Anatomy Quiz: Sliding Filament Theory And Contraction Cycle
20 questions · exam conditions
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Sliding Filament Theory And Contraction CycleQuestion 1 of 20

Which molecule is necessary for cross-bridge detachment and prevents sustained rigid binding in living muscle?

ATP
Calcium
Acetylcholine
Troponin
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Anatomy Quiz

Anatomy Quiz: Sliding Filament Theory And Contraction Cycle

Practice Sliding Filament Theory And Contraction Cycle in Anatomy with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Sliding Filament Theory And Contraction Cycle, giving you a quick way to practice the rules, question types, and explanations that matter most for Anatomy.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

Which molecule is necessary for cross-bridge detachment and prevents sustained rigid binding in living muscle?

  1. ATP (correct answer)
  2. Calcium
  3. Acetylcholine
  4. Troponin
Explanation: This question tests understanding of the sliding filament theory and contraction cycle in muscle physiology, focusing on preventing rigid binding. The sliding filament theory explains how muscles contract by the sliding of actin filaments over myosin filaments, requiring calcium ions and ATP. In this question, ATP's necessity for detachment is highlighted. The correct choice identifies ATP for cross-bridge detachment. A common distractor might pick calcium, but it's for initiation. To help students, discuss ATP depletion effects. Use cycle charts to emphasize detachment.

Question 2

Which event directly exposes myosin-binding sites on actin during the sliding filament process?

  1. ATP hydrolysis on myosin
  2. Calcium binding to troponin (correct answer)
  3. Calcium reuptake into the sarcoplasmic reticulum
  4. Release of acetylcholine into the synaptic cleft
Explanation: This question tests understanding of the sliding filament theory and contraction cycle in muscle physiology, focusing on exposing myosin-binding sites on actin. The sliding filament theory explains how muscles contract by the sliding of actin filaments over myosin filaments, requiring calcium ions and ATP. In this question, the direct trigger for site exposure is emphasized. The correct choice points to calcium binding to troponin, causing tropomyosin to shift. A common distractor might suggest ATP hydrolysis, but that's for the power stroke. To help students, explain regulatory proteins' roles. Use animations to show the binding and shifting process.

Question 3

During the power stroke phase of the cross-bridge cycle, myosin heads undergo a conformational change from high-energy to low-energy state. What happens to the ADP and inorganic phosphate (Pi) that were bound to the myosin head during this transition?

  1. Both ADP and Pi remain bound to maintain the low-energy myosin conformation
  2. ADP is released first, followed by Pi release, which triggers the power stroke
  3. Pi is released first during the power stroke, while ADP remains bound until ATP binding (correct answer)
  4. Both ADP and Pi are released simultaneously at the end of the power stroke
  5. Pi and ADP combine to reform ATP while still bound to the myosin head
Explanation: When you encounter questions about muscle contraction, focus on the precise sequence of molecular events in the cross-bridge cycle. The power stroke is the critical moment when myosin converts chemical energy into mechanical work. During the power stroke, the myosin head undergoes a dramatic conformational change that pulls the actin filament. This change is triggered by the release of inorganic phosphate (Pi) from the myosin-ADP-Pi complex. Once Pi is released, the myosin head snaps into its low-energy position, generating the force that moves actin. Importantly, ADP remains bound to the myosin head throughout this power stroke and only gets released afterward, when the myosin head is in its rigor state attached to actin. Option A is wrong because both nucleotides don't remain bound - Pi is released to trigger the conformational change. Option B reverses the actual sequence - ADP release doesn't trigger the power stroke; Pi release does. Option D incorrectly suggests simultaneous release, but the releases are sequential and serve different functions in the cycle. The correct answer is C because Pi release initiates the power stroke by allowing the conformational change, while ADP stays bound until ATP eventually binds to release the myosin head from actin. Remember this sequence: Pi release triggers power stroke → ADP release occurs later → ATP binding causes detachment. The timing of these molecular events determines the mechanical output of muscle contraction.

Question 4

During muscle contraction, if ATP becomes completely depleted while myosin heads remain bound to actin filaments, what physiological state will result and why?

  1. Muscle relaxation, because myosin heads cannot bind to actin without ATP present
  2. Rigor mortis-like state, because ATP is required for myosin head detachment from actin (correct answer)
  3. Continuous contraction cycles, because stored calcium maintains troponin-tropomyosin activation
  4. Immediate muscle stretching, because actin filaments lose their structural integrity without ATP
  5. Normal contraction strength, because creatine phosphate will substitute for ATP in all reactions
Explanation: Questions about muscle contraction and ATP depletion test your understanding of the molecular mechanism of muscle fiber function, particularly the cross-bridge cycle between actin and myosin filaments. During normal muscle contraction, myosin heads bind to actin, pivot to pull the filaments past each other, then must detach to reset for the next cycle. This detachment step is crucial and requires ATP binding to the myosin head. When ATP binds, it causes a conformational change that releases the myosin head from actin. Without ATP, myosin heads become permanently locked onto actin filaments in what's called a cross-bridge state. This explains why answer B is correct - ATP depletion while myosin heads are bound creates a rigor mortis-like state because the myosin heads cannot detach from actin without ATP. Answer A incorrectly suggests muscle relaxation would occur. While ATP is indeed needed for myosin binding, the question specifies that heads are already bound when ATP runs out - they become stuck in this position. Answer C misunderstands the role of calcium and ATP - even with calcium present to keep troponin-tropomyosin activated, no contraction cycles can continue without ATP for myosin detachment. Answer D incorrectly focuses on actin structural integrity, but actin filaments remain intact without ATP; the problem is specifically with myosin head detachment. Remember this key principle: ATP has two critical roles in muscle contraction - providing energy for the power stroke and enabling myosin head detachment. Without detachment, muscles lock up regardless of calcium availability.

Question 5

In a laboratory experiment, researchers apply a drug that prevents tropomyosin from moving away from myosin-binding sites on actin, even in the presence of calcium and troponin. What would be the most likely result on muscle contraction?

  1. Enhanced contraction strength because tropomyosin directly participates in cross-bridge formation
  2. Normal contraction because calcium binding to troponin is sufficient for contraction
  3. Complete inhibition of contraction because myosin heads cannot access actin binding sites (correct answer)
  4. Delayed contraction initiation but normal strength once tropomyosin degrades naturally
  5. Weaker contraction because some myosin heads can still bind through gaps in tropomyosin
Explanation: When you encounter questions about muscle contraction mechanisms, focus on the molecular interactions that must occur in proper sequence for contraction to happen. The sliding filament theory depends on myosin heads binding to specific sites on actin, and this binding is tightly regulated by the troponin-tropomyosin complex. In normal muscle contraction, calcium binds to troponin C, causing a conformational change that shifts tropomyosin away from myosin-binding sites on actin. This exposure allows myosin heads to bind to actin and form cross-bridges, initiating the power stroke that generates contraction. If a drug prevents tropomyosin from moving away from these binding sites, the fundamental mechanism of contraction is blocked at its most basic level. Answer C is correct because without access to actin binding sites, myosin heads cannot form cross-bridges, completely preventing contraction regardless of calcium presence. Answer A is wrong because tropomyosin doesn't directly participate in cross-bridge formation—it's a regulatory protein that controls access to binding sites. Answer B misses the critical point that calcium binding to troponin is only the first step; tropomyosin must still move to expose binding sites. Answer D is incorrect because the drug specifically prevents tropomyosin movement regardless of natural degradation, and the effect would be complete inhibition, not just delay. For anatomy and physiology exams, remember that muscle contraction questions often test your understanding of the sequential steps required. Each step must occur properly—blocking any single step can prevent the entire process.

Question 6

During excitation-contraction coupling, calcium ions are released from the sarcoplasmic reticulum and bind to troponin C. However, a student notices that in some muscle fibers, calcium is present but no contraction occurs. Which of the following best explains this observation?

  1. Calcium concentration is sufficient but troponin I is preventing tropomyosin movement despite calcium binding
  2. Troponin C is saturated with calcium, but ATP depletion prevents cross-bridge formation entirely
  3. Calcium is binding to troponin C, but troponin T is not properly connected to tropomyosin (correct answer)
  4. The calcium release is occurring, but actin and myosin filaments are not properly aligned
  5. Calcium is present but the sarcoplasmic reticulum is simultaneously reuptaking it too rapidly
Explanation: When you encounter questions about muscle contraction failure despite calcium presence, focus on the structural integrity of the troponin-tropomyosin regulatory complex. This system requires all three troponin subunits to work together properly. The correct answer is C because troponin T serves as the critical structural link between the troponin complex and tropomyosin. Even when calcium successfully binds to troponin C, if troponin T cannot properly connect to tropomyosin, the conformational change needed to expose myosin-binding sites on actin won't occur. Think of troponin T as the mechanical linkage that transmits the calcium signal from troponin C to tropomyosin movement. Option A is incorrect because if calcium is bound to troponin C and the complex is structurally intact, troponin I's inhibitory effect would be overcome. Troponin I only blocks contraction when calcium levels are low. Option B misses the point entirely. While ATP depletion would prevent cross-bridge cycling, the scenario describes calcium binding to troponin C but no contraction - this suggests a regulatory problem, not an energy problem. Option D focuses on filament alignment issues, but sarcomere organization problems would affect force generation rather than the initial calcium-triggered exposure of binding sites. Remember: excitation-contraction coupling failures often involve the regulatory protein complex rather than calcium availability. When studying muscle physiology, pay special attention to how troponin subunits work as an integrated system - each has a distinct role that must function properly for contraction to occur.

Question 7

A researcher observes muscle fibers under different conditions and measures the following: Condition A shows maximum overlap between thick and thin filaments with optimal force generation. Condition B shows reduced overlap with longer sarcomere length and decreased force. Which principle of the sliding filament theory explains the force difference between these conditions?

  1. Force generation depends on the total number of myosin molecules present in each thick filament
  2. Force production is directly proportional to the degree of overlap between actin and myosin filaments (correct answer)
  3. Longer sarcomeres contain more ATP per unit length, enhancing contraction strength significantly
  4. Force generation increases as the distance between Z-lines decreases due to mechanical advantage
  5. Maximum force occurs when actin filaments are stretched to their optimal length for binding
Explanation: When you encounter questions about muscle contraction and force generation, focus on the fundamental relationship between filament overlap and contractile strength in the sliding filament theory. The key principle here is that muscle force production directly correlates with how much the thick (myosin) and thin (actin) filaments overlap within each sarcomere. In Condition A, maximum overlap means more myosin heads can form cross-bridges with actin binding sites, generating optimal force. In Condition B, the longer sarcomere length reduces this overlap, so fewer cross-bridges can form simultaneously, resulting in weaker contraction. This makes choice B correct. Choice A is incorrect because while myosin molecules are essential for contraction, the total number per thick filament remains constant regardless of sarcomere length. The difference in force comes from how many can actively engage, not how many exist. Choice C contains a major misconception—longer sarcomeres don't contain more ATP per unit length, and ATP availability isn't the limiting factor in this scenario. Choice D reverses the relationship; force generation depends on optimal filament overlap, not simply shorter Z-line distances. Overly shortened sarcomeres actually produce less force due to filament interference. For anatomy and physiology exams, remember that muscle contraction questions often test your understanding of the sliding filament mechanism. Always consider how structural changes (like sarcomere length) affect functional outcomes (like force production). The sweet spot for muscle force occurs at optimal resting length where filament overlap maximizes cross-bridge formation.

Question 8

A muscle physiologist studies the sequence of molecular events during a single cross-bridge cycle. She notes that after myosin binds strongly to actin and before the power stroke occurs, there is a brief intermediate state. What characterizes this intermediate state in terms of nucleotide binding and myosin head conformation?

  1. Myosin head is in high-energy state with ATP bound, preparing for immediate detachment
  2. Myosin head is in high-energy state with ADP + Pi bound, primed for power stroke (correct answer)
  3. Myosin head is in low-energy state with no nucleotides bound, maximizing actin affinity
  4. Myosin head is in intermediate-energy state with only ADP bound, stabilizing actin interaction
  5. Myosin head is rapidly cycling between high and low energy states with variable nucleotide binding
Explanation: When you encounter questions about muscle contraction, focus on the cross-bridge cycle's precise sequence of molecular events. The key is understanding how nucleotide binding states control myosin's energy level and its affinity for actin. The intermediate state described occurs right after myosin forms a strong bond with actin but before the power stroke begins. At this moment, myosin exists in a high-energy conformation with both ADP and inorganic phosphate (Pi) still bound from the previous ATP hydrolysis. This creates a "cocked" myosin head that's primed and ready to execute the power stroke. The energy stored in this configuration will soon drive the conformational change that pulls the actin filament. Choice A is incorrect because ATP binding actually causes myosin to detach from actin - this happens at the end of the cycle, not during the intermediate state before the power stroke. Choice C misses the mark because the no-nucleotide state (rigor complex) occurs after ADP release, well after the power stroke is complete. Choice D incorrectly suggests only ADP is bound, but the phosphate must still be present at this stage - Pi release actually triggers the power stroke itself. Remember that the cross-bridge cycle follows a strict sequence: ATP hydrolysis energizes myosin → myosin binds actin → Pi release triggers power stroke → ADP release → ATP binding causes detachment. Questions about muscle physiology often test whether you can pinpoint exactly where in this cycle specific molecular events occur.

Question 9

A student observes that when calcium is released in skeletal muscle, the I-band shortens while the A-band length remains constant during contraction. Which aspect of the sliding filament theory does this observation directly support?

  1. Myosin filaments slide past actin filaments by changing their individual lengths significantly
  2. Actin and myosin filaments maintain constant lengths while sliding past each other (correct answer)
  3. The H-zone disappears because actin filaments dissolve into component proteins
  4. Tropomyosin molecules physically shorten to pull actin filaments toward the M-line
  5. Cross-bridge formation causes both thick and thin filaments to contract longitudinally
Explanation: When you encounter questions about muscle contraction and sarcomere changes, focus on what happens to the different bands and zones during the sliding filament process. The observation described perfectly demonstrates the core principle of sliding filament theory. During contraction, the I-band (which contains only thin actin filaments) shortens as actin filaments slide deeper into the A-band. Meanwhile, the A-band (the full length of thick myosin filaments) stays constant because the myosin filaments themselves don't change length. This directly supports option B - the filaments maintain their individual lengths while sliding past each other like telescoping parts. Option A is incorrect because myosin filaments don't significantly change their individual lengths during contraction. The sliding filament theory specifically states that filaments slide, not stretch or contract themselves. Option C misrepresents what happens in the H-zone - it narrows or disappears because actin filaments slide further into it, not because actin dissolves. The actin filaments remain intact throughout contraction. Option D incorrectly describes tropomyosin's role - tropomyosin moves aside to expose binding sites when calcium is present, but it doesn't physically shorten to create the pulling force. The key insight is that muscle contraction results from filaments sliding past each other while maintaining their structural integrity, not from the filaments themselves changing length. Remember this distinction: the sarcomere shortens, but the individual protein filaments within it do not.

Question 10

How does ATP contribute to muscle contraction after the power stroke has occurred?

  1. ATP binds to myosin, causing it to detach from actin (correct answer)
  2. ATP binds actin, causing it to detach from myosin
  3. ATP blocks calcium release from the sarcoplasmic reticulum
  4. ATP lengthens the sarcomere by pulling Z discs apart
Explanation: This question tests understanding of the sliding filament theory and contraction cycle in muscle physiology, focusing on ATP's post-power stroke role. The sliding filament theory explains how muscles contract by the sliding of actin filaments over myosin filaments, requiring calcium ions and ATP. In this question, ATP's detachment function is emphasized. The correct choice states ATP binds myosin to detach it from actin. A common distractor might reverse binding to actin, but that's incorrect. To help students, sequence the cycle steps. Use animated models for visual clarity.

Question 11

In an experimental setup, muscle fibers are exposed to calcium and ATP, but myosin heads are chemically modified so they cannot hydrolyze ATP to ADP + Pi. What would be the immediate consequence for the cross-bridge cycle?

  1. Myosin heads would bind to actin but remain in low-energy state unable to perform power stroke
  2. Myosin heads would detach from actin immediately and be unable to rebind for subsequent cycles (correct answer)
  3. Normal cross-bridge cycling would continue using stored ADP and Pi from previous ATP hydrolysis
  4. Myosin heads would bind to actin and detach repeatedly but without generating force
  5. Cross-bridge formation would be completely prevented because ATP hydrolysis is required for actin binding
Explanation: Understanding the cross-bridge cycle requires knowing the precise role ATP plays at each step. The cycle depends on ATP not just as an energy source, but specifically on the myosin head's ability to hydrolyze ATP into ADP + Pi to "cock" the myosin head into its high-energy state. Here's what normally happens: myosin hydrolyzes ATP to ADP + Pi (while detached), creating a high-energy myosin head that can bind to actin. After binding and performing the power stroke, the myosin head releases ADP + Pi and remains tightly bound to actin. Only when a new ATP molecule binds does myosin detach from actin to begin another cycle. If myosin heads cannot hydrolyze ATP, they cannot transition to the high-energy state needed to bind actin in the first place. Even with calcium present to expose binding sites on actin, the myosin heads remain in their low-energy, actin-detached state. This makes answer B correct—the heads would detach (or remain detached) and be unable to rebind for subsequent cycles. Answer A is wrong because myosin heads cannot bind to actin when in the low-energy state—they need the high-energy conformation from ATP hydrolysis first. Answer C is incorrect because any stored ADP + Pi would be immediately released once calcium triggers the conformational changes, and no new high-energy states could form. Answer D is wrong because without ATP hydrolysis, there's no binding-detaching cycle at all. Remember: ATP hydrolysis is the prerequisite for actin binding, not just the energy source for force generation.

Question 12

Identify the sequence of events in the cross-bridge cycle after calcium exposes actin binding sites.

  1. Reactivation → detachment → attachment → pivoting
  2. Attachment → pivoting → detachment → reactivation (correct answer)
  3. Detachment → pivoting → attachment → reactivation
  4. Pivoting → attachment → reactivation → detachment
Explanation: This question tests understanding of the sliding filament theory and contraction cycle in muscle physiology, focusing on cross-bridge cycle sequence after site exposure. The sliding filament theory explains how muscles contract by the sliding of actin filaments over myosin filaments, requiring calcium ions and ATP. In this question, the post-calcium steps are highlighted. The correct choice lists attachment, pivoting, detachment, reactivation. A common distractor might start with detachment, disrupting the flow. To help students, mnemonic devices for order. Use interactive simulations for practice.

Question 13

What role do calcium ions play in allowing actin and myosin to interact during contraction?

  1. They bind troponin and move tropomyosin away from actin binding sites (correct answer)
  2. They bind myosin and split ATP for energy
  3. They prevent cross-bridge formation to stop contraction
  4. They are stored in mitochondria and released during exercise
Explanation: This question tests understanding of the sliding filament theory and contraction cycle in muscle physiology, focusing on calcium's role in actin-myosin interaction. The sliding filament theory explains how muscles contract by the sliding of actin filaments over myosin filaments, requiring calcium ions and ATP. In this question, calcium's binding to troponin is highlighted. The correct choice explains it moves tropomyosin away from actin sites. A common distractor might claim calcium binds myosin, but it doesn't. To help students, detail calcium's pathway. Use 3D models to show molecular shifts.

Question 14

During muscle relaxation, calcium is actively pumped back into the sarcoplasmic reticulum by Ca²⁺-ATPase pumps. If these pumps were selectively inhibited while calcium release continued normally, what would happen to the regulatory proteins and muscle contraction state?

  1. Tropomyosin would remain displaced and sustained contraction would occur (correct answer)
  2. Troponin would become oversaturated causing muscle paralysis
  3. Calcium would bind irreversibly to actin bypassing normal regulation
  4. Muscle would relax normally because calcium binding is reversible
  5. Alternating contraction and relaxation would occur from fluctuating calcium
Explanation: Questions about muscle contraction regulation test your understanding of the calcium-troponin-tropomyosin system and what happens when normal processes are disrupted. Think through the sequence: calcium release → binding to troponin → tropomyosin displacement → contraction → calcium removal → relaxation. In this scenario, calcium continues to be released normally but cannot be pumped back into the sarcoplasmic reticulum due to inhibited Ca²⁺-ATPase pumps. This means calcium levels in the sarcoplasm remain elevated. With abundant calcium present, troponin C continues to bind calcium, keeping troponin in its "on" configuration. This maintains tropomyosin in the displaced position, leaving myosin-binding sites on actin continuously exposed. The result is sustained, uncontrolled contraction. Answer A correctly describes this outcome - tropomyosin remains displaced due to persistent calcium-troponin binding, causing sustained contraction. Answer B is incorrect because troponin doesn't become "oversaturated" or cause paralysis; it simply maintains its calcium-bound state. Answer C misunderstands the mechanism - calcium doesn't bind directly to actin, and the normal regulatory pathway (through troponin) is still functioning, just continuously activated. Answer D fails because while calcium binding is indeed reversible, the problem is that calcium cannot be removed from the system due to pump inhibition, so reversal cannot occur. Remember: muscle relaxation requires active calcium removal, not just reversible binding. When you see questions about pump inhibition, always trace through what happens when the "off switch" mechanism fails.

Question 15

How does ATP contribute to muscle contraction by preparing myosin for another cycle?

  1. ATP hydrolysis re-cocks the myosin head into a high-energy position (correct answer)
  2. ATP binds troponin to expose actin binding sites
  3. ATP directly transports calcium out of the muscle fiber membrane
  4. ATP converts actin into myosin to increase force production
Explanation: This question tests understanding of the sliding filament theory and contraction cycle in muscle physiology, focusing on ATP's preparation of myosin. The sliding filament theory explains how muscles contract by the sliding of actin filaments over myosin filaments, requiring calcium ions and ATP. In this question, ATP's role in recocking is highlighted. The correct choice explains ATP hydrolysis re-cocks the myosin head. A common distractor might say ATP binds troponin, but that's calcium. To help students, detail energy states. Use energy diagrams to show high-energy configuration.

Question 16

What initiates the release of calcium ions from the sarcoplasmic reticulum after a nerve signal arrives?

  1. Myosin binding to actin at rest
  2. An action potential spreading along sarcolemma into T-tubules (correct answer)
  3. ATP binding to troponin and splitting immediately
  4. Mitochondria releasing calcium into the sarcoplasm
Explanation: This question tests understanding of the sliding filament theory and contraction cycle in muscle physiology, focusing on calcium release after nerve signaling. The sliding filament theory explains how muscles contract by the sliding of actin filaments over myosin filaments, requiring calcium ions and ATP. In this question, the action potential's spread into T-tubules is highlighted. The correct choice identifies it as initiating SR calcium release. A common distractor might suggest myosin binding at rest, but that's prevented. To help students, connect synapse to fiber. Use neural-muscle pathway diagrams.

Question 17

During muscle contraction, a researcher measures the length of the A band, I band, and H zone at different stages. Which combination of changes would be observed when comparing a relaxed sarcomere to a maximally contracted sarcomere?

  1. A band decreases, I band decreases, H zone decreases
  2. A band remains constant, I band decreases, H zone decreases (correct answer)
  3. A band increases, I band remains constant, H zone decreases
  4. A band remains constant, I band remains constant, H zone increases
Explanation: According to sliding filament theory, during contraction, thick and thin filaments slide past each other without changing length. The A band (length of thick filaments) remains constant because myosin filaments don't shorten. The I band (region with only thin filaments) decreases as actin filaments slide deeper into the A band. The H zone (region with only thick filaments) decreases as actin filaments overlap more with myosin. Choice A incorrectly suggests the A band shortens. Choice C incorrectly suggests the A band lengthens and I band stays constant. Choice D describes relaxation, not contraction.

Question 18

In the sliding filament model, what role do calcium ions play during skeletal muscle contraction?

  1. They bind troponin, moving tropomyosin to expose actin binding sites (correct answer)
  2. They break down ATP to power the myosin head
  3. They directly pull actin filaments toward the Z disc
  4. They prevent myosin from attaching to actin during contraction
Explanation: This question tests understanding of the sliding filament theory and contraction cycle in muscle physiology, focusing on the role of calcium ions in enabling cross-bridge formation. The sliding filament theory explains how muscles contract by the sliding of actin filaments over myosin filaments, requiring calcium ions and ATP. In this question, the regulatory function of calcium is highlighted, as it binds to troponin to initiate contraction. The correct choice explains that calcium binds troponin, moving tropomyosin to expose actin binding sites, allowing myosin to attach. A common distractor might claim calcium breaks down ATP or pulls actin, but calcium's primary role is regulatory, not energetic or mechanical. To help students, break down the troponin-tropomyosin complex and its interaction with calcium. Use labeled diagrams to illustrate the exposure of binding sites on actin filaments.

Question 19

Use the graph to answer the question. The graph shows the relationship between sarcomere length and tension development in skeletal muscle. A student notes that at point X, tension is reduced compared to the optimal length. What structural explanation accounts for this tension reduction according to sliding filament theory?

  1. Excessive overlap causes actin filaments from opposite sides to interfere with each other (correct answer)
  2. Myosin filaments begin to buckle under compression, reducing their binding efficiency significantly
  3. Troponin-tropomyosin complexes become less responsive to calcium at shorter sarcomere lengths
  4. Z-lines physically restrict myosin head movement, preventing complete power stroke execution
  5. ATP diffusion becomes limited in compressed sarcomeres, reducing available energy for contraction
Explanation: At very short sarcomere lengths (point X represents the descending limb), actin filaments from opposite halves of the sarcomere begin to overlap and interfere with each other, reducing the effective binding sites available for myosin heads. This is the structural basis for reduced tension at short lengths. Choice B is incorrect because myosin filaments are rigid and don't buckle significantly. Choice C is wrong because troponin-tropomyosin sensitivity doesn't change with length. Choice D is incorrect because Z-lines don't directly restrict myosin head movement. Choice E is wrong because ATP diffusion is not significantly limited at shorter lengths.

Question 20

Refer to the diagram. A muscle fiber is stimulated and calcium is released, but ATP levels are sufficient only for myosin heads to complete the power stroke phase before becoming depleted. At what numbered step in the cross-bridge cycle would the process become arrested?

  1. Step 1, because ATP is required for initial myosin head activation and actin binding
  2. Step 2, because ATP hydrolysis is necessary for the power stroke to occur
  3. Step 3, because ATP is needed for the conformational change during power stroke
  4. Step 4, because ATP binding is required for myosin head detachment from actin
Explanation: D