Cell Membrane Features

Abstract

The biological membranes provide much of the functionality of the organisms of which they are a part. From intracellular organelles to extracellular cell–cell interactions, membranes provide the structures necessary for biological function and organisation. Many structural and functional features are common among membranes in archaea, bacteria, eukaryotic cells and enveloped viruses. The fundamental architecture of all biological membranes is built upon the lipid bilayer. Thus, all membranes have properties in common. Membrane proteins confer a myriad of specific functions expressed by cell membranes. Plasma membranes determine the cell boundary and the interactions of the cell with its environment. Intracellular membranes compartmentalise cells into different functional units with differing internal compositions. The composition of the compartments is maintained by essential transport functions in concert with permeability control. Signal transduction is enabled by membranes. Membrane fusion provides a mechanism for intracellular vesicular transport and enveloped virus entry into cells.

Key Concepts

  • The hydrophobic effect, which governs the behaviour of hydrocarbons in water, controls the structures formed by membrane lipids and membrane proteins.
  • Many properties of membranes are conferred by lipid bilayers that form the basic structure for all biological membranes.
  • Integral membrane proteins have a portion of their mass buried in the lipid bilayer.
  • Transmembrane proteins have specialised organisations of amino acids in their primary sequence such that linear sequences of hydrophobic amino acids enable the protein to span the lipid bilayer.
  • Peripheral membrane proteins are much like soluble proteins in structure, but bind to membranes.
  • Cellular functions, including compartmentalisation, transport, intracellular trafficking, enveloped viral entry, signal transduction, endocytosis, phagocytosis, action potentials and RNA processing, are provided by membranes.
  • Specialised cell membranes have specialised functions for cells.

Keywords: membrane protein; lipid bilayer; membrane transport; signal transduction; membrane fusion; plasma membrane; organelle membranes

Figure 1. Schematic representation of a lipid bilayer. The circles represent the polar headgroups of the lipids, and the lines connected to the circle represent the hydrophobic hydrocarbon chains of the lipids. These amphipathic molecules are dual nature: one end is hydrophilic and the other end is hydrophobic. They organise so as to limit the exposure of the hydrophobic portions to the aqueous phase that is found on both sides of the membrane.
Figure 2. Schematic representation of the incorporation of a transmembrane protein into a lipid bilayer. The cylinders represent hydrophobic transmembrane α helices, the dark lines are loops of the polypeptide chain that connect the helices, and the lipids are represented as in Figure.
Figure 3. Schematic representation of the classes of membrane proteins. The darker shaded regions are the hydrophobic portions of these membrane proteins, and the clear horizontal box represents the lipid bilayer. The transmembrane proteins are exposed on both sides of the membrane; anchored membrane proteins penetrate only one half of the lipid bilayer; associated membrane proteins bind to transmembrane proteins as part of a complex; and skeletal membrane proteins form a network underneath the plasma membrane that can give shape to a cell.
Figure 4. Three‐dimensional atomic structure of an archaeal oligosaccharyltransferase, a transmembrane protein. The top portion is a mixture of α‐helix (purple cylinders) and β‐sheet (yellow flat arrows) and is located outside the membrane. The bottom portion is a bundle of hydrophobic α‐helices that is buried in the lipid bilayer (Matsumoto ., ).
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Further Reading

Alberts B, Johnson A, Lewis J, et al. (2007) The Molecular Biology of the Cell, 5th edn. New York, NY: Garland Press.

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

Yeagle PL (2012) The Structure of Biological Membranes, 3rd edn. Boca Raton, FL: CRC Press.

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Yeagle, Philip L(Mar 2015) Cell Membrane Features. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001261.pub3]