Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

Passive transport is best described as a process that occurs without the use of cellular energy. It allows substances to move across cell membranes from an area of higher concentration to an area of lower concentration through natural diffusion. This movement relies on the inherent kinetic energy of the molecules rather than on an external energy source like ATP.

Because passive transport relies on concentration gradients, it is a fundamental biological process that allows for the exchange of nutrients and waste products without expending energy. Water, gases, and small nonpolar molecules typically enter or exit cells via passive transport mechanisms, such as simple diffusion, facilitated diffusion, and osmosis, which is the diffusion of water specifically.

The other options refer to active processes that require energy input, such as using ATP or other forms of energy to move substances against their concentration gradient, which is contrary to the passive nature of the correct answer.