A Level Biology Practice Exam

What is facilitated diffusion?

1. Diffusion that occurs through the cell wall

2. Passive movement of substances without energy input

3. Diffusion across a plasma membrane through protein channels

4. Active transport of substances across a membrane

The correct answer is: Diffusion across a plasma membrane through protein channels

Facilitated diffusion is defined as the process in which substances move across a plasma membrane with the help of protein channels or carriers that facilitate their passage. This mechanism is crucial for transporting molecules that cannot easily diffuse through the lipid bilayer due to their size or polarity, such as glucose and certain ions. During facilitated diffusion, the movement is driven by the concentration gradient of the substance, meaning that molecules move from an area of higher concentration to an area of lower concentration without the expenditure of energy. This contrasts with active transport, which requires energy input to move substances against their concentration gradient. The role of protein channels is essential as they provide a pathway for these substances, effectively increasing the permeability of the cell membrane to specific molecules while still abiding by the principles of passive transport. Therefore, the correct understanding of facilitated diffusion emphasizes its reliance on both the membrane's structural components and the concentration gradient driving the process.