2.Which is true for gap junction
(1) It is made of connexin protein
(2) Allows free movement of very large molecules across cells
(3) Made up of single subunit of cylindrical connexions
(4) Exclusively involved in movement of gases

Gap Junction Structure

Gap junctions are specialized intercellular connections that form channels between adjacent cells, permitting the direct flow of ions and small molecules. The primary structural component is the connexin protein. Six connexin subunits assemble into a ring structure called a connexon, and two connexons (one from each cell) dock to create a complete gap junction channel.​

Connexins have four transmembrane domains, with cytoplasmic and extracellular loops. In humans, over 20 types of connexins exist, and channels may be homomeric or heteromeric depending on the specific combination of connexins used.​

Connexin Proteins: The True Building Block

Connexin proteins are essential for gap junction formation. Each connexon is made up of six connexin molecules, creating a cylinder-shaped hemichannel. Two such hemichannels align across adjacent cell membranes to form a complete gap junction channel. These channels are exclusively formed by connexins in vertebrates, although other protein families, like pannexins, exist, but serve other functions.​

Accurate Statement:

• “Gap junctions are made of connexin protein.” This is the foundation for their channel structure and function.​

Function and Selectivity of Gap Junctions

Gap junctions allow the passage of ions and small molecules—typically less than 1.5 kDa in size—between cells. This passage enables rapid electrical signaling and metabolic cooperation, particularly in tissues such as the heart, brain, and muscles. The pore diameter of the gap junction channels excludes very large molecules, restricting transfer to small chemicals, sugars, nucleotides, and ions.​

Incorrect Statement:

• “Allows free movement of very large molecules across cells.” This is not correct; only small molecules and ions can pass through the tightly regulated gap junction pores.​

Subunit Structure

Each gap junction channel is not made of a single subunit; it consists of twelve connexin proteins, six from each cell forming two connexons. The structure is distinctly multimeric, forming a complex channel rather than being composed of a single subunit.​

Incorrect Statement:

• “Made up of single subunit of cylindrical connexions.” This is inaccurate; each gap junction is formed from multiple subunits assembled as a complex.​

Role in Cell Physiology

Gap junctions chiefly facilitate intercellular communication through the passage of ions and small signaling molecules, not exclusively gases. While some gases may diffuse through, their main physiological role is in the transfer of ions (for electrical activity) and small biomolecules (for metabolic coordination).​

Incorrect Statement:

• “Exclusively involved in movement of gases.” Gap junctions function in broader cellular communication, with a major role in ion conductance and signaling, rather than only gas transfer.​

Key Facts at a Glance

Importance in Health and Disease

Gap junctions play crucial roles in maintaining coordinated cell responses. They are vital in cardiac muscle for synchronized contractions, in the brain for neural network function, and in tissues for cell growth and differentiation. Mutations in connexin genes have been linked to several diseases, including deafness, certain skin disorders, and neurodegenerative conditions.​

Summary

• The only true statement among the options about gap junctions is that they are made of connexin protein.​

• Gap junctions allow passage of small molecules and ions, not large molecules or exclusively gases.​

• Their structure comprises multiple connexin proteins, forming complex channels, and they are fundamental to intercellular signaling and tissue health.