IB Biology Topic B2.1.1: Lipid Bilayers as the Basis of Cell Membranes Notes

Phospholipid Bilayers Form the Basis of All Cell Membranes

1. The lipid bilayer forms the foundation of every cell membrane.
2. It defines the boundary between a cell and its surroundings.
3. It also separates internal compartments inside eukaryotic cells.

Phospholipids Are Amphipathic, With a Water-Loving Head and Water-Hating Tails

1. Hydrophilic head: the polar phosphate group forms the head, which is attracted to water.
2. Hydrophobic tails: two nonpolar hydrocarbon chains form the tails, which avoid water.
3. Having both a water-loving and a water-hating part makes the molecule amphipathic.

Hydrophobic Interactions Make Phospholipids Self-Assemble Into a Bilayer

1. In water, phospholipids spontaneously arrange themselves into a bilayer.
2. The hydrophobic tails cluster together on the inside, away from water.
   1. This clustering lowers the free energy of the system, so it happens on its own.
3. The hydrophilic heads face outward and form hydrogen bonds with the water on both sides.

The Bilayer Is Stable and Self-Sealing

1. The result is a stable, self-sealing bilayer.
2. If it is disrupted, the bilayer reassembles on its own, which keeps the membrane intact.

The hydrophobic core that forms inside the bilayer also acts as a barrier to many substances, which is covered in the next subtopic.

Bilayers Also Contain Other Amphipathic Lipids Besides Phospholipids

1. Phospholipids are the main component, but other amphipathic lipids are also present in the bilayer.
   1. Cholesterol sits between phospholipids in animal cell membranes.
   2. Glycolipids have carbohydrate groups attached and sit on the membrane surface.