Advanced Placement (AP) Biology Practice Exam

What type of transport describes the movement of molecules from a high concentration to a low concentration?

1. Active transport

2. Facilitated diffusion

3. Diffusion, passive transport

4. Bulk transport

The correct answer is: Diffusion, passive transport

The movement of molecules from a region of higher concentration to a region of lower concentration is known as diffusion, which is a type of passive transport. This process does not require cellular energy (ATP) because it relies on the natural tendency of molecules to spread out evenly in a given space until equilibrium is reached. This concept is essential in biological systems, where it plays a pivotal role in processes like gas exchange in the lungs and the transport of nutrients and waste products across cell membranes. While facilitated diffusion also describes the movement of molecules down their concentration gradient, it specifically involves the assistance of transport proteins to help larger or polar molecules cross the membrane. However, the broad definition of diffusion encompasses both simple diffusion (which does not require assistance) and facilitated diffusion. Hence, the answer emphasizes the fundamental concept of movement along the concentration gradient as passive transport, distinguishing it from processes that require energy, such as active transport or bulk transport.