Roles of membrane components in transport and structure

Steps:

• Label 2 represents a channel or carrier protein, enabling facilitated diffusion of specific molecules across the membrane.
• Label 4 represents cholesterol or phospholipids, which maintain membrane fluidity by affecting packing of lipid tails.
• Label 1 likely depicts a phospholipid bilayer, which allows simple diffusion but not specific ion diffusion without proteins.
• Label 3 likely depicts a glycoprotein, involved in cell recognition but not antigen-specific immunity in this context.

Why D is correct:

• Facilitated diffusion (statement 2) requires protein channels/carriers, and fluidity (statement 4) is regulated by lipids like cholesterol, matching typical labels.

Why the others are wrong:

• A: Statement 1 is incorrect; ions require facilitated diffusion via proteins, not simple diffusion.
• B: Only 1 is insufficient; 2 and 4 are also accurate for their labels.
• C: Statement 3 is unrelated; antigens are recognized by immune proteins, not general membrane labels.

Final answer: D