CBSE Class 11 Biology Chapter 22: Locomotion and Movement NCERT Solutions

Solution: A sarcomere, the fundamental contractile unit of a muscle fiber, is characterized by specific bands and lines. It extends between two successive Z-lines. The diagram of a sarcomere includes:

1. Z-line: A thin, elastic protein band that bisects each I-band and serves as an anchor for the thin filaments.

2. I-band (Isotropic band): The region containing only thin filaments (actin). It appears lighter under a microscope and is bisected by the Z-line.

3. A-band (Anisotropic band): The region containing thick filaments (myosin) that overlap with thin filaments. It appears darker under a microscope and its length remains constant during contraction.

4. H-zone: The central part of the A-band where only thick filaments are present, and there is no overlap with thin filaments. It appears as a lighter region within the A-band.

5. M-line: A thin line in the middle of the H-zone where the thick filaments are held together.

During contraction, the thin filaments slide over the thick filaments, causing the I-band and H-zone to shorten, while the A-band length remains unchanged.

Solution: The sliding filament theory is a widely accepted explanation for muscle contraction. It proposes that during contraction, the muscle shortens because the thin filaments (primarily actin) slide past the thick filaments (primarily myosin) within each sarcomere. This sliding action pulls the Z-lines closer together, thereby shortening the sarcomere and the entire muscle fiber. The lengths of the individual actin and myosin filaments do not change; rather, their degree of overlap increases. This process is driven by the interaction between the myosin heads and the actin filaments, facilitated by the hydrolysis of ATP.

Solution: Muscle contraction is a complex process that involves several sequential steps:

1. Initiation of Signal: Muscle contraction begins when a signal (nerve impulse) travels down a motor neuron and reaches the neuromuscular junction (motor end plate), which is the synapse between the neuron and the muscle fiber's sarcolemma.
2. Neurotransmitter Release: At the neuromuscular junction, the motor neuron releases a neurotransmitter, acetylcholine (ACh), into the synaptic cleft.
3. Action Potential Generation: Acetylcholine binds to receptors on the sarcolemma, generating an action potential that propagates along the muscle fiber membrane and into the T-tubules.
4. Calcium Ion Release: The action potential triggers the sarcoplasmic reticulum (a specialized endoplasmic reticulum in muscle cells) to release stored calcium ions ($Ca^\{2+\}$) into the sarcoplasm (muscle cell cytoplasm).
5. Activation of Actin Sites: The released calcium ions bind to troponin molecules located on the actin filaments. This binding causes a conformational change in the troponin-tropomyosin complex, moving tropomyosin away from the myosin-binding sites on the actin filaments.
6. Cross-bridge Formation: With the binding sites on actin now exposed, the energized myosin heads (from the thick filaments) can attach to actin, forming cross-bridges.
7. Power Stroke: The myosin head pivots, pulling the actin filament towards the center of the sarcomere. This movement is known as the power stroke and is powered by the hydrolysis of ATP.
8. Cross-bridge Detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin.
9. Reactivation of Myosin Head: The ATP molecule is hydrolyzed to ADP and inorganic phosphate, which re-energizes and cocks the myosin head, preparing it for another cycle.
10. Relaxation: Muscle contraction continues as long as calcium ions are present and ATP is available. Relaxation occurs when the nerve impulse stops, calcium ions are pumped back into the sarcoplasmic reticulum, and the binding sites on actin are covered by tropomyosin again, preventing cross-bridge formation.