Question

Of the 4 categories of molecules passing the membrane, list the categories from MOST permeable to LEAST permeable across the lipid bilayer. Give examples for each category and briefly explain why the last group is least permeable to the membrane.

Answer 1

The order of membrane permeability from most to least is: small nonpolar molecules, small uncharged polar molecules, large uncharged polar molecules, and lastly ions and charged molecules. The hydrophobic interior of the lipid bilayer hinders the permeability of ions and charged molecules, making them least permeable.

Step-by-step explanation:

The permeability of various molecules through the cell membrane's lipid bilayer can be ordered from most permeable to least permeable as follows:

1. Small nonpolar molecules: e.g., oxygen (O2) and carbon dioxide (CO2). These readily dissolve in the lipid bilayer without assistance due to their nonpolar nature.
2. Small uncharged polar molecules: e.g., water (H2O) and glycerol. They can sometimes pass through the membrane but not as easily as nonpolar molecules.
3. Large uncharged polar molecules: e.g., glucose and amino acids. Their size and polarity generally require them to use transport proteins to move across the membrane.
4. Ions and charged molecules: e.g., Na+, K+, Ca2+, and Cl−. These are the least permeable due to their charge and attraction to water molecules, which are incompatible with the hydrophobic interior of the lipid bilayer.

The last group, ions and charged molecules, is the least permeable because the hydrophobic interior of the membrane repels the hydrophilic (water-attracting) nature of these charged particles, substantially decreasing their ability to penetrate the nonpolar lipid bilayer without specialized structures such as ion channels or carriers.

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