Phospholipids are the building blocks of cell membranes. These molecules have a water-loving head and two water-hating tails, giving them unique properties. This structure allows them to form bilayers, creating a barrier between the inside and outside of cells.

Phospholipids come in different types, like glycerophospholipids and sphingomyelins. Their composition affects membrane fluidity and function. Understanding phospholipids is key to grasping how cells maintain their structure and control what goes in and out.

Phospholipid Structure and Function

Structure of phospholipids

• Phospholipids are amphipathic molecules having a hydrophilic head and hydrophobic tails
  • Hydrophilic head contains a phosphate group and a variable polar molecule (choline, ethanolamine, serine, or inositol)
  • Hydrophobic tails composed of two fatty acid chains, can be saturated or unsaturated (palmitic acid, oleic acid)
• Glycerol backbone serves as the central structure of phospholipids
  • Glycerol is a three-carbon molecule with hydroxyl groups at positions 1, 2, and 3
  • Fatty acids esterified to the hydroxyl groups at positions 1 and 2 of the glycerol backbone
  • Phosphate group esterified to the hydroxyl group at position 3 of the glycerol backbone

Glycerophospholipids vs sphingomyelins

• Glycerophospholipids most common type of phospholipids
  • Contain a glycerol backbone with two fatty acid chains and a phosphate group
  • Examples include phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI)
• Sphingomyelins type of phospholipid containing a sphingosine backbone instead of glycerol
  • Sphingosine is an 18-carbon amino alcohol with a long, unsaturated hydrocarbon chain
  • Single fatty acid attached to the amino group of sphingosine via an amide bond (ceramide)
  • Phosphocholine group esterified to the terminal hydroxyl group of sphingosine

Phospholipids in cell membranes

• Phospholipids spontaneously form bilayers in aqueous environments due to their amphipathic nature
  • Hydrophilic heads face the aqueous environment on both sides of the bilayer
  • Hydrophobic tails face the interior of the bilayer, away from water (hydrophobic effect)
• Cell membranes composed of a phospholipid bilayer with embedded proteins and other lipids (cholesterol)
  • Phospholipid bilayer provides a barrier between the intracellular and extracellular environments
  • Hydrophobic core of the bilayer prevents passage of polar and charged molecules (ions, most proteins)
• Phospholipid bilayer is selectively permeable, allowing passage of small, nonpolar molecules (oxygen, carbon dioxide, steroids)
  • Larger or polar molecules require specialized transport proteins to cross the membrane (ion channels, carrier proteins)
• Fluidity of the phospholipid bilayer influenced by composition of fatty acids and presence of other lipids
  1. Unsaturated fatty acids and shorter chain lengths increase membrane fluidity (cis double bonds create kinks)
  2. Cholesterol modulates membrane fluidity by intercalating between phospholipids and restricting their motion (liquid-ordered state)

Membrane dynamics and organization

• Phospholipids can form various structures in aqueous environments (micelles, bilayers)
• Lipid rafts are specialized membrane microdomains enriched in cholesterol and sphingolipids
• Membrane curvature is influenced by the shape and distribution of phospholipids
• Phospholipid asymmetry in membranes is maintained by specialized enzymes that prevent spontaneous flip-flop between leaflets