Question

The function of ATP during muscle contraction is to a. cause an allosteric change in myosin so it detaches from actin. b. provide the energy necessary for the movement of the crossbridge. c. expose the myosin-binding sites on the thin filaments. d. do all of the above. e. do a and b only

Short answer

The correct answer is e) do a and b only: ATP causes an allosteric change in myosin so it detaches from actin and provides the energy necessary for the movement of the crossbridge.

Step-by-step solution

Understanding Muscle Contraction

First, it's important to understand that muscle contraction is a complex process that relies on the interaction between proteins, energy sources, and molecular binding sites. Myosin and actin are two key proteins involved in this process. Briefly, myosin heads bind to actin filaments when a muscle needs to contract, forming crossbridges.

Role of ATP in Muscle Contraction

ATP (adenosine triphosphate) is the primary energy source for muscle contraction. The hydrolysis of ATP provides the necessary energy for the power stroke during which the myosin crossbridge moves and causes muscle contraction. Additionally, ATP is required when the myosin head needs to detach from the actin filament. Without ATP, the myosin would stay stuck to actin, freezing the muscle in a contracted state.

Evaluate the Options

Considering the roles ATP plays in muscle contraction described in the previous step, let's evaluate the options. Option a) is correct because ATP does cause an allosteric change in myosin allowing it to detach from actin. Option b) is also correct, as ATP provides the energy for crossbridge movement. However, option c) is not correct because the exposure of myosin-binding sites on thin filaments is primarily brought about by the removal of tropomyosin, regulated by calcium ions, not ATP.

Select the Correct Answer

Given that both option a) and b) are correct, but option c) is not, the correct answer is e) do a and b only.

Key concepts

Muscle Contraction Process

Muscle contraction is a fascinating and complex event that allows us to move our bodies with precision and strength. It involves a cycle where muscle fibers shorten and generate force. This process begins with a signal from the nervous system that arrives at a muscle cell. Calcium ions are then released within the muscle cell which leads to an exposure of binding sites on actin, a thin filament.

Myosin heads, which are part of a thick filament, latch onto these newly exposed sites on the actin filament, forming what is known as a crossbridge. By pivoting, myosin heads pull the actin filaments towards the center of the sarcomere - the basic unit of a muscle - causing the muscle to contract. This action is often referred to as the power stroke.

However, for this contraction to cease and the muscle to relax, myosin heads must release actin. This is where adenosine triphosphate, or ATP, plays a crucial role. ATP binds to the myosin head, causing it to release actin and allowing the muscle to return to its resting state. In summary, a continual supply of ATP is vital for the muscles to contract and relax in a well-controlled manner.

Myosin and Actin Interaction

At the core of muscle contraction lies the interaction between myosin and actin, the two proteins responsible for the cellular mechanism behind this process. Myosin, with its characteristic head and tail structure, serves as the motor protein capable of converting chemical energy into mechanical work.

Binding and Power Stroke

These myosin heads, which are energized by ATP, bind to specific sites on the actin filaments. Upon binding, ATP is hydrolyzed, and the energy released from this reaction propels the myosin head to perform a power stroke. This action slides the actin filament alongside the myosin, resulting in contraction.

Detachment and Reactivation

After the power stroke, a new molecule of ATP attaches to the myosin head, prompting it to detach from actin and recock into its original energized position. The cycle of attachment, stroke, and detachment repeats as long as ATP is available and the muscle is stimulated, creating successive contractions that lead to muscle shortening and movement.

Energy Source for Muscle Contraction

For muscles to contract, an immediate and reliable source of energy is required, and ATP is the essential molecule that fulfills this role.

ATP Hydrolysis and Energy Release

The hydrolysis of ATP, which means breaking down the molecule into adenosine diphosphate (ADP) and inorganic phosphate (Pi), provides the energy necessary for myosin heads to carry out their power strokes during muscle contraction. Without this energy, muscles would not be able to contract or maintain tension.

Replenishing ATP

Since ATP is used up rapidly during muscle activity, it must be continuously replenished. This replenishment comes from several biological pathways including creatine phosphate breakdown, anaerobic glycolysis, and aerobic respiration, depending on the intensity and duration of muscle activity.

To conclude, ATP not only triggers the contraction process by enabling the myosin-actin crossbridges but also allows muscles to relax by breaking those connections. It's the fundamental energy currency that enables all types of muscle movements from quick, explosive actions to sustained endurance exercises.