101. In the cross bridge cycle, ATP is directly required to:
ⓐ. attach calcium permanently to troponin
ⓑ. detach the myosin head from actin
ⓒ. shorten the actin filament itself
ⓓ. form new sarcolemmas around myofibrils
Correct Answer: detach the myosin head from actin
Explanation: ATP is essential in the cross bridge cycle because binding of ATP to the myosin head causes it to release actin. Without ATP, the myosin head remains attached, and normal cycling cannot continue. This is why ATP is not only an energy source but also a functional requirement for separation of actin and myosin. The detachment step is necessary so that the myosin head can reset and begin another cycle. If this step fails, repeated contraction and relaxation cannot occur properly. This makes ATP central to the dynamic nature of muscle action.
102. Hydrolysis of ATP in a muscle fibre mainly provides energy for:
ⓐ. energizing the myosin head into a high-energy state
ⓑ. shortening the actin filament chemically
ⓒ. forming new $Z$ lines between sarcomeres
ⓓ. converting troponin into tropomyosin
Correct Answer: energizing the myosin head into a high-energy state
Explanation: ATP is hydrolysed at the myosin head, and the released energy is used to convert the head into an energized state. In this condition, the myosin head becomes ready to bind with actin and participate in the next step of the cross bridge cycle. This energized position is often described as a cocked or high-energy state. The hydrolysis does not shorten actin or create new sarcomere structures. Instead, it prepares the motor protein for force generation.
103. A cross bridge is formed when:
ⓐ. calcium binds directly to myosin and bends it
ⓑ. tropomyosin joins the thick filament surface
ⓒ. ATP seals the space between two actin filaments
ⓓ. an energized myosin head attaches to an exposed site on actin
Correct Answer: an energized myosin head attaches to an exposed site on actin
Explanation: A cross bridge refers to the connection formed when a myosin head binds to an active site on actin. This binding can occur only when the actin sites are exposed by the calcium-dependent shift in the troponin-tropomyosin complex. The myosin head must also be in its energized state for effective interaction. Once this attachment occurs, the myosin head can generate force and pull the thin filament. This actin-myosin link is one of the most central events in muscle contraction. Repeated formation and breaking of these cross bridges produces contraction.
104. During contraction of a sarcomere according to the sliding filament theory:
ⓐ. the $A$ band disappears completely
ⓑ. the $I$ band becomes broader
ⓒ. the $I$ band and $H$ zone decrease in width
ⓓ. the $Z$ lines move farther apart
Correct Answer: the $I$ band and $H$ zone decrease in width
Explanation: As thin filaments slide inward during contraction, the overlap between thin and thick filaments increases. Because of this increased overlap, the regions containing less overlap become narrower. The $I$ band, which mainly contains only thin filaments, becomes shorter. The $H$ zone, which is the central region of the thick filament band, also decreases in width. The $A$ band, however, remains essentially unchanged because the length of the thick filaments does not change. This pattern is an important structural sign of sliding filament action.
105. Repeated cross bridge cycling in a contracting muscle can continue only as long as:
ⓐ. myofibrils keep increasing in number
ⓑ. $Ca^{2+}$ remains available and ATP is supplied
ⓒ. the sarcolemma converts into sarcoplasm
ⓓ. all thin filaments lose their actin molecules
Correct Answer: $Ca^{2+}$ remains available and ATP is supplied
Explanation: For the cross bridge cycle to continue, two conditions are especially important. Calcium must remain available so that the active sites on actin stay exposed through the troponin-tropomyosin system. ATP must also be present so that myosin heads can detach, reset, and become re-energized for another cycle. If calcium is removed, the regulatory proteins block the actin sites again. If ATP is absent, the cycle cannot proceed normally because myosin cannot separate and reset. Continuous contraction therefore depends on the continued presence of both calcium and ATP.
106. Which observation would directly contradict the sliding filament theory of muscle contraction?
ⓐ. The overlap between thin and thick filaments increases during contraction
ⓑ. The sarcomere becomes shorter while the filaments retain their own lengths
ⓒ. The $Z$ lines move closer to each other during contraction
ⓓ. The actin and myosin filaments themselves become permanently shorter
Correct Answer: The actin and myosin filaments themselves become permanently shorter
Explanation: The sliding filament theory states that thin and thick filaments do not shorten during contraction. Instead, they slide past one another so that the overlap between them increases. Because of this increased overlap, the sarcomere shortens and the muscle contracts. The protein filaments themselves remain essentially the same length. So any statement saying that actin and myosin become shorter directly goes against the theory.
107. A muscle fibre has sufficient ATP, but the level of $Ca^{2+}$ in the sarcoplasm remains very low. Which event is most directly prevented?
ⓐ. Hydrolysis of ATP by the myosin head
ⓑ. Exposure of active sites on actin for cross bridge formation
ⓒ. Presence of myosin in the thick filament
ⓓ. Formation of thin and thick filaments in the fibre
Correct Answer: Exposure of active sites on actin for cross bridge formation
Explanation: Calcium is required to bind with troponin on the thin filament. This binding causes the troponin-tropomyosin complex to shift and uncover the active sites on actin. If calcium remains low, those active sites stay blocked and myosin cannot attach properly. ATP may still be present, but without exposed actin sites the cycle cannot begin effectively. This shows that calcium and ATP do not do the same job in contraction. Calcium controls access to actin, while ATP supports the cycle of myosin activity.
108. Which option best compares the roles of $Ca^{2+}$ and ATP in skeletal muscle contraction?
ⓐ. $Ca^{2+}$ exposes actin sites through troponin, while ATP helps detach and re-energize the myosin head
ⓑ. $Ca^{2+}$ forms the thick filament, while ATP forms the thin filament
ⓒ. $Ca^{2+}$ shortens actin, while ATP shortens myosin
ⓓ. $Ca^{2+}$ replaces troponin, while ATP replaces tropomyosin
Correct Answer: $Ca^{2+}$ exposes actin sites through troponin, while ATP helps detach and re-energize the myosin head
Explanation: Calcium and ATP are both essential in muscle contraction, but they perform different functions. Calcium binds to troponin and causes a shift that exposes the active sites on actin. ATP, on the other hand, is needed for the myosin head to detach from actin and then become energized again after hydrolysis. Without calcium, the binding sites remain blocked. Without ATP, the cycle cannot continue properly even if binding sites are available.
109. In an experimental setup, ATP is added to myosin heads that are already attached to actin. What is the immediate effect?
ⓐ. Troponin is removed from the thin filament
ⓑ. The thin filament becomes a thick filament
ⓒ. The myosin heads detach from actin
ⓓ. The $Z$ lines move farther apart
Correct Answer: The myosin heads detach from actin
Explanation: One of the direct effects of ATP binding to the myosin head is release of the actin-myosin connection. This detachment is necessary so that the myosin head can reset and participate in another cycle. If ATP did not bind, the myosin head would remain attached for longer and repeated cycling would not proceed normally. It highlights that ATP is required not just for energy, but also for proper separation of myosin from actin. This makes ATP essential for continued muscle function.
110. Which event depends most directly on ATP hydrolysis rather than on ATP binding alone?
ⓐ. Exposure of actin active sites by tropomyosin movement
ⓑ. Binding of $Ca^{2+}$ to troponin
ⓒ. Formation of the sarcolemma around the muscle fibre
ⓓ. Conversion of the myosin head into a high-energy state for the next cycle
Correct Answer: Conversion of the myosin head into a high-energy state for the next cycle
Explanation: ATP binding and ATP hydrolysis play related but different roles in the cross bridge cycle. Binding of ATP to myosin causes detachment from actin, while hydrolysis of ATP provides energy to place the myosin head into its energized state. That energized state prepares the head for another productive interaction with actin. So the event most directly linked with ATP hydrolysis is re-energizing the myosin head. It is not only a fuel source but part of a precise cyclic mechanism.
111. Assertion: Repeated cross bridge cycling cannot continue in the absence of ATP even if $Ca^{2+}$ remains available.
Reason: ATP is required for both detachment of myosin from actin and re-energizing the myosin head.
ⓐ. Both Assertion and Reason are true, and the Reason correctly explains the Assertion
ⓑ. Both Assertion and Reason are true, but the Reason does not explain the Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and the Reason correctly explains the Assertion
Explanation: The assertion is true because calcium alone cannot keep the contraction cycle running. Even if calcium continues to expose the active sites on actin, the myosin heads still need ATP to detach and then become energized again. The reason is also true and directly explains why the absence of ATP stops repeated cycling. Without ATP, the cycle cannot proceed from one attachment event to the next in a normal way. It shows that both are necessary, but for different parts of the mechanism.
112. A mutant skeletal muscle fibre has normal actin, normal myosin, and adequate ATP, but the active sites on actin can never be uncovered. Which step is most directly blocked?
ⓐ. Presence of thick filaments in the sarcomere
ⓑ. Hydrolysis of ATP by the myosin head
ⓒ. Formation of cross bridges between actin and myosin
ⓓ. Existence of troponin on the thin filament
Correct Answer: Formation of cross bridges between actin and myosin
Explanation: Cross bridge formation requires the myosin head to bind to an exposed active site on actin. If those active sites remain covered, the myosin heads cannot attach properly even if ATP is present and the filaments themselves are normal. This means the major block occurs at the stage of actin-myosin interaction. Exposure of actin sites is a necessary condition for the cycle to begin effectively. Without that exposure, contraction cannot proceed in the normal sliding-filament manner.
113. Which statement best describes excitation of a skeletal muscle fibre?
ⓐ. It is the permanent shortening of the actin filament
ⓑ. It is the initial stimulation that makes the muscle fibre ready to contract
ⓒ. It is the complete return of the muscle to its resting length
ⓓ. It is the storage of ATP inside the sarcomere
Correct Answer: It is the initial stimulation that makes the muscle fibre ready to contract
Explanation: Excitation is the event that starts the contraction process in a muscle fibre. It refers to the stimulatory change that activates the fibre and prepares it for the next steps of contraction. This stage comes before calcium release, cross bridge activity, and shortening of the sarcomere. It should not be confused with relaxation, which restores the resting condition after contraction. The idea is important because contraction is not a single event but a sequence of linked changes. Excitation is the opening step in that sequence.
114. After a skeletal muscle fibre is excited, the next key event is the release of:
ⓐ. myosin from the thick filament
ⓑ. ATP from the $Z$ line
ⓒ. $Ca^{2+}$ from the sarcoplasmic reticulum
ⓓ. actin from the thin filament
Correct Answer: $Ca^{2+}$ from the sarcoplasmic reticulum
Explanation: Once the muscle fibre is excited, $Ca^{2+}$ is released into the sarcoplasm from the sarcoplasmic reticulum. This release is one of the most important early steps in muscle contraction. Calcium does not come from the $Z$ line or from the contractile proteins themselves. Instead, it is stored and then released from the specialized internal membrane system of the fibre. This event links excitation with the beginning of actin-myosin interaction. Without calcium release, contraction cannot proceed in the normal way.
115. The main importance of $Ca^{2+}$ release during muscle contraction is that it:
ⓐ. allows the thin filament system to become available for interaction with myosin
ⓑ. breaks the thick filament into smaller active units
ⓒ. converts the sarcolemma into a contractile membrane
ⓓ. shortens the myosin filament directly by chemical action
Correct Answer: allows the thin filament system to become available for interaction with myosin
Explanation: Released $Ca^{2+}$ plays a regulatory role in contraction rather than acting as a structural filament. Its presence helps expose the active sites on actin through the thin filament regulatory system. Once those sites become available, myosin can attach and contraction can move forward. Calcium therefore acts like a key step between excitation and actual mechanical pulling. It does not directly cut, shorten, or destroy the filaments. Its main role is to make proper actin-myosin interaction possible.
116. What is meant by the power stroke in skeletal muscle contraction?
ⓐ. The release of calcium from the sarcoplasmic reticulum
ⓑ. The formation of the outer membrane around the fibre
ⓒ. The recovery of the muscle after complete relaxation
ⓓ. The pulling action by the myosin head that moves the thin filament inward
Correct Answer: The pulling action by the myosin head that moves the thin filament inward
Explanation: The power stroke is the force-generating step of the contraction process. During this event, the myosin head pulls the thin filament inward toward the center of the sarcomere. This pulling action contributes directly to shortening of the sarcomere and therefore to contraction of the muscle. The power stroke is not the same as excitation or calcium release, which occur earlier in the sequence. It is the mechanical event that produces actual movement of the filaments. That is why it is considered a central step in muscle contraction.
117. Which event is most directly associated with muscle relaxation?
ⓐ. Continuous exposure of active sites on actin
ⓑ. Return of $Ca^{2+}$ into the sarcoplasmic reticulum
ⓒ. Repeated inward sliding of thin filaments
ⓓ. Increased overlap of thick and thin filaments
Correct Answer: Return of $Ca^{2+}$ into the sarcoplasmic reticulum
Explanation: Muscle relaxation begins when $Ca^{2+}$ is removed from the sarcoplasm and taken back into the sarcoplasmic reticulum. As the calcium level falls, the active sites on actin are no longer kept exposed in the same way, so cross bridge activity declines. This allows the muscle fibre to return toward its resting condition. Relaxation is therefore not just the absence of movement, but an active reversal of the contraction-supporting state. The reuptake of calcium is one of the most important events in that reversal. It marks the transition from contraction toward rest.
118. In a relaxed skeletal muscle fibre, the concentration of free $Ca^{2+}$ in the sarcoplasm is generally:
ⓐ. low because much of it has been taken back into the sarcoplasmic reticulum
ⓑ. high because it is continuously released during rest
ⓒ. zero because calcium is absent from the muscle fibre
ⓓ. unchanged because calcium has no role in contraction
Correct Answer: low because much of it has been taken back into the sarcoplasmic reticulum
Explanation: In the relaxed state, free $Ca^{2+}$ in the sarcoplasm is kept low because calcium is stored again in the sarcoplasmic reticulum. This low calcium condition helps prevent continued actin-myosin interaction. The muscle fibre is therefore maintained in a non-contracting state. Calcium is not absent from the fibre, but it is no longer freely available in the sarcoplasm in the same way as during contraction. Changes in calcium level help control the switching between those two states.
119. Which sequence correctly represents the major events of muscle contraction and relaxation?
ⓐ. Relaxation $\rightarrow$ excitation $\rightarrow$ calcium release $\rightarrow$ power stroke
ⓑ. Power stroke $\rightarrow$ excitation $\rightarrow$ relaxation $\rightarrow$ calcium release
ⓒ. Excitation $\rightarrow$ calcium release $\rightarrow$ power stroke $\rightarrow$ relaxation
ⓓ. Calcium release $\rightarrow$ relaxation $\rightarrow$ excitation $\rightarrow$ power stroke
Correct Answer: Excitation $\rightarrow$ calcium release $\rightarrow$ power stroke $\rightarrow$ relaxation
Explanation: Muscle activity follows an organized sequence rather than occurring randomly. First the muscle fibre is excited, which starts the process. This is followed by release of $Ca^{2+}$, allowing the contractile machinery to become active. Then the power stroke occurs, producing the mechanical pulling that shortens the sarcomere. Finally, relaxation follows as calcium is taken back and the fibre returns toward rest.
120. Which statement best distinguishes contraction from relaxation in skeletal muscle?
ⓐ. In contraction, calcium is returned to storage; in relaxation, calcium is released
ⓑ. In contraction, myofibrils disappear; in relaxation, they are rebuilt
ⓒ. In contraction, the fibre loses all ATP; in relaxation, ATP is created from calcium
ⓓ. In contraction, available $Ca^{2+}$ supports filament interaction; in relaxation, removal of $Ca^{2+}$ helps stop it
Correct Answer: In contraction, available $Ca^{2+}$ supports filament interaction; in relaxation, removal of $Ca^{2+}$ helps stop it
Explanation: One of the clearest differences between contraction and relaxation is the availability of $Ca^{2+}$ in the sarcoplasm. During contraction, calcium is present in a way that allows actin and myosin interaction to continue. During relaxation, calcium is taken back into the sarcoplasmic reticulum, and this helps stop that interaction. The contrast is therefore based on a change in regulatory conditions, not on destruction of filaments or disappearance of myofibrils. Calcium availability is a major switch between active contraction and relaxation.
