Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
Chapter 20 — Locomotion and Movement belongs to the Human Physiology unit of CBSE Class 11 Biology. Movement and locomotion are fundamental properties of living organisms. While movement refers to any change in position of a body part, locomotion is the voluntary movement that results in a change of place or location of the entire organism. All locomotion involves movement, but not all movement is locomotion.
This chapter covers the types of movement seen in organisms, the structure and mechanism of muscle contraction, the organisation of the human skeletal system, classification of joints, and common musculoskeletal disorders. Understanding these concepts is essential for NEET and board examinations alike.
Key Definitions
1. Movement — A change in position or posture of any body part or the whole organism; a general property of all living cells. 2. Locomotion — Voluntary movement that results in a change of place or location of the entire organism (e.g., walking, swimming, flying). 3. Amoeboid movement — Movement brought about by pseudopodia formed by the streaming of protoplasm; seen in Amoeba, macrophages, and leukocytes (WBCs). 4. Ciliary movement — Movement produced by the coordinated beating of cilia; seen in Paramoecium, respiratory tract epithelium, and fallopian tubes. 5. Muscular movement — Movement caused by the contraction of muscle fibres using contractile proteins (actin and myosin); the primary mechanism for locomotion in higher animals. 6. Sarcomere — The functional unit of a myofibril; the region between two adjacent Z lines; contains the complete set of thick and thin filaments needed for contraction. 7. Sliding filament theory — The theory that muscle contraction occurs by the sliding of actin (thin) filaments over myosin (thick) filaments, shortening the sarcomere without changing filament lengths. 8. Motor unit — A single motor neuron and all the muscle fibres it innervates; the smallest functional unit of muscle contraction. 9. Osteoporosis — An age-related disorder characterised by reduced bone mass and increased susceptibility to fractures; common in post-menopausal women. 10. Arthritis — Inflammation of one or more joints, causing pain, swelling, and stiffness; includes osteoarthritis (wear and tear) and rheumatoid arthritis (autoimmune). 11. Gout — Inflammation of joints caused by the deposition of uric acid crystals; commonly affects the big toe.
Detailed Concepts
Types of Movement
Living organisms exhibit several types of movement, each driven by different structures and mechanisms:
| Type | Mechanism | Examples |
|---|---|---|
| Amoeboid | Pseudopodia — cytoplasmic streaming and extension | Amoeba, macrophages, WBCs (leukocytes) |
| Ciliary | Cilia beating in coordinated metachronal waves | Paramoecium, respiratory tract epithelium, fallopian tube lining |
| Flagellar | Whip-like flagellar movement | Spermatozoa, Euglena |
| Muscular | Contractile proteins (actin + myosin) in muscle fibres | Skeletal muscles, heart, visceral organs |
In humans, locomotion is achieved primarily through muscular movement — the coordinated contraction of skeletal muscles acting on bones across joints. Ciliary and amoeboid movements serve internal functions such as mucus clearance and immune cell migration.
Muscle Types — Detailed Comparison
The human body contains three types of muscle tissue, each specialised for a distinct function:
| Feature | Skeletal (Striated) | Smooth (Non-striated) | Cardiac |
|---|---|---|---|
| Location | Attached to bones via tendons | Walls of internal organs (stomach, intestine, blood vessels, urinary bladder) | Heart only |
| Striations | Present (distinct) | Absent | Present (but less distinct) |
| Control | Voluntary (conscious) | Involuntary (autonomic) | Involuntary (autonomic) |
| Nuclei | Multinucleate (peripheral) | Uninucleate (central) | Uninucleate (central) |
| Fibre shape | Long, cylindrical, unbranched | Spindle-shaped, fusiform | Branched, cylindrical |
| Speed of contraction | Fast | Slow | Intermediate, rhythmic |
| Fatigue | Fatigues easily | Fatigues slowly | Does not fatigue (lifelong rhythmic contraction) |
| Intercalated discs | Absent | Absent | Present (gap junctions for rapid impulse spread) |
| Sarcoplasmic reticulum | Well-developed | Poorly developed | Moderately developed |
| Energy requirement | High | Low | High |
| Gap junctions | Absent | Present (in visceral smooth muscle) | Present (in intercalated discs) |
Key distinctions to remember:
- Only skeletal muscle is voluntary.
- Only cardiac muscle has intercalated discs.
- Smooth muscle is the only type that is non-striated.
- Cardiac muscle is the only type that does not fatigue.
Muscle Structure — Sarcomere
A skeletal muscle is composed of muscle fascicles, each fascicle contains muscle fibres (muscle cells), and each fibre contains myofibrils. Myofibrils are made of repeating units called sarcomeres — the functional unit of muscle contraction.
Sarcomere landmarks:
- Z line (Z disc) — The boundary of the sarcomere; anchors the thin (actin) filaments. One sarcomere extends from one Z line to the next.
- A band (Anisotropic band) — The dark band; contains thick (myosin) filaments throughout, with overlapping thin (actin) filaments at the edges.
- I band (Isotropic band) — The light band; contains only thin (actin) filaments; bisected by the Z line.
- H zone — The central region of the A band where only thick (myosin) filaments are present (no overlap with actin).
- M line — The midline of the sarcomere (centre of the H zone); connects the thick filaments to maintain their alignment.
- Titin — A giant elastic protein that connects the M line to the Z line; provides elasticity and keeps thick filaments centred.
Sarcomere structures at a glance:
| Structure | Description | Content |
|---|---|---|
| A band | Dark, anisotropic band | Overlapping actin + myosin (edges) + only myosin (centre) |
| I band | Light, isotropic band | Thin filaments (actin) only |
| H zone | Central A band (no overlap zone) | Only thick filaments (myosin) |
| M line | Midline of sarcomere | Connects thick filaments centrally |
| Z line | Boundary of sarcomere | Anchors thin filaments; defines sarcomere |
| Titin | Elastic protein filament | Connects M line to Z line; provides elasticity |
Proteins of the myofilaments:
- Myosin (thick filament): Each myosin molecule has a tail and two globular heads. The heads form the cross-bridges that bind to actin and generate force. Myosin also has ATPase activity — it hydrolyses ATP to provide energy for contraction.
- Actin (thin filament): A filamentous (F-actin) polymer of globular (G-actin) monomers, wound in a double helix. Two regulatory proteins are associated with actin:
- Tropomyosin — Covers the myosin-binding sites on actin in the resting state.
- Troponin — A three-subunit complex; TnC binds Ca2+, TnI inhibits actin-myosin interaction, TnT binds tropomyosin. When Ca2+ binds to troponin, tropomyosin shifts to expose binding sites.
Sliding Filament Theory of Muscle Contraction
The sliding filament theory, proposed independently by Hugh Huxley and Andrew Huxley in 1954, explains how muscle contraction occurs without the thick and thin filaments themselves shortening. Instead, the thin filaments slide over the thick filaments, reducing the sarcomere length.
Step-by-step mechanism:
1. Signal from motor neuron — A motor neuron sends an action potential to the muscle fibre. At the neuromuscular junction, the neurotransmitter acetylcholine (ACh) is released into the synaptic cleft and binds to receptors on the muscle fibre membrane (sarcolemma).
2. Action potential spreads — The action potential travels along the sarcolemma and penetrates deep into the muscle fibre via T-tubules (transverse tubules), which are invaginations of the sarcolemma.
3. Calcium release — The action potential triggers the sarcoplasmic reticulum (SR) to release stored Ca2+ ions into the sarcoplasm (cytoplasm of the muscle fibre).
4. Calcium initiates contraction — Ca2+ binds to troponin (specifically the TnC subunit). This causes a conformational change in the troponin complex, which shifts tropomyosin away from the myosin-binding sites on actin. The binding sites are now exposed.
5. Cross-bridge cycle — This is the core mechanism of contraction:
- Cross-bridge formation: The energised myosin head (charged with ATP hydrolysis products) binds to the exposed site on actin, forming a cross-bridge.
- Power stroke: The myosin head pivots and pulls the actin filament toward the M line (centre of the sarcomere). ADP and inorganic phosphate (Pi) are released during this step.
- Detachment: A new molecule of ATP binds to the myosin head, causing it to detach from actin.
- Re-cocking (re-energising): The myosin head hydrolyses the ATP to ADP + Pi. The energy released re-cocks the myosin head back to its energised, high-energy position, ready for the next cycle.
- The cycle repeats as long as Ca2+ and ATP are available.
6. Changes during contraction:
- The I band shortens (actin filaments slide inward).
- The H zone disappears (actin filaments reach the centre of the A band).
- The A band remains the same length (the thick filament length does not change).
- The Z lines move closer together (sarcomere shortens).
7. Relaxation — When the neural signal stops, Ca2+ is actively pumped back into the sarcoplasmic reticulum (using ATP). As Ca2+ concentration drops, tropomyosin re-covers the myosin-binding sites on actin. Cross-bridge formation stops, and the muscle relaxes.
Summary of ATP roles in muscle contraction:
- ATP hydrolysis energises the myosin head (re-cocking).
- ATP binding detaches the myosin head from actin.
- ATP powers the Ca2+ pump (SERCA) that returns Ca2+ to the sarcoplasmic reticulum during relaxation.
Skeletal System
The adult human skeletal system consists of 206 bones. It is divided into two major divisions:
Axial vs. Appendicular Skeleton:
| Division | Components | Details |
|---|---|---|
| Axial skeleton | Skull (22 bones) | 8 cranial bones + 14 facial bones; mandible is the only movable bone of the skull |
| Vertebral column (33 vertebrae) | 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused into sacrum), 4 coccygeal (fused into coccyx) | |
| Ribs (12 pairs = 24 ribs) | True ribs (1-7): directly attached to sternum via costal cartilage; False ribs (8-10): attached to sternum via common cartilage of rib 7; Floating ribs (11-12): no sternal attachment | |
| Sternum (1 bone) | Flat bone in the midline of the anterior chest wall | |
| Appendicular skeleton | Pectoral girdle (4 bones) | 2 clavicles (collar bones) + 2 scapulae (shoulder blades) |
| Upper limbs (60 bones) | Each side: humerus (1), radius (1), ulna (1), carpals (8), metacarpals (5), phalanges (14) | |
| Pelvic girdle (2 bones) | 2 hip bones (each formed by the fusion of ilium, ischium, and pubis); acetabulum is the socket for the femoral head | |
| Lower limbs (60 bones) | Each side: femur (1), tibia (1), fibula (1), patella (1), tarsals (7), metatarsals (5), phalanges (14) |
Key points about the skull:
- Cranial bones: frontal (1), parietal (2), temporal (2), occipital (1), sphenoid (1), ethmoid (1) = 8 bones.
- The mandible (lower jaw) is the only movable bone of the skull and forms the temporomandibular joint.
- The skull rests on the atlas (first cervical vertebra, C1).
Key points about the vertebral column:
- Cervical vertebrae (7): C1 (atlas) — supports the skull, allows nodding; C2 (axis) — allows rotation of the head.
- Thoracic vertebrae (12): articulate with the ribs.
- Lumbar vertebrae (5): largest; bear the maximum body weight.
- Sacrum: 5 fused vertebrae; articulates with the pelvic girdle.
- Coccyx: 4 fused vertebrae; vestigial tailbone.
Key points about the ribs:
- True ribs (1-7): Directly connected to the sternum via their own costal cartilages.
- False ribs (8-10): Connected to the sternum indirectly — their costal cartilages join the cartilage of rib 7.
- Floating ribs (11-12): Only connected to the vertebral column; no sternal attachment at all.
Key points about the appendicular skeleton:
- The femur is the longest and strongest bone in the human body.
- The acetabulum (in the pelvic girdle) is a deep socket that articulates with the head of the femur to form the hip joint.
- The glenoid cavity (in the scapula) is a shallow socket that articulates with the head of the humerus to form the shoulder joint.
Joints
Joints are points of contact between bones, between bone and cartilage, or between bone and teeth. They are classified based on the degree of movement they allow and the type of tissue connecting the bones:
| Joint Type | Structure | Movement | Examples |
|---|---|---|---|
| Fibrous | Fibrous connective tissue between bones | Immovable (synarthroses) | Sutures of the skull |
| Cartilaginous | Cartilage between bones | Slightly movable (amphiarthroses) | Intervertebral discs, pubic symphysis |
| Synovial — Ball and socket | Ball-shaped head fits into a cup-shaped socket | Multiaxial (movement in all directions) | Shoulder (humerus-scapula), Hip (femur-pelvis) |
| Synovial — Hinge | Convex surface of one bone fits into concave surface of another | Uniaxial (movement in one plane, like a door hinge) | Elbow, Knee, Ankle |
| Synovial — Pivot | Peg-like process rotates in a ring | Rotational movement | Atlas-Axis (C1-C2, allows head rotation) |
| Synovial — Gliding | Flat or slightly curved articular surfaces slide over each other | Minimal movement (sliding only) | Between carpals, between tarsals |
| Synovial — Saddle | Two saddle-shaped articular surfaces, concave-convex in opposite directions | Biaxial (movement in two planes) | Thumb (carpometacarpal joint) |
Features of synovial joints (most common and most freely movable):
- Articular capsule — Encloses the joint; has an outer fibrous capsule and inner synovial membrane.
- Synovial fluid — Secreted by the synovial membrane; lubricates the joint and nourishes the articular cartilage.
- Articular cartilage — Hyaline cartilage covering the bone ends; reduces friction and absorbs shock.
- Menisci (articular discs) — Fibrocartilage pads in some synovial joints (e.g., knee) that improve fit and absorb shock.
- Ligaments — Connect bone to bone; provide stability and limit excessive movement.
Disorders of the Muscular and Skeletal Systems
| Disorder | Description | Cause/Risk Factors |
|---|---|---|
| Arthritis | Inflammation of joints causing pain, swelling, stiffness, and reduced mobility | Osteoarthritis (degeneration of articular cartilage with age/wear); Rheumatoid arthritis (autoimmune — immune system attacks synovial membrane) |
| Osteoporosis | Reduced bone mass and density; bones become brittle and prone to fractures | Ageing, calcium/vitamin D deficiency, oestrogen decline (post-menopausal women), sedentary lifestyle |
| Gout | Joint inflammation due to deposition of uric acid (monosodium urate) crystals | Excess uric acid production or reduced excretion; commonly affects the metatarsophalangeal joint of the big toe |
| Tetany | Muscle spasms, cramps, and convulsions due to low blood Ca2+ | Hypocalcaemia (parathyroid dysfunction, vitamin D deficiency) |
| Muscular dystrophy | Progressive degeneration and weakening of skeletal muscle | Genetic disorder (X-linked recessive); Duchenne muscular dystrophy is the most common and severe form |
Osteoarthritis vs. Rheumatoid arthritis:
- Osteoarthritis is a degenerative joint disease — cartilage wears down over time; more common in weight-bearing joints (knees, hips) of older adults.
- Rheumatoid arthritis is an autoimmune disease — the immune system attacks the synovial membrane, causing chronic inflammation; can affect people of any age and often involves multiple joints symmetrically.
Important Diagrams
The following diagrams are frequently asked in CBSE and NEET examinations. Practice drawing and labelling them:
1. Sarcomere (relaxed state) — Label: Z line, A band, I band, H zone, M line, thick filaments (myosin), thin filaments (actin). Show the arrangement of filaments in the resting sarcomere.
2. Sarcomere (contracted state) — Label the same structures. Show that the I band has shortened, the H zone has disappeared, the Z lines have moved closer together, and the A band remains the same length.
3. Sliding filament theory (cross-bridge cycle) — Draw the sequence: (a) energised myosin head binds to actin, (b) power stroke — actin pulled toward M line, (c) ATP binds and myosin detaches, (d) ATP hydrolysis re-cocks the myosin head.
4. Human skeleton — Label the axial skeleton (skull, vertebral column, ribs, sternum) and appendicular skeleton (pectoral girdle, pelvic girdle, upper and lower limbs).
5. Types of synovial joints — Illustrate ball and socket (hip/shoulder), hinge (elbow/knee), pivot (atlas-axis), and gliding (intercarpal) joints with labelled diagrams.
Common Mistakes
1. Saying the A band shortens during contraction — The A band stays the SAME length. Only the I band shortens and the H zone disappears. The thick filament (myosin) does not change length.
2. Confusing actin (thin/I band) with myosin (thick/A band) — Actin is the thin filament found in the I band; myosin is the thick filament found in the A band. Do not swap them.
3. Forgetting the role of Ca2+ — Calcium is essential for contraction. Ca2+ binds to troponin, causing tropomyosin to shift and expose the myosin-binding sites on actin. Without Ca2+, contraction cannot occur.
4. Writing that smooth muscle is striated — Smooth muscle is NON-STRIATED. Only skeletal muscle and cardiac muscle are striated (and cardiac striations are less distinct).
5. Confusing true, false, and floating ribs — True ribs (1-7) attach directly to the sternum; false ribs (8-10) attach indirectly; floating ribs (11-12) have no sternal attachment at all.
6. Saying intervertebral discs allow free movement — Intervertebral discs are cartilaginous joints (slightly movable only). They permit limited movement between adjacent vertebrae, not free movement.
7. Forgetting ATP's dual role — ATP serves two critical functions: (a) it provides energy for the power stroke when hydrolysed by myosin, and (b) it detaches the myosin head from actin when it binds. Without ATP, cross-bridges remain locked (rigor mortis).
Exam Tips
1. Sarcomere diagram — Always draw the sarcomere in both relaxed and contracted states. Clearly show the changes: I band shortens, H zone disappears, A band unchanged, Z lines move closer. Label all structures.
2. Sliding filament theory — Explain the cross-bridge cycle step-by-step. Emphasise the roles of Ca2+ (binds troponin, shifts tropomyosin) and ATP (energises myosin head AND detaches cross-bridge). This is a very frequently asked long-answer question.
3. Skeletal system — State the total bone count (206) and the axial/appendicular division. Know the vertebral formula (7C + 12T + 5L + 5S + 4Co = 33). Mention the mandible as the only movable skull bone.
4. Joint types — Give at least one specific example for each type of synovial joint. Ball and socket (hip, shoulder), hinge (elbow, knee), pivot (atlas-axis), gliding (intercarpal), saddle (thumb).
5. Muscle comparison table — Always include: striations, control (voluntary/involuntary), number of nuclei, fatigue resistance, location, and the presence of intercalated discs. These are the most frequently compared features.
6. Disorders — For osteoporosis, mention post-menopausal risk and calcium/vitamin D deficiency as key factors. For arthritis, distinguish osteoarthritis (degenerative) from rheumatoid arthritis (autoimmune). For gout, specify uric acid crystal deposition.
7. Ribs classification — Know the numbers: 7 true + 3 false + 2 floating = 12 pairs (24 ribs total). Be precise — do not confuse the numbering.
8. Vertebral column — Remember the atlas (C1) allows nodding, the axis (C2) allows rotation. The sacrum (5 fused) and coccyx (4 fused) are often asked.
9. Key terms — Use precise terminology: "sarcolemma" (not cell membrane), "sarcoplasm" (not cytoplasm), "sarcoplasmic reticulum" (not endoplasmic reticulum) when referring to muscle fibres.
Quick Revision
- Movement types: Amoeboid (pseudopodia), ciliary (cilia/flagella), muscular (actin + myosin).
- Muscles: Skeletal (striated, voluntary, multinucleate), smooth (non-striated, involuntary, uninucleate), cardiac (striated, involuntary, uninucleate, intercalated discs).
- Sarcomere: Z line to Z line; A band (myosin + overlap region), I band (actin only), H zone (myosin only).
- Sliding filament theory: Actin slides over myosin; Ca2+ binds troponin, tropomyosin shifts, cross-bridge cycle operates, ATP powers contraction and detachment.
- Contraction changes: I band shortens, H zone disappears, A band unchanged, Z lines move closer.
- Axial skeleton: Skull (22), vertebral column (33), ribs (12 pairs), sternum (1).
- Appendicular skeleton: Pectoral girdle (2 clavicles + 2 scapulae), upper limbs (60 bones), pelvic girdle (2 hip bones), lower limbs (60 bones).
- Joints: Fibrous (immovable), cartilaginous (slightly movable), synovial (freely movable — ball and socket, hinge, pivot, gliding, saddle).
- Disorders: Arthritis (osteo/rheumatoid), osteoporosis (reduced bone mass), gout (uric acid crystals), tetany (low Ca2+), muscular dystrophy (genetic degeneration).