Flagellum Bacterial Cell: Function & Definition Video

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  • 0:00 Bacterial Motility & Flagella
  • 1:25 Structure & Synthesis
  • 2:15 Function
  • 3:40 Finding Food
  • 4:45 Examples
  • 5:20 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

In this lesson, we'll explore the flagellum, which is the structure that bacteria use for motility. We'll also examine the main characteristics of this specialized structure.

Bacterial Motility

What did you do the last time you felt hungry? You probably got up, walked into your kitchen, and grabbed a snack. The muscles in your legs coordinated the movement of your bones, propelling you in the direction you wanted to travel.

Now, imagine you're microscopically small, without any legs, muscles, or bones. How do you move to find food?

Bacteria are tiny, typically single-celled organisms that require food and nutrients just like any other living organism. Some bacteria are unable to move and are referred to as immotile. These bacteria must rely on environmental factors, like water flow, to provide the food and nutrients they need. Other bacteria have specialized structures that allow movement within the environment. These bacteria are referred to as motile, or capable of motion.


Flagella (singular: flagellum) are long, thin, whip-like appendages attached to a bacterial cell that allow for bacterial movement. Bacterial cells are typically between 0.1 micrometers and 50 micrometers in diameter, but average around 2 micrometers. Flagella can be several times longer than the cell, averaging 10 micrometers in length. Some bacteria have a single flagella protruding from one end of the cell, while others have many flagella surrounding the entire cell.

Structure and Synthesis

What exactly is a flagellum? The long, filamentous portion of the flagellum, known as the filament, is composed of a protein called flagellin. These proteins form long chains that give the flagellum a helical shape.

Close to the bacterial cell membrane, the flagellum gets wider and forms the hook, which attaches the long filament to the cell at the motor. The motor is a series of protein rings that span the cell membrane, anchoring the flagellum to the cell, and providing movement to the flagellum.

When a bacterium builds its flagellum, the motor is first synthesized in the membrane. Once the motor is complete, the hook is synthesized and pushed through the motor rings. Starting with the tip of the filament, the length of the flagellum is synthesized, one piece at a time, and slowly pushed through the rings until it reaches full size.


You'll notice that we refer to the base of the flagellum as a motor. This term accurately describes how the flagellum works. If you imagine an electric mixer, like the one you use in baking, there's a cord that supplies electricity to spin a motor, which then translates that spin to the attached mixing beaters.

The flagellum is very similar! The motor is plugged into the cell membrane, where it can be powered by capturing the energy of chemical gradients. This turns the flagellum motor, and the spin is translated to the rest of the flagella. The flagellum is able to spin up to 1,500 times per minute, and the spinning of the flagellum filament results in a whip-like motion that propels the cell forward.

Despite being so small, a bacterial cell powered by flagella can be faster than a cheetah. In actual numbers, the cheetah is able to run about 110 kilometers per hour. The bacteria can only reach a speed of 0.00017 kilometers per hour. Based on pure speed, the race isn't even close, but let's adjust for the huge size difference. At 110 kilometers per hour, the cheetah is able to move about 25 body lengths every 1 second. The bacteria, at 0.00017 kilometers per hour, is able to move 60 cell lengths every second! That's quite impressive for a single cell with no muscles or bones.

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