Final answer:
Simple diffusion is the movement of small, uncharged, and hydrophobic molecules like oxygen and carbon dioxide across the cell membrane down their concentration gradient without the need for energy.
Step-by-step explanation:
Simple diffusion is a process in which molecules move from an area of higher concentration to an area of lower concentration without the input of energy.
This movement is down their concentration gradient and is a part of passive transport. Molecules that can pass through the cell membrane via simple diffusion include small, uncharged, and hydrophobic substances such as oxygen and carbon dioxide.
These molecules can freely cross the cell membrane's lipid bilayer due to their size and nonpolar nature. Polar molecules and charged ions, however, usually require assistance to traverse the cell membrane through mechanisms such as facilitated diffusion or active transport.