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 Part A of the illustration shows a relaxed muscle fiber. Two zigzagging Z lines extend from top to bottom. Thin actin filaments extend left and right from each Z line. Between the Z lines is a vertical M line. Thick myosin filaments extend left and right from the M line. The thick and thin filaments partially overlap. The A band represents the length that the thick filaments extend from both sides of the M line. The I band represents the part of the thin filaments that does not overlap with the thick filaments. Part B shows a contracted muscle fiber. In the contracted fiber, the thick and thin filaments completely overlap. The A band is the same length as in the uncontracted muscle, but the I band has shrunken to the width of the Z line.
When (a) a sarcomere (b) contracts, the Z lines move closer together and the I band gets smaller. The A band stays the same width and, at full contraction, the thin filaments overlap.

When a sarcomere shortens, some regions shorten whereas others stay the same length. A sarcomere is defined as the distance between two consecutive Z discs or Z lines; when a muscle contracts, the distance between the Z discs is reduced. The H zone—the central region of the A zone—contains only thick filaments and is shortened during contraction. The I band contains only thin filaments and also shortens. The A band does not shorten—it remains the same length—but A bands of different sarcomeres move closer together during contraction, eventually disappearing. Thin filaments are pulled by the thick filaments toward the center of the sarcomere until the Z discs approach the thick filaments. The zone of overlap, in which thin filaments and thick filaments occupy the same area, increases as the thin filaments move inward.

Atp and muscle contraction

The motion of muscle shortening occurs as myosin heads bind to actin and pull the actin inwards. This action requires energy, which is provided by ATP. Myosin binds to actin at a binding site on the globular actin protein. Myosin has another binding site for ATP at which enzymatic activity hydrolyzes ATP to ADP, releasing an inorganic phosphate molecule and energy.

ATP binding causes myosin to release actin, allowing actin and myosin to detach from each other. After this happens, the newly bound ATP is converted to ADP and inorganic phosphate, P i . The enzyme at the binding site on myosin is called ATPase. The energy released during ATP hydrolysis changes the angle of the myosin head into a “cocked” position. The myosin head is then in a position for further movement, possessing potential energy, but ADP and P i are still attached. If actin binding sites are covered and unavailable, the myosin will remain in the high energy configuration with ATP hydrolyzed, but still attached.

If the actin binding sites are uncovered, a cross-bridge will form; that is, the myosin head spans the distance between the actin and myosin molecules. P i is then released, allowing myosin to expend the stored energy as a conformational change. The myosin head moves toward the M line, pulling the actin along with it. As the actin is pulled, the filaments move approximately 10 nm toward the M line. This movement is called the power stroke, as it is the step at which force is produced. As the actin is pulled toward the M line, the sarcomere shortens and the muscle contracts.

When the myosin head is “cocked,” it contains energy and is in a high-energy configuration. This energy is expended as the myosin head moves through the power stroke; at the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the cross-bridge formed is still in place, and actin and myosin are bound together. ATP can then attach to myosin, which allows the cross-bridge cycle to start again and further muscle contraction can occur ( [link] ).

Questions & Answers

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rab Reply
a part of a organism
Devinayasha
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Mmesoma Reply
a temporary protrusion of the surface of an ameboid cell for movement and feeding.
Black
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Francess
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Shaaibu
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bix
biology_it's science that study of living things
Black
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Black
is a branch of science whith deals the study of living things
AZHKIR
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Mikaela Reply
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Ubon
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Sweety
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Aben
reproducing specifically : the process by which plants and animals give rise to offspring and which fundamentally consists of the segregation of a portion of the parental body by a sexual or an asexual process and its subsequent growth and differentiation into a new individual.
Black
is the process where by organism produce their new organism of the same species from those who a ready in existence.
Prudent
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Abigail
unicellular orgnism* meaning consisting of a single cell
Black
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Aben
Is a single celled organism
Brian
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Tamara Reply
antibiotics is any medication that stop the growth of bacteria
onuoha
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This is a group of molecules produced by a living organism
Odion
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It Reply
Now Mr. Niger Dac M;movement R;respiration N;nutrition G;growth E;excretion R;reproduction D;death A;adaptation C;competition
Odion
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Odion
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Mesumbe Reply
ccc
Peace
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Emmanuel
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Emmanuel
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debby
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Diamond Reply
Cardiac muscle
Divya
cardiac or myocardia muscle
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Jemima
Cardiac muscle
Dr
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yimam
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Emmanuel
It is an initial to represent life processes of organisms. M means movement, R for reproduction, N stands for nutrition, I for irritability or sensitivity,G means growth , E for excretion and R stands for respiration.
Quartey
cardiac muscle
Stanisla
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it is the chemical break down of insoluble food substances such as fatty acid to soluble substances which are then used for body processes
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sadeeq Reply
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sadeeq
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Dolla
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Dolla
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Quartey
artery is type of blood vessel which carries blood away from heart...
shams
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shams
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Sweety Reply
Georges Lemaître
Mr

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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