Difference Between Channel and Carrier Proteins

 Main Difference – Channel vs Carrier Proteins

The cell membrane is semi-permeable to the molecules which pass through it.  The movement of ions, small molecules or macromolecules across a membrane is facilitated by membrane transport proteins. Channel and carrier proteins are two types of transport proteins found in the cell membrane, which facilitate diffusion and active transport mechanisms. The main difference between channel and carrier proteins is that channel proteins have a fixed conformation in the cell membrane whereas carrier proteins flip between two conformations while transporting molecules.

This article explains, 

1. What are Channel Proteins
       – Characteristics, Structure, Role
2. What are Carrier Proteins
       – Characteristics, Structure, Role
3. What is the difference between Channel and Carrier Proteins

What are Channel Proteins

Channel proteins are one of the two classes of membrane transport proteins. They are intrinsic proteins, which span the entire cell membrane. Therefore, one side of the protein is exposed to the extracellular fluid while the other side is exposed to the cytosol. The two exposing domains of the protein are hydrophilic. The hydrophobic channel is embedded in the lipid bilayer. Selected, water soluble molecules move across the membrane passively through the aqueous pore of the channel proteins. The concentration or the electrochemical gradient of the solute molecule determines the direction of the flow as well as the rate of the transportation of that particular solute molecule.

Figure 1: Protein channel

Aquaporins are a type of channel proteins, which allow water molecules to pass through the membrane at a very high rate. GLUT4 and aquaporins are examples of channel proteins, which are involved in the facilitated diffusion. Primary active transport through sodium/potassium pump (Na+/K+ ATPase) and proton/potassium pump (H+/K+ ATPase) in primary active transport, as well as the antiporters like sodium/calcium exchanger and symporters like SGLT2, are examples for channel proteins, which are involved in active transport. Some channel proteins are opened at all the times. But, others are ‘gated,’ controlling the opening and closing of the channel. In some tissues, sodium and chloride ions pass freely through open channels. But in cells involved in the transmission of electrical impulses, gated channel proteins transport sodium, calcium and potassium ions.          

What are Carrier Proteins

Carrier proteins are the other type of transport molecules found in the cell membrane. They bind with large, selected molecules like proteins from one side of the membrane and release the molecules to the other side. The binding of the molecule to the carrier protein changes the conformation of the latter. Carrier proteins transport molecules against the concentration gradient of the transporting molecule. Hence, carrier proteins require cellular energy for their action. In contrast, some carrier proteins transport molecules through the concentration gradient by passive transport as well. The rate of the transportation by carrier proteins is very low when compared to channel proteins. Channel proteins and carrier proteins are shown in figure 2.

Figure 2: Channel proteins and carrier proteins

Difference Between Channel Proteins and Carrier Proteins

Type

Channel Proteins: Channel proteins transport ions.

Carrier Proteins: Carrier proteins transport molecules.

Shape

Channel Proteins:  Channel proteins are fixed.

Carrier Proteins: Carrier proteins flip between two conformations.

Transport Core

Channel Proteins: Channel proteins contain a pore, facilitating the transport of molecules.

Carrier Proteins: Carrier proteins do not contain a core inside the protein.

Mechanism

Channel Proteins: Solute molecules diffuse through the pores of channel proteins.

Carrier Proteins: Solute molecules are bound to the carrier protein in one side and released from the other side.

Transport Rate

Channel Proteins: Channel proteins have high transport rates.

Carrier Proteins: Carrier proteins have very low transport rates compared to channel proteins.

Solute-bound Conformations

Channel Proteins: Channel proteins do not bind with solute molecules it transports.

Carrier Proteins: Carrier proteins consist of alternative solute-bound conformations.

Lipo/Glycoproteins

Channel Proteins: Channel proteins are lipoproteins.

Carrier Proteins: Carrier proteins are glycoproteins.

Synthesis

Channel Proteins: Channel proteins are synthesized in the rough endoplasmic reticulum.

Carrier Proteins: Carrier proteins are synthesized in the free ribosomes in the cytoplasm.

Type of Molecules Transported

Channel Proteins: Channel proteins only transport water soluble molecules.

Carrier Proteins: Carrier proteins transport both water soluble and insoluble molecules.

Conclusion

Channel proteins and carrier proteins are the two types of membrane transport proteins found in the cell membrane. Both types of proteins are involved in passive transport through facilitated diffusion and active transport through cotransporters like uniporters, antiporters, and symporters. Transport proteins are specific to the molecules which are transported through them. Channel proteins are capable of transporting molecules at a very high rate compared to carrier proteins. The main difference between channel proteins and carrier proteins is their mechanisms of transporting molecules across the membrane.

Reference:
1. “Facilitated transport – Boundless Open Textbook.” Boundless. 26 May 2016. Web. 16 May 2017. <https://www.boundless.com/biology/textbooks/boundless-biology-textbook/structure-and-function-of-plasma-membranes-5/passive-transport-65/facilitated-transport-332-11469/>.

Image Courtesy:
1. “Figure 05 02 04” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Scheme facilitated diffusion in cell membrane-en” By LadyofHats Mariana Ruiz Villarreal – Own work. (Public Domain) via Commons Wikimedia