Passive transport across a cell membrane requires:

Passive transport is a fundamental biological process where substances move across a cell membrane without the expenditure of energy. This movement occurs down the concentration gradient, meaning that molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached. This process relies solely on the natural tendency of molecules to spread out in space.

In the context of the given answer, which indicates that passive transport requires no energy and involves movement down a concentration gradient, it’s key to understand that this mechanism is driven by the inherent kinetic energy of the molecules themselves. Substances such as oxygen, carbon dioxide, and water can passively move across cell membranes by simple diffusion.

Other processes, like facilitated diffusion through proteins, do involve protein channels or carriers but still fall under the broader category of passive transport because they do not require energy input. While facilitated diffusion does permit certain polar or charged substances to cross membranes, it remains a passive process as it also follows the concentration gradient.

On the contrary, systems that require energy input or involve the active movement of particles against their concentration gradient are classified as active transport, which is not applicable to passive transport mechanisms. Thus, understanding these concepts clarifies why the assertion that passive transport occurs without energy input and along the concentration gradient is