Flagella: Types, Function & Structure

Cell mobility is a key component for the survival for many single-cell organisms, and it can be important within more advanced animals as well. Cells use flagella for locomotion to look for food and to escape danger. The whiplike flagella can be rotated to promote motion via a corkscrew effect, or they can act like oars to row cells through liquids.

Flagella are found in bacteria and in some eukaryotes, but those two types of flagella have a different structure.

A bacterial flagellum helps beneficial bacteria move through the organism and helps disease-causing bacteria to spread during infections. They can move to where they can multiply, and they can avoid some of the attacks from the immune system of the organism. For advanced animals, cells such as sperm move with the aid of a flagellum.

In each case, the motion of the flagella permits the cell to move in a general direction.

The Structure of Prokaryotic Cell Flagella Is Simple

Flagella for prokaryotes such as bacteria are made up of three parts:

  1. The filament of the flagellum is a hollow tube made of a flagellar protein called flagellin.
  2. At the base of the filament is a flexible hook that couples the filament to the base and acts as a universal joint.
  3. The basal body is made up of a rod and a series of rings that anchor the flagellum to the cell wall and the plasma membrane.

The flagellar filament is created by transporting the protein flagellin from cell ribosomes through the hollow core to the tip where the flagellin attaches and makes the filament grow. The basal body forms the motor of the flagellum, and the hook gives the rotation a corkscrew effect.

Eukaryotic Flagella Have a Complex Structure

The motion of eukaryotic flagella and those of prokaryotic cells is similar, but the structure of the filament and the mechanism for rotation are different. The basal body of eukaryotic flagella is anchored to the cell body, but the flagellum lacks a rod and disks. Instead, the filament is solid and is made up of pairs of microtubules.

The tubules are arranged as nine double tubes around a central pair of tubes in a 9 + 2 formation. The tubules are made up of linear protein strings around a hollow center. The double tubes share a common wall while the central tubes are independent.

Protein spokes, axes and links join the microtubules along the length of the filament. Instead of a motion created at the base by rotating rings, the flagellum motion comes from interaction of the microtubules.

Flagella Work Through Rotational Motion of the Filament

Although bacterial flagella and those of eukaryotic cells have a different structure, they both work through a rotational movement of the filament to propel the cell or move fluids past the cell. Shorter filaments will tend to move back and forth while longer filaments will have a circular spiral motion.

In bacterial flagella, the hook at the bottom of the filament rotates where it is anchored to the cell wall and plasma membrane. The rotation of the hook results in a propeller-like motion of the flagella. In eukaryotic flagella, the rotational motion is due to the sequential bending of the filament.

The resulting motion can be whiplike in addition to rotational.

The Prokaryotic Flagella of Bacteria Are Powered by a Flagellar Motor

Under the hook of bacterial flagella, the the base of the flagellum is attached to the cell wall and the cell's plasma membrane by a series of rings surrounded by protein chains. A proton pump creates a proton gradient across the lowest of the rings, and the electrochemical gradient powers rotation through a proton motive force.

When protons diffuse across the lowest ring boundary due to the proton motive force, the ring spins and the attached filament hook rotates. Rotation in one direction results in a controlled forward motion of the bacterium. Rotation in the other direction makes the bacteria move in a random tumbling fashion.

The resulting bacterial motility combined with the change in direction of rotation produces a kind of random walk that allows cell to cover a lot of ground in a general direction.

Eukaryotic Flagella Use ATP to Bend

The base of the flagellum of eukaryotic cells is firmly anchored to the cell membrane and the flagella bend rather than rotate. Protein chains called dynein are attached to some of the double microtubules arranged around the flagella filaments in radial spokes.

The dynein molecules use energy from adenosine triphosphate (ATP), an energy storage molecule, to produce bending motion in the flagella.

The dynein molecules make the flagella bend by moving the microtubules up and down against each other. They detach one of the phosphate groups from the ATP molecules and use the liberated chemical energy to grab one of the microtubules and move it against the tubule to which they are attached.

By coordinating such bending action, the resulting filament motion can be rotational or back and forth.

Prokaryotic Flagella Are Important for Bacterial Propagation

While bacteria can survive for extended periods in the open air and on solid surfaces, they grow and multiply in fluids. Typical fluid environments are nutrient-rich solutions and the interior of advanced organisms.

Many of these bacteria, such as those in the gut of animals, are beneficial, but they have to be able to find the nutrients they need and avoid dangerous situations.

Flagella allow them to move toward food, away from dangerous chemicals and to spread when they multiply.

Not all bacteria in the gut are beneficial. H. pylori, for example, is a flagellated bacterium that causes stomach ulcers. It relies on flagella to move through digestive system mucus and avoid areas that are too acid. When it finds a favorable space, it multiplies and uses flagella to spread out.

Studies have shown that the H. pylori flagella are a key factor in the infectiousness of the bacteria.

Bacteria can be classified according to the number and location of their flagella. Monotrichous bacteria have a single flagellum at one end of the cell. Lophotrichous bacteria have a bunch of several flagella at one end.

Peritrichous bacteria have both lateral flagella and flagella at the ends of the cell while amphitrichous bacteria can have one or several flagella at both ends.

The arrangement of the flagella influences how rapidly and in what way the bacterium can move.

Eukaryotic Cells Use Flagella to Move Inside and Outside Organisms

Eukaryotic cells with a nucleus and organelles are found in higher plants and animals but also as single-celled organisms. Eukaryotic flagella are used by primitive cells to move around, but they can be found in advanced animals as well.

In the case of single-cell organisms, the flagella are used to locate food, to spread and to escape from predators or unfavorable conditions. In advanced animals, specific cells use a eukaryotic flagellum for special purposes.

For example, the green algae Chlamydomonas reinhardtii uses two algal flagella to move through the water of lakes and rivers or soil. It relies on this motion to spread after reproducing and is widely distributed around the world.

In higher animals, the sperm cell is an example of a mobile cell using eukaryotic flagellum for motion. This is how sperm move through the female reproductive tract to fertilize the egg and begin sexual reproduction.

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