Question

In muscle cells, myosin molecules continue moving along actin molecules as long as a. ATP is present and troponin is not bound to \(\mathrm{Ca}^{2+}\) b. ADP is present and tropomyosin is released from intracellular stores. c. ADP is present and intracellular acetylcholine is high. d. ATP is present and intracellular \(\mathrm{Ca}^{2+}\) is high.

Short answer

d. ATP is present and intracellular \(\mathrm{Ca}^{2+}\) is high.

Step-by-step solution

Understanding muscle contraction mechanism

Muscle contraction occurs when muscle fibers generate tension (force) through their interaction of actin and myosin filaments. This interaction is governed by the sliding filament model, which includes the presence of ATP, calcium ions (\(\mathrm{Ca}^{2+}\)), troponin, and tropomyosin.

Evaluating option (a)

"a. ATP is present and troponin is not bound to \(\mathrm{Ca}^{2+}\)." In the muscle contraction process, calcium ions (\(\mathrm{Ca}^{2+}\)) play a crucial role by binding to the troponin complex on the actin filaments. This binding changes troponin's conformation and pulls tropomyosin away from the myosin-binding sites on actin, allowing myosin heads to connect with actin. Additionally, ATP is necessary for cross-bridge cycling between myosin heads and actin filaments. Thus, if ATP is present but troponin is not bound to \(\mathrm{Ca}^{2+}\), myosin molecules cannot continue moving along actin molecules.

Evaluating option (b)

"b. ADP is present and tropomyosin is released from intracellular stores." Here, the presence of ADP is mentioned, but ATP is the actual molecule needed for muscle contraction. Therefore, this statement cannot be correct.

Evaluating option (c)

"c. ADP is present and intracellular acetylcholine is high." As mentioned earlier, ATP, not ADP, is needed for muscle contraction. Acetylcholine is a neurotransmitter that activates muscle contraction, but its presence alone does not ensure that myosin molecules will continue moving along actin filaments. We also need ATP and calcium ions for this process to occur, so this statement is also incorrect.

Evaluating option (d)

"d. ATP is present and intracellular \(\mathrm{Ca}^{2+}\) is high." This statement indicates that there is sufficient ATP for cross-bridge cycling and a high concentration of intracellular calcium ions, which will facilitate their binding to troponin and trigger muscle contraction. This is the correct option as it describes the necessary conditions for myosin molecules to continue moving along actin molecules in muscle cells. Final Answer: d. ATP is present and intracellular \(\mathrm{Ca}^{2+}\) is high.

Key concepts

Sliding Filament Model

The 'Sliding Filament Model' is fundamental to understanding how muscles contract. Picture myosin and actin, two types of protein filaments, residing within the muscle fibers. The myosin filaments have tiny projections called 'myosin heads' that grasp and pull the actin filaments. This action is akin to a crew team powerfully rowing a boat—each stroke of their oars draws the boat forward, just like each pull of the myosin heads slides the actin closer, shortening the muscle fiber.

Imagine the muscle as a zipper; as the teeth interlock, the whole structure becomes compact. In the same way, as myosin pulls actin, the muscle cell contracts. The brilliance of this model lies in its cyclical nature – after pulling, myosin releases actin to grab it again further down the filament, powering another contraction cycle.

• Muscle contraction starts when the brain sends a signal.
• Myosin heads latch onto binding sites on the actin filaments.
• Once attached, the myosin heads pivot, pulling the actin filaments towards the center of the muscle cell.
• The myosin heads release, attach to a new site, and the process repeats.

This elegant dance is propelled by ATP and regulated by calcium ions, creating the rhythmic contraction and relaxation we call muscle movement.

ATP in Muscle Contraction

ATP, or Adenosine Triphosphate, is the lifeline of muscle contraction, as indispensable as fuel is to a car. Without ATP, even if the muscle is revved up by signals from the brain, it can't move, like a car with an empty gas tank. This molecule supplies the energy required for myosin heads to attach to and pull on the actin filaments, powering the muscle contraction.

During a muscle contraction, ATP has a crucial job of disconnecting myosin heads from the actin once they have completed their pull. It's as if ATP is the key that unlocks the myosin heads from the actin 'door.' When ATP attaches to the myosin heads, they release the actin, allowing the cycle of pulling and releasing to carry on. After its encounter with the myosin head, ATP is transformed into ADP (Adenosine Diphosphate), and in order to participate again in muscle contraction, it must be 'recharged' back into ATP.

• ATP binds to the myosin head, energizing it for action.
• The myosin head releases actin after a 'power stroke' is completed.
• ATP is split into ADP and a phosphate, gearing up the myosin for another cycle.

Calcium Ions in Muscle Contraction

Calcium ions (\( \text{Ca}^{2+} \) are the conductors orchestrating each step of muscle contraction. When a nerve impulse signals a muscle to contract, calcium ions rush from storage within the muscle cell and bind to a complex of proteins called 'troponin' that sits on the actin filaments. The binding of calcium to troponin is like a green light on a racetrack; it changes troponin's shape and shifts another protein, tropomyosin, that blocks the bonding sites for the myosin on the actin filament.

The clearance of tropomyosin exposes the binding sites and allows the myosin heads to attach to the actin — the starting pistol of the muscle contraction race. Without calcium ions, the binding sites would remain hidden, and contraction could not be triggered, akin to a race with no starting gun. Calcium's role doesn't end there; once the message to contract is over, calcium ions detach from troponin and are pumped back into their storage, allowing the muscle to relax.

• Calcium ions released in response to a nerve impulse.
• Calcium binds to troponin, triggering a shape change.
• Tropomyosin moves, exposing myosin binding sites on actin.
• The muscle contracts as myosin pulls on actin.
• Calcium ions are reabsorbed, and the muscle relaxes.