Channel Protein

Channel Protein Definition

A channel protein is a special arrangement of amino acids which embeds in the cell membrane, providing a hydrophilic passageway for water and small, polar ions. Like all transport proteins, each channel protein has a size and shape which excludes all but the most specific molecules. A generic channel protein is seen below, embedded within the membrane. Ions, the small green hexagons, travel through the channel protein. They move from an area of high concentration to an area with a lower concentration.

Types of Channel Protein

Non-gated

Facilitated Diffusion Channel Protein
Like the image above, a channel protein may exist in a state which stays open all the time. This is called a non-gated channel protein. These proteins allow ions and water to flow through the cell membrane, which is normally hydrophobic and would resist the passage of these molecules. A non-gated channel protein is needed whenever the balance of water and ions must be assisted by the constant passage of water and ions into or out of the cell. However, it is often a disadvantage to leave a channel open all the time. The second type of channel protein addresses this problem.

Gated

A gated channel protein remains closed, until it receives a special chemical or electrical signal. These channel proteins are extremely important in many cellular functions. The ability to gate an ion channel allows electrical energy to be built up inside the cell. Nerve function is entirely based on this fact. Channel proteins on the surface of nerve cells react to electrical signals created by the flooding of ions through the membrane next to them. As they open, ions spill through and continue the electrical disturbance. This passes a signal very quickly through the body. A gated channel protein reacting to a signal molecule can be seen in the image below.
Gated channel protein

Channel Protein Function

Depending on whether it is gated or non-gated, a channel protein has a slightly different function. A non-gated channel protein simple allows ions and water to flow freely from one side of a membrane to another. While these type of channels are not often found on the external cell membrane, they are more often found within organelles and places where ion gradients are not maintained.

When an ion gradient needs to be maintained, gated channel proteins serve the role of holding back the tide of ions until they are signaled to open. A closed channel acts as corked bottle. Water and ions move slowly through the plasma membrane, or not at all. If the channel protein is closed, they have little chance of obtaining an equilibrium. Cells use these proteins in many ways, everything from balancing their water content to actively building up charges.

Channel Protein Structure

Most channel proteins are made of several identical protein subunits which form a hydrophilic region in their center. Gated channels function by changing conformation upon receiving a signal, allowing access to the hydrophilic passageway. Non-gated channels are usually formed from identical subunits, which attach to each other in a circle. While the inside of the circle is hydrophilic, the amino acids exposed within the hydrophobic cell membrane are also non-polar. This helps to anchor the protein within the membrane. If the protein tried to slip out of the membrane, it would be pushed by polar forces back into place.

Channel Protein Example

When your muscles contract, this is the result of the action of gated channel proteins within your muscle cells. These cells respond to the neurotransmitter acetylcholine, which is present in high amount as the end of nerve cells. At the synapse or space where they release the neurotransmitter, the opposing nerve cell contains many channel proteins set to receive the signal. An electrical signal coming down the nerve (also driven by a type of channel protein) causes the acetylcholine to be released.

The neurotransmitter diffuses quickly across the synapse, and reaches channel proteins on the other side. Each channel protein opens, releasing sodium and potassium ions. The electrical disturbance travels down special channels within muscles, carrying the signal to each muscle cell. Here, another set of channel proteins is activated. These release sodium, causing the proteins actin and myosin to start their crawling motion against each other, contracting each cell. The full result is a full muscle contraction, moving a limb or operating a part of the body.

Channel Proteins and Carrier Proteins

There are four types of transport that occur within cells. Simple diffusion occurs with small gas molecules, such as oxygen and carbon dioxide, as well as many non-polar chemicals such as steroid hormones and medicinal drugs. These molecules have the right chemistry and size to pass right through the cell membrane.

More charged molecules, which are hydrophilic, have a hard time passing through the membrane. These include ions, water, and sugars such as glucose. Channel proteins carry out the majority of facilitated diffusion. While the chemicals are still moving in the direction of their concentration (from high to low), they are given a passageway through the cell membrane. This allows them to move at near diffusion speeds.

However, not all facilitated diffusion is carried out by channel proteins. Carrier proteins, proteins which bind and transport molecules across the membrane, are also involved in facilitated diffusion. Large molecules like glucose cannot pass through the narrow passageway created by channel proteins. Carrier proteins known as uniporters bind to glucose molecules one at a time. The binding action causes a conformational change in the protein, which causes it to deposit the molecule on the opposite side of the cell. These carrier proteins operate without energy, and move molecules down their concentration gradient.

When substances need to be moved against their concentration gradient, more complicated carrier proteins are needed. Active transport is the process of using a carrier protein and powering it with an interaction with ATP to move a molecule against the gradient. The energy is needed because molecules naturally want to diffuse, and spread out. It takes a lot of energy to move some ions and molecules, but is necessary for the way life has evolved. Other carrier proteins have evolved for cotransport. By transporting a molecule down its concentration gradient, another molecule can be moved against its gradient. This carrier protein type allows cells to transport materials using the ion gradient they build with other ATP carrier proteins.

The major difference between a channel protein and a carrier protein is stereospecificity. While channel proteins only allow certain sized molecules to pass, they do not bind the molecules. Carrier proteins have an active site, which the chemical to be transported must bind to. This site will bind specifically to only one molecule, and seeks to transport this molecule alone. The binding action is what allows the passage of the large molecule through the cell membrane.

Quiz

1. What is the difference between a channel protein and a carrier protein?
A. They move different types of molecules
B. A channel protein does not need energy
C. A channel protein does not bind the molecules it transports

Answer to Question #1
C is correct. Channel proteins are simply that: channels. Like a straw, or the drain on a tub, they simply allow water and ions to pass through them. While they can be gated or non-gated, they do not need energy to operate, but neither do uniporters nor other carrier proteins. Channel proteins and carrier proteins can move the same types of molecules.

2. A mutation in a person causes their ion channels to malfunction. Will this be a problem?
A. Yes
B. No
C. They can treat it

Answer to Question #2
A is correct. This is the cause of a terrible disease known as Cystic Fibrosis. Sufferers of this disease cannot regulate ions in their mucous membranes, causing suffocating phlegm buildups and organ failures at a young age. Channel proteins are very important for many other functions, and most of them are required to work for normal bodily functions.

3. In an experiment, a scientist separates two bodies of water with a thin phospholipid membrane, such as that found in a cell. He pours salt in one of the bodies of water. The membrane has channel proteins embedded. His control experiment is two bodies of water separated by the same membrane, but without the channel proteins. He adds salt to this control as well. Which of the following would you expect to happen?
A. In both experiments, the salt will quickly come to equilibrium between the bodies
B. In the control, equilibrium will come more slowly than the experimental membrane
C. The control will come to equilibrium faster

Answer to Question #3
B is correct. The control will come to equilibrium more slowly because it lacks a channel protein. These proteins will allow the ions in salt to distribute right through the membrane along with water. These two substances will work their way back and forth across the membrane until they are equal. Without the proteins, the membrane holds back water and ions, and the process will happen much more slowly, if it happens at all.

References

  • Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology (6th ed.). New York: W.H. Freeman and Company.
  • Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.
  • Widmaier, E. P., Raff, H., & Strang, K. T. (2008). Vander’s Human Physiology: The Mechanisms of Body Function (11th ed.). Boston: McGraw-Hill Higher Education.
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