Difference Between Porins and Aquaporins

The key difference between porins and aquaporins is that porins are water-filled pores and channels found in the membranes of bacteria and eukaryotes. Meanwhile, aquaporins are membrane proteins that form water-selective channels in living cells.

Different molecules go in and out of the cell across the plasma membrane in living organisms. Thus, there are different membrane proteins in the plasma membrane to maintain and regulate the molecules entering and going out from the cells. Porins and aquaporins are two such types of membrane proteins. Of these, the porins are water-filled pores that facilitate the transportation of hydrophilic molecules across the membrane. Meanwhile, aquaporins are water selective channels that specifically allow water to pass freely across the membrane.

1. Overview and Key Difference
2. What are Porins
3. What are Aquaporins
4. Similarities Between Porins and Aquaporins
5. Side by Side Comparison – Porins vs Aquaporins in Tabular Form
6. Summary

What are Porins?

Porins are water-filled channels found in membranes. Generally, porins are abundantly present in the outer membrane of gram-negative bacteria. Furthermore, porins are seen in gram-positive bacteria; specifically, in the outer membranes of mitochondria and chloroplasts of eukaryotes. They are proteins composed of beta-barrel proteins. Porins act as pores in order to facilitate the transportation of different types of molecules, especially hydrophilic molecules of various sizes and charges.

Difference Between Porins and Aquaporins

Figure 01: Porin

There are two types of porins as general and selective. General porins are not substrate specific while selective porins specifically prefer chemical species based on the threshold sizes of the porins, and the amino acid residues lining them. Structurally, the majority of porins are monomers. But there are some dimmers as well as oligomeric porins.

What are Aquaporins?

Aquaporins are membrane proteins that form water channels. They belong to the major intrinsic protein (MIP) family. Polypeptide chains of these proteins span the membrane six times. Moreover, they have amino and carboxy termini that face the cytoplasm. Aquaporins allow water to pass freely. But they do not allow ions or metabolites to pass through them. Aquaporins are seen in plant cells as well as in animal cells. Moreover, they are seen in bacteria. In plants, aquaporins are abundant in tonoplast (vacuolar membrane). But there are aquaporins in the plasma membrane too.

Figure 02: Aquaporin

When considering animal cells, aquaporins are present in the plasma membranes of specific cell types such as plasma membranes of red cells, as well as kidney proximal and collecting tubules to maximize permeability to water. In the human body, more than ten different types of aquaporins are present.

Aquaporins mainly control the water content of a cell, facilitating the water movement. However, they do not act as pumps. Water movement across the aquaporins takes place in response to an osmotic or hydrostatic gradient. The driving forces behind water movement across the aquaporins are hydraulic or osmotic in nature. Besides these, aquaporins are permeable to some very small uncharged solutes such as glycerol, CO₂, ammonia, and urea. However, they are impermeable to charged solutes.

What are the Similarities Between Porins and Aquaporins?

Porins and aquaporins are membrane proteins.
They facilitate the transport of molecules across the plasma membrane.
They pass molecules passively.
Both are found in bacteria as well as eukaryotes.

What is the Difference Between Porins and Aquaporins?

Porins are water-filled pores and channels found in the membranes of bacteria and eukaryotes. Meanwhile, aquaporins are a family of water channel molecules that facilitate water transport through cell membranes in response to osmotic gradients. So, this is the key difference between porins and aquaporins. Moreover, porins transport hydrophilic molecules of various sizes and charges across the membrane passively while aquaporins facilitate water movement across the plasma membrane freely.

Furthermore, another difference between porins and aquaporins is that the porins transport charged hydrophilic molecules. Meanwhile, all aquaporins are impermeable to charged solutes.

Summary – Porins vs Aquaporins

Porins are water-filled pores that transport hydrophilic molecules of various sizes and charges. Aquaporins are the main membrane protein channels that transport of water across the plasma membrane. They are small, very hydrophobic, intrinsic membrane proteins. Unlike porins, aquaporins are impermeable to charged molecules. Thus, this is the summary of the difference between porins and aquaporins.

Reference:

1.“Porin (Protein).” Wikipedia, Wikimedia Foundation, 12 Oct. 2019, Available here.
2. “Aquaporin.” Aquaporin – an Overview | ScienceDirect Topics, Available here.

Image Courtesy:

1. “1pho opm” By Andrei Lomize – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Aquaporin-Sideview” By Vossman – Own work (CC BY-SA 3.0) via Commons Wikimedia