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Active transmembrane movement processes require the cell to actively expend {{energy}} – which is most often derived from the phosphate bonds of the molecule {{ATP}} – to move substances.
Active transport systems use Integral membrane proteins to transport substances as is also needed for {{facilitated diffusion}}. However, in active transport, transport is {{against}} the concentration gradient which is in contrast to facilitated diffusion where the transport is {{down}} the concentration gradient.
Active transport consists of two sub-types.
In {{Primary}} Active Transport, the expenditure is directly linked to transport a single specific molecule (in which case the transport system is called a {{uniporter}})) or two molecules (in which case the transport system is a {{dual porter}}). The main reason for this form of transport is that the molecule has (or molecules have) to be transported against the {{concentration}} gradient and, in the case of ions, sometimes against the {{electrical}} gradient too.
The three classes of this form of active transport are the {{ion pumps}} that only transport ions; the {{ABC system}} that has a sequence of 3 proteins involved in the transport system; and the {{Group Translocation}} system that has a sequence of molecules involved in the transport.
The system that is used to transport only ions in turn consists of three classes, called {{P, V, and F}} pumps that allow require the breakdown of {{ATP}} for the transport of ions. All such pumps have {{two}} possible states where they are optimally conformed to transport one ion or transport the other. Of the three classes, the ones that transport only {{protons}} are the {{V, F not P}} pumps.
The other form of active transport, {{Secondary}} Active Transport, is a little more complex. Here, expenditure of energy is first used to transport a specific molecule (as above) to create a {{concentration}} gradient for that molecule. You then allow entry of that molecule using a {{facilitated diffusion}} system, generally because this molecule is an {{ion}} and charged particles don't traverse the membrane well. You couple the movement of the first molecule down it's concentration gradient together with movement of another substance against it's concentration gradient.
In cases where of dual transport, the transporter can be one that moves both molecules in the same direction in which case it is called a {{symporter}}. In other cases, it may move the two transported molecules in opposite directions, in which case it is an {{antiporter}}.