Explain the difference between passive and active transport.

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Passive and active transport are both biological terms and have to do with the movement of chemical species from one side of a semi-permeable membrane to another.  Passive transport is a type of transport that requires no external energy.  It is driven purely by natural, thermodynamic forces.  Chemicals naturally travel...

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Passive and active transport are both biological terms and have to do with the movement of chemical species from one side of a semi-permeable membrane to another.  Passive transport is a type of transport that requires no external energy.  It is driven purely by natural, thermodynamic forces.  Chemicals naturally travel from areas of high concentration to areas of low concentration.  So in passive transport, if a chemical species is built up into a high concentration on one side of a membrane, it will tend to pass through the membrane to the less concentrated side to even out the concentration on both sides of the membrane.  Types of passive transport include diffusion (the process described above) and osmosis (the movement of water molecules across a membrane).

Active transport, on the other hand, is the non-natural movement of chemicals along a concentration gradient.  In other words, it is the movement of chemicals from areas of low concentration to high concentration.  Since this goes against the laws of thermodynamics, external energy must be supplied in order to achieve active transport.  The most common energy source for active transport is ATP.  An example of active transport are ion channels that allow particular ions to pass through a membrane to build up in concentration, often for a particular future biochemical purpose.  The ion channel proteins consume ATP in order to move the ions through the channel.

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