Question

During muscle contraction, a. ATP is hydrolyzed when the myosin head is unattached. b. ADP and ? are released as the myosin head attaches to actin. c. ADP and ? release causes the head to change position and actin filaments to move. d. release of ATP causes the myosin head to return to resting position.

Short answer

During muscle contraction, (a) ATP binds to the unattached myosin head and is hydrolyzed, energizing the myosin head. (b) The myosin head attaches to the actin filament, releasing ADP and Pi. (c) This release causes a conformational change, called the power stroke, that moves the actin filament towards the center of the sarcomere, resulting in muscle contraction. (d) Finally, the binding of another ATP molecule allows the myosin head to return to its resting position, detaching from the actin filament.

Step-by-step solution

a. ATP is hydrolyzed when the myosin head is unattached.

In the first step of muscle contraction, ATP (adenosine triphosphate) binds to the unattached myosin head. This binding of ATP causes the myosin head to change its conformation and allows it to detach from the actin filament. Then, ATP is hydrolyzed into ADP (adenosine diphosphate) and an inorganic phosphate (Pi), which energizes the myosin head, preparing it for another cycle of contraction.

b. ADP and Pi are released as the myosin head attaches to actin.

Next, the myosin head, now energized, is ready to bind to a specific site on the actin filament called the myosin-binding site. When the myosin head binds to actin, it forms a cross-bridge, causing the release of ADP and Pi. This release allows the myosin head to firmly attach to the actin filament, which is an essential step for muscle contraction.

c. ADP and Pi release causes the head to change position and actin filaments to move.

Upon the release of ADP and Pi, the myosin head undergoes a conformational change called the power stroke. During the power stroke, the myosin head pivots, changing its angle relative to the actin filament, and pulls the actin filament to slide towards the center of the sarcomere (the basic contractile unit of muscle). This sliding movement of actin filaments shortens the sarcomere and generates force, resulting in muscle contraction.

d. Release of ATP causes the myosin head to return to the resting position.

After the power stroke, another ATP molecule binds to the myosin head, leading to the release of the myosin head from the actin filament, as mentioned in step a. This ATP binding allows the myosin head to return to its resting position, where it can be re-energized by ATP hydrolysis and repeat the entire process of muscle contraction if necessary.

Key concepts

Myosin Head

The myosin head is a crucial component in the process of muscle contraction. Think of it like a tiny lever or hook on the larger structure called a myosin filament. During muscle contraction, the myosin head plays an active role in pulling the actin filaments closer together. When ATP binds to the myosin head, it causes changes in the myosin's shape that are necessary for movement.
The myosin head attaches and detaches from the actin filament repeatedly, using energy from ATP to power its movements. This cyclical attachment and detachment are essential to the contraction process, as it moves the muscle fibers, leading to a contraction of the whole muscle.

ATP Hydrolysis

ATP hydrolysis is a key step in muscle contraction. When the myosin head is unattached, ATP binds to it, and this binding allows ATP to be split into ADP (adenosine diphosphate) and inorganic phosphate (Pi).
This reaction is called ATP hydrolysis and it releases energy which energizes the myosin head.
This energy is crucial as it allows the myosin head to perform the necessary conformational changes needed to bind to actin. This energized state of the myosin head is what enables it to pull the actin filament during contraction.

Actin Filament

Actin filaments are thin strands that interact with myosin to enable muscle contraction. These are made up of actin proteins arranged in a helical structure. They provide the track along which the myosin heads move.
When the myosin heads attach to the actin filaments, they form what is known as a cross-bridge. This plays a direct role in muscle contractions.
Actin filaments slide along or over the myosin filaments when myosin heads execute a power stroke. This movement is responsible for pulling the Z-lines of the sarcomere closer, shortening the overall length of the muscle cell and contributing to contraction.

Cross-Bridge Formation

Cross-bridge formation is a term used to describe the binding of the myosin head to the actin filament. It's a fundamental part of muscle contraction. This process begins when the energized myosin head attaches to the myosin-binding site on the actin filament.
As a cross-bridge forms, ADP and inorganic phosphate (Pi) are released from the myosin, triggering a change in the angle of the myosin head.
This motion, called the power stroke, pulls the actin filament towards the center of the sarcomere, which is essential for muscle contraction. This cycle of cross-bridge formation, power stroke, and detachment is repeated during muscle contraction, powered by continuous ATP consumption.