Which statement about slow-twitch and fast-twitch muscles is true?
A) One is smooth muscle and one is skeletal muscle.
B) They have different myosin variants because they express different genes for their myosin molecules.
C) One occurs only in mammals.
D) They typically have similar numbers of mitochondria per cell.
E) Those with low ATPase activity compensate by quickly recycling their actin-myosin bridges.

Question

Quizplus · Accepted Answer

Slow-twitch and fast-twitch muscles have different myosin variants because they express different genes for their myosin molecules. Slow-twitch fibers express a form of myosin with slower ATPase activity, while fast-twitch fibers express a form of myosin with faster ATPase activity. This difference in myosin variants contributes to the different contractile properties of slow-twitch and fast-twitch muscle fibers. The other statements are not true - A is incorrect because both slow-twitch and fast-twitch muscles are types of skeletal muscle, not smooth muscle; C is incorrect because both slow-twitch and fast-twitch muscles occur in multiple vertebrate species, not just mammals; D is incorrect because slow-twitch muscles typically have more mitochondria per cell compared to fast-twitch muscles, as they rely more on oxidative metabolism for energy production; and E is incorrect because slow-twitch muscles are able to sustain contractions for longer periods of time precisely because they have slower ATPase activity and thus require less ATP to maintain muscle tension.

Which Statement About Slow-Twitch and Fast-Twitch Muscles Is True