Flagella: Definition, Structure & Functions

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  • 0:01 What is a Flagellum?
  • 1:09 Structure & Types
  • 4:00 Functions Other Than…
  • 4:27 Lesson Summary
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Lesson Transcript
Instructor
Nicholas Gauthier
Expert Contributor
Maria Airth

Maria has a Doctorate of Education and over 20 years of experience teaching psychology and math related courses at the university level.

One of the more dramatic ways that single-celled organisms get around is a whip-like structure called a flagellum. Learn the ways that different organisms make use of their flagella to move around their microscopic world.

What is a Flagellum?

A flagellum is a whip-like structure that allows a cell to move. They are found in all three domains of the living world: bacteria, archaea, and eukaryota, also known as protists, plants, animals, and fungi. While all three types of flagella are used for locomotion, they are structurally very different.

Other cells have different means of locomotion. Cilia are similar to flagella in structure and function, but a cilium is shorter and moves differently. A ciliated cell usually has hundreds or thousands of cilia, which move in unison like little oars. Sometimes a cell will also use cilia to funnel food into an oral groove. Various species of paramecium employ cilia for both purposes.

Some cells get around by pushing cytoplasm into an extension of the cell membrane, forming a structure called a pseudopod. This is true of Amoebas and white blood cells in our bloodstream.

Structures and Types of Flagella

Far from being a simple hair-like structure, the eukaryotic flagellum has a complex cross-section. It is similar to a cilium in structure, though cilia generally move in a back and forth motion, as opposed to the corkscrew movement of a flagellum.

The eukaryotic flagellum is a long, rod-like structure that is surrounded by an extension of the cell membrane like a sheath. The bulk of the structure is a filament called an axoneme. Necessary materials are transported along the flagellum. The whole structure is anchored in a basal body, which is similar to a centriole in structure.

The axoneme has nine pairs of microtubules supporting it from within. These microtubule doublets surround two single microtubules. This arrangement is called the 9 + 2 structure. Eukaryotic cilia also have this structure; the cilia are simply shorter.

The nine-microtubule doublets have dynein arms that are powered by ATP. The arms cause the microtubules in each pair to slide against one another. This causes the flagellum to bend, allowing the cell to move. Radial spokes extend toward the central microtubules. Their role is not known, but they may play a role in stabilizing the flagellum.

The flagella of domains bacteria and archaea are different. They still move the cell, but they do so by rotating, rather than by bending from the inside like eukaryotic flagella.

The flagella of archaeans are superficially similar to those of bacteria. However, there are key differences. To begin with, the two types of flagella move differently. Bacterial flagella are powered by the flow of ions, usually hydrogen, but sometimes sodium. They are composed of individual fibers that rotate individually. By contrast, achaeal flagella appear to be powered by ATP, the same chemical that powers eukaryotic flagella. Archaeal fibers are also bundled and rotate as one.

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Additional Activities

Recreate the Flagella Project

Diagramming items is a common assignment in science courses to help students focus on the structures and functions of the item. However, without activating multiple parts of the brain, a simple drawn diagram is easily forgotten. In this activity, students will continue their research of flagella in order to use common everyday items to recreate each of the three types of flagella, thus solidifying their understanding of the structures and functions of each.

Materials

  • You may choose to limit your students' material lists to certain areas of the home or classroom. For example:
    • Use items you might find in your kitchen
    • Use items you might find in your bedroom
  • You may choose to supply students with a wide range of art supplies to allow for a one-session activity completion time frame
  • Printed images of cross sections of each of the flagellum discussed in the lesson (you may screen print from the video if desired)

Instructions

  • After watching the video lesson, spend some time reviewing the vocabulary terms from the lesson. Ask your students to define each of the following terms and describe their function(s):
    • flagellum
    • cilia
    • pseudopod
    • eukaryotic flagellum
    • axoneme
    • basal body
    • dynein arms
    • bacterial flagella
    • achaeal flagella
    • euglena
  • Tell your students that they are going to create one of these flagella using common items from a chosen area (you decide the area or supply the art materials if you choose).
  • Students may use the provided print out images of the flagella or may research their own images from which to work.
  • Instruct your students to include the following in their reconstructions:
    • Properly scaled representations of the parts of the chosen flagellum.
    • Labels/annotation that explains each part of the flagellum as well as its function within the system.
    • If using commonly found items, students must include support for why the chosen items would do the job of the part it represents.
  • Allow time for students to present their creations.

Extension

  • To increase the difficulty of this activity, it can be assigned as a long-term project in which students build recreations of all three of the discussed flagella. In this scenario, students should also compose a brief essay comparing and contrasting the different flagella types.

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