Cell and membrane transport

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Important molecules, necessary for the functioning of the cell, need to be transported from the external environment into the cytoplasm in the cell. The cell does this through the help of transmembrane proteins that aid the active transport and facilitated diffusion of these molecules in and out of the cell. Proteins within the plasma membrane, which aid the movement of substances across the cell membrane include transport proteins, receptors and enzymes. Transport proteins have a very high specificity for the molecule that they are transporting, for example the sodium potassium pump is only specific to sodium and potassium ions[1].

Also see intracellular Ca2+ transport.

The two main types of proteins that take a role in membrane transport (transport proteins) are carrier proteins and channel proteins, which span the bilayer and are specific to the solute wishing to be transported.

These two varieties have different ways in which they transport molecules. Carrier proteins, for example, deliver by conformational change. This is where the protein changes shape ifollowing the moleccule binding to the carrier protein, in order to release the molecule to the other side of the membrane. On the other hand, channel proteins work by creating hydrophilic pores in the membrane. This forms a passageway for mainly ions and other larger molecules, which operates much more efficiently[2]. In channel proteins, molecules simply diffuse from one side of the protein to the other via the channel protein, sliding down it's concentration via the pore. A third way, in which shipment of molecules across the cell membrane occurs, is by use of ATPases that are enzymes assisting in the hydrolysis of adenosine triphosphate into adenosine diphosphate and an inorganic phosphate molecule. This illustrates the importance of proteins needed to perform processes via transport across the cell membrane[3].


  1. Luckey M. Membrane Structural Biology, 2nd ed. USA: Cambridge University Press; 2008.
  2. Lodish H., Berk A., Zipursky S L., Matsudaira P., Baltimore D., Darnell J. Molecular Cell Biology. 4th Edition, New York: W H. Freeman. 2000.
  3. Alberts B., Johnson A., Lewis J., Raff M., Roberts K., Walter P. Molecular Biology of the Cell. 4th Edition, New York: Garland Science. 2002.

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