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Class 10 Class 12
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Write differences between
(a) Actin and Myosin



1. It is thin low molecular weight filamentous protein present in the sarcomere.

1. It is thick filamentous protein present in the sarcomere.

2. It occurs in two forms the monomeric G-actin and polymeric F-actin.

2. Each myosin molecule has two components—a tail and head. The tail is formed of heavy meromyosin (HMM) while the head is formed of light meroyosin LMM.

3. The contractile protein called tropomyosin.

3. The head has contractile property.

4. It does not act as ATPase

4. The head acts as ATPase.


Draw the diagram of a sarcomere of skeletal muscle showing different regions.


Match Column I with Column II :

A. Smooth muscle (i) Myoglobin
B. Tropomyosin (ii) Thin filament
C. Red Muscles (iii) Sutures
D. Skull (iv) Involuntary


Smooth muscle






Thin filament


Red Muscles








Describe the important steps in muscle contraction.

The important steps muscle contraction:

1. Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron and reach the neuromuscular junction. As a result, neurotransmitter (Acetyl choline) which generates an action potential in the sarcolemma.

2. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm.

3. Increase in Calcium 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.

4. The actin filaments are pulled. As a result, the H-zone reduces. It is at this stage that the contraction of the muscle occurs.

5. After muscle contraction, the myosin head pulls the actin filament and releases ADP along with inorganic phosphate. ATP molecules bind and detach myosin and the cross bridges are broken and decreases the calcium ions contraction. As a result, masking the actin filaments and leading to muscle relaxation.


Define sliding filament theory of muscle contraction.

The sliding filament theory explains the mechanism of muscle contraction. A proposed mechanism of muscle contraction in which the actin and myosin filaments of striated muscle slide over each other to shorten the length of the muscle fibres (see sarcomere). Myosin-binding sites on the actin filaments are exposed when calcium ions bind to troponin molecules in these filaments. This allows bridges to form between actin and myosin, which requires ATP as an energy source. Hydrolysis of ATP in the heads of the myosin molecules causes the heads to change shape and bind to the actin filaments. The release of ADP from the myosin heads causes a further change in shape and generates mechanical energy that causes the actin and myosin filaments to slide over one another