Locomotion and Movement — NCERT Class 11 Biology

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This page provides complete NCERT Solutions for CBSE Class 11 Biology Chapter 20 — Locomotion and Movement. All intext questions (organised by page number) and chapter-end exercise questions are answered in detail. Use the section headings to jump directly to the question you need help with, or read through the full page for a comprehensive revision of the chapter.

Key Concepts (Quick Recap)

• Types of Movement: Amoeboid (pseudopodia, e.g., macrophages, WBCs), ciliary (cilia, e.g., respiratory tract, fallopian tubes), muscular (skeletal, smooth, cardiac muscles)
• Muscle Types: Skeletal (voluntary, striated, multinucleated, attached to bones), Smooth (involuntary, non-striated, uninucleated, fusiform, in hollow organs), Cardiac (involuntary, striated, branched, uninucleated, in heart wall)
• Sliding Filament Theory: Muscle contraction occurs by sliding of actin (thin) filaments over myosin (thick) filaments, reducing sarcomere length. Myosin heads form cross-bridges with actin, pull actin inward using ATP energy
• Sarcomere: Functional unit of muscle contraction; bounded by Z lines; contains A band (myosin, dark), I band (actin only, light), H zone (myosin only, central A band), M line (middle of H zone)
• Skeletal System: 206 bones in adult human; divided into axial skeleton (80 bones — skull, vertebral column, ribs, sternum) and appendicular skeleton (126 bones — limbs and girdles)
• Axial Skeleton: Skull (29 bones — 8 cranial, 14 facial, 6 ear ossicles, 1 hyoid), vertebral column (33 vertebrae — 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 coccygeal), ribs (12 pairs), sternum (1)
• Appendicular Skeleton: Pectoral girdle (scapula + clavicle), pelvic girdle (hip bone = ilium + ischium + pubis), forelimbs, hindlimbs
• Joints: Fibrous (immovable, e.g., skull sutures), Cartilaginous (slightly movable, e.g., intervertebral discs), Synovial (freely movable — ball and socket, hinge, pivot, gliding, saddle, condyloid)
• Disorders: Arthritis (osteoarthritis, rheumatoid), osteoporosis, gout, muscular dystrophy, tetany, myasthenia gravis

Intext Questions

Page 307

1. Amoeboid movement: Movement using pseudopodia (false feet), as seen in macrophages, leukocytes (WBCs), and amoeba. The cytoplasm streams into pseudopodia, allowing the cell to change shape and move. This type of movement is also seen in embryonic cells during development.

2. Ciliary movement: Movement using cilia (short, hair-like projections). In the human body, ciliary movement is seen in the respiratory tract (trachea, bronchi) where ciliated epithelium moves mucus and trapped particles upward, and in the fallopian tubes (oviducts) where cilia help move the ovum toward the uterus.

3. Muscular movement: Movement caused by the contraction of muscle fibres. This is the most common type of movement in the human body and includes the movement of limbs, jaw, tongue, and internal organs (through smooth and cardiac muscle contraction).

1. Skeletal muscles: Also called striated or voluntary muscles. They are attached to bones by tendons and are responsible for body movement. They are multinucleated, have a striped (striated) appearance under the microscope, and are under voluntary control.

2. Smooth muscles: Also called non-striated or involuntary muscles. They are found in the walls of hollow organs (intestine, blood vessels, stomach, uterus, urinary bladder). They are spindle-shaped (fusiform), uninucleated, and are under involuntary control. They contract slowly and can sustain contraction for long periods.

3. Cardiac muscles: Found exclusively in the wall of the heart. They are striated but involuntary (the heart beats without conscious control). They are branched, uninucleated, and have intercalated discs that allow rapid transmission of electrical impulses. They contract rhythmically and tirelessly throughout life.

Page 312

• Z line (Z disc): The boundary of the sarcomere. Thin (actin) filaments are anchored to the Z line.
• I band (Isotropic band): The light band containing only thin (actin) filaments. It shortens during muscle contraction.
• A band (Anisotropic band): The dark band containing both thick (myosin) filaments and the overlapping regions of thin filaments. It remains the same length during contraction.
• H zone: The central region of the A band containing only thick (myosin) filaments, with no overlap of thin filaments. It shortens during contraction and disappears at maximum contraction.
• M line: The middle of the H zone, where the central region of the thick filaments is held together.

The arrangement of filaments follows the Z — I — A — H — A — I — Z pattern from one end of the sarcomere to the other.

Steps of muscle contraction:

1. Signal from motor neuron: A motor neuron sends a signal (action potential) to the muscle fibre at the neuromuscular junction. The neurotransmitter acetylcholine is released into the synaptic cleft.

2. Depolarisation and Ca2+ release: Acetylcholine binds to receptors on the muscle fibre membrane (sarcolemma), causing depolarisation. This depolarisation travels through the T-tubules (transverse tubules) to the sarcoplasmic reticulum, which releases stored Ca2+ ions into the sarcoplasm.

3. Cross-bridge formation: Ca2+ ions bind to troponin on the actin filament, causing a conformational change that moves tropomyosin away from the active binding sites on actin. This exposes the myosin-binding sites on actin.

4. Power stroke: The myosin head (already energised with bound ADP and Pi from ATP hydrolysis) binds to the exposed active site on actin, forming a cross-bridge. The myosin head then pivots (power stroke), pulling the actin filament toward the centre of the sarcomere (toward the M line). ADP and Pi are released.

5. Cross-bridge detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin. ATP is then hydrolysed to ADP and Pi, which re-cocks the myosin head (returns it to its high-energy position), ready for the next cycle.

6. Repeated cycling: Steps 3–5 repeat as long as Ca2+ and ATP are available, progressively sliding the actin filaments inward and shortening the sarcomere.

7. Relaxation: When the neural signal stops, acetylcholinesterase degrades acetylcholine. Ca2+ is pumped back into the sarcoplasmic reticulum by Ca2+-ATPase pumps (active transport). Troponin returns to its original shape, tropomyosin covers the active sites on actin, and cross-bridge cycling stops. The muscle relaxes.

Whole muscle: Each skeletal muscle is surrounded by a connective tissue sheath called the epimysium. The muscle is attached to a bone by a tendon (or aponeurosis for flat muscles).

Muscle fascicles (bundles): Inside the muscle, there are bundles of muscle fibres called fascicles. Each fascicle is surrounded by a connective tissue sheath called the perimysium.

Muscle fibres (cells): Each fascicle contains many muscle fibres (muscle cells). Each muscle fibre is a long, cylindrical, multinucleated cell surrounded by a connective tissue sheath called the endomysium. The cell membrane of a muscle fibre is called the sarcolemma, and its cytoplasm is called the sarcoplasm.

Myofibrils: Inside each muscle fibre are many parallel, unbranched myofibrils. Each myofibril is composed of repeating units called sarcomeres, which are the functional units of contraction.

Sarcomere: Each sarcomere contains thick (myosin) and thin (actin) filaments arranged in a characteristic pattern that gives the muscle its striated appearance. The A band (dark, myosin), I band (light, actin only), H zone (myosin only), and Z line (boundary) are the key landmarks.

Sarcoplasmic reticulum: A specialised smooth endoplasmic reticulum that stores and releases Ca2+ ions during muscle contraction and relaxation.

Page 319

(a) Joint: A joint (or articulation) is the point of contact between two or more bones, or between a bone and cartilage. Joints allow varying degrees of movement (from immovable to freely movable) and provide stability to the skeletal framework.

(b) Articulation: Articulation is another term for a joint — the place where two bones meet and interact. The type of articulation determines the range and type of movement possible at that location.

(c) Synovial fluid: Synovial fluid is a thick, viscous, egg-white-like fluid secreted by the synovial membrane into the synovial cavity of freely movable (synovial) joints. It provides lubrication to reduce friction between the articulating bone surfaces, supplies nutrients to the avascular articular cartilage, and absorbs shocks.

Chapter-End Exercise

• Z line (Z disc): The lateral boundary of the sarcomere. Thin (actin) filaments are anchored here.
• I band (Isotropic band): The light band that contains only thin (actin) filaments. It lies on either side of the Z line and shortens during contraction.
• A band (Anisotropic band): The dark band that contains thick (myosin) filaments and the overlapping region of thin filaments. It spans the entire length of the myosin filament and does not change length during contraction.
• H zone: The lighter central region of the A band where only myosin filaments are present (no overlap with actin). It shortens during contraction.
• M line: The central line within the H zone where the myosin filaments are held together by connective proteins.

During contraction, the actin filaments slide inward on both sides of the myosin, reducing the I band width and the H zone width, while the A band remains constant.

1. Neural signal initiation: A motor neuron in the central nervous system sends an electrical signal (action potential) to the muscle fibre through the neuromuscular junction.

2. Acetylcholine release: When the action potential reaches the axon terminal, it triggers the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft.

3. Muscle fibre depolarisation: ACh binds to nicotinic receptors on the motor end plate of the sarcolemma, causing depolarisation and generating an action potential in the muscle fibre.

4. Ca2+ release from sarcoplasmic reticulum: The action potential travels along the sarcolemma and into the T-tubules (transverse tubules), triggering the release of Ca2+ ions from the sarcoplasmic reticulum into the sarcoplasm.

5. Troponin-tropomyosin shift: Ca2+ binds to troponin-C on the thin filament, causing a conformational change in the troponin complex. This shifts tropomyosin away from the active binding sites on actin, exposing them for myosin head attachment.

6. Cross-bridge formation: The myosin head (which has been energised by ATP hydrolysis, with ADP and Pi still attached) binds to the exposed active site on actin, forming a cross-bridge.

7. Power stroke: The myosin head pivots and pulls the actin filament toward the centre of the sarcomere (M line). This is called the power stroke. ADP and Pi are released during this step.

8. Cross-bridge detachment: A new molecule of ATP binds to the myosin head, reducing its affinity for actin and causing the cross-bridge to detach.

9. Myosin head reactivation: ATP is hydrolysed to ADP and Pi by the ATPase activity of myosin. The energy released re-cocks the myosin head to its high-energy position, ready for the next cross-bridge cycle.

10. Repeated cycling: Steps 6–9 repeat as long as Ca2+ and ATP are available, progressively pulling the actin filaments inward and shortening the sarcomere.

11. Relaxation: When the neural signal stops, ACh is degraded by acetylcholinesterase. Ca2+ is actively pumped back into the sarcoplasmic reticulum by Ca2+-ATPase pumps. Troponin returns to its original shape, tropomyosin covers the active sites on actin, and cross-bridge cycling ceases. The muscle returns to its resting length (through elastic recoil and antagonistic muscle action).

1. Skull (29 bones):

• Cranial bones (8): Frontal (1), Parietal (2), Temporal (2), Occipital (1), Sphenoid (1), Ethmoid (1). These form the cranium, which protects the brain.
• Facial bones (14): Maxilla (2), Zygomatic (2), Mandible (1), Nasal (2), Palatine (2), Inferior nasal conchae (2), Lacrimal (2), Vomer (1). The mandible is the only movable bone of the skull.
• Ear ossicles (6): Malleus (2), Incus (2), Stapes (2) — the smallest bones in the body, found in the middle ear.
• Hyoid bone (1): A U-shaped bone in the neck that anchors the tongue and does not articulate with any other bone.

2. Vertebral column (33 vertebrae):

• Cervical vertebrae (7): C1–C7. The first cervical vertebra (atlas) articulates with the skull, and the second (axis) allows the head to rotate.
• Thoracic vertebrae (12): T1–T12. Each articulates with a pair of ribs.
• Lumbar vertebrae (5): L1–L5. The largest and strongest vertebrae, bearing the most body weight.
• Sacral vertebrae (5): Fused into a single sacrum.
• Coccygeal vertebrae (4): Fused into a single coccyx (tailbone).

The vertebral column protects the spinal cord, supports the head, and serves as an attachment for ribs and muscles.

3. Ribs (12 pairs = 24 ribs):

• True ribs (1st–7th pairs): Directly attached to the sternum through hyaline cartilage.
• False ribs (8th–10th pairs): Attached to the 7th rib's cartilage, not directly to the sternum.
• Floating ribs (11th–12th pairs): Not attached to the sternum at all; only connected to the vertebrae posteriorly.

Ribs protect the heart, lungs, and other thoracic organs.

4. Sternum (1):

A flat bone in the anterior midline of the thorax, also called the breastbone. It articulates with the clavicles and the first seven pairs of ribs.

1. Pectoral girdle (shoulder girdle) — 4 bones (2 each side):

• Scapula (shoulder blade): A large, flat, triangular bone on the posterior side of the thorax. It has the glenoid cavity (a shallow socket) that articulates with the head of the humerus.
• Clavicle (collarbone): A long, S-shaped bone that connects the scapula to the sternum. It is the only direct bony attachment between the upper limb and the axial skeleton.

2. Upper limb (forelimb) — 60 bones (30 each side):

• Humerus (1): The bone of the upper arm. The head articulates with the glenoid cavity of the scapula.
• Radius and Ulna (2): The bones of the forearm. The radius is on the lateral (thumb) side; the ulna is on the medial (little finger) side.
• Carpals (8): Small bones of the wrist arranged in two rows of four each (scaphoid, lunate, triquetral, pisiform, trapezium, trapezoid, capitate, hamate).
• Metacarpals (5): Bones of the palm, one for each digit.
• Phalanges (14): Finger bones — 2 in the thumb and 3 in each of the other four fingers.

3. Pelvic girdle (hip girdle) — 2 bones (1 each side):

• Each hip bone (innominate bone) is formed by the fusion of three bones: ilium (upper, largest part), ischium (lower, posterior part), and pubis (anterior, lower part). The two hip bones meet anteriorly at the pubic symphysis and posteriorly articulate with the sacrum. The acetabulum is the deep socket that articulates with the head of the femur.

4. Lower limb (hindlimb) — 60 bones (30 each side):

• Femur (1): The longest and strongest bone in the body, forming the thigh. Its head articulates with the acetabulum of the hip bone.
• Patella (1): The kneecap, a sesamoid bone embedded in the quadriceps tendon.
• Tibia and Fibula (2): The bones of the lower leg. The tibia (shinbone) is the larger, weight-bearing bone; the fibula is thin and does not bear weight.
• Tarsals (7): Ankle bones — talus, calcaneus (heel), navicular, three cuneiforms, cuboid.
• Metatarsals (5): Bones of the sole of the foot.
• Phalanges (14): Toe bones — 2 in the great toe and 3 in each of the other toes.

(a) Actin vs Myosin:

(b) Red muscles vs White muscles:

(c) Muscle vs Tendon:

1. Fibrous joints (Immovable / Synarthroses):

The bones are held together by fibrous connective tissue. There is no joint cavity, and no movement is possible.

• Sutures: Flat bones of the skull held together by a thin layer of fibrous tissue. Example: sagittal suture between the two parietal bones.
• Syndesmoses: Bones connected by an interosseous ligament. Example: the joint between the tibia and fibula at the distal end.
• Gomphoses: Peg-in-socket joints. Example: tooth in its alveolar socket (held by periodontal ligament).

2. Cartilaginous joints (Slightly movable / Amphiarthroses):

The bones are connected by cartilage. There is no joint cavity, and only slight movement is possible.

• Synchondroses: Bones connected by hyaline cartilage. Example: the joint between the first rib and the sternum (costochondral joint).
• Symphyses: Bones connected by fibrocartilage. Example: intervertebral discs between vertebrae (which provide flexibility and shock absorption), and the pubic symphysis between the two hip bones.

3. Synovial joints (Freely movable / Diarthroses):

These joints have a synovial cavity filled with synovial fluid, allowing a wide range of movement. They are the most common type of joint in the body.

Subtypes and examples:

• Ball and socket joint: Rounded head of one bone fits into a cup-like cavity of another. Allows movement in all directions (flexion, extension, abduction, adduction, rotation, circumduction). Example: Shoulder joint (humerus in glenoid cavity) and Hip joint (femur in acetabulum).
• Hinge joint: Convex surface of one bone fits into a concave surface of another. Allows movement in one plane (flexion and extension). Example: Elbow joint, Knee joint, Ankle joint.
• Pivot joint: Rounded end of one bone fits into a ring formed by another bone and a ligament. Allows rotation. Example: Atlas-axis joint (C1–C2, allows the head to rotate "no").
• Gliding joint: Flat or slightly curved surfaces slide over each other. Allows limited movement in multiple directions. Example: Between carpal bones (wrist), between tarsal bones (ankle).
• Saddle joint: Both articulating surfaces have concave and convex regions, like a rider in a saddle. Allows movement in two planes (flexion/extension, abduction/adduction) but not rotation. Example: Carpometacarpal joint of the thumb.
• Condyloid (Ellipsoid) joint: An oval-shaped projection fits into an elliptical cavity. Allows movement in two planes (flexion/extension, abduction/adduction) but no rotation. Example: Wrist joint (radius and carpal bones).

1. Arthritis: Inflammation of one or more joints, causing pain, swelling, stiffness, and reduced mobility. The two most common types are:

• Osteoarthritis: A degenerative joint disease where the articular cartilage gradually wears away, causing bones to rub against each other. It is more common in weight-bearing joints (knees, hips) and in older adults. Symptoms include joint pain, stiffness, and reduced range of motion.

• Rheumatoid arthritis: An autoimmune disorder where the body's immune system attacks the synovial membrane, causing inflammation, thickening of the synovial fluid, and eventual destruction of the articular cartilage. It often affects multiple joints symmetrically (especially small joints of hands and feet) and can occur at any age.

2. Osteoporosis: An age-related disorder characterised by decreased bone mass and increased bone porosity, making bones fragile and susceptible to fractures. It is more common in post-menopausal women due to decreased oestrogen levels (oestrogen helps maintain bone density by promoting calcium deposition). Risk factors include ageing, low calcium intake, lack of exercise, and smoking. Vertebrae and femur are most commonly affected.

3. Gout: A form of arthritis caused by the accumulation of uric acid crystals in the joints (especially the big toe — podagra). It causes sudden, severe pain, redness, swelling, and warmth in the affected joint. Uric acid crystals form when blood uric acid levels are high (hyperuricaemia), often due to excessive production or insufficient excretion by the kidneys.

4. Muscular dystrophy: A group of genetic disorders characterised by progressive degeneration and weakening of skeletal muscles. The most common type is Duchenne muscular dystrophy (DMD), which is X-linked recessive and affects boys. It is caused by mutations in the dystrophin gene. Symptoms include progressive muscle weakness, difficulty walking, and eventual loss of mobility. There is no cure, but physical therapy and supportive care can help manage symptoms.

5. Tetany: Rapid, involuntary muscle spasms (contractions) caused by low calcium levels (hypocalcaemia) in the body fluids. Low Ca2+ increases the excitability of muscles and nerves, leading to twitching, cramps, and spasms. It can be caused by hypoparathyroidism, vitamin D deficiency, or alkalosis.

6. Myasthenia gravis: An autoimmune disorder in which antibodies block or destroy acetylcholine receptors at the neuromuscular junction. This impairs the transmission of nerve signals to muscles, causing progressive muscle weakness and fatigue. Symptoms include drooping eyelids (ptosis), difficulty swallowing, double vision (diplopia), and weakness of limb muscles that worsens with activity and improves with rest. Treatment includes acetylcholinesterase inhibitors (e.g., neostigmine) and immunosuppressants.

Internal Links

• [Chapter 20 Guide — Locomotion and Movement](/school/cbse/class/11/biology/cbse-locomotion-and-movement)
• [All Class 11 Biology Solutions](/school/cbse/class/11/biology/solutions)
• [Class 11 Biology Subject Guide](/school/cbse/class/11/biology)