2.1 Human Movement and Muscles on the Molecular Scale

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the short-term energy storage molecules in the body. NADH can be used in reductions,

which generate the different chemical compounds needed for building cell parts. The

phosphate from ATP can be transferred to a variety of compounds in the cell, generat-

ing a bond so high in energy that with its release another high-energy bond, and thus

another needed compound, can be made. During the muscle contraction, a phosphate is

added to the myosin head, and its release allows the head to move 10 nm, creating the

force [8]. To be recharged, another phosphate is added to the myosin head.

Muscles require a lot of energy, which is where the glucagon granules come in.

Glucagon is easily split into its glucose components, and glucose is oxidized through

just a few reactions, generating ATP quickly (Figure 1.31). Fat is a lot slower; it is stored

in special fat cells far away from the muscle cells; signals thus have to travel via the

bloodstream to the fat cells first, and then fat or its components must travel back via

the blood stream, where any cell on its way can take some of that energy. This process

would be too slow and too diffuse to work for the quick-acting muscle cells. Therefore,

the two different energy storage molecules actually have different functions in the body.

Any signal that needs to act fast must also be short in duration. The cell accomplishes

this by destroying the acetylcholine very quickly, as well as by pumping the calcium

back into the sarcolemma immediately. This removes the signal on both ends of the

amplification chain simultaneously.

In summary, muscles contract because many high-energy myosin heads pull for-

ward 10 nm with force along a stiff fiber, actin. The energy of the myosin head comes

from the short-term energy-storage molecule ATP and is replenished as soon as it is used

up. This occurs in response to signals that start fast and end fast.

There is another type of motor movement on the molecular level: the transport of

cargo (organelles, chemical compounds) within cells (Figure 2.3) [9]. The outcome of

these two motor systems is very different: carrying an organelle from one end of the

cell to the other takes place on the nanoscale, while a muscle contraction and force gen-

eration takes place on the meter scale. Nevertheless, the actual molecules and force gen-

eration mechanisms are very similar [9].

For cellular transport, a motor protein “walks” on a microtubule, carrying its spe-

cific cargo. The walk of the motor consists of one of the stiff protein subunits binding to

the microtubule and using force to move for a short distance, using up ATP energy in the

process [10–13] (Figure 2.4, A-E) [10]. When the energy is used up, the subunit releases

the microtubule via a hinge, moving to the next location on the microtubule (“step”).

The process then repeats. Both kinesin and dynein have two subunits involved in this

stepping, so that one is always connected to the microtubule. Those steps, though, might

be coordinated in a different way [10]. (Figure 2.4, F-G).

The cargo might have to go into a different reaction, and this is possible: the mi-

crotubule is actually continuously made or polymerized on one end (the “+” end) and

broken down or depolymerized on the other end (the “–” end). There are specific mo-

tors that will only go from the + to the – end and others that work the other way around