Which of the following best explains why a glucose transporter is required to move glucose into the cell?

• A. Glucose is a large, polar molecule that cannot easily cross the lipid bilayer without assistance.
• B. Glucose is actively pumped out of the cell by the sodium-potassium pump.
• C. Glucose is a small, nonpolar molecule that diffuses freely through the membrane.
• D. Glucose dissolves in the lipid bilayer and enters the cell by simple diffusion.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the structure of the cell membrane. The cell membrane is composed of a phospholipid bilayer, which has hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads. This structure makes it selectively permeable to certain molecules.
2. Step 2: Analyze the properties of glucose. Glucose is a large, polar molecule, meaning it has an uneven distribution of electrical charge. Polar molecules generally cannot pass through the hydrophobic core of the lipid bilayer without assistance.
3. Step 3: Recognize the role of transport proteins. Transport proteins, such as glucose transporters, are embedded in the cell membrane and facilitate the movement of molecules that cannot diffuse freely through the lipid bilayer. These proteins provide a pathway for glucose to enter the cell.
4. Step 4: Eliminate incorrect options. For example, glucose is not actively pumped out of the cell by the sodium-potassium pump, as this pump specifically moves sodium and potassium ions. Additionally, glucose does not dissolve in the lipid bilayer or enter by simple diffusion because it is polar and large.
5. Step 5: Conclude that a glucose transporter is required because glucose is a large, polar molecule that cannot easily cross the lipid bilayer without assistance. This explains why facilitated diffusion is necessary for glucose transport into the cell.