In both the animal and plant kingdoms, cells must be able to communicate with each other to ensure survival. A number of channels and junctions exist that bridge cells and allow for substances and messages to cross between them. Two major examples include plasmodesmata and gap junctions, but they possess important differences.
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In both plants and animals, cells need a way to communicate with each other, to pass along important signals for immune response and to allow materials to flow across membranes to other cells. Gap junctions in animals and plasmodesmata plants are two similar types of channels, but they possess distinct differences from each other.
What Is a Gap Junction?
Gap junctions are a form of connecting channel found in animal cells. Plant cells do not possess gap junctions.
A gap junction is comprised of connexons, or hemichannels. Hemichannels are made by the endoplasmic reticulum of cells, and relocated to the cell membrane by the Golgi apparatus. These molecular structures are made from transmembrane proteins named connexins. Connexons line up to form a gap junction in between neighboring cells.
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Gap junctions serve as channels to allow in crucial substances such as small diffusible molecules, micro RNAs (miRNAs) and ions. Larger molecules like sugars and proteins cannot pass through these tiny channels.
Gap junctions must work at different speeds for communication between cells. They can open and close quickly when rapid response is needed. Phosphorylation plays a role in the regulation of gap junctions.
Types of Gap Junctions
So far, scientists have found three main types of gap junctions in animal cells. Homotypic gap junctions possess identical connexons. Heterotypic gap junctions are made of different types of connexons. Heteromeric gap junctions can either have identical connexons or different ones.
The Importance of Gap Junctions
Gap junctions work to allow certain materials to pass between neighboring cells. This is paramount to maintaining the health of an organism. For example, myocardial cells of the heart need rapid communication via ion flow in order to work properly.
Gap junctions are also essential to immune system responses. Immune cells use gap junctions to generate responses in healthy cells as well as infected or cancerous cells.
Gap junctions in immune cells allow calcium ions, peptides, and other messengers to pass through. One such messenger is adenosine triphosphate or ATP, which serves to activate immune cells. Calcium (Ca2+) and NAD+ each serve as signaling molecules related to cellular function throughout a cell’s life.
RNA is also allowed to cross through gap junctions, but the junctions prove to be selective about which miRNAs are allowed.
Gap junctions also are important in certain cancers and blood disorders such as leukemia. Researchers are still discerning how the communication between stromal cells and leukemic cells works.
Scientists seek to discover more information about different blockers of gap junctions, to enable the production of novel drugs that can help treat immune disorders and other diseases.
What Are Plasmodesmata?
Given the important role of gap junctions in animal cells, you might wonder if they also exist in plant cells. However, gap junctions are absent in plant cells.
Plant cells contain channels called plasmodesmata. Edward Tangl first discovered these in 1885. Animal cells do not harbor any plasmodesmata per se, but scientists have discovered a similar channel that is not a gap junction. There are a number of structural differences between plasmodesmata and gap junctions.
So what are plasmodesmata (plasmodesma if singular)? Plasmodesmata are tiny channels that bridge plant cells together. In this regard, they are quite similar to the gap junctions of animals’ cells.
However, in plant cells, plasmodesmata must cross primary and secondary cell walls to allow signals and materials across. Animal cells do not possess cell walls. So plants need a way to get through cell walls, since plant plasma membranes do not directly contact each other in plant cells.
Plasmodesmata are generally cylindrical and lined with plasma membrane. They possess desmotubules, narrow tubes made from smooth endoplasmic reticulum. The newly-formed primary plasmodesmata tend to cluster together. Secondary plasmodesmata develop as cells expand.
The Functions of Plasmodesmata
Plasmodesmata allow the passage of specific molecules between plant cells. Without plasmodesmata, necessary materials could not pass between the rigid cell walls of plants. Important materials that pass through plasmodesmata include ions, nutrients and sugars, signaling molecules for immune response, occasionally larger molecules like proteins and some RNAs.
They also generally serve as a kind of filter to prevent much larger molecules and pathogens. However, invaders can force the plasmodesmata to open up and override this defense mechanism of plants. This change in the permeability of plasmodesmata is just one example of their adaptability.
Regulation of Plasmodesmata
Plasmodesmata can be regulated. One prominent regulatory polymer is callose. Callose builds up around plasmodesmata and works to control what can enter them. Increased amounts of callose result in less movement of molecules through plasmodesmata. It does this by essentially squeezing the pore’s diameter. Permeability can be increased when there is less callose.
Sometimes larger molecules can pass through plasmodesmata, by widening their pore size or dilating them. This is unfortunately sometimes taken advantage of by viruses. Researchers are still learning about the exact molecular makeup of plasmodesmata and how they work.
Variations of Plasmodesmata
Plasmodesmata possess different forms in different roles in plant cells. At their most basic form, they are simple channels. However, plasmodesmata can make more advanced and branching channels. These latter plasmodesmata work more as filters that control movement depending on the plant tissue type. Some plasmodesmata work as sieve while others work as a funnel.
Other Types of Junctions Between Cells
In human cells, four types of intracellular junctions can be found. Gap junctions are one of these. The other three are desmosomes, adhering junctions and occluding junctions.
Desmosomes are little connectors needed between two cells that often endure exposure, such as epithelial cells. The connection is comprised of cadherins, or linker proteins.
Occluding junctions are also called tight junctions. They occur when two cells’ plasma membranes fuse. Not many substances can get through the occluding or tight junction. The resulting seal serves a protective barrier against pathogens; however, these can sometimes be overcome, opening up the cells to attack.
Adhering junctions can be found under occluding junctions. Cadherins connect these two kinds of junctions. Adhering junctions are adjoined via actin filaments.
Yet another connector is the hemidesmosome, which uses integrin rather than cadherins.
Recently, scientists have discovered that both animal cells and bacteria do contain similar cell membrane channels to plasmodesmata, which are not gap junctions. These are called tunneling nanotubes, or TNTs. In animal cells, these TNTs can allow vesicular organelles to move between cells.
While there are many differences between gap junctions and plasmodesmata, they both play a role in allowing intracellular communication. They pass cell signals, and they can be regulated to allow or refuse certain molecules to cross. Sometimes viruses or other disease vectors can manipulate them and alter their permeability.
As scientists learn more about the biochemical makeup of both kinds of channels, they can better adjust or make new pharmaceuticals that can prevent disease. It is clear that intracellular membrane-lined pores are prevalent in many species, and it seems likely that new channels have yet to be discovered in bacteria, plants and animals.