Why is the plasma membrane called a selectively permeable membrane?

Hint: The membrane which protects cells from the outer environment.

Complete answer:
The plasma membrane is a biological membrane that separates the interior of all cells from the outside environment which protects the cell from its environment.
It is also called a cell membrane.
The cell membrane is made up of lipid bilayer, proteins, and cholesterol.
The plasma membrane is selectively permeable to ions and organic molecules and regulates the movement of substances in and out of cells. It makes sure that only selected particles enter inside the cell.
The structure of phospholipid bilayer prevents random things from entering through the membrane and proteins act like doors, letting the right particles in and out.
The selectively permeable membrane is important for the cell because if not then it may let in harmful substances.
Cell membranes are semipermeable which helps molecules to move through them.
The phospholipid bilayer in the cell membrane has a hydrolytic center which results in dissolving the molecules which are not capable of passing through the membrane.
Water passes through the cell membrane through osmosis while molecules of oxygen and carbon dioxide pass through the membrane by diffusion.

Significance of plasma membrane
- They keep toxic substances out of the cell.
- They allow specific molecules to enter the cell.
- It provides a fixed environment inside the cell.
- It transports nutrients inside the cell.

Note: The cell membranes are semipermeable because they keep the inside of the cell at low temperatures. The substances move from high concentration to low concentration concerning the concentration gradient.