* Polar: Polar molecules have an uneven distribution of electrical charge, making them difficult to pass through the hydrophobic (water-fearing) interior of the phospholipid bilayer that forms the cell membrane.

* Large: Large molecules, regardless of their polarity, have difficulty fitting through the membrane's pores.

* Charged: Charged molecules (ions) are repelled by the hydrophobic interior of the membrane. They require specialized transport proteins to cross the membrane.

Examples of substances that are relatively impermeable to cell membranes:

* Sugars: Many sugars are polar molecules.

* Amino acids: Amino acids are both polar and charged.

* Proteins: Proteins are large and often contain charged or polar regions.

* Ions: Ions, such as sodium (Na+) and potassium (K+), are charged particles.

Note: While the cell membrane is selectively permeable, it's not entirely impermeable. Some small, uncharged molecules, like oxygen (O2) and carbon dioxide (CO2), can readily pass through the membrane. Other substances require specific transport mechanisms, such as active transport or facilitated diffusion, to cross the membrane.