MCAT Biology : Sarcomeres

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1 : Sarcomeres

Which two proteins are the major components of myofibrils, allowing for muscle fiber contraction?

Possible Answers:

None of these

Actin and myosin

Mylode and myosin

Lamelae and actin

Myosin and cartilage

Correct answer:

Actin and myosin

Explanation:

Myosin and actin are the two major proteins in muscle cells that allow for contraction. Actin is the thin filament; myosin is the thick filament. During muscle contractions the overlap between these two proteins results in a shorter muscle fiber, and a shorter muscle, that pulls on the tendon. The result is movement. The other answers contain other structural elements of muscles but are not the direct cause of muscle contraction.

Example Question #2 : Muscles And Myocytes

Which of the following proteins does not play a functional role in creating the force-tension curve of muscle contraction?

Possible Answers:

Titin

Actin

All of these are involved in creating the force-tension curve

Myosin

Correct answer:

All of these are involved in creating the force-tension curve

Explanation:

The force-tension curve is used to measure the optimal muscle length for maximum muscle contraction. This length corresponds to the optimal overlap of actin and myosin filaments to generate force. The length of actin and myosin filaments determines the minimum and maximum possible overlap. Titin is the protein responsible for the elasticity of the sarcomere after it is stretched past maximum actin-myosin overlap. Titin allows force production to exist at a maximum tension slightly beyond only actin and myosin, thus affecting the force-tension curve.

Example Question #3 : Musculoskeletal System And Muscle Tissue

What type of enzyme is myosin?

Possible Answers:

ADP synthase

ADP hydrolase

ATP hydrolase

ATP synthase

Correct answer:

ATP hydrolase

Explanation:

In addition to the subunits of myosin that link it to actin, myosin is also an ATP hydrolase, or ATPase. Myosin must hydrolyze ATP to ADP to allow for the power stroke that propels myosin forward on the actin polymers.

Example Question #1 : Musculoskeletal System And Muscle Tissue

Which of the following sections of a sarcomere does not shorten during contraction?

Possible Answers:

A band

I band

H zone

Distance between Z lines

Correct answer:

A band

Explanation:

Upon contraction, actin filaments will be pulled by myosin heads resulting in the shortening of the sarcomeres. The I band is composed of only actin filaments, and will begin to overlap with the myosin filaments, shortening the band. The A band, however, is the section composed of myosin filaments. Since this section is not altered by contraction, it stays the same length. Unlike the I band, the A band can contain regions of overlap without changing length.

The H zone, in contrast, refers to the region of myosin that is not overlapped by action. As the region of overlap increases, the H zone decreases. The distance between Z discs represents the total length of the sarcomere and must shorten in order for the muscle to contract.

Example Question #4 : Muscles And Myocytes

What structure marks the separation between two sarcomeres?

Possible Answers:

M-line

I-band

A-band

Z-disc

Correct answer:

Z-disc

Explanation:

Z-discs are the dividing points between sarcomeres. Actin filaments extend from this region and are joined together by several complex protein structures.

The M-line is the middle of the sarcomere, marking the central point of the myosin filaments. The I-band consists of actin filaments that are not overlapped by myosin; this region contains the Z-disc. The A-band marks the length of an entire thick filament (myosin), including the overlap region with actin.

Example Question #2 : Sarcomeres

What protein, present in sarcomeres, is responsible for the passive elasticity of muscle?

Possible Answers:

Titin

Collagen

Myosin

Actin

Correct answer:

Titin

Explanation:

Titin is a massive protein that spans the length of half of a sarcomere (from the Z-disc to the M-line) and allows for the passive elasticity of muscle. It is not directly involved in the process of contraction; that function is performed by actin and myosin.

Collagen proteins play an important role in providing tensile strength and building connective tissue throughout the body, but play only a minor role in the properties of muscle tissue in the extracellular matrix. Collagen is not found in the sarcomere.

Example Question #3 : Sarcomeres

Which of the following changes length during sarcomere contraction?

I. Thick filaments

II. Thin filaments

III. H zone

Possible Answers:

I, II, and III

III only

I and II

I and III

Correct answer:

III only

Explanation:

Recall that during sarcomere contraction, the myosin filaments attach to actin filaments and slide along the actin filaments. By this mechanism, the region of overlap between the fibers is increased and the total sarcomere length shortens. Neither actin, nor myosin actually change length; they simply move in relation to one another.

The H zone refers to the region of myosin at the center of the sarcomere that is not overlapped by actin. When the sarcomere shortens, the region of overlap increases and the H zone decreases.

Example Question #211 : Biology

Which of the following is true about sarcomeres?

Possible Answers:

Actin filaments are only found in the I band

The A band contains both actin and myosin filaments

The sarcomeres contribute to the striated appearance of smooth muscle cells

Sarcomeres are functional units of skeletal and smooth muscle cells

Correct answer:

The A band contains both actin and myosin filaments

Explanation:

Recall that sarcomeres are functional units of muscles that facilitate muscle contraction. Myosin heads bind to actin filaments and cause the filaments to overlap, shortening the sarcomere and, subsequently, the muscle.

Inside a sarcomere there are several regions. One such region is the I band, which consists of the actin filaments in the region where they are not superimposed by the myosin filaments. This means that the I band consists only of actin filaments; however, actin filaments aren’t exclusive to the I band. They are also found in other regions, such as A band. The A band is the region of the sarcomere that contains the myosin (thick) filaments, regardless of overlap. This means that myosin is exclusive to the A band, but that this region contains both actin and myosin due to overlap.

Sarcomeres are functional units of muscles, but they are only found in skeletal and cardiac muscle cells; smooth muscle cells do not contain sarcomeres. Actin and myosin filaments still cause the contraction seen in smooth muscle, but are not organized into alignment. This means that smooth muscle cells do not contract linearly and can essentially shrink in size during contraction, which can allow for things like constriction around organs and vessels.

Example Question #1 : Sarcomeres

A researcher observes a sarcomere through a microscope. He notices that a single myosin filament is forty micrometers long and that a single actin filament is fifty micrometers long. What can the researcher conclude from this information?

Possible Answers:

The I band is 

The I band is 

The A band is 

The A band is 

Correct answer:

The A band is 

Explanation:

To answer this question you need to understand the structural regions of the sarcomere. The I band, A band, and H zone are regions in a sarcomere that constitute of actin (thin) and myosin (thick) filaments. I band is the region of actin filaments that are not superimposed by myosin filaments. The H zone is the region of myosin filaments that are not superimposed by actin filaments. To calculate the length of the I band, you need the length of the myosin filament, the actin filament, and the H zone. Since we don’t have the length of H zone, we can’t solve for the length of I band. Essentially, without knowing the degree of overlap, we cannot determine the length of un-overlapped actin.

The A band is the region of the sarcomere that consists of the entire length of the myosin filament. The question states that the length of the myosin filaments is  micrometers; therefore, the length of the A band is  micrometers.

Example Question #5 : Sarcomeres

Which of the following is true about the organization of actin filaments and myosin in sarcomeres?

Possible Answers:

Prior to contraction, there is no overlap between actin and myosin

All of these answers

The degree of overlap of actin and myosin affects the overall contractile strength

Myosin filaments appear thinner than actin filaments 

Correct answer:

The degree of overlap of actin and myosin affects the overall contractile strength

Explanation:

The only choice that is actually true is that the degree of overlap of myosin and actin plays a role in contractile strength. If there is little to no overlap, contractile strength is low; however, if there is too much overlap then contractile strength is also low. This trend can be represented in a force-tension curve, which demonstrates that maximum force generation occurs when the sarcomere begins at equilibrium.

In a normal sarcomere there is always a small area of overlap of myosin and actin prior to contraction. Myosin appears thicker than actin, and is considered the "thick filament."

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