Statistical Physics in Biology

Molecular Motors & Cell Motility

Molecular Motors

1. Molecular motors:   

   • Kinesin and Dynein move in opposite directions along microtubules consuming ATP
   • Myosin moves on actin filaments
   • Animation of motion of kinesin on a microtubule

2. General scheme of motor transport:   

   • Asymmetry for directionality
   • Rectification of fluctuations by consumption of energy

3. Motion in a noisy environment   

    

A Brownian ratchet (Image from Wikimedia Commons)

View a ratchet and pawl animation

• Feynman’s ratchet and pawl:
• Simulation of a Brownian motor
• From the web-page of George Oster:
  • Wang, H., and G. Oster. “Ratchets, power strokes, and molecular motors.” (PDF) Applied Physics A 75, no. 315 (2002).
  • Bustamante, C., D. Keller, et al. “The Physics of Molecular Motors.” (PDF) Acc. Chem. Res. 34, no. 412 (2001).

1. Asymmetric hopping models:
   • Fisher, M. E., A. B. Kolomeisky. “The force exerted by a molecular motor.” PNAS 96, no. 6597 (1999).
   • Multiple internal states
   • Solution with two internal state
   • Einstein force, stalling force

Some Related Links

• Kinesin Home Page
• Dynein Home Page
• Myosin Home Page
• Kinesin Motors—the Movies
• Vale lab movies (e.g. MT gliding)
• The inner life of a cell
• Lecture at Delft by Gerrit E.W. Bauer 
• Capturing and viewing single molecules:
  • Optical tweezers
  • Fluorescent light microscopy (PDF)

Cell Motion

1. Brownian motion: 
   • Movies of milk droplets in water
   • Animated simulation (see also Applet 1, Applet 2)
   • Langevin description
   • Fokker-Planck description
   • Thermal equilibrium and the Einsten relation
2. Crawling cells
   • Remodelling of actin cycloskeleton
   • Crawling Neutrophil Chasing a Bacterium
   • More from the Theriot lab movie collection
   • Latex beads coated with ActA protein: motion and spontaneous symmetry breaking
3. Swimming cells:
   • Life at Low Reynolds Number (E.M. Purcell)
   • The scallop theorem: impossibility of reciprocal motion
   • Flagellar rotary motion (efficiency)
   • Chemotaxis by runs and tumbles—why?

Some Related Links

• Cell Locomotion
• Some parameters for diverse Cell Motion
• E. M. Purcell. “The efficiency of propulsion by a rotating flagellum.” (PDF) PNAS 94, (1997): 11307–11.
• Berg, Howard. “Motile Behavior of Bacteria.” Physics Today, January 2000, 24.

(Detailed Lecture Notes (PDF))