What kinds of molecules do not pass through the plasma membrane, and why?
The correct answer and explanation is:
Molecules that do not pass through the plasma membrane are typically large, charged, or polar. These molecules are unable to cross the hydrophobic lipid bilayer of the membrane due to the structure and properties of the membrane itself.
- Large molecules: Macromolecules such as proteins, nucleic acids (e.g., DNA and RNA), and polysaccharides are too large to pass through the membrane by simple diffusion. The membrane acts as a selective barrier, restricting the entry of large substances.
- Charged molecules: Ions, such as sodium (Na+), potassium (K+), and chloride (Cl-), are electrically charged. Since the interior of the plasma membrane is hydrophobic, charged molecules cannot pass through easily. The membrane’s lipid bilayer does not allow ions to diffuse freely due to electrostatic repulsion.
- Polar molecules: Water-soluble molecules that are polar, such as glucose and amino acids, also have difficulty crossing the membrane without assistance. Although water itself can pass through via specialized channels called aquaporins, larger polar molecules cannot. This is due to the inability of the lipid bilayer to interact with or accommodate the hydrophilic nature of these molecules.
To allow the transport of these molecules, the plasma membrane contains specific proteins, such as channels, transporters, or carriers, that facilitate the movement of large, charged, or polar molecules across the membrane. These proteins enable active and passive transport mechanisms, ensuring that essential substances, such as glucose and ions, can enter or exit the cell while maintaining the membrane’s integrity and selective permeability.
In summary, molecules that are large, charged, or polar struggle to pass through the plasma membrane without assistance, due to the hydrophobic, selectively permeable nature of the lipid bilayer. Specialized transport proteins help facilitate the movement of these substances into and out of cells.