Cilia and flagella

Some cells have cilia or flagella on their cell surface. What are these structures for? Well, whenever these structures are present one can be sure that there is movement going on.

Cells lining our respiratory tract for example use their cilia to move mucus and trapped particles towards the mouth. Movement is usually towards one direction, so in a sense the cilia act somewhat like escalators that move people or things upwards or downwards.

We are familiar of course with the flagellum of sperm cells. This propels the sperm as it moves along the reproductive tract of females.

Structurally, both cilia and flagella consist of microtubules that are arranged in a specific manner together with associated proteins dynein and kinesin. They only differ in length and number as well as in the kind of movement. Cilia are shorter and more numerous than flagella. Ciliary motion is also more like the power stroke in swimming while flagellar movement is a wavelike motion.

By the way, ciliated unicellular organisms like the Paramecium use their cilia not only for moving about but also for moving food towards their oral groove or "mouth".

microtubules

So far I have only mentioned the functions of the microfilaments as part of the cell’s bones and muscles or cytoskeleton. To even up matters, I’ll talk about the microtubules today.

The microtubules consist mainly of the protein tubulin which has 2 phases, the alpha and beta tubulin. These tubulin molecules form a tube like structure that can elongate at one end and shorten at the other. This is a continuously occurring process so the microtubules and also the microfilaments are always in a state of dynamic instability. That means that nothing is permanent with the cell’s cytoskeleton.

Microtubules serve as scaffolding inside cells and act as “tracks” on which cells can move organelles, chromosomes, vesicles and other things inside. In other words, they act like bullet trains inside cells. Microtubules are also responsible for the movement of cilia and flagella. Imagine that, molecules that can act as scaffolding, train, and propeller at the same time! Yessiree, those are your microtubules.

In order to do their function however, microtubule need to associate with proteins like dynein and kinesin. These two serve as motors to power the movement of microtubules. If something goes wrong with these motors, then any of the movements mentioned above will not be possible. Sperm cells for example will be immotile if dynein is absent in their flagellum.

cytokinesis

Cytokinesis is another activity that is generated by the cytoskeleton, particularly the microfilament.

After the chromosomes of a cell separate during anaphase, the microfilaments together with their associated protein, myosin, create a contractile ring somewhere near the middle of a cell. This ring tightens like a purse string until finally the cell is divided into two. This division completes the final stage of mitosis wherein two new cells with the same chromosome number are formed.

Separation of chromosomes and cytokinesis have to be properly coordinated so that the chromosome number of each generation of cells remain the same. If the timing of these two processes is off, we can end up with cells that have abnormal chromosome number or cells that can develop into cancerous ones.

amoeboid movement

Amoeboid movement is one example of how the microfilaments (the cell’s muscles) function. It is the same kind of movement that is involved in phagocytosis or “cell eating” which is the subject of my post last Nov. 19.

Biology – Online dictionary defines it as “A crawling-like type of movement in which the cell forms temporary cytoplasmic projections called pseudopodia (false feet) towards the front of the cell”.

Aside from the Amoeba, other cells that exhibit amoeboid movement are: neutrophils and macrophages (our professional phagocytes, remember?), monocytes (another kind of white blood cell), Kupffer cell of the liver, as well as cancer cells. Yes, cancer cells. This is the way by which cancer cells metastasize or spread to other parts of the body.

cell bones and muscles

The cell has its own bones and muscles called the cytoskeleton. This cytoskeleton has 3 major components: microfilaments, intermediate filaments and microtubules.

These major components are usually associated with other proteins. The association enables them to do several functions like formation of scaffolding inside the cell, ciliary or flagellar movement on the cell surface and internal cell movements like chromosomal movement during mitosis.

A summary of the functions of the cytoskeleton is shown in the table below.

Microfilaments Intermediate Filaments Microtubules
Muscle contraction Support and tensile strength Cell motility
(cilia and flagella)
Amoeboid movement Maintenance of cell shape Chromosome movement
Cell locomotion Formation of nuclear lamina Movement of
and scaffolding organelles
Cytoplasmic streaming Strengthening of nerve cell axons Determination of cell shape
Cell division (cytokinesis) Keeping muscle fibers in Maintenance of cell
register shape
Maintenance of cell shape


I will talk about each of these functions in a future post.