Active transport is when molecules move across a cell membrane from a lower concentration to a higher concentration. This takes energy, often from adenosine triphosphate (ATP). Active transport is done so cells get what they need, such as ions, glucose and amino acids.
In general, molecules move from an area of higher concentration to an area of lower concentration. To get molecules into the cell against the concentration gradient, work must be done. The work is done in special proteins which act as ports in the cell membrane. The imports must come through the ports: they cannot get through the bilipid layer of the cell membrane.
Types of portsEdit
There are three main types of protein ports in cell membranes: 
- Uniporters: they use energy from ATP to pull molecules in.
- Symporters: they use the movement in of one molecule to pull in another molecule against gradient.
- Antiporters: one substance moves against its gradient, using energy from the second substance (mostly Na+, K+ or H+) moving down its gradient.
- "The importance of homeostasis". Science. BBC. Retrieved 15 December 2014.
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- Bruce Alberts; et al, Molecular Biology of the Cell, 5th ed. (New York: Garland Science, 2008), pp. 619–631