Question

A cell membrane is very specific about what it allows across. How does this help the cell?

Answer 1

It is better for the cell if most molecules are prevented from entering.

Answer 2

Answer: it allows the passage of small molecules for nutrition and metabolism

Explanation:

A cell membrane  constitutes the physical edge, that keeps the cellular content separated from the extracellular environment, and delimits the shape of the cell.

 The lipid bilayer of the membrane acts as a barrier that separates two aqueous media: the medium, where the cell lives, and the cellular internal environment. The cells require nutrients from the outside and must eliminate waste substances from the metabolism. In this way, the membrane keeps their internal environment stable. The membrane has a selective permeability, since it allows the passage of small molecules, which are lipophilic; but regulates the passage of non-lipophilic molecules. Then, most of the water-soluble ions and molecules which are unable to spontaneously cross this barrier,  require the concurrence of special carrier proteins or protein channels.