Transport energetics: what passive and facilitated diffusion do NOT require Considering membrane transport mechanisms, which requirement is <em>not</em> associated with passive diffusion or facilitated diffusion across biological membranes?

Introduction / Context:
Cells use multiple transport strategies to exchange substances with their environment. Distinguishing passive and facilitated diffusion from active transport is fundamental in physiology, pharmacology, and microbiology.

Given Data / Assumptions:

• Passive diffusion: solutes cross membranes down their electrochemical gradients without proteins.
• Facilitated diffusion: membrane proteins (channels/carriers) assist, but transport remains down-gradient.
• Active transport: requires energy to move solutes against gradients.

Concept / Approach:
Neither passive nor facilitated diffusion requires direct metabolic energy. Both processes rely on the existing gradient as the driving force. By definition, they cannot accumulate solute against that gradient; doing so would require coupling to an energy source (ATP hydrolysis, ion gradients, or group translocation).

Step-by-Step Solution:
Identify requirements listed: metabolic energy, uphill movement, and accumulation against gradient.Assign them to “active transport,” not to passive/facilitated diffusion.Conclude that all listed requirements are not features of passive or facilitated diffusion.

Verification / Alternative check:
Carrier-mediated passive systems (for example, GLUT transporters) saturate and are selective but still move glucose down its gradient without energy expenditure.

Why Other Options Are Wrong:

• A–D each describe an energy-requiring or uphill component inconsistent with passive/facilitated diffusion.

Common Pitfalls:
Assuming that the involvement of a carrier automatically means energy use; facilitated diffusion is carrier-mediated but not energy-coupled.

Final Answer:
All of the above are not required for passive/facilitated diffusion