Items 203-204
203. All of the following processes directly involve the binding/or hydrolysis of ATP EXCEPT
(A) assembly of actin monomers into microfilaments
(B) elongation of microtubules via addition of tubulin subunits
(C) movement of cilia
(D) transport of vesicles along axonal microtubules
(E) dissociation of myosin cross bridges from thin filaments in cardiac muscles
204. Which of the following correctly states deference between the way in each smooth and cardiac muscle cells contract?
(A) Smooth muscle cells are innervated, whereas cardiac muscles are not.
(B) The myosin’s found in cardiac muscles do not form thick filaments as readily as do those is smooth muscle.
(C) Cardiac muscle contains actin, but smooth muscle does not.
(D) Smooth muscle has a much less extensive sarcoplasmic reaticulum than doe’s cardiac muscle.
(E) While smooth muscle bundles contain connective tissue fibers, cardiac muscles do not.
ANSWER AND TOTURIAL ON ITEMS 203 AND 204
The answers are: 203-B; 204-D. ATP is the immediate energy source for the contraction of skeletal and cardiac muscle, as well as in smooth muscle. The myosins of smooth muscle cells from thick filaments, but mach less readily than in skeletal and cardiac muscle. In each case, the binding of ATP to myosin causes dissociation of myosin cross bridges from adjacent thin filaments and is followed by the rapid hydrolysis of ATP yielding ADP and Pi , both of which remain bound to the myosin head until the head can reassociate with actin. G-actin binds ATP and this is hydrolysed to ADP and Pi when the actin monomer polymerizes to from F-actin. Neither cardiac nor smooth muscles have the extensive innervations characteristic of skeletal muscle.
While contraction of all muscles is regulated by calcium levels, smooth muscle does not have the extensive sarcoplasmic reticulum found in the striated muscles. All three of muscle contains thin filaments made of actin and, in all three, muscles cells (fibers) are held together by extracellular connective tissue fibers, mostly collagen. Enzymes, such energy as dynein and kinesin, which cause vesicles to move along microtubules, utilize ATP as the energy source for such movements. Tubulins all bind GTP (not ATP), which is hydrolysed to GDP when microtubules are formed. Articulating microscope
