B2.1.1 Lipid bilayers as the basis of cell membranes Notes

Lipid Bilayers Are The Basis of Cell Membranes

1. The lipid bilayer forms the foundation of all cell membranes, defining the boundary between a cell and its environment.
2. This versatile structure controls the passage of substances into and out of the cell while maintaining compartmentalization within eukaryotic cells.

The Essential Structure of the Lipid Bilayer

1. The lipid bilayer consists of two layers of phospholipids and other amphipathic molecules.
2. These specialized molecules self-assemble into a bilayer due to their unique chemical properties:
   1. Hydrophilic heads (polar regions) face outward toward water.
   2. Hydrophobic tails (non-polar regions) cluster inward, away from water.

Structure of a Phospholipid

1. Hydrophilic Head: The polar phosphate group forms the head, which readily interacts with water.
2. Hydrophobic Tails: The nonpolar hydrocarbon chains make up the tails which avoid water.
3. This amphipathic nature is key to the behavior of phospholipids in water.
4. When introduced into an aqueous environment, they spontaneously arrange themselves to shield their hydrophobic tails while exposing their hydrophilic heads to water.

Formation of the Lipid Bilayer

How Does This Work?

1. Hydrophobic Interactions: The hydrophobic tails cluster together, avoiding contact with water and reducing the system's free energy.
2. Hydrophilic Interactions: The hydrophilic heads form hydrogen bonds with water molecules, stabilizing the bilayer structure.
3. The result is a stable, self-sealing bilayer.
4. If disrupted, the bilayer will spontaneously reassemble, a property critical for maintaining the integrity of cell membranes.

Why Lipid Bilayers Are Perfect Barriers

1. Selective Permeability

1. The bilayer’s hydrophobic core makes it selectively permeable:
2. Small Nonpolar Molecules: These can diffuse freely through the bilayer (e.g., oxygen and carbon dioxide).
3. Large or Polar Molecules: These are blocked by the hydrophobic core and require specialized transport proteins to cross.

2. Fluidity and Flexibility

1. The lipid bilayer is fluid, allowing phospholipids to move laterally within the layer.
2. This fluidity enables the membrane to adapt its shape, repair itself when damaged, and facilitate processes like cell division and vesicle formation.

The Role of Other Amphipathic Lipids

1. While phospholipids are the primary components of the bilayer, other amphipathic lipids contribute to its function:
   1. Cholesterol: Found in animal cell membranes, cholesterol fits between phospholipids, modulating membrane fluidity and stability.
   2. Glycolipids: These lipids, with attached carbohydrate groups, play roles in cell recognition and signaling.