The plasma membrane is a thin, flexible layer that surrounds the cell. It is known as the selectively permeable membrane because it allows some molecules to pass through while preventing others.


The plasma membrane is composed of a phospholipid bilayer, which means it is made up of two layers of phospholipid molecules. These phospholipids have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. The heads face outward, while the tails face inward.


Embedded within the phospholipid bilayer are various proteins that perform different functions. Some of these proteins act as channels or pores, allowing certain molecules to pass through the membrane. Other proteins act as carriers, transporting molecules across the membrane.


There are two main types of membrane transport: passive and active.


Passive transport occurs when molecules move across the membrane without the input of energy. This includes diffusion, osmosis, and facilitated diffusion.


Active transport requires energy to move molecules across the membrane. This includes primary and secondary active transport.


The plasma membrane is selectively permeable because it allows certain molecules to pass through while preventing others. This is due to the structure of the phospholipid bilayer and the proteins embedded within it.


Several factors can affect the permeability of the plasma membrane, including:

• Molecule size


• Molecule charge


• Molecule concentration


• Presence of transport proteins

In conclusion, the plasma membrane is known as the selectively permeable membrane because it allows some molecules to pass through while preventing others. This is due to the structure of the phospholipid bilayer and the proteins embedded within it. Several factors can affect the permeability of the membrane, including molecule size, charge, concentration, and the presence of transport proteins.

Plasma Membrane is semi permeable membrane because it allows or permits the entry and exist of some materials in and out of the cell. It also prevents movements of some other materials therefore it is also called selectively permeable membrane.