Which process moves substances from high to low concentration across a membrane without energy input?

Which process moves substances from high to low concentration across a membrane without energy input?

• A. Passive Transport
• B. Active Transport
• C. Facilitated Diffusion
• D. Osmosis

Answer: (A)

Moving substances down a concentration gradient without using energy is the essence of passive transport. In passive transport, molecules spread out from where they’re more concentrated to where they’re less concentrated simply because of their natural kinetic energy; no cellular energy (like ATP) is required. This umbrella term fits best because it includes the different ways substances can cross a membrane without energy input: simple diffusion directly through the lipid bilayer, facilitated diffusion via membrane proteins, and osmosis—the diffusion of water. The question’s broad description points to this general process rather than one specific mechanism. Active transport, by contrast, requires energy to move substances against their gradient. Osmosis is a specific form of diffusion for water, and facilitated diffusion is a specific form of passive transport—both are energy-free, but the broader label that covers all these energy-free movements is passive transport.

Moving substances down a concentration gradient without using energy is the essence of passive transport. In passive transport, molecules spread out from where they’re more concentrated to where they’re less concentrated simply because of their natural kinetic energy; no cellular energy (like ATP) is required.

This umbrella term fits best because it includes the different ways substances can cross a membrane without energy input: simple diffusion directly through the lipid bilayer, facilitated diffusion via membrane proteins, and osmosis—the diffusion of water. The question’s broad description points to this general process rather than one specific mechanism.

Active transport, by contrast, requires energy to move substances against their gradient. Osmosis is a specific form of diffusion for water, and facilitated diffusion is a specific form of passive transport—both are energy-free, but the broader label that covers all these energy-free movements is passive transport.