refers to the ability of the plasma membrane to selectively allow the passage of certain substances while restricting the movement of others. This property is vital for the cell's survival as it enables the regulation of the internal environment and the exchange of essential molecules with the external environment.

Entry of Selective Substances

The correct option 'A' states that the plasma membrane allows the entry of selective substances. This is indeed true. The phospholipid bilayer of the membrane forms a hydrophobic barrier that prevents the free movement of most molecules. However, the presence of specific transport proteins, such as channel proteins and carrier proteins, allows the passage of certain substances across the membrane.

- Channel proteins: These proteins form channels or pores in the membrane, creating a passageway for ions and small molecules to move across. Examples include ion channels and aquaporins that facilitate the movement of specific ions and water, respectively.

- Carrier proteins: These proteins bind to specific molecules and undergo a conformational change to transport them across the membrane. This process is known as facilitated diffusion. Glucose transporters are an example of carrier proteins that allow the entry of glucose into the cell.

Exit of Selective Substances

Although not mentioned in the given options, it is important to note that the plasma membrane also allows the exit of selective substances. Active transport processes, such as exocytosis and active transport pumps, enable the removal of specific molecules and waste products from the cell.

- Exocytosis: This process involves the fusion of transport vesicles with the plasma membrane, resulting in the release of cellular materials outside the cell. It is responsible for the secretion of proteins, hormones, and other molecules.

- Active transport pumps: These proteins utilize energy, usually in the form of ATP, to transport specific molecules against their concentration gradient. Examples include the sodium-potassium pump that maintains the electrochemical gradient across the cell membrane.

Conclusion

In summary, the plasma membrane is indeed semi-permeable, allowing the entry of selective substances through various transport proteins. It also enables the exit of specific molecules through active transport processes. The correct option, 'A,' accurately describes the characteristics of the plasma membrane.