Biology Class 11 NCERT Solutions: Chapter 20 Locomotion and Movement Part 1

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Q: 1. The diagrammatic representation of a sarcomere is as follows:




Q: 2. The sliding filament theory explains the process of muscle contraction during which the thin filaments slide over the thick filaments, which shortens the myofibril.


Each muscle fibre has an alternate light and dark band, which contains a special contractile protein, called actin and myosin respectively. Actin is a thin contractile protein present in the light band and is known as the I-band, whereas myosin is a thick contractile protein present in the dark band and is known as the A-band. There is an elastic fibre called line that bisects each I-band. The thin filament is firmly anchored to the line. The central part of the thick filament that is not overlapped by the thin filament is known as the H-zone.

During muscle contraction, the myosin heads or cross bridges come in close contact with the thin filaments. As a result, the thin filaments are pulled towards the middle of the sarcomere. The Z line attached to the actin filaments is also pulled leading to the shortening of the sarcomere. Hence, the length of the A- band remains constant as its original length and the I-band shortens and the H-zone disappears.’

Sliding filament theory

Sliding Filament Theory

Q: 3. Describe the important steps in muscle contraction.


During skeletal muscle contraction, the thick filament slides over the thin filament by a repeated binding and releases of myosin head along the filament. This whole process occurs in a sequential manner.

Image of the cycle in muscle contraction

Image of the Cycle in Muscle Contraction

Step 1:

Muscle contraction is initiated by signals that travel along the axon and reach the neuromuscular junction or motor end plate. Neuromuscular junction is a junction between a neuron and the sarcolemma of the muscle fibre. As a result, Acetylcholine (a neurotransmitter) is released into the synaptic cleft by generating an action potential in sarcolemma.

Step 2:

The generation of this action potential releases calcium ions from the sarcoplasmic reticulum in the sarcoplasm.

Step 3:

The increased calcium ions in the sarcoplasm leads to the activation of actin sites. Calcium ions bind to the troponin on actin filaments and remove the tropomyosin, wrapped around actin filaments. Hence, active actin sites are exposed and this allows myosin heads to attach to this site.

Step 4:

In this stage, the myosin head attaches to the exposed site of actin and forms cross bridges by utilizing energy from ATP hydrolysis. The actin filaments are pulled. As a result, the H-zone reduces. It is at this stage that the contraction of the muscle occurs.

Step 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.

Step 6:

This process of formation and breaking down of cross bridges continues until there is a drop in the stimulus. As a result, the concentration of calcium ions decreases, thereby masking the actin filaments and leading to muscle relaxation.

Q: 4. Write true or false. If false change the statement so that it is true.

(A) Actin is present in thin filament

(B) H-zone of striated muscle fibre represents both thick and thin filaments.

(C) Human skeleton has 206 bones.

(D) There are 11 pairs of ribs in man.

(E) Sternum is present on the ventral side of the body.


(A) True

(B) False

H -zone of striated muscle fibre is the central part of the thick filament that is not overlapped by the thin filament.

(C) True

(D) False

There are 12 pairs of ribs in a man.

(E) True

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