Which Ions Power a Bacterial Flagellar Motor?

A bacterium can swim by spinning a tiny propeller called a flagellum. But what powers the motor that turns it? For many bacteria, the answer is a flow of hydrogen ions across the cell membrane. Others use sodium ions instead.

The key is not simply that ions cross the membrane. It is that they cross through specific motor proteins, releasing stored energy that the motor converts into rotation.

The short answer: protons or sodium ions

An ion is an atom or molecule with an electrical charge. The two main ions used to power bacterial flagellar motors are:

Both carry a positive electrical charge. Which one a motor uses depends on the bacterium and the motor’s protein machinery.

Most bacterial flagellar motors are powered by protons; some are powered by sodium ions.

That answers which ions matter for the motor. A bacterial membrane also contains many other transport systems, so this is not a complete list of all the ions that can cross it.

Why the membrane matters

The cell membrane is a thin, mostly fatty barrier that separates the cell’s interior from its surroundings. Charged particles cannot easily pass straight through its fatty interior. They generally need a route provided by a membrane protein.

This barrier lets a cell maintain different conditions on its two sides. For the flagellar motor, two differences are especially important:

• Ion concentration: there may be more of the relevant ion on one side than on the other.
• Electrical voltage: the two sides may differ in electrical potential, which influences the movement of charged particles.

Together, these create an electrochemical gradient—a difference in both chemical and electrical conditions that can drive ions across the membrane.

Think of a reservoir with a controlled outlet

Water held behind a dam stores energy. When it flows through an outlet, some of that energy can do useful work.

An ion gradient offers a similar opportunity. The membrane helps preserve the difference between the two sides, while protein pathways provide controlled routes across it. For an operating flagellar motor, the driving gradient typically sends its working ions from outside the cell toward the inside.

The comparison is useful, but incomplete: ions respond not only to differences in concentration, but also to electrical forces. Maintaining this stored energy requires the cell’s ongoing metabolic activity.

Where the ions cross: the stator

The ions do not pass through the long flagellar filament itself. They cross the membrane through pathways in proteins belonging to the motor’s stator.

The names help you picture the machinery:

• The stator is the stationary part of the motor.
• The rotor is the part that turns.
• The flagellum is the external structure driven by that rotation.

Stator proteins sit in the membrane around the rotor. As the appropriate ions pass through them, ion interactions drive changes in the proteins that help exert torque—a turning force—on the rotor.

You can summarize the energy transfer in four steps:

1. The cell maintains an electrochemical gradient.
2. Protons or sodium ions move down that gradient through the stator.
3. The stator converts energy from that movement into a turning force.
4. The rotor spins, driving the flagellum.

A molecular motor, not a miniature waterwheel

It is tempting to imagine ions striking tiny blades. That is not the best picture. The motor works through molecular interactions: ions interact with particular sites in stator proteins, and those interactions are coupled to mechanical motion.

The essential idea is simple:

The membrane stores an energy difference; the stator provides a route for ions to cross and machinery that turns their movement into rotation.

So when you ask which ions cross the membrane to power bacterial swimming, remember protons most often, sodium ions in some motors—and the stator as the place where ion flow becomes turning force.