Final answer:
Membranes are more permeable to nonpolar molecules due to the hydrophobic nature of their lipid bilayer structure. Polar and ionized molecules are repelled by the hydrophobic core of the membrane. This selective permeability allows membranes to control the passage of molecules in and out of cells.
Step-by-step explanation:
The primary reason why membranes are more permeable to nonpolar molecules than to most polar and ionized molecules is their structure. Membranes are made up of a lipid bilayer, which consists of two layers of phospholipids. These phospholipids have a hydrophilic head and a hydrophobic tail, meaning that they are attracted to water on one end and repelled by it on the other.
Nonpolar molecules, such as oxygen and carbon dioxide, are able to easily pass through the hydrophobic interior of the membrane because they have no charge and are attracted to the hydrophobic tails of the phospholipids. In contrast, polar and ionized molecules, such as water and ions, have charges or partial charges and are repelled by the hydrophobic core of the membrane.
This selective permeability allows membranes to control what enters and exits the cell, ensuring that only certain molecules are able to pass through.