Channel Proteins: What,Function,Types,Examples, Exhaustive FACTS

The channel proteins are one of the most important membrane transport proteins in cell. Here we are going to discuss the channel proteins functions, types, examples and all exhaustive facts.

Channel proteins are one of the most important membrane transport proteins found in the cell of all living beings. These proteins are able to open a pore or a channel through which specific cellular molecules can be transported across the cell membrane.

The channel proteins transport specific molecules like cellular water and polar ions across the membrane through diffusion process. The molecules are transported down to their concentration gradient in cell.

What Is A Channel Protein?

In simple words, pore forming integral proteins which transport substances down the concentration gradient are the channel proteins.

The channel proteins are a special kind of membrane transport protein which provides passageway for water and polar ions and transports them down to the concentration gradient of the cell. These are actually the integral proteins of cell membranes, serving as hydrophilic channels of water and specific polar ions.

A channel protein is actually a multi subunit protein assembly. That means several protein subunits (homologous proteins) such as α, β, γ and more subunits get involved together and work as an individual pore forming channel protein.

With a high selectivity nature, the channel proteins can transport millions of ions within seconds. These channels are mostly open in both intracellular and extracellular environments and transport water and polar ions through facilitated diffusion processes towards the concentration gradient.

The channel proteins have different diameters according to their types. They are exclusively integral membrane proteins that cover the whole membrane around a specific pore or channel. That’s how by remaining embedded within the plasma membrane which is highly hydrophobic in nature these channel proteins open hydrophilic pathways for some highly specific polar ions or water molecules down to the concentration gradient.

To know more about monomers read our article on Monomer Examples: Detailed Insights And Facts

Channel Protein Function

The channel protein performs several functions inside the cell. It plays an important role in the movement of body parts also. Let’s have a closer look at functions of channel proteins.

• The main function of channel proteins is to help in transportation of some specific molecules (water molecules or small polar ions) through the hydrophobic plasma membrane.
• It opens a hydrophilic pore or  channel from which the water substance or polar ions can be passed through easily.
• By controlling the flow of water, the channel proteins can balance the water substance inside the plasma membrane. That’s how it can also maintain the stability of interior parts of the cell.
• The gated channel proteins also maintain the ion gradient of the cell and help to obtain equilibrium state. It holds the ion until some signal for opening of the channel is received.
• By controlling the ion exchange mechanisms it can actively build up electrical changes within the cell.
• The channel proteins also have some contribution in signal transduction procedure of cells.

The non gated channels provide a hydrophilic pathway from which water and polar ions can freely pass through within the hydrophobic plasma membrane by diffusion. In this process, any kind of energy is not  required. The molecules are transferred from a higher concentration area to a lower concentration area naturally.

The gated channel proteins are mostly situated at the plasma membrane of membrane bound organelles of the cell. Hence they contribute in maintaining the water balance and electrical ion stability of the interior environment under that particular organelles (homeostasis).

How Does A Channel Protein Work?

The channel proteins play a very important role in maintaining the balance of the cellular environment. Let’s have a closer look at how it works step by step.

When the intracellular water or polar ions are required to be transported through the plasma membrane, the channel protein triggers and opens a channel for transportation. After transporting that molecules can also trigger some other activities and that’s how the channel proteins contribute to the signal transduction procedure.

For example, muscle contraction is also stimulated by the channel proteins. When a muscle is to be contracted, a nerve cell brings an electrical signal for it and releases a neurotransmitter, acetylcholine. The acetylcholine diffuses and reaches the channel proteins nearby. That channel proteins release sodium and potassium ions. This ion secretion also involves another channel protein activation. The sodium and potassium ions trigger the actin and myosin proteins to contract. This is how the channel proteins work together and stimulates the muscles contraction procedure.

In the case of water channels, mostly known aquaporins play a major role in maintaining the water balance in the interior environment of the cell. From bacteria to plan every cell contains aquaporin water channels. These channels block all other substances apart from the water molecules, and stimulate transportation of water in or out of the cell by diffusion. 

Channel Protein Types

There are various types of channel proteins found inside the cells. About 300 types of channel proteins are present in the cells of the inner ear. Based on the presence of gates the channel proteins are differentiate into two types such as-

• Non Gated Channel Proteins
• Gated Channel Proteins

Non Gated Channel Proteins

In case of non gated channel proteins, the pore or channel remains open in both internal and external environments all time. That means it can transport water and specific ions through it freely. It is also known as leak channel as it passes out molecules constantly. The non gated channel proteins are generally situated at the membrane of organelles or in some specific places where ion gradient maintenance is not necessary.

Gated Channel Proteins

A gated channel protein remains closed most of the time until it receives any signal for opening the channel. After opening of the channel specific polar ions can pass through this. Nerve cells play a major role to trigger the channel proteins. The nerve cells bring an electrical signal to the channel proteins. The proteins react to that and open the channel.

Based on the gating process the gated channel proteins further differentiate into some other categories. Let’s have a closer look at them.

• Voltage Gated Ion Channels
• Ligand Gated Ion Channels
• Lipid Gated Ion Channels
• Others

Voltage Gated Ion Channels

The voltage gated ion channels are such kind of channel proteins, which triggers by changing the electrical membrane potential. These channels directionally propagate electrical signals. Voltage Gated ion channels are strictly ion specific and transport only  sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl−) ions through the diffusion process.

These ion channels are made of three major parts, such as a voltage sensor, the pore or conducting pathway, and the gate. Some most common voltage gated ion channels are Voltage-gated sodium channels, Voltage-gated calcium channels, Voltage-gated potassium channels, transient receptor potential channels, cyclic nucleotide-gated channels, Voltage-gated proton channels, etc.

Ligand Gated Ion Channels

Ligand Gated ion channels or ionotropic receptors are special kinds of ion channels that respond to a chemical messenger (ligand) binding process to open and close the passageway. When a nerve cell brings an electrical massage it releases a neurotransmitter, acetylcholine, near the ion channel. After that the neurotransmitter binds to a receptor (ligand gated ion channel) and makes conformational changes in the structure of that protein. This causes opening of that ion channel and transporting specific polar ions like Na+, K+, Ca2+, Cl−,etc.

The ligand gated ion channels are catagorized into three superfamilies such as cys-loop receptors, ionotropic glutamate receptors and ATP-gated channels. There are some other ligand gated ion channels examples can be found like cation-permeable acid sensing ion channels (ASICs), ATP-gated P2X receptors, and the anion-permeable γ-aminobutyric acid-gated GABAA receptor,etc.

Ligand Gated ion channels from Wikimedia

Lipid Gated Ion Channels

The lipid gated ion channels are regulated by lipid binding molecules resulting in the opening of the channel passageway. An anionic signaling lipid molecule binds to the transmembrane domain of plasma membrane. Phosphatidylinositol 4,5-bisphosphate gated channel, Phosphatidic acid gated channel, Phosphatidylglycerol gated channel, mechanosensitive ion channel are the most common lipid gated ion channels.

Others

Apart from the types mentioned above,there are many ion channels present. They also consist of some similarities to the previously discussed ion channels. 

Inward rectifier potassium ion channels, Calcium-activated potassium channels are one of the most common ion channels present for potassium transport. Light-gated channel which triggers by electromagnetic radiation (photon). channelrhodopsin-1, channelrhodopsin-2 are most common light gated ion channels. Mechanosensitive ion channels which are influenced by pressure, stretch, displacements. It is mostly found in bacteria and archaea cells.

There are some other ion channels also present like Cyclic nucleotide-gated channels, Temperature-gated channels, etc.

Channel Protein Examples

There are various different kinds of channel proteins present in every living organism. Let’s have a closer look at some most common examples of channel proteins.

• Aquaporins
• Chloride channel proteins
• Calcium channel proteins
• Potassium channel proteins

Aquaporins

The water channel proteins of the cell membrane are called aquaporins. From bacteria to fungi, plants ,animals all contain aquaporins for water transportation through the plasma membrane. The aquaporins are gated channel proteins that allow transport of water by simple diffusion process. It is the main water pumping system of the cell. Aquaporins main function is to transport water in or out of the cell. It prevents polar ions and other solutes from the passage. There are thirteen aquaporins present in mammals. 

Aquaporins from Wikimedia

Chloride channel proteins

The chloride channel proteins are mostly under the family of m voltage gated ion channels or ligand gated ion channels. The main function of this is to regulate the pH of the cell, stimulate organic solute transportation, help in cell migration, proliferation and differentiation process. This protein is made up of 1 to 12 transmembrane segments.

Calcium channel proteins

The calcium channel proteins are mostly under the family of voltage gated ion channels and ligand gated ion channels. These channel proteins conduct calcium ion transportation through the cell membrane. The calcium channel proteins are mostly found in skeletal muscles, bones(osteoblasts), brain, peripheral nervous system, spines, neurons, etc. The main function of calcium channel proteins is to transport calcium ions to their destinations. It releases calcium ions and stimulates signal transduction procedures. 

Potassium channel proteins

Potassium channel proteins are mostly distributed channel proteins found in all living organisms. It regulates various kinds of cellular functions actively. It allows potassium ions diffusion down to electrochemical gradients. It directly influences cardiac action potential, maintains vascular tone, stimulates hormone secretion process (insulin hormone from beta cell of pancreas),etc.

Apart from these there are also many Channel proteins that are widely distributed in the cells of living beings such as sodium channel proteins, proton channel proteins, etc. 

Channel Protein Pathway

The role of channel proteins in the membrane transport system of cells is immense. The channel proteins allow transportation of water and specific polar ions across the cell membrane. Let’s discuss the channel proteins functioning pathway step by step.

• At first when cellular water or polar ions need to be transported to another side of the cell for functioning, the concentration gradient allows them to diffuse through the cell membrane. 
• The polar ions are repelled by the hydrophobic nature of the plasma membrane and cannot diffuse through it.
• Then membrane transport proteins (channel proteins) are required,which provides the hydrophilic passageway for transportation of the water substances and polar ions.
• Incase of non-gated channel proteins the facilitated diffusion process aided, the channel protein forms a hydrophilic pore or channel for constant transportation. 
• In case of gated channel proteins, to open a channel some kind of chemical, electrical or biological signaling is required.
• Some specific cellular activities then trigger that particular channel protein, causing opening of a channel and ion transportation across the cell membrane is processed.
• After the end of the transportation process when the channel is not needed more, the external signal is stopped and the channel will be closed.

So these are the steps by which the channel proteins function and stimulate several signal transduction procedures.

Is Channel Protein Transportation Active Or Passive?

The channel protein transportation process is exclusively passive. It transports specific molecules across the cell membrane towards their concentration gradient via passive diffusion process only.

In case of active transport the molecules need to be transferred across the cell membrane against their concentration gradient. This process needs chemical energy also. That’s why the channel proteins cannot proceed via an active transport mechanism; they only transport molecules by a passive diffusion process.

Facilated Diffusion process from Wikimedia

Channel Proteins Passive Transport

The channel proteins only transport molecules by passive transport mechanism or facilitated diffusion process.

Most of the polar molecules or ions are unable to move across the plasma membrane for its hydrophobic nature. That’s why the channel proteins provide a hydrophilic passageway for that specific molecule to pass through it by a facilitated diffusion process. In this process no energy consumption is required. The polar ions move down to their concentration gradient easily.

In this process the polar ions move from higher concentration area to lower concentration area until it becomes equal.

Is ATP Synthase A Channel Protein?

ATP synthase is a protein which acts as an enzyme in mitochondria as well as a channel protein conducted proton transportation. ATP synthase is a channel protein that opens a channel for proton transportation in mitochondria.

The ATP synthase is a protein which forms adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi) during cellular respiration process in mitochondria. During this process the flow of electrons creates an electrochemical gradient by difference in proton (H+) concentration. This time the ATP synthase is activated and serves as an integral channel protein, it opens a channel for proton transportation across the cell membrane.

To know more read our article on Adenosine nucleoside and nucleoside phosphoramidite | Overview of important aspects

Carrier Protein vs Channel Protein

The carrier proteins and channel proteins both are the integral membrane transport proteins that help in transportation of specific molecules across the plasma membrane of a cell. With several similarities, these two kinds of transport proteins also possess various differences too.

Why Are Channel And Carrier Proteins Specific To One Molecule?

The channel proteins are very specific to one kind of molecule they transport. The gated ion channels remain closed until it receives any biological signal from the cell. After that it opens a pore or hydrophilic channel for that specific molecule or several similar molecules and transports it across the membrane.

The carrier proteins are also very specific to one molecule. Naturally it needs to bind with a solute it transported. After binding it has a conformational change in its structure and transports that one molecule across the cell membrane. 

Are Channel Proteins Amphipathic?

An Amphipathic molecule means a special kind of molecule which has both polar and nonpolar regions in it. The channel proteins have both polar and nonpolar parts in its structure, that’s why all channel proteins are amphipathic in nature.

This amphipathic nature of channel proteins help them to transfer polar ions and cross the polar and nonpolar region of plasma membrane of the cell.

Why Are Channel Proteins Amphipathic?

The channel proteins are amphipathic because they contain both polar and non polar regions in their structure.

The amphipathic nature is necessary for their functioning. The channel proteins attach to the cell membrane (hydrophobic region) and provide a hydrophilic passageway for transportation. The polar ions interact with its hydrophilic passageway and pass through it.

Why Does Water Need A Channel Protein?

Water substances use channel proteins called aquaporins for their Transportation across plasma membranes. About 6 types of aquaporins are expressed in just our kidney area.

The water is a polar molecule so when it tries to cross the plasma membrane it gets repelled by the hydrophobic nature of the membrane. It also faces several blockages and disturbance in the pathway and the transportation rate will be falling down. That’s why the water needs a channel protein that is aquaporins for transportation across the plasma membrane.

There are thirteen aquaporins found in mammals. Which opens a hydrophilic pore for the polar water substance and helps in transportation without interacting with the hydrophobic areas of plasma membrane.

Are Channel Proteins Hydrophobic Or Hydrophilic?

The channel proteins are amphipathic in nature that means they contain both polar and nonpolar regions in their structure.

The channel protein provides a hydrophilic pathway or channel exposed in both intracellular and extracellular environments for conducting transportation processes. It transports some specific polar ions or molecules through its hydrophilic channel, avoiding them to interact with the hydrophobic areas of the plasma membrane.

To know more about eukaryotic cells read our article on Eukaryotic Cells Examples: Detailed Insights

As a whole we can say that channel proteins are one of the most important parts of the membrane transport system of a cell. It consists of several different protein subunits and is present in cells of all living beings. It plays a significant role in passive transportation of molecules and stimulates various signal transduction procedures. We also state the difference between carried proteins and channel proteins and many more. Hope it will be informative to you.

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Piyali Das

Hello, I am Piyali Das, pursuing my Post Graduation in Zoology from Calcutta University. I am very passionate on Academic Article writing. My aim is to explain complex things in simple way through my writings for the readers.