Membrane transport

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 <a href="Proteins">proteins</a>. Without these, membranes would only be permeable to some gases and small, <a href="Water">water</a> soluble molecules  Lodish H et al. (2012) Molecular Cell Biology, 6th Edition, New York: WH Freeman. pg.473.  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. <a href="ATP-powered pumps">ATP-powered pumps</a> use energy from <a href="ATP">ATP</a> <a href="Hydrolysis">hydrolysis</a> in order to move <a href="Ions">ions</a> or <a href="Molecules">molecules</a> against their concentration gradient. However Channel Proteins are passive as they allow the movement of ions through the membrane down their concentration gradientAlberts B., Johnson A., Lewis J., Raff M., Roberts K., Walter P. (2008) Molecular Biology of The Cell, 5th Edition, New York: Garland Science.

Transporters are split into three groups; <a href="Uniporters">Uniporters</a>, which transport a single molecule down a concentration gradient, <a href="Symporters">Symporters</a>, 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 <a href="Antiporters">Antiporters</a>, 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) Lodish H et al. (2012) Molecular Cell Biology, 6th Edition, New York: WH Freeman. pg.475.