Lipids are molecular building blocks that form the most fundamental structures in biology, including the serum lipoproteins, the membranes of cells and cellular organelles and the membranes of enveloped viruses. Many lipids spontaneously form lipid bilayers, the fundamental architectural feature of all biological membranes. A nearly uncountable catalogue of lipid molecules is found in nature, their individual structures imparting a wide variety of properties essential to the structures and functions of biological membranes and lipoproteins in all life forms. Lipids regulate, often in specific ways, the functionality of biological membranes, through the properties of the lipid bilayer and by binding to membrane proteins. Lipids are at the centre of membrane fusion events essential for intracellular transport, endocytosis and viral infection of cells by enveloped viruses. Lipids are the metabolic precursors of some hormones and signalling molecules in cells.

Key Concepts

  • Phospholipids, as well as many other lipids, spontaneously form bilayer structures in water, as governed by the hydrophobic effect.
  • Lipid bilayers provide the fundamental architecture upon which all biological membranes are built.
  • Lipid bilayers in biological membranes are in the liquid crystal state, permitting lateral diffusion of components including proteins and lipids, and internal flexibility essential to membrane enzyme function.
  • Lipid bilayers determine many of the properties of biological membranes including their limited permeability.
  • Thousands of different lipid species are found in biological membranes, but they can be organised into a few major lipid classes.
  • Unsaturation in the hydrocarbon chains of lipid bilayers supports the liquid crystal state of bilayers in biology, essential to the proper functioning of membrane enzymes.
  • Although bilayers are inherently stable, defects in the bilayer structure are required to initiate critical membrane fusion events.
  • Individual membrane lipids play specific biological roles in membrane biology by modulating membrane enzyme activity through binding to membrane proteins.
  • Some lipids are second messengers in intracellular signalling systems.

Keywords: lipid; bilayer; membrane; micelle; hydrophobic effect; liquid crystal; membrane fusion; lipid–protein interactions; phospholipid; sphingomyelin; glycolipid; cholesterol; lipoproteins; membrane dynamics; membrane proteins

Figure 1. The chemical structure of phosphatidylcholine.
Figure 2. The geometry of some phospholipids may be approximated to a cylinder.
Figure 3. The lipid bilayer structure.
Figure 4. The structure of a sphingomyelin (a) and a glycolipid (b).
Figure 5. Diagram showing the stalk structure, which is thought to form the intermediate in a fusion event between two lipid bilayers.
Figure 6. A schematic representation of the enzymatic action of the common phospholipases (A2, C and D).


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Further Reading

Vance DE and Vance J (2008) Biochemistry of Lipids, Lipoproteins and Membranes, 5th edn. Elsevier: Amsterdam.

Yeagle PL (2016) The Membranes of Cells, 3rd edn. Academic Press: San Diego, CA. ISBN: 978‐0‐12‐800047‐2.

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Yeagle, Philip L(Jul 2020) Lipids. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000711.pub4]