Chapter 12 Sliding Filament Theory #3
What causes the myosin heads to go from their tilted state to their upright, high energy state?
Energy transferred from ATP to the myosin head will return to it's high energy state
Where is myosin found in skeletal muscle cells?
In THICK filaments (crossbridges)
What is the FUNCTION of tropomyosin?
In a unstimulated muscle the position of the tropomyosin covers the binding siteson the actin subunits & prevent myosin binding.
Each subunit on actin contains binding sites for...
myosin heads
After a sarcomere has contracted fully and the calcium ion concentration within the cytosol decreases, what happens within the sarcomere?
Contraction Ends
According to the sliding filament theory, when a muscle cell contracts, the thin filaments slide past the thick filaments and the sarcomere shortens.
Thin Filaments (myofilament); Actin -actin monomers can bind with myosin (actin has a myosin-binding site), -they are arranged in an orderly, repeating fashion. -sometimes called contractile protein -made of G actin (globular proteins). -G actins form strands called F actin (fiberous protein). -two F actins arrange into a double helix to form actin strands found in thin filaments. Thick Filaments -myosin, contractile protein -made of hundreds of myosin molecules -look like two golf clubs wrapped together, a 1 tail and 2 heads together called CROSSBRIDGES (they bridge the gap between the thick and thin filaments).
What are the 2 parts to a myosin molecule
Tail Heads called CROSSBRIDGE -these are said to be the "POWER STROKE" for muscle contraction.
What two binding sites found on the myosin heads?
1.) ATP binding site 2.) Actin binding site
What 3 protein molecules are the thin filaments made of?
1.) Actin 2.) Tropgomyosin 3.) Troponin
Roles of ATP in Muscle Contraction
1.) Energizes the power stroke of the cross bridge. 2.) Disconnects the myosin head fm the binding site on actin at the conclusion of a power stroke. 3.) Actively transports calcium ions into the SR.
A Single Cross Bridge Cycle
1.) Presence of calcium ions in the cytosol trigger the exposure of binding sites on actin. 2.) Myosin bind to actin. 3.) The power stroke occurs. 4.) ATP binds to the myosin head and the myosin head disconnects from actin. 5.) ATP is hydrolyzed,leading to the re-energizing and repositioning of the myosin head. 6.) Calcium ions are transported back into the SR.
Summary of Sliding Filament
1.) The influx of calcium triggering the exposure of binding sites on actin. 2.) The binding of myosin to actin forming a cross bridge. 3.) The power stroke of the cross bridge that causes the sliding of the thin filaments. 4.) The binding of ATP to the myosin head which results in the myosin head disconnecting from Actin. 5.) The hydrolysis of ATP which leads to the re-energizing and repositioning of the Myosin head. 6.) The transport of calcium ions back into the sarcoplasmic reticulum. Multiple Crossbridge cycling prevents all of the cross bridges from either being connected or disconnected at the same time.
During the contraction of a muscle cell, what is happening.......
A.) the length of the sarcomere SHORTENS. B.) the position of the Z lines w/ repect tone another- the Z lines get closer to each other. C.) the length gof the thin filament DOESN'T CHANGE. D.) the length gof the thick filament DOESN'T CHANGE. E.) the width of the H zone changes.
What causes the myosin heads to disconnect from the actin?
ATP molecule must bind to it's site on the myosin head. This then releases the myosin head fm actin, which TRIGGERS the hydolysis of ATP molecule into ADP and Pi.
What causes the release of calcium ions into the cytosol from the terminal cisternae?
Action potential
What causes the tropomyosin to move away from the myosin binding site?
After action potential the Terminal Cisternae releases Calcium ions to bind to troponin, this then drags the tropomyosin strand off the binding sites
What is required to move the calcium ions from the cytosol back into the SR (sarcoplasmic reticulum)
An active transport of calcium involves specialized Ion pumps in the membrane of SR. These pumps must be energized by ATP.
What causes the myosin binding sites on actin to be exposed?
An energized myosin head can bind to it forming a cross bridge.
What causes the tropomyosin to cover back over the actin binding site?
Calcium is actively transported from the cytosol into the "SR ion pump." As the calcium is removed, the tropin-tropomyosin complex again covers the binding sites on actin.
Whatisit called when the myosin heads flex, pulling the filament inward toward the center of the sarcomere?
The Power Stroke.
What part of the myosin molecule has a hinge that allows for vertical movement so that the myosin head can bind to actin?
The TAIL
What happens to the myosin head when the power stroke occurs?
The chemical energy of ATP has been transferred into mechanical energy of a contraction.
TWO Regulatory Proteins in Thin filaments are Tropomyosin and Tropin
Tropomyosin -can block myosin-binding sites in muscles at rest. Tropin -complex of 3 proteins that: 1.) attaches to Actin 2.) binds to Tropomyosin 3.) contains a site to which calcium ...................ions can bind reversibly. This is the site that triggers muscle contraction by causing troponin to move tropomyosin aside, thereby exposing the myosin-binding sites on the actin molecules.
What is the function of troponin?
Troponin moves tropomyosin aside.
