Cell membrane is selectively permeable because

The cell membrane is described as selectively permeable because it functions as a gatekeeper, regulating the traffic of substances into and out of the cell. It allows the entry of essential materials like nutrients and the exit of waste products while preventing the movement of other substances [1]. This selectivity is primarily due to the phospholipid bilayer, which acts as a barrier to large polar molecules and ions, while allowing small, nonpolar molecules to diffuse freely [1]. Specific transport proteins, such as carrier and channel proteins, further mediate this process by facilitating the movement of specific molecules that cannot cross the lipid interior on their own [1]. This controlled movement is vital for maintaining cellular homeostasis and the internal composition necessary for life processes [1].

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 2: The Invisible Living World: Beyond Our Naked Eye > Activity 2.3: Let us investigate > p. 12