Phospholipid bilayers

How can the fluid-mosaic model be described?

• Fluid because

• Phospholipids in bilateral can move within the plane of the membrane

• Mosaic because

• Various different kinds of components are embedded within the phospholipid bilayer

• Including transmembrane proteins such as channel & carrier proteins

• Glycolipid

• Glycoprotein receptors

• Cholesterol

Why are phospholipid bilayers effective barriers

• phospholipid fatty acids are hydrophobic

• they prevent movement of charged, larger or polar substances

• Because these substances can’t interact with the hydrophobic fatty acid chains

• The phospholipids are also very close together, so large molecules can’t freely diffuse across the membrane

List evidence for the fluid-mosaic model of biological membranes

Two layers of phospholipids -

• area of total lipid from RBC was double that of RBC cell membrane

Protein in cell membrane-

• electron micrographs show electron dense layer in membrane

Proteins are distinct & independent structures that span the membrane-

• Freeze fracture electron micrographs show distinct globular structures that span membrane

• These disappear when protein is removed from cell before procedure

Phospholipids are mobile & not in static positions

• Differently labelled proteins from 2 diff cells end up mixed together when the cells are fused (showing they can move)

How do phospholipids form bilayers?

• Membranes are made of 2 layers of phospholipids

• That have a hydrophilic phosphate head & hydrophobic fatty acid tails

• The fatty acids face away from water & towards each other

• While the heads face towards water on both sides of the bilayer