Why do flagella form a bundle only when they rotate counterclockwise during chemotaxis?

Question

During Chemotaxis in bacteria with flagella, the flagellar rotation dictates how the cell moves. If the flagella rotate counterclockwise, then they form a bundle at one end of the cell (---O) and propel it forward in a coordinated fashion. However, if the flagella rotate clockwise, then each flagella acts independently to push the cell in many different directions. My question is, why doesn't clockwise flagellar rotation simply cause the bundle to form on the opposite end of the cell? (O---)

Answer 1

1. You have stated something like "However, if the flagella rotate clockwise, then each flagella acts independently to push the cell in many different directions..."

NO, this is not what happens. When Flagella rotate clockwise, they are NOT moving or travelling, they TUMBLE in place, they do this to change their direction of movement.

 

2.Now to answer your question of "WHY doesn't clockwise flagellar rotation simply cause the bundle to form on the opposite end of the cell? ?"

Think of mechanism: which is complicated and involves too many and various proteins. Flagella cannot just rotate and move or tumble, or "simply cause the bundle...."

I'll try explaining simply and in short, and only for prokaryotes and the ones that use Na⁺pump. The mechanism of flagellar movement in prokaryotes is different from eukaryotic flagella.

 

The flagellum consists of three parts:

1. the filament (helical propeller),
2. the hook (universal joint), and
3. the basal structure (rotary motor).

At the base of each flagellum, a reversible rotary motor, is embedded in the cell envelope. It is powered by H⁺ or Na⁺ motive force. The flagella act just like propellers on a boat. The motor consists of two parts:

1. the rotating part, (rotor) that is connected to the hook and the filament, and the
2. non rotating part, (stator) that conducts coupling ion and is responsible for energy conversion.

The direction of flagellar rotation determines the nature of bacterial movement:

To move forward, the flagella rotate counter clockwise by bending at their hooks. This forms a rotating bundle that propels them forward. At this time, the flagella is driving the cell forward towards the attractant or away from the deterring signal.

 

The flagella is able to switch between the clockwise and counterclockwise directions through Fli proteins, which senses where the attractant/deterrent is, and switches the direction of rotation for the flagella in response to the chemical signal (therefore performing chemotaxis).

 

The flagella is able to rotate in either direction by generating torque in the basal body. Within the basal body is a specialized protein called the mot protein, which generates torque to propel the circular flagellar motion.

Clockwise rotation of the flagella disrupts the bundle and the cell tumbles.

 

When the cell swims, the flagellar filament converts rotary motion into thrust Formation of a helical structure. This is achieved by a mixture of the protofilaments of two distinct conformations, the R- and L-type,

The bacterium moves when the helix rotates as the filament is in the shape of rigid helix.

 

P.S I have a schematic i was trying to post here, but i'm unable to.