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The purpose of this essay is to compare and contrast the cellular organelles, cilia and flagella. According to (Russell et al., 2008) flagella (singular: flagellum) and cilia (singular: cilium) are thin, self-propagating structures found on the surfaces of cells. These organelles are typically found on certain specialised cells.
Within this assignment, I shall be examining the relative similarities and differences between the structures of these organelles, and how their structure relates to the roles that they are required to perform. In addition to this, the specific mechanisms which drive the movement of both structures when performing their functions will also be addressed.
Through direct comparison and contrast, I aim to determine whether cilia and flagella are in fact more similar than different, or whether their distinctions render them unlike on the whole.
It is established that there are two main types of cilia; motile, and that of non-motile. There are also three main types of flagella; bacterial, archaeal and eukaryotic. However, for the purpose of this essay, and in order to conform to the word limit, the two types of cilia will be grouped as eukaryotic cilia. Eukaryotic flagella will remain the same, whilst bacterial and archaeal flagella shall be grouped as prokaryotic flagella.
To begin with, the structure of eukaryotic cilia shall be compared with that of eukaryotic flagella. Cilia are found on eukaryotic cells and are approximately 0.2Âµ in diameter and up to 25Âµ in length. In comparison, eukaryotic flagella are of a similar diameter, but tend to be of a greater length. They may extend any length up to 1000Âµ (Toole, 1999).
Another difference between these two organelles is that eukaryotic flagella are found to be present in far fewer numbers when compared to eukaryotic cilia. Typically, only one or two flagella are located on a cell as is the case with the green alga: Chlamydomonas (Brooker et al., 2011). (Aizawa, 2009) states that the term 'monotrichous' pertains to an organism with only one flagellum, whereas organisms with two or more flagella are 'multitrichous'. In contrast, eukaryotic cilia usually completely cover the surfaces they are found upon; for example, epithelial cells in the human respiratory tract have a 'density of 10^9 cilia/cm2' which equates to approximately 200 cilium organelles per individual cell, according to (Toole, 1999).
A great similarity between both eukaryotic cilia and eukaryotic flagella is that they both consist of a bundle of microtubules running throughout its length. This assortment of microtubules extends from a centriole-like structure known as a basal body, and is arranged in a 9+2 complex. They are referred to as a 9+2 complex as they consist of 9 outer doublets of microtubules with 1 central doublet. (Russell et al., 2008). Collectively, this system of microtubules is known as an axoneme and is present in both eukaryotic cilia and eukaryotic flagella (Stevens and Lowe, 2005).
To add force to the notion that both eukaryotic cilia and eukaryotic flagella are almost identical with regards to their internal structure, both have a similar arrangement of internal proteins. In cilia, the protein dynein is found at intervals of 24nm. This can be referred to as the "molecular motor" as it is the underlying mechanism which results in the propagation and movement of the organelle. The linking protein, nexin, can also be found at intervals of 86nm. The purpose of these is to hold the microtubules in place. Finally, radial spokes are present from each of the 9 outer doublets of microtubules, and serve to attach themselves to the central pair of microtubules; their purpose being to aid the waveform which comes about as a result of the actions of dynein (Stevens & Lowe, 2005). Eukaryotic flagella have an exceptionally similar structure to cilia (Alberts et al., 1997).
With regards to prokaryotic flagella, in comparison to eukaryotic cilia and flagella, there are more notable differences to be observed. The main variation between the two is that the structure is altogether completely different. The axonemal 9+2 microtubular arrangement found in eukaryotic cilia and eukaryotic flagella is not present in prokaryotic flagella. Although prokaryotic flagella maintain a basal body as part of its structure, other components are dissimilar; the other main components being a filament, a sheath and a hook (Bhat and Shimkets, 2009).
Another significant observation is the difference in internal proteins. With eukaryotic cilia and flagella, the microtubules consist of molecules of the protein tubulin (Stevens & Lowe, 2005). In contrast, the filament of prokaryotic flagella consists of the protein flagellin, with the internal proteins dynein and nexin also not being present (Indge, 2000).
A further difference to be noticed is that the size of prokaryotic flagella are smaller than that of eukaryotic cilia and flagella, with their diameter ranging between approximately 15-25nm (Margulis, 1980).
The structure of an organelle is often heavily related to its function, as is the case with eukaryotic cilia and eukaryotic flagella. This is also true of prokaryotic flagella. It has already been established that eukaryotic cilia and eukaryotic flagella are structurally very alike and therefore it should come as no surprise that their functions are also closely related. The two organelles are responsible for aiding movement of the cell they are attached to (Brooker et al., 2008).
According to (Alberts et al., 1997), eukaryotic cilia are the main mode in which some protozoa are able to move. Cilia enable protozoa, such as Paramecium, to move by sweeping fluid over its external surface (Brooker et al., 2011) & (Alberts et al., 1997).
Examples of other cells facilitated by cilia are epithelial cells located in the trachea of the human respiratory system and in the fallopian tubes of mammals (Stevens and Lowe, 2005). In the trachea, cilia sweep unwanted substances, such as mucus and trapped dirt, away to be expelled from the lungs. The cilia forming the lining of the oviduct wall also perform the role of movement, in this case the movement of an ovum along the oviducts (Alberts et al., 1997). 0020
Eukaryotic flagella have a similar function to eukaryotic cilia in the sense that they too enable the movement of some protozoa. One of the most important examples of where eukaryotic flagella are utilised are the spermatozoa. Here, sperm cells are propelled forward due to the action of the eukaryotic flagella (Alberts et al., 1997) & (Brooker et al., 2011).
Prokaryotic flagella are also functionally designed to propel cells through fluid, as is the case with some Gram-negative bacteria (Brooker et al., 2011).
It can thus be said that a common function of eukaryotic cilia and flagella, and prokaryotic flagella, is their purpose to propel the cell or protozoa that they are attached to as a form of locomotion. However, eukaryotic cilia and flagella differ from prokaryotic flagella as they also have the function of propagating the movement of external substances such as food particles or dirt particles (Alberts et al., 1997).
I will now move on to touch upon the mechanisms by which the organelles in question perform their functions. The dynein 'molecular motor' proteins within eukaryotic cilia and flagella allow for them to move in the characteristic manner in which they do. Activation of the dynein protein molecules results in the sliding of the axonemal microtubules. However, due to the presence of the linking protein nexin, and the radial spokes, sliding of the microtubules is prohibited, resulting in a bending action (Russell et al., 2008).
Since both eukaryotic cilia and flagella both have very similar structures, it may be presumed that their movements would follow to the same effect, yet there are slight differences in the beating of the structures. Whilst eukaryotic cilia move via a series of undulations, eukaryotic flagella move in a more whip-like manner, due to bending of the flagella taking place throughout its length, from the base to the tip (Russell et al., 2008).
Prokaryotic flagella cannot act in the same way as we have already established the absence of both dynein motor proteins and core microtubules. Rather, prokaryotic flagella operate in a rotating manner due to a revolving basal body embedded in the cell membrane which acts as the driving motor. The motion created by this mechanism of action can be likened to that of a propeller of a boat (Russell et al., 2008).
In the above paragraphs, information has been gathered concerning the structure, function and the mechanisms of action of both eukaryotic cilia and flagella, and prokaryotic flagella. Taking all of these factors into account, it is evident that eukaryotic cilia have more in common with eukaryotic flagella than they do differences. To add to this, I have acknowledged that there are marked differences between the structure and mechanism of action between eukaryotic cilia and flagella, and prokaryotic flagella. However, the fact that the essential purpose for each of them as being motile structures for locomotion leads me to conclude that cilia and flagella are more similar than different.