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Explanation:
Definition
Active transport is the process of transferring substances into, out of, and between cells, using energy. In some cases, the movement of substances can be accomplished by passive transport, which uses no energy. However, the cell often needs to transport materials against their concentration gradient. In these cases, active transport is required.
Active transport requires energy to move substances from a low concentration of that substance to a high concentration of that substance, in contrast with the process of osmosis. Active transport is most commonly accomplished by a transport protein that undergoes a change in shape when it binds with the cell’s “fuel,” a molecule called adenosine triphosphate (ATP).
For example, one type of active transport channel in the cell membrane will bind to the molecule it is supposed to transport – such as a sodium ion – and hold onto it until a molecule of ATP comes along and binds to the protein. The energy stored in ATP then allows the channel to change shape, spitting the sodium ion out on the opposite side of the cell membrane. This type of active transport directly uses ATP and is called “primary” active transport.
Another type of active transport is “secondary” active transport. In this type of active transport, the protein pump does not use ATP itself, but the cell must utilize ATP in order to keep it functioning. This will be explained in more depth in the section on Symport Pumps below.
Lastly, active transport can be accomplished through processes called endocytosis and exocytosis. In exocytosis, a cell moves something outside of itself in large quantities by wrapping it in a membrane called a vesicle and “spitting out” the vesicle. In endocytosis, a cell “eats” something by wrapping and re-forming its membrane around the substance or item.
Each type of active transport is explained in more detail below.
Types of Active Transport
Antiport Pumps
Antiport pumps as an example of active transport
Active transport by antiport pumps
Antiport pumps are a type of transmembrane co-transporter protein. They pump one substance in one direction, while transporting another substance in the opposite direction. These pumps are extremely efficient because many of them can use one ATP molecule to fuel these two different tasks.
One important type of antiport pump is the sodium-potassium pump, which is discussed in more detail under “Examples of Active Transport.”
Symport Pumps
Symport pumps take advantage of diffusion gradients to move substances. Diffusion gradients are differences in concentration that cause substances to naturally move from areas of high to low concentration.
In the case of a symport pump, a substance that “wants” to move from an area of high concentration to low concentration down its concentration gradient is used to “carry” another substance against its concentration gradient.
One example of a symport pump – that of the sodium-glucose transport protein – is discussed below under “Examples of Active Transport.”
Sympoter pump as an example of active transport
Active transport by symporter pumps
Endocytosis
In the third type of active transport, large items, or large amounts of extracellular fluid, may be taken into a cell through the process of endocytosis.
