Which type of transport involves movement through a membrane without requiring energy?

The selection of passive transport as the correct answer highlights a fundamental concept in cellular physiology. Passive transport refers to the movement of substances across a cell membrane without the expenditure of energy, typically occurring along a concentration gradient. This means that molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached.

There are several key processes that fall under passive transport, including diffusion, osmosis, and facilitated diffusion. In diffusion, small, nonpolar molecules can pass directly through the lipid bilayer of the cell membrane. Osmosis specifically pertains to the movement of water. Facilitated diffusion uses specific transport proteins to assist substances that cannot easily cross the membrane, but importantly, it does not require energy since it still operates by moving down the concentration gradient.

In contrast, active transport requires energy, often in the form of ATP, because it involves moving substances against their concentration gradient, from an area of lower concentration to one of higher concentration. Endocytosis refers to a process where the cell membrane engulfs substances to bring them into the cell, which also requires energy.

Understanding these differences is critical for comprehending how cells maintain homeostasis and manage the internal environment.