Define simple diffusion and give an example relevant to biology.

Define simple diffusion and give an example relevant to biology.

• A. Movement of molecules from high to low concentration through a membrane without energy; example: O2 moving into cells
• B. Movement of molecules against the gradient requiring energy
• C. Movement through vesicles using vesicular transport
• D. Movement of ions through ion channels with energy

Answer: (A)

Simple diffusion is the passive movement of small, nonpolar molecules across a cell membrane from a region of higher concentration to a region of lower concentration, and it requires no energy input. Oxygen is a classic example: O2 moves from outside the cell, where its concentration is higher, into the cell where it’s lower, crossing the phospholipid bilayer directly without help from transport proteins. This is why the option describing O2 moving into cells best fits simple diffusion. The other descriptions refer to processes that involve energy or other mechanisms—moving against a gradient uses energy (active transport), vesicular transport moves substances in or out via vesicles, and moving ions through channels with energy would not be simple diffusion (it implies energy use or a different transport form).

Simple diffusion is the passive movement of small, nonpolar molecules across a cell membrane from a region of higher concentration to a region of lower concentration, and it requires no energy input. Oxygen is a classic example: O2 moves from outside the cell, where its concentration is higher, into the cell where it’s lower, crossing the phospholipid bilayer directly without help from transport proteins.

This is why the option describing O2 moving into cells best fits simple diffusion. The other descriptions refer to processes that involve energy or other mechanisms—moving against a gradient uses energy (active transport), vesicular transport moves substances in or out via vesicles, and moving ions through channels with energy would not be simple diffusion (it implies energy use or a different transport form).