Carrier Protein - Definition, Function and Examples | Biology Dictionary (2022)

Carrier Protein Definition

Carrier proteins are proteins that carry substances from one side of a biological membrane to the other. Many carrier proteins are found in a cell’s membrane, though they may also be found in the membranes of internal organelles such as the mitochondria, chloroplasts, nucleolus, and others.

Carrier proteins and channel proteins are the two types of membrane transport proteins.

While channel proteins are exactly what they sound like – proteins that open channels in the cell membrane, allowing molecules to flow in and out along their concentration gradient – carrier proteins are only open to one side of the membrane in question at a time.

While a sodium-potassium channel may simply open and allow ions to flow from one side to the other, for example, the carrier protein known as the sodium-potassium pump binds to ions on one side of the membrane, then changes shape to carry them through to the other side without opening a channel.

This makes carrier proteins useful for active transport, where a substance needs to be carried against its concentration gradient in a direction it would not normally flow.

However, carrier proteins can also be used for facilitated diffusion, a form of passive transport.

Carrier proteins typically have a “binding site” which will only bind to the substance they’re supposed to carry. The sodium-potassium pump, for example, has binding sites that will only bind to those ions.

Once the carrier protein has bound to a sufficient quantity of its target substance, the protein changes shape to “carry” the substance from one side of the membrane to the other. A textbook example of this process is the action of the sodium-potassium pump, illustrated below:

(Video) Carrier Proteins

Some carrier proteins require no energy sources but the diffusion gradient that their substrate “wants” to pass down, making them a form of passive transport. Others may require energy in the form of ATP, or may perform “secondary active transport,” where the transport of one substance against its diffusion gradient is powered by a different diffusion gradient that is created by ATP-using carrier proteins.

We will discuss examples of all of passive, active, and secondary active transport using carrier proteins below in the “examples” section.

Function of Carrier Protein

Carrier proteins are some of the most common proteins in the world, and some of the most important to sustaining life. A cell’s ability to perform the functions of life depends on its ability to maintain a difference between the intracellular and extracellular environment.

That’s where carrier proteins come in.

Within our own bodies, the action of all of our nerve cells is powered by the sodium-potassium gradient that is created by the sodium-potassium pump. This carrier protein binds to ions of sodium on one side of the membrane, and ions of potassium on the other side. Then the carrier protein binds with ATP, and uses the energy of ATP to pump these ions across the cell membrane in opposite directions.

It is ultimately this sodium-potassium gradient that allows our nerve cells to fire, which is what allows us to move, think, perceive the world around us, and even keep our hearts beating.

Carrier proteins which transport protons across the mitochondrial membrane to create a concentration gradient there are also responsible for the creation of most of the ATP made by eukaryotic cells. The mitochondria use the enzyme ATP synthase to turn the energy of that concentration gradient into the energy of ATP.

Some of the common purposes served by carrier proteins include:

  • Creating ion gradients which allow nerve cells to function
  • Creating ion gradients which allow the mitochondria to function
  • Creating ion gradients which allow chloroplasts to function in photosynthesis
  • Transporting large molecules such as sugars and fats in and out of cells
  • Many other tasks not listed here

Types of Carrier Proteins

Active Transport

Active transport carrier proteins require energy to move substances against their concentration gradient. That energy may come in the form of ATP that is used by the carrier protein directly, or may use energy from another source.

(Video) A Level Biology Revision "Membrane Proteins"

Many active transport carrier proteins, such as the sodium-potassium pump, use the energy stored in ATP to change their shape and move substances across their transportation gradient.

Pumps which practice “secondary active transport,” are sometimes referred to as “coupled carriers.” These pumps use the “downhill” transport of one substance to drive the “uphill” transport of another.

“Coupled carriers” like the sodium-glucose cotransport protein do end up costing the cell energy, because the cell must use ATP to maintain the sodium concentration gradient that this carrier uses as its energy source. But the carrier protein does not use ATP directly.

Other carrier proteins, such as some that are found in bacteria and in organelles such as mitochondria and chloroplasts, might use energy sources directly from the environment without requiring ATP.

Facilitated Diffusion

Carrier proteins can also carry substances in a “downhill” direction – that is, carry them down their concentration gradient, in the direction that the substance “wants” to go.

One example is the valinomycin potassium carrier, which binds to potassium ions and changes shape to release them on the other side of the membrane.

Examples of Carrier Proteins

Sodium-Potassium Pump

The sodium-potassium pump uses ATP to transport both sodium and potassium ions against their transportation gradient.

The protein binds to sodium ions inside the cell, while simultaneously binding to potassium ions inside the cell. Once it has bound to a sufficient number of ions on both sides, it binds to a molecule of ATP. By releasing the energy stored in ATP, it changes shape to move both sets of ions to the opposite side of the membrane.

The sodium-potassium pump is crucial for the nerve function of animals, and is estimated to use about 20-25% of all the ATP in the human body!

This is because nerve cells fire using electrochemical signals – which are created by moving charged particles, i.e. sodium and potassium ions, from one side of the nerve cell membrane to the other very quickly. These potentials can only be created if there is an extreme difference in concentration between sodium and potassium ions inside the cells vs. outside them.

One reason why diseases like anorexia and cholera can be so dangerous is that extreme dehydration or malnutrition can disrupt the amount of sodium and potassium available to our cells, disrupting this gradient. In extreme cases these ionic imbalances can cause the nerve cells that power our heart muscles to fail.

(Video) Channel Proteins

This is also why diseases that effect the kidneys, which control how we export or retain ions in our urine, can be dangerous. A rare side effect of diabetes, for example, is hypokalemia – not enough potassium in the blood, which can disrupt the function of the nerve cells driving the heart muscle.

Glucose-Sodium Cotransport

The glucose-sodium cotransport protein is a good example of a protein that uses “secondary active transport, by “indirectly” using ATP.

In the example above, we discussed how the cell uses ATP to maintain the sodium and potassium gradients between the inside and outside of the cell. Generally, cells try to keep a higher concentration of sodium outside, and a higher concentration of potassium inside.

So to power the glucose-sodium pump, the cell allows a couple of sodium ions inside along with the glucose. The carrier protein binds to both the glucose molecule – which doesn’t “want” to move inside the cell – and the two sodium ions, which do want to move down their concentration gradient into the cell.

The energy of the sodium ions “wanting” to get into the cell overrides the glucose’s resistance, and all three particles are moved into the cell together.

This means more work for the sodium-potassium pump in the cell membrane, which will have to use ATP to pump the sodium back out in order to preserve this vital gradient. But the glucose-sodium cotransport protein does not use ATP itself – it only takes advantage of the energy of ATP indirectly.

This type of secondary active transport is called “symport,” from the Greek words “sym” for “together” and “port” for “transport.” Symport transports two substances together in the same direction in order to assure that they both get transported.

Valinomycin: A Passive Transport Carrier

Valinomycin is a protein that binds to potassium and carries it across the cell membrane down its concentration gradient, in the direction that the potassium “wants” to move.

It is found in the cell membranes of strep bacteria, who use it when they “want” to move potassium out of their cells. Its high degree of selectivity for potassium only gives it an advantage over other means to accomplish this transport, which might be more likely to move other ions such as sodium.

If you think “valinomycin” sounds like the name of an antibiotic, you’re right! Valinomycin is also used as an antibiotic to fight bacteria like strep, because artificially introducing it to bacteria can destroy their electrochemical gradient.

For the same reason, valinomycin can also be a powerful neurotoxin: if it gets into nerve cells, it can dangerously disrupt their sodium-potassium gradient too!

(Video) Channel vs Carrier protein | Chapter 1 Cell structure and Function

  • Active transport – Transport that moves a substance against its concentration gradient and requires the cell to expend energy to perform this task.
  • Membrane protein – A protein found within the membrane of a cell, which usually has both hydrophilic and hydrophobic domains to anchor itself firmly with respect to the hydrophobic membrane interior and the hydrophilic intracellular and extracellular fluid.
  • Passive transport – Transport that moves a substance down its concentration gradient. Passive transport requires no energy expenditure, since it is moving substances in the direction that they “want” to go.

Quiz

1. Which of the following is NOT a difference between carrier proteins and channel proteins?
A. Channel proteins are open on both sides of the membrane at once, while carrier proteins are only open to one side of the membrane at a time.
B. Channel proteins allow substances to flow through them freely, while carrier proteins have binding sites for specific atoms and molecules.
C. Channel proteins perform passive transport, while carrier proteins perform active transport.
D. None of the above.

Answer to Question #1

C is correct. While carrier proteins are capable of performing active transport, they can also perform passive transport. Valinomycin, for example, passively transports potassium down its concentration gradient. It is used instead of a channel because it is highly selective and transports potassium ions only.

2. Which of the following is NOT an example of a carrier protein?
A. A sodium-potassium channel protein.
B. The proton pump in the membrane of a chloroplast.
C. Bacteriorhodopsin.
D. All of the above.

Answer to Question #2

A is correct. While the sodium-potassium pump is a carrier protein, the sodium-potassium channel is a different protein which is – as the name suggests – a channel protein, not a carrier protein!

3. Which of the following is NOT true of carrier proteins?
A. They undergo a shape change to move substances from one side of the membrane to the other.
B. They are open to both sides of the cell membrane at once.
C. They can bind to more than one target substance.
D. They will transport any substance that is of the right size, shape, or charge.
E. B and D.

Answer to Question #3

(Video) MEMBRANE PROTEINS - Types and Functions

E is correct. While carrier proteins can bind to more than one target substance – such as the sodium-potassium pump, or the sodium-glucose cotransport protein – their binding sites are highly specific. The binding sites on the sodium-potassium pump, for example, must distinguish between sodium and potassium ions to ensure that they transport each in the right direction! This requires a high degree of specificity because sodium and potassium are both small, positively-charged ions.

FAQs

What is carrier protein give example? ›

An example of a carrier protein is the sodium potassium pump. This carrier protein uses ATP to move three sodium ions out of the cell and two potassium ions into the cell.

What is the carrier protein function? ›

Carrier proteins bind specific solutes and transfer them across the lipid bilayer by undergoing conformational changes that expose the solute-binding site sequentially on one side of the membrane and then on the other.

Where is the carrier protein? ›

Examples. Every carrier protein, especially within the same cell membrane, is specific to one type or family of molecules. For example, GLUT1 is a named carrier protein found in almost all animal cell membranes that transports glucose across the bilayer.

What are carrier proteins called? ›

Carrier proteins (also called carriers, permeases, or transporters) bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane (Figure 11-3).

What is difference between carrier protein and channel protein? ›

Carrier proteins are proteins that bind to molecules or ions on one side of the membrane and release them on the other. Channel proteins create holes/pores that penetrate the membrane, enabling target molecules or ions to flow through via diffusion without interfering with one another.

Is carrier protein active or passive? ›

They are essential proteins that carry chemicals across the membrane/cell in both the direction against and down the concentration gradient. It requires energy while transferring substances against the concentration gradient. Carrier proteins are both active and passive.

What are three types of transport proteins? ›

Answer and Explanation: The three types of transport proteins are option (c) the channel, ATP-powered pumps, and carrier. A channel protein employs either facilitated diffusion or active transport to allow proteins and other molecules to pass through the cell. Carrier proteins carry substances in and out of the cell.

What type of cell transport uses carrier proteins? ›

Active transport uses carrier proteins, not channel proteins. These carrier proteins are different than the ones seen in facilitated diffusion, as they need ATP in order to change conformation.

Do carrier proteins use energy? ›

The carrier protein uses the energy of the sodium gradient to drive the transport of glucose molecules.

What is a carrier molecule? ›

A carrier molecule is typically involved in the transport of other biological compounds such as proteins, DNA or RNA, electrons, or protons including ions. For example, carrier proteins can transport other molecules such as ions, sugar, fat, or peptides through the cell membrane.

What is a carrier biology? ›

A carrier is a person who can pass an inherited (genetic) disease on to their children but who does not have the disease. The person can also pass on carrier status. Some diseases are caused by changes in a person's chromosomes or genes.

Which structures are classified as carrier proteins? ›

Carrier proteins are proteins that carry substances from one side of a biological membrane to the other. Many carrier proteins are found in a cell's membrane, though they may also be found in the membranes of internal organelles such as the mitochondria, chloroplasts, nucleolus, and others.

What are carriers in cell membrane? ›

Carriers are membrane proteins that complement the structural features of the molecules transported. They bind to the chemicals in order to move them across the cell membrane. Energy is consumed because the transport proceeds against the concentration gradient.

Is a proton pump a carrier protein? ›

proton pump A carrier protein or complex of proteins that transports protons (H +) across biological membranes. Such pumps use energy, for example ATP, to establish a high concentration of protons on one side of the membrane compared to the other.

What are the 3 types of carrier proteins? ›

Types of Carrier Proteins

Depending on the energy source, the carrier proteins may be classified as (1) ATP-driven, (2) electrochemical potential-driven, or (3) light-driven.

What are examples of channel proteins? ›

Calcium, chloride, potassium and sodium ion channels are some examples of channel proteins. Aquaporins are a specific type of channel proteins that are involved in facilitated diffusion.

What is carrier biology? ›

A carrier is a person who can pass an inherited (genetic) disease on to their children but who does not have the disease. The person can also pass on carrier status. Some diseases are caused by changes in a person's chromosomes or genes.

What is a carrier molecule? ›

A carrier molecule is typically involved in the transport of other biological compounds such as proteins, DNA or RNA, electrons, or protons including ions. For example, carrier proteins can transport other molecules such as ions, sugar, fat, or peptides through the cell membrane.

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