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.
Process of Active Transport
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 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 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.”
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.
In endocytosis, the cell uses proteins in its membrane to fold the membrane into the shape of a pocket. This pocket forms around the contents to be taken into the cell. The pocket grows until it is pinched off, re-forming the cell membrane around it and trapping the pocket and its contents inside the cell. These membrane pockets, which carry materials inside of or between cells, are called “vesicles.”
The folding of the cell membrane is accomplished in a mechanism similar to the antiport transport of potassium and sodium ions. Molecules of ATP bind to proteins in the cell membrane, causing them to change their shape. The conformational changes of many proteins together change the shape of the cell membrane until a vesicle is created.
In receptor-mediated endocytosis, a cell’s receptor may recognize a specific molecule that the cell “wants” to take in, and form a vesicle around the area where it recognizes the molecule. In other types of endocytosis, the cell relies on other cues to recognize and engulf a particular molecule.
Exocytosis is the opposite of endocytosis. In exocytosis, the cell creates a vesicle to enclose something inside the cell, for the purpose of moving it outside of the cell, across the membrane. This most commonly occurs when a cell wants to “export” an important product, such as cells that synthesize and export enzymes and hormones that are needed throughout the body.
In eukaryotic cells, protein products are made in the endoplasmic reticulum. They are often packaged by the endoplasmic reticulum into vesicles and sent to the Golgi apparatus.
The Golgi apparatus can be thought of as a cellular “post office.” It receives packages from the endoplasmic reticulum, processes them, and “addresses” them by adding molecules that will be recognized by receptors on the membrane of the cell intended to receive the product.
The Golgi apparatus then packages the finished “addressed” products into vesicles of its own. These vesicles move toward the cell membrane, dock, and fuse with it, allowing the vesicle membrane to become part of the cell membrane. The vesicle’s contents are then spilled into the extracellular space.
Examples of Active Transport
Sodium Potassium Pump
One of the most important active transport proteins in animals is the sodium-potassium pump. As animals, our nervous system functions by maintaining a difference in ion concentrations between the inside and outside of nerve cells.
It is this gradient that allows our nerve cells to fire, creating muscle contractions, sensations, and even thoughts. Even our heart muscle relies upon these ion gradients to contract!
The ability of the sodium-potassium pump to transport potassium into cells while transporting sodium out of cells is so important that some estimates suggest we spend a total of 20-25% of all the energy we get from food just performing this one task! In neurons, a great majority of the cell’s energy is used to power sodium-potassium pumps.
This might sound like a lot of energy, but it is an important and monumental task; it is this pump that allows us to move, think, pump blood throughout our bodies, and perceive the world around us.
Sodium-Glucose Transport Protein
A famous example of a symport pump is that of the sodium-glucose transport protein. This protein binds to two sodium ions, which “want” to move into the cell, and one glucose molecule, which “wants” to stay outside of the cell. It represents an important method of sugar transport in the body, required to provide energy for cellular respiration.
The natural diffusion of sodium ions inside the cell facilitates the movement of glucose into the cell. Glucose can be carried into the cell with sodium without the transport protein expending ATP. However, ATP must be utilized by the sodium-potassium pump elsewhere in the cell to keep up the sodium gradient in place. Without the sodium gradient, sodium-glucose transport could not function.
White Blood Cells Destroying Pathogens
An important example of endocytosis is the process by which white blood cells “eat” pathogens. When white blood cells recognize a foreign object inside the body, such as a bacterium, they fold their cell membrane around it to take it into their cytoplasm.
They then merge the vesicle containing the invader with a lysosome – a vesicle containing strong chemicals and enzymes that can break down and digest organic matter. They have essentially just created a cellular “stomach” to “digest” the invader!