biology : class11 A Band : Structure, Contractile Proteins, Muscle Fibre : Skeletal Muscle MCQ Questions and Answers
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A Band
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MCQ Questions
1. Which structural feature most directly defines the A band of a skeletal muscle sarcomere?
A.
The entire region occupied by thick filaments, including portions overlapped by thin filaments
B.
The region containing only thin filaments between two Z discs
C.
The central region containing only actin filaments
D.
The interval between adjacent M lines
Correct Answer:
The entire region occupied by thick filaments, including portions overlapped by thin filaments
Explanation:
The A band corresponds to the full length of the thick myosin filaments. It includes both the regions where thick and thin filaments overlap and the central region containing thick filaments alone.
2. During contraction of a skeletal muscle fibre, which change specifically characterizes the A band?
A.
It shortens because the thick filaments themselves become shorter
B.
It lengthens because thin filaments slide toward the M line
C.
Its length remains essentially constant while the overlap with thin filaments increases
D.
It disappears as the Z discs move apart
Correct Answer:
Its length remains essentially constant while the overlap with thin filaments increases
Explanation:
According to the sliding-filament mechanism, thick filaments do not change length. Therefore, the A band remains essentially constant even though thin filaments slide inward and overlap the thick filaments more extensively.
3. A sarcomere is observed before and during maximal contraction. Which pattern of band changes would be expected if the A band is identified correctly?
A.
A band shortens, whereas the H zone and I band remain unchanged
B.
A band remains constant, whereas the H zone and I band become narrower
C.
A band becomes wider, whereas the H zone becomes wider
D.
A band disappears, whereas the I band becomes occupied only by thick filaments
Correct Answer:
A band remains constant, whereas the H zone and I band become narrower
Explanation:
The A band represents the length of the thick filament and therefore remains constant. Increased actin-myosin overlap reduces the portions containing only thick filaments or only thin filaments, narrowing the H zone and I band.
4. If the length of a thick filament is 1.6 micrometres, what is the approximate length of the A band in a sarcomere containing that filament?
A.
0.8 micrometres
B.
1.2 micrometres
C.
1.6 micrometres
D.
3.2 micrometres
Correct Answer:
1.6 micrometres
Explanation:
The A band extends across the entire length of the thick filament. Thus, when the thick filament is approximately 1.6 micrometres long, the A band is also approximately 1.6 micrometres long.
5. A microscopic region contains thick filaments throughout its length, but thin filaments overlap them only near the two ends. Which interpretation is most accurate?
A.
The entire region is an A band, with a central H zone inside it
B.
The central region alone is the A band, while the ends are I bands
C.
The region is an I band because thin filaments are present at its ends
D.
The region is an H zone because overlap occurs at both ends
Correct Answer:
The entire region is an A band, with a central H zone inside it
Explanation:
The A band includes the complete length of the thick filaments, including both overlap regions and the central non-overlap region. The central thick-filament-only portion is the H zone within the A band.
6. Why does the A band appear relatively dark in electron micrographs of striated muscle?
A.
It contains a high concentration of thick myosin filaments that produce strong electron density
B.
It contains only the lightly staining ends of actin filaments
C.
It lacks organized protein filaments and therefore absorbs more stain
D.
It is formed exclusively by the Z discs, which are the darkest structures
Correct Answer:
It contains a high concentration of thick myosin filaments that produce strong electron density
Explanation:
The A band is anisotropic and appears dark because it contains the thick myosin filaments. Regions where myosin and actin overlap can also contribute to its dense appearance.
7. A mutation prevents thin filaments from sliding inward but does not alter thick-filament length. What would happen to the A band during attempted contraction?
A.
It would shorten because force generation is blocked
B.
It would lengthen because thin filaments remain outside the thick-filament region
C.
It would remain at approximately the same length because thick-filament length is unchanged
D.
It would be replaced by an enlarged H zone
Correct Answer:
It would remain at approximately the same length because thick-filament length is unchanged
Explanation:
The A band is determined by the length of the thick filaments, not by the degree of sliding. Even if thin-filament movement is prevented, the thick filaments and therefore the A band retain their length.
8. Which statement best distinguishes the A band from the H zone within a sarcomere?
A.
The A band contains thick filaments throughout its length, whereas the H zone is only its central thick-filament-only region
B.
The A band contains only thin filaments, whereas the H zone contains both filament types
C.
The A band lies between two sarcomeres, whereas the H zone forms their boundary
D.
The A band changes length during contraction, whereas the H zone does not
Correct Answer:
The A band contains thick filaments throughout its length, whereas the H zone is only its central thick-filament-only region
Explanation:
The A band encompasses the whole thick filament, including overlap with actin. The H zone is a narrower central portion of the A band where thick filaments are not overlapped by thin filaments.
9. In a contracted sarcomere, the H zone is no longer visible, but the region containing thick filaments can still be identified. Which conclusion follows?
A.
The A band has also disappeared
B.
The A band remains because it includes the thick-filament region even when overlap becomes extensive
C.
The sarcomere has become an I band
D.
The thick filaments have completely depolymerized
Correct Answer:
The A band remains because it includes the thick-filament region even when overlap becomes extensive
Explanation:
With strong contraction, thin filaments may overlap the entire central thick-filament region, causing the H zone to disappear. The A band remains because the thick filaments themselves still occupy that region.
10. Two sarcomeres contain thick filaments of different lengths but are examined at the same relative degree of contraction. Which comparison of their A bands is most appropriate?
A.
Both A bands must have the same length because contraction level is the same
B.
The A band of each sarcomere corresponds to the length of its own thick filaments
C.
The shorter thick filament produces a longer A band because overlap is greater
D.
A band length depends only on the distance between adjacent Z discs
Correct Answer:
The A band of each sarcomere corresponds to the length of its own thick filaments
Explanation:
A band length is determined by thick-filament length. The degree of overlap affects the H zone and I band, but it does not redefine the A band as a function of Z-disc spacing.
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