2.2—Cell membrane structure and models

A cell-surface membrane is a phospholipid bilayer with hydrophilic phosphate heads facing water and hydrophobic fatty-acid tails facing inward. Proteins, cholesterol and carbohydrate chains give the membrane a fluid but selective structure.

Small non-polar molecules can diffuse through the hydrophobic core; ions and larger polar molecules require channel or carrier proteins. Cholesterol helps limit excessive fluidity and permeability.

Oxygen can cross the bilayer down its concentration gradient, whereas a charged sodium ion needs a membrane protein. A receptor protein can also convert an external signal into a cell response.

When explaining movement, identify the substance, its gradient, the membrane barrier and the protein or energy requirement. “Partially permeable” describes selectivity, not an absolute gate for every molecule.

The membrane is not a rigid wall and not an open sieve. Protein number, temperature and lipid composition can change the observed rate.