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How a Kinesin Walks

Page history last edited by Ben Geller 4 years, 8 months ago

7.2.P3

 

We know a lot of movement takes place within cells. In order to function properly, cells need to move things - ions, molecules, even whole organelles. But what drives this movement?  Active transport can move stuff in cells over long distances much more quickly than through diffusion alone, but it requires energy.  But how exactly is ATP used to generate movement? Let's look at a kinesin as an example of active transport and use your understanding of energy transformations to explain what it means to say that a cell "uses ATP to fuel molecular movement."

 

 A kinesin is a molecular motor protein with two motor head domains and a tail where the cargo binds. It looks something like this:

 

Scientists have found that the kinesin moves in a hand-over-hand fashion, similar to how humans walk, as shown in the following frames (the grey and black track represents the microtubule the kinesin walks along). The exact mechanism is still controversial, but according to one model, we can break the movement down into the following steps: In frame (a) both motor heads are bound to the microtubule. Then, in frame (b) an ATP molecule binds with one of the heads of the kinesin, causing strain on the motor protein (like a compressed spring). In frame (c) ATP is hydrolyzed and the protein moves in the forward direction. 

 

 

 

 

1) Act out a movie (with your body or hands) of how the kinesin walks along the track so you can get a feel for describing the movement.

 

 

 

 

2) In order to make sense of the role ATP is playing you will need to keep track of the energy transformations that take place during this process. First, define the system. That is, identify the component parts and the associated forms of energy that you think are important. Then, for each frame of the process, use energy bar charts and the principle of conservation of energy to keep track of changes in energy. 

 

 

 

 

 

3) Where does the energy that allows the kinesin to move forward come from? Make sure your bar charts are consistent with your answer.

 

 

 

 

4) In Q2 & Q3, has the energy conservation principle been satisfied?  If not, how can you modify the definition of your system in each frame so that it is?  If you make that change, what happens to your energy bar charts?

 

 

 

 

5) The energy released by ATP hydrolysis is around 50 kJ/mol.  Each "step" the kinesin moves the cargo approximately 8.5 nm.  If each step of the kinesin requires one ATP, how much energy does it take to move a vesicle all the way down your leg?

 

 

 

6) Finally, discuss with your group what it means to say that a cell "uses ATP to fuel molecular movement".

 

 

 

 

Vashti Sawtelle, Ben Dreyfus, Julia Gouvea, and Chandra Turpen 1/23/12

 

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