Plasmodesmata: Plant communication network
Plant cells are surrounded by dense cell walls, yet they are not isolated from one another. Connection between them is provided by plasmodesmata—fine cytoplasmic channels that literally pierce the cell walls through and through.
The lumen of each such channel is carefully lined with plasmalemma (cell membrane), making the transition from one cell to another smooth and continuous. The main function of plasmodesmata is to physically connect neighboring cells and ensure tightly regulated transport of substances.
Due to the numerous plasmodesmata piercing the tissues, the symplast is formed—a global aggregate of all plant cells united into a single continuous system.
Animal gap junctions (Macula communicans)
Animal cells lack a cell wall, but they also require rapid information exchange. To this end, there are gap junctions, which are localized in most tissues of the body. They can be found in epithelial and connective tissues, cardiac muscle, and nervous tissue.
A characteristic feature of such a junction is the extreme apposition of membranes of adjacent cells. The distance between the areas of the plasmalemma is only 2–4 nm.
In this narrow space, the membranes are pierced by special protein complexes called connexons.
The architecture of the connexon is strictly ordered:
- It consists of exactly 6 transmembrane proteins. These structural units are named connexins.
- Connexins are arranged in a circle, surrounding the central hydrophilic pore.
- The diameter of this pore is 1.5 nm.
Function and clinical significance of gap junctions
The main task of connexons is to ensure efficient "communication" between neighboring cells. Ions and small hydrophilic molecules can freely pass through the hydrophilic pores with a diameter of 1.5 nm.
If the function of gap junctions is impaired, cell-to-cell signaling inevitably suffers. A prime example of such pathology is cardiac arrhythmia.
Normally, heart muscle cells (cardiomyocytes) must contract synchronously, transmitting ionic signals through gap junctions. When connexon function in cardiomyocytes is impaired, this coordinated signal transmission fails, ultimately leading to a dangerous heart rhythm disorder.
Briefly: answers to topical questions
What factors regulate the opening and closing of the connexon pore?
The opening and closing of the connexon pore are regulated by membrane potential and various humoral factors. Connexons are voltage-gated channels, so changes in electrical charge affect their conformation.
Factors that close the channel and reduce its permeability:
- Increase in intracellular calcium ion concentration (changes protein conformation).
- Changes in pH (medium acidification, carbon dioxide accumulation).
- Toxic factors.
- Depolarization of either of the adjacent cells.
Factors promoting channel opening:
- Depolarization of the presynaptic membrane, which transitions the pore from a closed to an open state.
Remembering the composition of a gap junction is easy using the "rule of six": 1 CONNEXON (complex) = 6 CONNEXINS (proteins) forming the pore.
Class and exam questions
Where are gap junctions localized?
They are found in most animal tissues: epithelial, connective, nervous, and cardiac muscle.
What is the symplast?
This is the aggregate of all plant cells that are physically interconnected by cytoplasmic channels—plasmodesmata.
What passes through the connexon pore?
Ions and small hydrophilic molecules are able to pass through the hydrophilic pore with a diameter of 1.5 nm.
What else to study on this topic
- Mechanism of regulated substance transport through plasmodesmata
- Types of transmembrane connexin proteins in different tissues
- The process of ionic signal transmission between cardiomyocytes
- The role of gap junctions in nervous tissue (electrical synapses)
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