Structure of a lipid bilayer
A lipid bilayer is a membrane mainly composed of lipid molecules, usually a phospholipids (See Figure 1). Phospholipids are formed from 3 components :- (1) 2 fatty acids tails – these are hydrophobic; (2) a negatively-charged hydrophilic phosphate group; and (3) a glycerol backbone.
The bilayer structure is favourable energetically because the hydrophobic fatty acid tails cluster together to exclude water, where the hydrophilic head groups (consisting of the phosphate group and glycerol backbone) are on the two surfaces of the membrane, which allows them to have contact with the surrounding water. This structure also allows the membrane to be polarised, which plays an important role in transport across the membrane. When a molecule has both a hydrophilic and hydrophobic nature, it is said to be amphiphatic. (1)
In addition to phospholipids, the bilayer membrane consists of a number of different other molecules:-
• Cholesterol – aids in fluidity, as it disrupts the orderly packing of the phospholipids.
• Channel proteins, which allow specific molecules to pass through them.
• Carbohydrates, which are found either attached to a protein (forming a glycoprotein) or a lipid (forming a glycolipid). Glycolipids on the cell surface act as markers for cellular recognition, whilst glycoproteins are important in immune cell recognition. (1)
What kind of molecules can and cannot pass easily across a lipid bilayer?
Figure 1 clearly shows the different molecules that can and cannot transport easily across the plasma membrane. All lipid soluble membranes (hydrophobic molecules and small uncharged molecules) can pass easily through the membrane in both directions. The movement of these molecules is dependent only on their concentrations inside and outside the cell, and these molecules are transported due to the effects of concentration gradients
References: (3) Baynes JW, Dominiczak MH: Medical Biochemistry; 2nd Edition, 2005, Elsevier Mosby: PA, USA. (4) Elliot WH, Elliot DC: Biochemistry and Molecular Biology; Third Edition, 2005, Oxford University Press: New York, USA (5) Meisenberg G, Simmons WH: Principals of Medical Biochemistry; 2nd Edition, 2006, Mosby Elsevier: PA, USA. (9) Devlin TM; Editor: Textbook of Biochemistry :with Clinical Correlations; 6th Edition, 2006, Wiley-Liss: Hoboken, N.J., USA. (2) Karp G: Cell and Molecular Biology; 3rd Edition, 2005: Wiley, NY USA. (3) Elliot WH, Elliot DC: Biochemistry and Molecular Biology; Third Edition, 2005, Oxford University Press: New York, USA, Page 263. (6) Alberts et al: Molecular Biology of the Cell; 4th Edition, 2002, Garland Science: New York, USA.