Which property of the plasma membrane allows selective permeability?

The phospholipid bilayer is integral to the concept of selective permeability in the plasma membrane. It consists of two layers of phospholipids, where the hydrophilic (water-attracting) heads face outward towards the aqueous environment, while the hydrophobic (water-repelling) tails are oriented inward, away from water. This arrangement creates a semi-permeable barrier that allows certain substances to pass freely while restricting others.

Small nonpolar molecules, such as oxygen and carbon dioxide, can easily dissolve in the lipid layer and pass through, whereas larger or polar molecules require specific transport mechanisms. This selective permeability is essential for maintaining the internal environment of the cell and regulating the substances that enter and exit, which is crucial for cellular function and homeostasis.

Other elements like the fluid mosaic model describe the dynamic nature of the membrane, while the presence of carbohydrates is related to recognition sites for cell signaling rather than selective permeability. The thickness of the membrane does not play a direct role in determining which molecules can pass through, as it is more about the composition of the phospholipid bilayer itself.