Which molecules primarily aid in transporting substances across the plasma membrane?

Integral proteins primarily facilitate the transport of substances across the plasma membrane due to their structural characteristics and functional roles. These proteins are embedded within the lipid bilayer of the membrane and can extend across the entire membrane, forming channels or transporters.

Their presence allows for the selective passage of various molecules, including ions and larger polar substances that cannot diffuse freely through the hydrophobic lipid bilayer. Some integral proteins act as channels that create hydrophilic pathways for specific ions, while others function as carriers that undergo conformational changes to move substances from one side of the membrane to the other.

This unique ability of integral proteins to interact with both the internal and external environments of the cell makes them essential for maintaining cellular homeostasis and facilitating essential processes such as signaling, nutrient uptake, and waste removal. The other molecules mentioned—cholesterol, carbohydrates, and phospholipids—have different roles in maintaining membrane structure and stability, but they do not primarily function as transport molecules across the plasma membrane.