Transport Protein Definition, Function, & Example

Transport Protein Definition

Transporter proteins are proteins that transport substances across biological membranes. Transport proteins reside within the membrane itself, where they form a transport channel or mechanism to allow their substrate to move from one side to the other.

The substances carried by these proteins can include ions such as sodium and potassium; Sugars like glucose; Proteins and messenger molecules; and many more.

Transport proteins generally perform two types of transport: “facilitated diffusion”, in which a transport protein simply creates an opening for a substance to diffuse along its concentration gradient; and “active transport”, in which the cell expends energy to move a substance against its concentration gradient.

What is Transport Protein?

A transport protein (also known as a transmembrane pump, transporter, companion protein, acid transport protein, cation transport protein, or anion transport protein) is a protein that has the function of moving other materials within an organism. Transporter proteins are crucial for the growth and life of all living things. There are different types of carrier proteins.

Carrier proteins are proteins that participate in the movement of ions, small molecules, or macromolecules, like another protein, through a biological membrane. Carrier proteins are integral membrane proteins; that is, they exist within and pass through the membrane through which they transport substances. Proteins can aid the movement of substances through facilitated diffusion (i.e., passive transport) or active transport. These movement mechanisms are known as carrier-mediated transport. Each carrier protein is designed to recognize only one substance or a group of very similar substances. Research has correlated defects in certain transporter proteins with certain diseases. A membrane transport protein (or simply transporter) is a membrane protein that functions as such a carrier.

A vesicular transport protein is a transmembrane or membrane-associated protein. Regulates or facilitates the movement of cellular content through vesicles.

What is the Function of Transport proteins?

Life as we know it depends on the ability of cells to selectively move substances when they need them. Certain important molecules, such as DNA, must be kept in the cell at all times; But other molecules such as ions, sugars, and proteins may need to move in and out for the cell to function properly.

Each transport protein is designed to transport a specific substance as needed. For example, some channel proteins only open when they receive the correct signal, allowing the substances they carry to flow through when needed. Active transporters can often also be “turned on and off” by messenger molecules.

By moving substances across membranes, transport proteins make everything from nerve impulses to cellular metabolism possible.

Without transport proteins, for example, the sodium-potassium gradient that allows our nerves to fire would not exist.

What are the Types of Transport Proteins?


As the name suggests, “channel” or “pore” proteins open holes in the membrane of a cell.

These proteins are characterized by being open to both the intracellular and the extracellular space at the same time. In contrast, carrier proteins are only open to the inside or outside of a cell at any given time.

Channels or pores are typically designed in such a way that only a certain substance can pass through.

For example, voltage-gated ion channels often use charged amino acids that are precisely spaced to attract your desired ion while repelling all the others. The desired ion can flow through the channel while other substances cannot.

Voltage-gated ion channels are good examples of transport proteins that act on demand. Voltage-gated ion channels, often found in neurons, open in response to changes in the electrochemical potential of a membrane.

When closed, the voltage-controlled channel does not allow ions to pass through the cell membrane. But when open, it lets through large amounts of ions very quickly, allowing the cell to rapidly change its membrane potential and trigger a nerve impulse.

Carrier Proteins

Carrier proteins are transport proteins that are only open on one side of the membrane at a time.

They are often designed this way because they transport substances against their concentration gradient. Simultaneous opening on both sides of the membrane could allow these substances to simply flow back along their concentration gradient and destroy the work of the carrier protein.

To do their job, transporter proteins generally use energy to change their shape.

The sodium-potassium pump, for example, uses the energy of ATP to change its shape from being open to the intracellular solution to being open to the extracellular solution. This allows you to collect inside the cell and release them outside the cell and vice versa.

Other carrier proteins can use other energy sources, such as existing concentration gradients, to achieve “secondary active transport.” This means that its transport is possible thanks to the energy exerted by the cell, but the protein itself does not use ATP directly.

How is that possible? These transporter proteins often use the energy of a substance that wants to “descend” its concentration gradient to change its shape. The same change in shape makes it possible to transport a substance that “does not want to move at the same time.”

A good example is the sodium-glucose transporter protein, which uses the sodium concentration gradient, originally generated by the sodium-potassium pump, to move glucose against its concentration gradient.

We discuss the sodium-potassium pump and the sodium-glucose transport protein in detail below.

What are the Examples of Transport Proteins?

Examples of Transport Proteins

The Sodium-Potassium Pump

The best-known example of a primary active transport protein is the sodium-potassium pump. It is this pump that creates the ion gradient that allows neurons to fire.

The sodium-potassium pump begins with its sodium-binding sites toward the interior of the cell. These points attract and hold the sodium ions in place.

When each of its three sodium binding sites has bound to a sodium ion, the protein binds to an ATP molecule and cleaves a phosphate group on ADP +. Protein uses the energy released in the process to change its shape.

Now the sodium binding sites are in front of the extracellular solution. They release all three sodium ions out of the cell, while the protein’s potassium binding sites bind two potassium ions.

When both potassium binding sites are full, the protein returns to its original shape. Now the potassium ions are released into the cell and the empty sodium binding sites can bind more sodium ions.

For every ATP that this pump uses, it carries three positively charged ions out of the cell, while only carrying two back into the cell. This creates an electrochemical gradient, with the inside of the cell negatively charged compared to the outside solution. It also creates a sharp concentration gradient with much more potassium inside the cell and much more sodium outside the cell.

When it’s time for a nerve cell to be activated, the cell’s powerful electrical and chemical gradients allow the cell to create a huge and instantaneous change by opening its voltage-gated ion channels.

Sodium-Glucose Transport Proteins

The sodium-glucose transporter protein uses secondary active transport to move glucose into cells. They are active in intestinal cells and kidney cells, which have to transport glucose to the body systems against the concentration gradient.

This process requires energy because the affected cells have a higher concentration of glucose than the extracellular fluid. Therefore, it would be impossible for glucose to diffuse into cells on its own; You have to spend energy.

In this case, the energy comes from the sodium concentration gradient. Thanks to the action of the sodium-potassium pump, there is much more sodium outside the cell than inside it. Therefore, there is a strong concentration gradient that favors the passage of sodium into the cell.

This concentration gradient can be considered as a kind of “stored energy”. The sodium-potassium pump extracts energy from ATP and converts it into this concentration gradient, which can then be used for other purposes, such as sodium-glucose transporter protein.

Transport Protein Definition, Function, & Example

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