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, water soluble molecules  [1].  There are two types of Transport proteins, these are Carrier (Transporter) Proteins and Channel Proteins. The former is an active process as it requires ATP. ATP-powered pumps use energy from ATP hydrolysis in order to move ions or molecules against their concentration gradient. However Channel Proteins are passive as they allow 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, 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) [3]. Symporters and Antiporters are also known as co-transporters due to the aided movement of two or more differing molecules simultaneously. 

Transporters work by undergoing a conformational change when a solute binds, allowing them to pass through the membrane. This means that this form of transport is highly interactive. Contrastingly, channels interact far less with the solutes; they form aqueous pores in which specific solutes pass through the bilayer. [4]

References

  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. Lodish H et al. (2012) Molecular Cell Biology, 6th Edition, New York: WH Freeman. pg.475
  4. Alberts, B., et al. (2007) Molecular Biology of the Cell. 5th Edition. New York:Garland Science.

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