What is a defining feature of the lipids in the plasma membrane?

The defining feature of the lipids in the plasma membrane is that they make the membrane impermeable to water, ions, and large molecules. This characteristic is primarily due to the structure of phospholipids that form the bilayer of the membrane. The bilayer is composed of hydrophobic (water-repelling) tails that face inward, away from the aqueous environment, and hydrophilic (water-attracting) heads that face outward towards the water.

This configuration prevents the free passage of polar substances, including ions and large molecules, which cannot easily cross the hydrophobic core of the membrane. Instead, the plasma membrane exhibits selective permeability, allowing certain small nonpolar molecules to pass through while restricting the movement of those that are charged or large.

Other choices present contrasting or misleading information. The notion that lipids are hydrophilic and allow all substances to pass freely is incorrect, as it overlooks the selective permeability function of the membrane. The idea that lipids are only present in animal cells ignores the fact that plant cells and many microorganisms also contain lipids in their membranes, such as phospholipids and sterols. Lastly, the assertion that all lipids in the membrane are exclusively saturated fats is inaccurate, as biological membranes typically contain