final EXAM 2009 - Name: _KEY_ SC/BPHS 2090 Final Exam (Lew)...

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Name: _______KEY___________________ SC/BPHS 2090 Final Exam (Lew) Page 1 of 4 I NSTRUCTIONS . Be sure to write your name above. Read the question carefully, think, then write your answer in the lined space (front and back of this page). Excessive length is not encouraged. When finished, please hand in your answer separate from your exam booklet. Q UESTION . A bacterial flagella is very flexible. When the motor hook is rotating, it forms a helical coil shape, which then causes the bacteria to swim. Notably, because of its flexibility, a number of flagella --each attached to separate motors-- may intertwine to create a thick helical coil. But for this question, we will assume that the bacteria has only one flagellum O NE . Would a bacteria swim faster if the flagellum was stiff, always in the shape of a helical coil as shown in the diagram? Explain. T WO . Could a bacteria swim if the flagellum was whipped back and forth by a reciprocating motor, to create a two-dimensional sinusoidal shape with constantly changing phase? Explain. One. As long as the shape (length, wavelength and pitch angle) of the stiff flagellum is the same as the shape of the flexible flagellum, then the speed of the bacteria will be the same, since the net force available for propelling the bacteria will be the same. Work is required to ‘shape’ the flexible flagellum into a helical coil, because of its elasticity, so the propulsion efficiency of the stiff flagellum will be higher. A stiff flagellum has one major drawback: tumbling (to change the swimming direction) would not be possible. Reversing the motor would simply cause the bacteria to go backwards. Two. In the required reading by Purcell, his fascinating exposition on Life at Low Reynolds Number , he argues that the it would be possible if the flagellum was flexible: “You see, you
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Name: _______KEY___________________ SC/BPHS 2090 Final Exam (Lew) Page 2 of 4 can’t row a boat at low Reynolds number in molasses —if you are submerged— because the stiff
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final EXAM 2009 - Name: _KEY_ SC/BPHS 2090 Final Exam (Lew)...

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