Question

small non - polar molecules
lipid bilayer (plasma membrane)
extracellular fluid
cytoplasm
time
which of the following best describes the phenomenon depicted in this figure?
active transport of molecules moving against their concentration gradient
exocytosis of molecules contained in membrane - bound vesicles
simple diffusion of molecules moving down their concentration gradient
facilitated diffusion of molecules requiring a protein channel embedded in the phospholipid bilayer

Explanation:

The figure shows small non - polar molecules moving from an area of higher concentration (extracellular fluid) to an area of lower concentration (cytoplasm) across the lipid bilayer (plasma membrane) without the need for a protein channel or energy input. This is the definition of simple diffusion where molecules move down their concentration gradient. Active transport requires energy and moves against the concentration gradient. Exocytosis involves the release of molecules from the cell in membrane - bound vesicles. Facilitated diffusion requires a protein channel for the movement of molecules.

Answer:

simple diffusion of molecules moving down their concentration gradient