MCAT Biology : Biology

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #251 : Biology

Skeletal muscle fibers are not all contracted at once by the same action potential. Instead, muscle fibers are divided into clusters that can range from two to two thousand cells. All of these muscle fibers are innervated by the same neuron; the muscle fibers and the neuron that innervates them are collectively referred to as a motor unit.

When the neurotransmitter attaches to the sarcolemma of the muscle fibers, it stimulates the release of calcium. What is the primary role of calcium in skeletal muscle?

Possible Answers:

Spreading the action potential throughout the muscle fibers

Cocking the myosin head

Allowing myosin to attach to actin

Creating a gradient for ATP production, which will power the muscle contractions

Correct answer:

Allowing myosin to attach to actin

Explanation:

Acetylcholine will stimulate sodium channels on the sarcolemma, which will consequently trigger the release of calcium ions from the sarcoplasmic reticulum. The calcium will then attach to troponin, which pulls tropomyosin away from the active site on actin. With the active site available, myosin heads are able to attach to the actin filament.

Example Question #252 : Biology

Which of the following proteins binds to calcium released from the sarcoplasmic reticulum?

Possible Answers:

ATP hydrolase

Tropomyosin

Calsequestrin

Troponin

Correct answer:

Troponin

Explanation:

Troponin binds free calcium once it is released from the sarcoplasmic reticulum, causing a conformational change in tropomyosin. This change exposes the myosin binding site on actin, allowing for cross-bridge formation and contraction.

Example Question #253 : Biology

Which of the following motor units are recruited first in a muscle contraction?

Possible Answers:

Large motor units

All motor units are activated simultaneously

Intermediate motor units

Small motor units

Correct answer:

Small motor units

Explanation:

Small motor units, typically consisting of one nerve and a few muscle cells, are recruited first. As the muscle contracts for a longer period of time or is required to lift a heavier load, intermediate and large muscle motor units are recruited. As intermediate and large muscle motor units are recruited, more action potentials begin to fire, releasing more calcium from the sarcoplasmic reticulum and increasing the overall strength of the muscle contraction.

Example Question #254 : Biology

A toxin prevents calcium from being actively pumped back into the sarcoplasmic reticulum. What would you expect to be a consequence of this toxin's presence in the body?

Possible Answers:

Muscles would be unable to contract

Myosin would always be able to attach to actin

Acetylcholine would constantly be released at the neuromuscular junction

Myosin would be unable to enter into a high energy position before the contracting motion

Troponin would be unable to move tropomyosin off of the active site on actin

Correct answer:

Myosin would always be able to attach to actin

Explanation:

Before a contraction, calcium is released from the sarcoplasmic reticulum and attaches to troponin. Troponin will then remove tropomyosin from the active site on actin where myosin is able to attach.

If calcium is never pumped back into the sarcoplasmic reticulum, the active site on actin will stay exposed, which allows myosin to attach at all times.

Note that calcium is also responsible for initiating acetylcholine release from the neuron at the neuromuscular junction; however, this process involves extracellular calcium ions and is not linked to the sarcoplasmic reticulum.

Example Question #32 : Muscle Stimulation And Contraction

What is the correct movement within the sarcomere during contraction?

Possible Answers:

The I-band, the H-zone, and the A-band get smaller while the Z-lines move apart.

The I-band and the H-zone get smaller, the Z-lines move closer together, but the A-band does not change.

The I-band and the H-zone get smaller, the Z-lines move closer together, and the A-band gets wider.

The I-band gets smaller, the Z-lines move closer together, and the H-zone gets wider.

None of the above

Correct answer:

The I-band and the H-zone get smaller, the Z-lines move closer together, but the A-band does not change.

Explanation:

During muscle contraction, the I-band, the H-zone, and the area between the Z-lines get decreased in length. The A-band remains in constant length. Overall, the sarcomere constricts in size, allowing the muscle to contract.

Example Question #255 : Biology

The sliding filament model of muscle contraction states all of the following EXCEPT __________.

Possible Answers:

the actin slides toward the M-line, causing the muscle fiber to contract

a cross-bridge forms between actin and the myosin head

ATP hydrolysis causes the myosin head to unbend

during the power stroke, the myosin head bends

when calcium is present, myosin head detaches from actin

Correct answer:

when calcium is present, myosin head detaches from actin

Explanation:

The sliding filament model of muscle contraction states that as the cross-bridge forms between actin and myosin, the myosin head bends (the power stroke), causing actin to move (slide) in the direction of the M-line. When all the actin filaments slide toward the M line like this, the muscle fiber contracts. Calcium is needed for the binding of myosin head to actin. ATP binding leads to the detachment of myosin head from actin. ATP hydrolysis is needed for the unbending of myosin head.

Example Question #256 : Biology

Contraction of the muscle cell is initiated by calcium ions bonding to which of the following cell components?

Possible Answers:

Actin

Myosin

Tropomyosin

Troponin

Correct answer:

Troponin

Explanation:

The mechanism for troponin and tropomyosin’s interaction with calcium is comparable to a bike chain with a lock and key. Tropomyosin is the “chain” that blocks the binding sites on actin from the myosin heads. The calcium ion acts like a key to “unlock” troponin and move tropomyosin off of actin’s binding sites. This allows myosin heads to bind to actin and complete their power stroke.

Example Question #1 : Types Of Muscle Cells

Which answer correctly mactches an organ or structure with the type of muscle found there?

Possible Answers:

The stomach—cardiac muscle

The heart—skeletal muscle

The aorta—cardiac muscle

The small intestine—skeletal muscle

The bladder—smooth muscle

Correct answer:

The bladder—smooth muscle

Explanation:

Smooth muscle is found is the bladder, intestines, blood vessels, and a lot of other places that have involuntary motion. Skeletal muscle connects bones and muscles and allows us to move things voluntarily. Cardiac muscle is found in one place—the heart; therefore, the only correct match is the bladder to smooth muscle.

Example Question #1 : Types Of Muscle Cells

How many different types of muscle tissue are there in the human body?

Possible Answers:

Two

None of these

One

Three

Four

Correct answer:

Three

Explanation:

There are three classifications of muscle tissue: skeletal, cardiac, and smooth. A major distinction between these groups is that only skeletal muscle can be voluntarily controlled. Smooth muscle (such as that around blood vessels) and cardiac muscle (in the heart) are not consciously controlled. Each muscle type is unique to its specific function.

Example Question #2 : Types Of Muscle Cells

Which muscle tissue type allows for voluntary control?

Possible Answers:

None of these

Skeletal

Smooth

Rough

Cardiac

Correct answer:

Skeletal

Explanation:

Skeletal muscle is the only tissue type that allows for voluntary control; cardiac and smooth do not. Unlike the muscles in your arm, you cannot simply will the other muscle types to work. Think about it: you cannot stop your heart from pumping simply by trying to.

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