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Home  >>  JEEMAIN and AIPMT  >>  Biology  >>  Human Physiology
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Explain step 1 and step 2:

image
( a ) Step 1: Increase in Ca++ level --> ATP hydrolysis, the myosin head binds active sites on actin --> SLIDING. Step 2: The myosin, releases the ADP and P1 --> relaxed state (A new ATP binds and the cross-bridge is broken) --> The ATP is again hydrolysed by the myosin head --> FORMATION OF CROSS BRIDGE
( b ) Step 1: Increase in Ca++ level --> binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin --> ATP hydrolysis, the myosin head binds active sites on actin to FORM CROSS BRIDGE. Step 2: The myosin, releases the ADP and P1 --> relaxed state (A new ATP binds and the cross-bridge is broken --> BREAKING OF CROSS BRIDGE
( c ) Step 1: Increase in Ca++ level --> binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin --> ATP hydrolysis, the myosin head binds active sites on actin to FORM CROSS BRIDGE. Step 2: The myosin, releases the ADP and P1 --> relaxed state (A new ATP binds and the cross-bridge is broken) --> The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation (step 1 ) and breakage (step 3) is repeated causing further SLIDING.
( d ) Step 1: Increase in Ca++ level --> ATP hydrolysis, the myosin head binds active sites on actin --> BREAKING OF CROSS BRIDGE. Step 2: The myosin, releases the ADP and P1 --> relaxed state (A new ATP binds and the cross-bridge is broken) --> The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further SLIDING.
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(C) is the right answer.

Increase in Ca++ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. This pulls the attached actin filaments towards the centre of ‘A’ band. The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. The myosin, releasing the ADP and P1 goes back to its relaxed state. A new ATP binds and the cross-bridge is broken (Figure 20.4). The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding. The process continues till the Ca++ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments. This causes the return of ‘Z’ lines back to their original position, i.e., relaxation. The reaction time of the fibres can vary in different muscles.
answered Mar 26, 2014 by pady_1
 
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