Explains how cell membranes resist stretching while bending and shearing easily, using the lipid bilayer as a two-dimensional fluid with a cytoskeleton beneath it. It covers area stretching, shear and bending in a summary table, membrane tension and lysis in red blood cells, viscoelastic recovery, the bilayer couple hypothesis for cell shape, methods such as micropipette aspiration and optical tweezers, and disorders including hereditary spherocytosis and sickle cell disease.
Topic 26 of 30 in the MU Sofia syllabus for Biophysics (Medicine, Year 2). The free AceMed note covers it in 8 sections, about 332 words (3 min of reading), with 15 flashcards.
In this note
Introduction
The membrane as a two-dimensional fluid
Types of deformation
Membrane tension and lysis
Viscoelastic behavior
Red blood cell shape
Methods of investigation
Medical importance
Introduction
Cell membranes must be strong enough to hold a cell together and flexible enough to let it change shape. A red blood cell about 8 µm wide, for example, squeezes through capillaries narrower than itself. Membrane mechanics describes how membranes respond to these forces.
The membrane as a two-dimensional fluid
The lipid bilayer behaves like a thin fluid sheet: lipids move freely within its plane, but the sheet resists being stretched. Because the bilayer alone has no resistance to shear, cells use a cytoskeleton underneath it to keep their shape.
Types of deformation
Membranes bend and shear easily but hardly stretch. This is why a red blood cell can fold through a capillary while its surface area stays almost constant.
Membrane tension and lysis
Stretching raises the membrane tension. A bilayer breaks at a tension of about 10 mN/m. Red blood cells have spare surface area thanks to their biconcave shape, so they can deform a lot without being stretched. When a cell swells in a hypotonic solution, this spare area runs out and the membrane breaks, which is called hemolysis.
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