Which is an example of facilitated diffusion?

• A. Fat soluble molecules passing through the plasma membrane with the concentration gradient
• B. Water traveling across the plasma membrane with the help of channel proteins called aquaporins
• C. K+ being moved across the plasma membrane against its concentration gradient

Answer: (B)

Verified step by step guidance

1. Understand the concept of facilitated diffusion: Facilitated diffusion is a type of passive transport that allows substances to cross membranes with the assistance of special transport proteins. It does not require energy and moves substances down their concentration gradient.
2. Identify the role of channel proteins: In facilitated diffusion, channel proteins or carrier proteins help specific molecules or ions to pass through the plasma membrane. These proteins provide a pathway for the molecules to move across the membrane.
3. Consider the movement of water: Water molecules can move across the plasma membrane through specialized channel proteins known as aquaporins. This process is a classic example of facilitated diffusion, as it involves the passive movement of water molecules down their concentration gradient.
4. Differentiate from active transport: Active transport, unlike facilitated diffusion, requires energy to move substances against their concentration gradient. For example, the movement of K+ ions against their concentration gradient would require energy, typically in the form of ATP.
5. Apply the concept to the given examples: Evaluate each example to determine if it involves passive movement with the help of proteins. Water traveling across the plasma membrane with the help of aquaporins fits the criteria for facilitated diffusion, as it involves passive transport with the aid of channel proteins.