Membrane transport

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Transport across the membrane can be divided into 2 sub-divisions, macrotransfer, which is the bulk movement of molecules across the cell membrane, and microtransfer, which is the movement of 1 or a few molecules across the membrane. Examples of macrotransfer include exocytosis and endocytosis. Microtransfer includes both passive transport and active transport, both use a range of different membrane transport proteins.

Membrane transport occurs through the use of membrane transport proteins. Without these, membranes would only be permeable to some gases and small molecules[1], due to the semi-permeability phospholipid bilayer structure of the membrane. There are two types of Transport proteins, these are Carrier (Transporter) Proteins and Channel Proteins.

Carrier Proteins

The former can be an active or passive process. ATP-powered pumps are an example of Primary active transport as they use energy from ATP hydrolysis in order to move ions or molecules against their concentration gradient. However Channel-mediated facilitated diffusion is where carrier proteins have a role in passive transport, allowing the movement of ions through the membrane down their concentration gradient[2].

Transporters are split into three groups; Uniporters, which transport a single molecule down a concentration gradient, for example, the transport of calcium ions into mitochondria via the Ca2+ uniporters to control the rate of energy production[3], Symporters, which transport a molecule against its concentration gradient through the transport of other molecules down their electrochemical gradient (same direction of travel across membrane), and Antiporters, which also use the transport of other molecules down their electrochemical gradient to transport other molecules (opposite directions of travel of molecules across the membrane)[4]. Symporters and Antiporters are also known as co-transporters due to the aided movement of two or more different molecules simultaneously.

Channel Proteins

Channel can either form aqueous pores in which specific solutes pass through the bilayer[5] or become gated. The pore or ungated channel proteins allow the movement of molecules such as water across cell membrane while gated channels require some signal in order to be activated and opened. Voltage-gated Sodium and Potassium channels are critical for neuronal and muscle communication. The influx of Sodium ions and efflux of Potassium ions generates action potential underlying the delivery of neuronal information and muscle contraction.


  1. Lodish H et al. (2012) Molecular Cell Biology, 6th Edition, New York: WH Freeman. pg.473
  2. Alberts B., Johnson A., Lewis J., Raff M., Roberts K., Walter P. (2008) Molecular Biology of The Cell, 5th Edition, New York: Garland Science.
  3. Kirichok Y; Krapivinisky G; Clapham DE. The mitochondrial calcium uniporter is a highly selective ion channel. Nature 2004; 427: 360-364.
  4. Lodish H et al. (2012) Molecular Cell Biology, 6th Edition, New York: WH Freeman. pg.475
  5. Alberts, B., et al. (2007) Molecular Biology of the Cell. 5th Edition. New York: Garland Science.
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