In diffusion, why do substances move from high to low concentration, and what membrane property facilitates diffusion?

In diffusion, why do substances move from high to low concentration, and what membrane property facilitates diffusion?

• A. They diffuse down their concentration gradient; the lipid bilayer's selective permeability and transport proteins facilitate diffusion.
• B. They diffuse against their concentration gradient; diffusion requires active transport.
• C. They diffuse down their concentration gradient; the nucleus regulates diffusion.
• D. Diffusion requires energy input from ATP; membranes do not influence diffusion.

Answer: (C)

Diffusion is the passive spreading of molecules from areas of higher concentration to areas of lower concentration, driven by random molecular motion. It does not require energy. When diffusion occurs across a cell membrane, the membrane’s selective permeability determines what can pass: small, nonpolar molecules can diffuse directly through the lipid bilayer, while polar or charged substances often need transport proteins to move down their concentration gradient. This combination—diffusion down the gradient and facilitation by the membrane’s permeability and transport proteins—best explains why diffusion happens and how the membrane supports it.

Diffusion is the passive spreading of molecules from areas of higher concentration to areas of lower concentration, driven by random molecular motion. It does not require energy. When diffusion occurs across a cell membrane, the membrane’s selective permeability determines what can pass: small, nonpolar molecules can diffuse directly through the lipid bilayer, while polar or charged substances often need transport proteins to move down their concentration gradient. This combination—diffusion down the gradient and facilitation by the membrane’s permeability and transport proteins—best explains why diffusion happens and how the membrane supports it.