Which of the following is an example of facilitated diffusion across a cell membrane?

• A. Movement of glucose into a cell via a carrier protein
• B. Sodium ions being pumped out of the cell using ATP
• C. Water moving through the membrane by osmosis
• D. Oxygen passing directly through the lipid bilayer

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the concept of facilitated diffusion. Facilitated diffusion is a type of passive transport where molecules move across the cell membrane with the help of specific carrier or channel proteins, without requiring energy (ATP). It is used for molecules that cannot pass directly through the lipid bilayer due to their size, polarity, or charge.
2. Step 2: Analyze the first option: 'Movement of glucose into a cell via a carrier protein.' Glucose is a large polar molecule that cannot pass directly through the lipid bilayer. It requires a carrier protein to facilitate its movement across the membrane, making this an example of facilitated diffusion.
3. Step 3: Analyze the second option: 'Sodium ions being pumped out of the cell using ATP.' This describes active transport, not facilitated diffusion, because it requires energy (ATP) to move sodium ions against their concentration gradient.
4. Step 4: Analyze the third option: 'Water moving through the membrane by osmosis.' Osmosis is the passive movement of water molecules across a membrane, but it does not involve carrier or channel proteins, so it is not facilitated diffusion.
5. Step 5: Analyze the fourth option: 'Oxygen passing directly through the lipid bilayer.' Oxygen is a small, nonpolar molecule that can diffuse directly through the lipid bilayer without the need for carrier proteins, so this is an example of simple diffusion, not facilitated diffusion.