What are Cilia?
The cilia are a series of short, mobile, numerous extensions of the plasma membrane that line the cell surface of some eukaryotic organisms.
The cilia have an internal structure made up of proteins and microtubules that allow the cell to move and transport materials over the epithelia, as well as the movement of fluids both in the respiratory tract and in the reproductive system.
The movements of the cilia are rhythmic and coordinated, they can be illustrated as the movement that the wheat fields make when they are shaken by the wind. This movement is possible because the cilia receive energy from proteins in the form of ATP, and allow the transport of single-cell cells and particles.
The cilia perform several important functions thanks to their rhythmic movements, such as protecting against the attack of microorganisms in the respiratory tract by allowing the expulsion of particles accumulated in the mucosa, such as dust.
Also, in the reproductive system the cilia make it possible for the egg to move from the fallopian tubes to the uterus. They also move the water around the gills, among others.
On the other hand, the cilia share some characteristics with the flagella, which are few structures (1 or 2) in the cells of eukaryotic organisms, which allow their movement, however, they are a structure with different functions.
Cilia are structures that can move and allow the movement of various fluids and particles, hence they can perform the following functions:
- Generate small currents of motion near the plasma membrane that attract food.
- Regulate the nervous system.
- Allow fluid displacement.
- Allow the displacement of particles located on its surface.
- Allow the propulsion of protic unicellular organisms.
- Allow the movement of mucosa in the airways.
- Allow the movement of gametes in the reproductive system.
- Regulate the water balance of the excretory organs.
- Filter the particles that pass through the gills.
Cilia have a diameter of approximately 0.25 μm and a length between 5 and 50 μm. Here is how the cilia are structured:
Axoneme or stem: they are composed of two simple central microtubules that are surrounded by 9 doublets of external microtubules, this arrangement is known as (9 + 2). Microtubules allow the cilia to move and are associated with proteins called molecular motors (kinesin and dynein).
The doublets of the central microtubules contain nexin. On the other hand, in the 9 outer microtubule doublets, two microtubules can be distinguished:
- Microtubule A: contains 13 protofilaments and is complete. From this microtubule, two arms with protein dynein are attached to microtubule B. This union allows the cilia to move.
- Microtubule B: contains 10 protofilaments, of which it shares three with microtubule A.
Transition zone: there is a change in the axoneme structure of (9 + 2) with the structure of the basal corpuscle (9 + 0). In this process the central microtubules disappear, so that the external doublets become triplets.
Basal or centriole corpuscle: it is located below the cytoplasmic membrane. It contains nine triplets and lacks the pair of the central microtubules, that is (9 + 0). It is a cylinder that is located at the base of the cilium and allows the union of the axoneme with the cell, as well as the organization of microtubules.
In general, the microtubules are anchored to the basal corpuscle by the ciliary roots that extend into the cell, which gives more stability to the movements of the cilia.