Cilia (singular cilium) and flagella (singular flagellum) are flexible extensions of the membrane of certain cells. The main purpose of these organelles is to aid in motility, or movement, of the organism they are attached to. Sometimes cilia help move along substances external to the cell. They are made from the same basic components, but differ subtly in their construction and thus in their appearance.
Imagine the picture of cilia and flagella to be like the fin of a shark or the oars of a boat. Only in an aqueous, or liquid, medium can cilia and flagella function effectively.
Thus the bacteria that have these structures can tolerate or thrive in wet environments. Eukaryotic flagella, such as those of sperm cells, differ substantially in composition and organization from prokaryotic flagella, but despite having evolved in different ways, their purpose is the same: to move the cell.
Cilia and flagella themselves consist of specific kinds of proteins and are anchored to the cell proper in a number of ways depending on the nature of the parent organism. Microtubules in general play a major role in the ongoing activity within cells, whereas what cilia and flagella do deals with events that are external to cells.
A Review of the Cell
The cell is the basic unit of life being the smallest entity that displays all of the properties formally associated with the process of life. Many organisms consist of only a single cell; almost all of these come from the classification called Prokaryota. Other organisms are classified as Eukaryota, and most of these are multicellular.
All cells have, at a minimum, a cell membrane, cytoplasm, genetic material in the form of DNA (deoxyribonucleic acid) and ribosomes. Eukaryotic cells, which are capable of aerobic respiration, have many other components as well, including a nucleus around the DNA and other membrane-bound organelles such as mitochondria, chloroplasts (in plants) and the endoplasmic reticulum.
Both prokaryotic cells and eukaryotic cells have flagella, whereas only eukaryotes have cilia. The flagella attached to bacteria are used to move the single-celled organism about, whereas the flagella and cilia of eukaryotic cells, which extend from the cell membrane but are not a part of it, participate in both locomotion and other functions.
What Are Microtubules?
Microtubules interact with the organelles and other components of eukaryotic cells. They are one of the three types of protein filaments found in these cells, the others being actin filaments or microfilaments, which are the thinnest of the three filaments, and intermediate filaments, which have a diameter greater than actin filaments but smaller than microtubules.
These three filaments make up the cytoskeleton, which serves the same basic purpose as the bony skeleton in your own body: It provides integrity and structural support, and its components also aid in mechanical processes within the cell, such as movement and cell division.
Microtubules, which are made of proteins appropriately called tubulins, are what form the mitotic spindle during mitosis in eukaryotic cells. These fibers connect to parts of paired chromosomes and pull them apart toward the poles of the cell.
Structures called centrioles, which themselves are made of microtubules, sit at both cell poles during mitosis and are responsible for synthesizing the mitotic spindle fibers.
What Cells Feature Cilia and Flagella?
Bacterial cells feature flagella in a number of characteristic arrangements and styles.
- Monotrichous bacteria, such as Vibrio cholerae, have one flagellum ("mono-" = "only"; "trich-" = "hair").
- Lophotrichous bacteria have multiple flagella fanning out from the same spot on the bacteria, marked by a polar organelle.
- Amphitrichous bacteria have one flagellum at each end, allowing for rapid direction changes.
- Peritrichous bacteria, such as E. coli, have various flagella pointing in all different directions.
The important flagella in eukaryotes are those that propel sperm cells, the male sex cells or gametes.
Eukaryotes feature a variety of cilia types, however. Cilia in the respiratory tract help move along mucus in a slow sweeping or "brush-like" manner. Cilia in the uterus and Fallopian tubes are needed to move an egg that has been fertilized by a sperm in the direction of the uterine wall, where it can implant itself and eventually grow into a mature organism.
Structure of Cilia and Flagella
Cilia and flagella are really no more than different forms of the same structure. While cilia are short and usually appear in rows or groups and flagella are long and often stand-alone organelles, there is no definitive reason a given example of one could not be relabeled as the other.
Both structures adhere to the same assembly format, which is the commonly cited – but somewhat misleading – "9 + 2" scheme.
This means that in each structure, a ring of nine microtubule elements surrounds a core of two microtubule elements. The central pair are enclosed in a sheath that is connected to the nine "ring" microtubule elements by radial spokes, while these outer nine tubes are connected to each other by proteins called dyneins.
Each of the nine ring microtubules is actually a doublet, one with 13 proteins forming the tube and one with 10. The two central microtubules also have 13 proteins. The 9 + 2 structure that forms the bulk of a cilium or flagellum is called an axoneme.
Cell Membrane Connections
The two central microtubules of a eukaryotic flagellum insert into the cell membrane at a plate near the surface. This plate sits above a centriole-like structure called a basal body.
These are cylindrical, like cilia and flagella themselves, but contain a nine-member ring of microtubules that have three subunits each, rather than the two each seen in the axoneme. The two central tubes of the axoneme end in the "transition zone" above the basal body and below the axoneme.
How Do Cilia Function?
Some cilia move the entire organism, whereas others move external matter, as described above. Some cilia function instead as sensory protuberances. Cilia usually project outward from the cell a distance of about 5 to 10 millionths of a meter. Those primarily concerned with movement of the cell are called "motile" cilia, and these beat mainly in one direction, more or less together. The motion of other kinds of cilia appears more random.
In both cilia and flagella, the motion of the extension is usually "whip-like," or back-and-forth, like the flickering tail of a tadpole. This is accomplished mainly using the dynein proteins between microtubules on the outside of the axoneme. The motion involves individual microtubule elements "sliding" past one another, causing the whole structure to bend in a given direction.
How Do Flagella Function?
When flagella beat in an aqueous medium, they generate a wave of energy that moves in that medium, and this in turn propels the organism along in the case of bacteria. Different bacteria, as noted, use different arrangements and numbers of flagella. Not covered before is the fascinating spirochete, a kind of bacteria that has a doubly-anchored flagella, with one insertion at one end and one at the other. When this structure beats, the result is spiral-like motion of the flagella.
The anchor in the cell of a bacterial flagellum differs from that of its eukaryotic counterpart. These flagella are powered by "motors" that sit inside this anchor, with the motion of the flagella itself being generated remotely, just like a propeller shaft moves thanks to the engine housed in the boat hull rather than resulting from processes in the shaft proper.
Also, in each of the nine microtubule doublets of a single eukaryotic flagellum, the two subunits are connected by proteins called nexins. These can cause each doublet to bend when activated, and when enough doublets bend the same way the axoneme as a whole responds and moves accordingly.