What kind of proteins allow specific molecules or ions to cross cell membranes?

The correct choice is channel proteins because they facilitate the passive transport of specific ions and molecules across the cell membrane. These proteins form pores or channels within the membrane that allow for the selective passage of substances, often ions such as sodium, potassium, calcium, and chloride.

Channel proteins are integral membrane proteins, and their selective permeability is crucial for maintaining cellular homeostasis and enabling essential processes, such as nerve impulse transmission and muscle contraction. By providing a hydrophilic pathway through the hydrophobic lipid bilayer of the membrane, channel proteins allow specific molecules and ions to flow in and out of the cell without the need for energy, making them essential for functions such as signal transduction and maintaining osmotic balance.

In contrast, carrier proteins also help transport molecules across cell membranes, but they typically undergo a conformational change when binding a substrate and often require energy input for active transport. Structural proteins serve primarily to provide support and structure to the cell, contributing to the overall shape and stability rather than facilitating the movement of substances. Membrane proteins is a broader category that includes both channel and carrier proteins, but it does not specify the function related to the transport of specific molecules, which makes it less precise in this context.