Question

In a 300 word essay, describe the cross bridge cycle. Make sure to include the action of actin, myosin, troponin, calcium, and ATP. Please Do NOT Copy and Paste

Answer 1

The contraction of a skeletal muscle produces the force required to move the skeleton. Cross bridge cycle is a series of molecular events that triggers a contraction. The functional unit in a contraction in skeletal muscles is known as the sarcomere. A sarcomere shortens when myosin head in thick myofilaments form cross bridges with actin molecules and thin myofilaments. The formation of a cross bridge is initiated when calcium ions are released from the sarcoplasmic reticulum and they bind to troponin. This binding of calcium ions causes troponin to change shape which causes tropomyosin to move away from myosin binding sites on actin, allowing the myosin head to bind actin and form a "cross-bridge". Before a cross bridge cycle can begin it is very important for the myosin head to be activated which happens when ATP binds to the myosin head and is hydrolyzed to ADP and inorganic phosphate (pi). This hydrolysis releases energy that activates myosin head and forcing it to a cocked position.

The cross bridge cycle can be divided into 4 steps:

1. Cross bridge formation: The activated myosin head binds to the actin forming a cross bridge and pi is released making the bond between actin and myosin stronger.

2. The power stroke: ADP is liberated and the activated myosin head pivots sliding the thin myofilament towards the middle of the sarcomere.

3. Cross bridge detachment: When another ATP binds to the myosin head, the bond between actin and myosin weakens and the myosin head detaches.

4. Reactivation of the myosin head: Another ATP molecule binds to the myosin head and is hydrolyzed to ADP and inorganic phosphate (pi) reactivating myosin head returning it to the cocked position.

As long as the binding sites on actin remain exposed the cross bridge cycle continues. Along with which the thin myofilaments are pulled towards each other and the sarcomere shortens leading to contraction of the entire muscle. Cross bridge cycle ends when calcium ions are actively transported back to the sarcoplasmic reticulum and troponin returns to its original shape and tropomyosin glides over and covers the myosin binding site on actin.