Flagellum in bacteria are very similar to those found in archaea. They consist of two parts: a central rod with a pair of appendages at its tip called a “tail” and a membrane or sheath around the tail. The flagellar apparatus consists of many specialized proteins that work together to propel the cell forward (or backward) along the length of the rod. These proteins are arranged into a complex network of filaments that move the cell forward.
In addition, flagella have the ability to rotate relative to the cell’s body. They are driven by a reversible chemical motor that converts the chemical energy into mechanical energy.
This rotary engine is powered by the hydrolysis of ATP (adenosine triphosphate) molecules. These energy-rich molecules are combined with H2O by the dissociation of the terminal phosphate group. Dissociation releases a lot of free energy, which is used to power the flagellar motor. The ATP is broken down into ADP (adenosine diphosphate) and an inorganic phosphate group. The waste product is used up in the rotary engine to convert the chemical energy into mechanical rotary motion.
flagellum structure in eukaryota
Eukaryotic flagella are made up of many different types of proteins that form a complex meshwork. The key to this meshwork is the dynein arms, which surround the entire bundle of microtubules that make up the flagellum.
These dynein arms are also known as the “motors” of the flagellum. They convert the chemical energy from the hydrolysis of ATP into mechanical energy that rotates the flagellum. The dynein arms are an interesting type of motor protein that has the ability to move in two directions while staying anchored to the same place. They have the ability to perform “back-and-forth” motion by using a ratchet-and-pawl mechanism. This motion causes the flagellum to rotate in one direction. The dynein arms are also responsible for the bending of the flagellum, since some are straight and some are bent. The bending is due to the dynein arms being anchored at different points along the length of the flagellum. The dynein arms that are anchored closer to the cell’s body are responsible for bending the flagellum tip. The flagellar motor is also complex. It consists of multiple motors, which are capable of moving in both directions. These multiple motors are interlinked together, so that they can rotate together. This interlinking of multiple motors is necessary for the one-way ratchet-and-pawl mechanism. This mechanism allows only one way of motion, which causes the flagellum to rotate in one direction. The motors themselves are powered by the hydrolysis of ATP. ATP is “fired” into the motors, causing them to rotate. As the motors rotate they cause the whole flagellum to turn.
Both types of flagella are used for different purposes. For instance, the bacterial flagellum is used to move bacteria around, while the eukaryotic flagellum is used to eject waste material and also to swim.
The eukaryotic flagellum starts off as a ciliary axonemal, which is a bundle of microtubules. These microtubules have a dynein arms anchored at intervals along them. The dynein arms are responsible for the motion of the flagellum. The ciliary axonemal is used for very different functions to the bacterial flagellum. They both work in different ways and have other features that distinguish them from each other.
(C) 1997-2017 Chris P. Schuster
Last modified: September 5th, 2016
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Sources & references used in this article:
- The archaeal flagellum: a different kind of prokaryotic motility structure (NA Thomas, SL Bardy, KF Jarrell – FEMS microbiology reviews, 2001 – academic.oup.com)
- Flagella in prokaryotes and lower eukaryotes (DF Blair, SK Dutcher – Current opinion in genetics & development, 1992 – Elsevier)
- Archaeal type IV pilus-like structures—evolutionarily conserved prokaryotic surface organelles (M Pohlschroder, A Ghosh, M Tripepi… – Current opinion in …, 2011 – Elsevier)
- Prokaryotic motility structures (SL Bardy, SYM Ng, KF Jarrell – Microbiology, 2003 – microbiologyresearch.org)