Membrane transport fundamentals: Passive transport of molecules across biological membranes lacks which requirement?

Introduction / Context:
Cells move substances across membranes using passive and active mechanisms. Passive transport relies on gradients and intrinsic molecular motion, while active transport consumes metabolic energy to move solutes against gradients.

Given Data / Assumptions:

• Passive processes include simple diffusion, facilitated diffusion, and osmosis.
• Temperature-dependent molecular motion underlies diffusion.
• Active transporters (e.g., pumps) require ATP or ion-gradient energy.

Concept / Approach:
By definition, passive transport proceeds down a concentration or electrochemical gradient without direct cellular energy input. Random thermal motion and selective permeability are prerequisites; ATP hydrolysis is not.

Step-by-Step Solution:

Verification / Alternative check:
Carrier-mediated facilitated diffusion (e.g., GLUT transporters) operates without ATP consumption, confirming the principle.

Why Other Options Are Wrong:

• Internal/vibrational energy: these are inherent to all molecules at physiological temperature and drive diffusion.
• Gradient and selective permeability: necessary for passive flux and specificity.

Common Pitfalls:
Assuming that all carrier-mediated processes are active; many carriers are passive and only channels/carriers change conformation without energy expenditure.

Final Answer:
An input of metabolic energy (for example, ATP hydrolysis)