# Explain how membrane proteins facilitate the transport of glucose into a cell that lacks glucose transporters.

> IB DP Biology Higher Level (2023 syllabus) — B2.1 Membranes and membrane transport · Explain · 4 marks

> Glucose is a polar, hydrophilic molecule that cannot diffuse freely across the phospholipid bilayer. Some cells rely entirely on protein-mediated transport to obtain glucose from the extracellular fluid.

## Mark scheme (4 marks)

1. Glucose cannot cross the hydrophobic core of the phospholipid bilayer by simple diffusion because it is polar/hydrophilic.
2. Channel proteins or carrier proteins (intrinsic/integral membrane proteins) provide a hydrophilic pathway through the membrane.
3. Facilitated diffusion via carrier proteins involves a conformational change in the protein that transfers glucose from one side of the membrane to the other.
4. Net movement of glucose occurs down its concentration gradient (from high to low concentration) without the expenditure of ATP, until equilibrium is reached.

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