What is the primary characteristic of passive transport?

• A. Moves molecules against their concentration gradient
• B. Requires energy to function
• C. Moves molecules down their concentration gradient
• D. Transport is limited to small non-polar molecules

Answer: (C)

The primary characteristic of passive transport is that it moves molecules down their concentration gradient. This means that substances naturally flow from areas of higher concentration to areas of lower concentration without the need for energy input. This process occurs until equilibrium is reached, where the concentration of the molecules is equal on both sides of the membrane.

Passive transport is fundamental for various physiological processes, such as the absorption of nutrients and the removal of waste products. Molecules like oxygen, carbon dioxide, and certain small non-polar molecules can diffuse freely across cell membranes due to this gradient-driven mechanism.

In contrast, active transport involves moving molecules against their concentration gradient and requires energy, typically in the form of ATP. Additionally, while passive transport can indeed occur with small non-polar molecules, it is not limited to just those types; larger or polar molecules may move through specific types of facilitated diffusion channels. Thus, the essence of passive transport lies in its reliance on existing concentration gradients rather than energy expenditure for movement.