What is the Difference Between Porins and Aquaporins

The main difference between porins and aquaporins is that porins are transport proteins that occur in biological membranes, whereas aquaporins are water channels that allow the movement of water.

Porins and aquaporins are two types of channel proteins that form large pores in the cell membrane, outer membranes of cell organelles, such as mitochondria and plastids, and bacteria. 

Key Areas Covered

1. What are Porins
     – Definition, Structure, Function
2. What are Aquaporins
     – Definition, Structure, Function
3. Similarities Between Porins and Aquaporins
     – Outline of Common Features
4. Difference Between Porins and Aquaporins
     – Comparison of Key Differences

Key Terms

Porins, Aquaporins

What are Porins

Porins are protein molecules that occur in the outer membrane of cells, organelles, and bacteria. Generally, fungal, plant, mammalian cells, and other vertebrate cell membranes contain porins. Organelles such as mitochondria and chloroplast also contain porins. In addition, the outer membrane of Gram-negative bacteria and some Gram-positive bacteria contain porins. The most significant feature of porins is the transport of molecules through passive diffusion.

Figure 1: Sucrose Porin

Porins are water-filled pores that occur through the membrane from the exterior to the periplasm. Apart from that, the majority of porins are monomers. Some are dimeric, while some are octameric. Furthermore, the primary function of porins is the passive transport of hydrophilic molecules of various sizes and charges through the membrane. Porins are important for the transport of some nutrients and substrates. Meanwhile, toxins and other wastes are transported away from the cell, avoiding toxic accumulation inside the cell.   

What are Aquaporins

Aquaporins are a type of porins that especially transport water molecules across the membrane. They are also a type of large integral membrane proteins. They are made up of alpha-helical domains. Generally, the transport of water molecules occurs through either simple diffusion or osmosis. The transport of water occurs through a concentration gradient from a high-water concentration to a low water concentration. However, the process of simple diffusion is really slow due to the presence of polarity on the water molecule.

Figure 2: Aquaporin

Moreover, aquaporins are tetramers, and each monomer contains a single pore. However, mammalian aquaporins are involved in multiple physiological processes, including salivary gland function and kidney function. Therefore, they are responsible for several clinical disorders, such as kidney dysfunction and brain edema, and loss of vision. On the contrary, aquaporins do not transport molecules that have a charge, such as protons. Therefore, they conserve the electrochemical potential across the cell membrane.    

Similarities Between Porins and Aquaporins

• Porins and aquaporins are two types of channel proteins that occur in biological membranes.
• They are beta-barrel proteins that occur across the cell membrane.
• They form large pores in the outer membrane of cells, cell organelles, and bacteria.
• Their main function is to allow the passive diffusion of molecules through the membrane.

Difference Between Porins and Aquaporins

Definition

Porins refer to the beta-barrel proteins that cross a cellular membrane, acting as a pore through which molecules can diffuse, while aquaporins refer to water channels that form pores in the cell membranes that selectively conduct water molecules through the membrane.

Significance

Porins are transport proteins, while aquaporins are types of transport proteins.

Structure

Porins can be monomers, dimers, or octamers, while aquaporins are tetramers.

Made up of

Moreover, porins are made up of beta sheets, while aquaporins are made up of alpha-helical domains.

Transportation

Porins transport ions and small molecules, while aquaporins transport water molecules.

Type of Transportation

Porins transport molecules with different sizes and charges, while aquaporins transport molecules with different polarities, such as water molecules.

Mechanism of Transport

Furthermore, porins transport molecules through passive diffusion, while aquaporins transport water through simple diffusion or osmosis.

Conclusion                                                 

In brief, porins and aquaporins are two types of transport proteins that transport different types of molecules across the membrane. They also undergo passive diffusion. Generally, porins transport different types of molecules across the membrane, such as ions and small molecules. On the other hand, aquaporins are a type of porins that especially transport water molecules across the cell membrane. Therefore, the main difference between porins and aquaporins is the type of molecules they transport.                                                         

References:                

1. “Porins.”National Center for Biotechnology Information.  
2. “Membrane proteins: Aquaporins — channels without ions.” Current Biology.

Image Courtesy:

1. “Sucrose specific porin 1A0S” By en:User:Zephyris – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “AQP1” By Peter Wolber – Own Work (CC BY-SA 3.0) via Commons Wikimedia