Roles of molecules in cell surface membranes

Steps:

• Identify typical membrane components: phospholipids/cholesterol stabilize, proteins enable transport/signaling, channels allow diffusion.
• Match labels to functions: W likely cholesterol for stability, X pump for active transport, Y receptor for signaling, Z channel for ion diffusion.
• Evaluate options against biological roles: diffusion uses passive channels, active transport requires energy pumps.
• Confirm C aligns with ion channels facilitating passive diffusion without energy.

Why C is correct:

• Ion channels, like Z, enable facilitated diffusion of ions down concentration gradients per Fick's law of diffusion.

Why the others are wrong:

• A: W (cholesterol) maintains fluidity/stability, not direct control.
• B: Y (receptor protein) receives signals, but signaling involves broader pathways.
• D: X (carrier/pump) uses ATP for active transport against gradients.

Final answer: C