Chapter 20-Locomotion and Movement

NCERT Solutions for Class 11 Biology Chapter 20 – Locomotion and Movement

Have you ever thought about what happens inside your body when you pick up a pen, walk across a room, or simply blink? All of these actions involve the coordinated effort of muscles and bones, which is exactly what Chapter 20 of Class 11 Biology is about. Locomotion and Movement covers the types of movement, the structure and working of muscle fibres, the sliding filament theory, the types of joints, and common skeletal and muscular disorders. 

This chapter carries good marks in both CBSE board exams and NEET, and students often find diagrams of sarcomere and joints challenging. The NCERT Solutions for Class 11 Biology Chapter 20, available on Myclass24, break down every concept systematically so that you can understand the mechanism of muscle contraction and answer diagram-based questions with accuracy. Must check the CBSE resources and NCERT Solutions. 

Download PDF – NCERT Solutions for Class 11 Biology Chapter 20 Locomotion and Movement

The free PDF for NCERT Solutions Class 11 Biology Chapter 20 is available on Myclass24. It contains detailed answers for all textbook questions, well-labelled diagrams of sarcomere structure and different types of joints, and quick-revision tables. The PDF is mobile-optimised for easy reading on smartphones and tablets during your last-minute preparation.

Chapter 20 Locomotion and Movement – Detailed Notes and Key Facts

Movement is one of the fundamental features of living organisms. In animals, movement can range from movement of limbs, jaws, tongue, and eyelids to the movement of substances within the body. Locomotion is a more specific term that refers to voluntary movement from one place to another. Not all movements result in locomotion, for example, the beating of cilia in the respiratory tract moves mucus but does not cause locomotion of the organism itself.

There are three types of movements seen in cells and organisms. Amoeboid movement is seen in macrophages and leucocytes, where pseudopodia are formed for movement. Ciliary movement occurs in the respiratory tract and the oviduct. Muscular movement is responsible for locomotion and most voluntary body movements in vertebrates. Check out NCERT Solutions for Class 11 Biology and NCERT Solutions for Class 11 for the rest of the chapters.

Muscles are the primary organs responsible for movement. The human body has three types of muscle tissue. Skeletal muscles, also called striated or voluntary muscles, are attached to bones and are responsible for locomotion. Smooth muscles, also called non-striated or involuntary muscles, are found in the walls of hollow organs like the gut, uterus, and blood vessels. Cardiac muscle is a specialised striated but involuntary muscle found exclusively in the heart wall.

Each skeletal muscle is made up of many muscle bundles called fascicles, which in turn contain muscle fibres. Each muscle fibre is a multinucleated, elongated cell enclosed by a membrane called the sarcolemma. The cytoplasm of the muscle fibre, called sarcoplasm, contains many thread-like myofibrils running parallel to each other. Each myofibril shows alternate light (I-band, isotropic) and dark (A-band, anisotropic) bands. The light band contains only thin actin filaments, while the dark band contains thick myosin filaments and parts of actin filaments. The centre of the I-band has a Z-line (Z-disc), and the centre of the A-band has an M-line (M-band). The H-zone in the A-band has only myosin. The basic contractile unit of a myofibril is the sarcomere, defined as the region between two consecutive Z-lines.

The thin filaments are made of F-actin (filamentous actin), which is a polymer of G-actin (globular actin) monomers. Tropomyosin runs along the grooves of actin and blocks the myosin-binding sites. Troponin is a complex of three proteins that, along with tropomyosin, regulates actin-myosin interaction. The thick filaments are made of myosin, which consists of meromyosin subunits. Each myosin has a globular head with ATPase activity and an actin-binding site.

Sarcomere Structure – Key Zones

The sliding filament theory, proposed by Hugh Huxley and Andrew Huxley (independently) in 1954, explains muscle contraction. When a nerve impulse arrives at the neuromuscular junction, acetylcholine is released, generating an action potential across the sarcolemma. This spreads through the T-tubules into the sarcoplasmic reticulum, causing the release of calcium ions into the sarcoplasm. Calcium ions bind to troponin, causing a conformational change in the troponin-tropomyosin complex. Tropomyosin shifts, exposing the active sites on actin. The myosin head binds to actin, forming a cross-bridge. Using ATP (hydrolysed by myosin ATPase), the myosin head undergoes a power stroke, pulling the actin filament towards the centre (M-line). After the stroke, a new ATP binds, the cross-bridge detaches, and the cycle repeats. This causes the actin filaments to slide over the myosin filaments, shortening the sarcomere and the overall muscle.

When nerve stimulation stops, calcium is actively pumped back into the sarcoplasmic reticulum. Troponin covers the active sites on actin, cross-bridges cannot form, and the muscle relaxes. Rigor mortis is the stiffness of muscles after death due to the absence of ATP, which prevents cross-bridge detachment.

The human skeleton is divided into the axial skeleton (80 bones: skull, vertebral column, ribs, sternum) and the appendicular skeleton (126 bones: limb bones and girdles). The skull has 22 bones, including 8 cranial bones protecting the brain. The vertebral column has 33 vertebrae in children, which fuse to 26 in adults (7 cervical, 12 thoracic, 5 lumbar, 1 sacrum with 5 fused, 1 coccyx with 4 fused). There are 12 pairs of ribs. The first 7 pairs are true ribs (attached to sternum), pairs 8–10 are false ribs (attached to the 7th rib cartilage), and pairs 11–12 are floating ribs (no anterior attachment).

Axial vs Appendicular Skeleton

Joints are points where two or more bones meet. Fibrous joints (synarthrosis) are immovable, like the sutures between skull bones. Cartilaginous joints (amphiarthrosis) allow limited movement, like the joints between vertebrae and the pubic symphysis. Synovial joints (diarthrosis) are freely movable and are the most common type. Synovial joints contain synovial fluid in a joint cavity surrounded by a synovial membrane. Types of synovial joints include ball-and-socket (shoulder, hip), hinge (knee, elbow), pivot (atlas-axis for head rotation), gliding (wrist bones), saddle (thumb), and condyloid joints.

Common skeletal and muscular disorders include osteoporosis (decreased bone density, common in older women), arthritis (inflammation of joints, of two main types: osteoarthritis and rheumatoid arthritis), gout (uric acid deposits in joints), myasthenia gravis (an autoimmune disorder affecting neuromuscular junctions causing muscle weakness), muscular dystrophy (progressive degeneration of skeletal muscles), and tetany (rapid muscle spasms due to low calcium).

Common Disorders – Quick Reference

For complete NCERT Solutions, chapter-wise PDFs, previous year NEET questions on Locomotion and Movement, and expert-verified answers, visit Myclass24. The content is regularly updated to match the latest CBSE syllabus and NTA NEET pattern.