Active Transport - Physiology
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Introduction
Active transport is reliant on carrier proteins and thus follows the same rules as facilitated diffusion in that they are specific have a maximum rate and are subject to competition. Crucially they transport substances against their concentration gradient and so require energy to work.
Different Types of Active Transport
Primary Active Transport
In primary active transport the main source of energy is ATP. The ATP is hydrolyzed to ADP by the protein releasing energy which powers the transport.
Example - Sodium
This allows the transport of sodium ions from the inside to the outside of the cell. This process requires energy as there is a lower concentration within the cell than outside. However it is essential to maintain the correct internal environment of the cell. The system also has to overcome the electrical gradient involved as the interior of cells is negatively charged thus attracting sodium.
Development of Potential Energy
When sodium is pumped out of cells against a concentration gradient it gathers potential energy. If ion channels are present to allow the sodium to re-enter the cell passively then this energy is released. It is this energy which is used to power secondary Active Transport.
Secondary Active Transport
The energy from this process is derived from the transport of another ion usually Na+ in a direction which reduces its potential energy. The released energy powers the process.
Uses
Sometimes secondary active transport transports things in the same direction as the sodium ions. For example glucose from the intestine and amino acids are transported this way. In other cases sodium flows into a cell to provide energy to remove other ions such as H+ and Ca2+