Sugar molecules move into a cell against a concentration gradient by the process of

• A. Passive diffusion
• B. Facilitated diffusion
• C. Osmosis
• D. Active transport

Answer: (D)

Moving a substance against its concentration gradient requires energy, so the cell uses active transport to bring glucose into the cell. Sugar molecules are large and polar, so they can’t cross the lipid bilayer by simple diffusion. They need a transport protein that uses energy to move them uphill, either directly from ATP or by exploiting another ion’s gradient. That energy-dependent mechanism fits active transport.

Passive diffusion would let sugars move from high to low concentration without energy. Facilitated diffusion also uses a transporter but still moves substances down their gradient, not against it. Osmosis involves water moving across the membrane, not sugars.