Why can’t charged molecules pass through the lipid cell membrane?

Charged molecules, including ions and polar molecules, cannot pass through the lipid cell membrane easily due to the structural and chemical properties of the phospholipid bilayer. Here are the key reasons:

Hydrophobic Interior

The interior of the phospholipid bilayer is composed of the hydrophobic (water-fearing) tails of the phospholipid molecules. These tails are made up of fatty acid chains that are non-polar, meaning they do not have a charge. This non-polar environment repels polar or charged particles, such as ions and polar molecules, because "like dissolves like" - non-polar molecules cannot easily interact with polar or charged molecules45.

Hydrophilic-Hydrophobic Dichotomy

The phospholipid molecules have a hydrophilic (water-loving) head and a hydrophobic tail. While the hydrophilic head can interact with water and polar molecules, the hydrophobic tails create a barrier that prevents polar and charged molecules from passing through. The hydrophobic tails are arranged in such a way that they face inward, away from the aqueous environment, making it difficult for charged molecules to cross the membrane24.

Size and Charge

Large polar molecules and ions are not only repelled by the hydrophobic interior but also face a size barrier. The lipid bilayer is not permeable to these molecules due to their size and charge, which makes it impossible for them to diffuse through the membrane without the assistance of transport proteins12.

Need for Transport Proteins

To overcome this barrier, cells use transport proteins embedded within the membrane. These proteins can facilitate the passage of charged and large polar molecules across the membrane, either through active transport or facilitated diffusion12.