# Explain how the structure of the cell surface membrane is related to its function in controlling the movement of substances into and out of the cell.

> IB DP Biology Standard Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks

## Mark scheme (4 marks)

1. The phospholipid bilayer forms a hydrophobic core/barrier that restricts the passage of water-soluble (polar/charged) molecules or ions.
2. Intrinsic/integral (channel or carrier) proteins span the bilayer and allow specific substances to cross that cannot pass through the phospholipid bilayer directly.
3. The fluid/flexible nature of the bilayer (fluid-mosaic model) allows proteins to move laterally, enabling dynamic regulation of transport.
4. Glycoproteins / glycolipids act as receptor or recognition molecules on the outer surface, enabling cell signalling or selective uptake via receptor-mediated processes such as endocytosis.

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