Active transport

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Active transport involves the use of proteins that don't just passively facilitate the transport of substances across the cell membrane, but require the use of cellular energy(usually ATP) to actively pump substances into or out of the cell. The animation represents the action of a sodium-potassium pump found in the cell membrane of neurons. This protein pumps sodium ions(red squares) from the inside to the outside of the neuron and pumps potassium ions(green squares)in the opposite direction. Notice the cell must use ATP(purple) for this process. As the sodium fits onto a site on the protein, a phosphate is transferred to the protein providing energy to kick the sodium ion to the outside and the potassium ion to the inside. This process sets up a high concentration of sodium ions outside the cell and a high concentration of potassim ions inside the cell. This concentration difference across the membrane is important for the generation of thenerve impulses by which neurons transmit information from on end of the neuron to the other. So even if you're zoned out in front of the tube your neurons are actively working to maintain this concentration difference...just in case they nave to send any information. 

Active transport is used to: 

1. Generate charge gradients. For example in the mitochondrion, hydrogen ion pumps pump hydrogen ions into the intermembrane space of the organelle as part of making ATP. 

2. Concentrate ions, minerals and nutrients inside the cell that are in low concentration outside. 

3. Keep unwanted ions or other molecules out of the cell that are able to diffuse through the cell membrane. 

In all these cases the key is that active transport uses energy to send substances against the direction they would travel by simple diffusion: that is from a region of low concentration to a region of high concentration. 

Created7/9/99 Revised 01/04/00