Explain how the structure of a membrane transport protein enables it to move specific substances across a phospholipid bilayer.

IB DP Biology Standard Level (2023 syllabus) — B1.2 Proteins · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The protein has a specific tertiary shape determined by its amino‑acid sequence, giving it a channel or binding site that is complementary in shape and charge to one particular substrate.
The channel or pore is lined with hydrophilic residues, allowing polar or charged molecules to pass through.
The hydrophobic side chains of the protein face the bilayer core, anchoring the protein in the membrane.

Examiner tips

Use the word ‘specific’ to show the protein recognises only one substrate.
Mention both the hydrophilic channel and the hydrophobic anchoring residues.
Show the relationship between structure and function in a single sentence.
Use the exact terminology: tertiary structure, channel/pore, hydrophobic core, hydrophilic lining.

Common mistakes

Confusing the protein’s overall shape with the shape of the substrate; the answer must link the protein’s shape to substrate specificity.
Omitting the hydrophobic anchoring residues or the hydrophilic lining of the channel.
Using vague terms like ‘structure’ without specifying tertiary shape or channel/pore.

Mark scheme (4 marks)

The protein has a specific tertiary (three-dimensional) shape / conformation determined by its amino acid sequence
The protein contains a channel or pore / binding site that is complementary in shape (and charge) to one specific substance / substrate
Hydrophobic R-groups / amino acids face outward / are embedded in the hydrophobic core of the bilayer, anchoring the protein in the membrane
Hydrophilic R-groups line the channel / pore, allowing polar or charged substances to pass through the hydrophobic bilayer that they could not otherwise cross

Key terms in this question

phospholipid bilayer