Class 11 Biology Notes Chapter 20 (Locomotion and movement) – Biology Book
Detailed Notes with MCQs of Chapter 20: Locomotion and Movement. This is a crucial chapter, not just for understanding how we and other organisms move, but also because questions frequently appear from this section in various government exams. Pay close attention to the details, especially regarding muscle structure, the mechanism of contraction, and the skeletal system.
Chapter 20: Locomotion and Movement - Detailed Notes
1. Introduction
- Movement: A significant feature of living beings. It's a change in position of a body part relative to the whole body (e.g., movement of limbs, jaws, tongue).
- Locomotion: Voluntary movement resulting in a change of place or location. All locomotion involves movement, but not all movement is locomotion (e.g., beating of the heart is movement, not locomotion).
- Purpose of Locomotion: Search for food, shelter, mate, suitable breeding grounds, favourable climatic conditions, escape from predators.
- Types of Movement in Human Cells:
- Amoeboid: Performed by pseudopodia (false feet) formed by protoplasmic streaming. Seen in macrophages and leucocytes (WBCs) in blood. Also involves cytoskeletal elements like microfilaments.
- Ciliary: Occurs in internal tubular organs lined by ciliated epithelium. Coordinated movement of cilia helps move substances. Examples: Removal of dust particles in the trachea, passage of ova through the female reproductive tract (fallopian tubes).
- Muscular: Involves contractile muscle tissue. Responsible for movement of limbs, jaws, tongue, and overall locomotion.
2. Muscle
- Specialized tissue of mesodermal origin.
- Properties: Excitability, contractility, extensibility, elasticity.
- Types of Muscles:
- Skeletal Muscle:
- Associated with skeletal components.
- Striated appearance (striped) due to alternating light and dark bands.
- Voluntary control (under conscious control of the nervous system).
- Primarily involved in locomotion and changes in body posture.
- Multinucleated (syncytium), unbranched fibers.
- Visceral Muscle (Smooth Muscle):
- Located in the inner walls of hollow visceral organs (e.g., alimentary canal, reproductive tract, blood vessels).
- Non-striated (smooth) appearance.
- Involuntary control (not under conscious control).
- Slow, sustained contractions.
- Spindle-shaped, uninucleated cells.
- Cardiac Muscle:
- Muscle of the heart wall (myocardium).
- Striated appearance.
- Involuntary control.
- Branched fibers, uninucleated (usually).
- Contain intercalated discs (junctions between cells) allowing rapid spread of impulse and function as a unit (syncytium). Never fatigues.
- Skeletal Muscle:
3. Skeletal Muscle: Detailed Structure
- Organized structure: Muscle -> Fascicles (bundles of muscle fibers) -> Muscle Fiber (muscle cell) -> Myofibrils -> Myofilaments (Actin & Myosin).
- Muscle Fiber (Cell):
- Elongated, cylindrical cell.
- Covered by plasma membrane called Sarcolemma.
- Cytoplasm is called Sarcoplasm, containing numerous nuclei (syncytium) and mitochondria (sarcosomes).
- Endoplasmic reticulum is called Sarcoplasmic Reticulum (storehouse of Calcium ions, Ca++).
- Contains parallelly arranged Myofibrils.
- Myofibrils:
- Have characteristic striations (dark A-bands and light I-bands).
- I-Band (Isotropic Band): Light band, contains only thin filaments (Actin). Bisected by a dense line called the Z-line.
- A-Band (Anisotropic Band): Dark band, contains both thick (Myosin) and thin (Actin) filaments overlapping.
- H-Zone: Lighter region in the middle of the A-band where only thick filaments (Myosin) are present (thin filaments do not extend here in relaxed state).
- M-Line: A thin fibrous membrane in the middle of the H-zone, holding thick filaments together.
- Sarcomere: The functional unit of contraction. It is the portion of the myofibril between two successive Z-lines.
4. Structure of Contractile Proteins
- Actin (Thin Filament):
- Composed of two intertwined 'F' (filamentous) actins, which are polymers of 'G' (globular) actin monomers.
- Associated proteins:
- Tropomyosin: Two filamentous proteins running close to the F-actin grooves, masking the active binding sites for myosin on actin in the resting state.
- Troponin: A complex of three globular proteins distributed at regular intervals on tropomyosin.
- Troponin I (TnI): Inhibits actin-myosin interaction.
- Troponin T (TnT): Binds to tropomyosin.
- Troponin C (TnC): Binds calcium ions (Ca++).
- Myosin (Thick Filament):
- A polymerised protein. Each monomeric myosin ('Meromyosin') has two parts:
- Heavy Meromyosin (HMM): Globular head with a short arm. Projects outwards at regular distances and angles. Contains ATPase enzyme activity and binding sites for ATP and Actin. Forms the 'cross bridge'.
- Light Meromyosin (LMM): Forms the tail of the myosin molecule.
- A polymerised protein. Each monomeric myosin ('Meromyosin') has two parts:
5. Mechanism of Muscle Contraction (Sliding Filament Theory)
- Proposed by Andrew Huxley and Rolf Niedergerke, and Hugh Huxley and Jean Hanson.
- States that muscle contraction occurs by the sliding of thin filaments (Actin) over thick filaments (Myosin).
- Steps:
- Signal Transmission: A signal (action potential) travels from the Central Nervous System (CNS) via a motor neuron to the neuromuscular junction (motor end plate).
- Neurotransmitter Release: Acetylcholine (neurotransmitter) is released at the synapse, binding to receptors on the sarcolemma.
- Action Potential Generation: This generates an action potential in the sarcolemma, which spreads through the muscle fiber via T-tubules.
- Calcium Release: The action potential triggers the release of Ca++ ions from the sarcoplasmic reticulum into the sarcoplasm.
- Activation of Actin: Ca++ binds to Troponin C (TnC). This causes a conformational change in the Troponin-Tropomyosin complex, uncovering the active sites on Actin for myosin binding.
- Cross-Bridge Formation: The energized myosin head (containing ADP + Pi from previous ATP hydrolysis) binds to the exposed active site on actin, forming a cross-bridge.
- Power Stroke (Sliding): Binding triggers the myosin head to pivot/bend, pulling the actin filament towards the center of the sarcomere (M-line). ADP and Pi are released during this step. This is the power stroke.
- Cross-Bridge Detachment: A new ATP molecule binds to the myosin head, causing the detachment of the myosin head from the actin filament.
- Reactivation of Myosin Head: The ATPase activity of the myosin head hydrolyzes the ATP into ADP and Pi, releasing energy. This energy 're-cocks' or energizes the myosin head, returning it to its high-energy conformation, ready for another cycle.
- Cycle Repeats: Steps 6-9 repeat as long as Ca++ levels remain high and ATP is available, causing further sliding.
- Changes during Contraction:
- Sarcomere shortens.
- I-bands shorten.
- H-zone disappears.
- A-bands retain their length.
- Relaxation:
- When nerve stimulation ceases, Ca++ is actively pumped back into the sarcoplasmic reticulum.
- Ca++ detaches from Troponin C.
- Tropomyosin again masks the active sites on actin.
- Cross-bridges detach, and the muscle fiber returns to its resting length (due to elasticity).
6. Muscle Fatigue and Oxygen Debt
- Repeated activation can lead to muscle fatigue due to the accumulation of lactic acid (formed during anaerobic breakdown of glycogen when oxygen supply is inadequate).
- Oxygen Debt: The extra oxygen required after strenuous exercise to metabolize the accumulated lactic acid and restore the resting metabolic state.
- Muscle Fiber Types:
- Red Fibers (Slow-twitch): High myoglobin content (oxygen storing pigment), abundant mitochondria, rich blood supply. Aerobic metabolism. Slow contraction rate, resistant to fatigue. (e.g., postural muscles).
- White Fibers (Fast-twitch): Low myoglobin content, fewer mitochondria, depend on anaerobic metabolism (glycolysis). Fast contraction rate, fatigue quickly. Sarcoplasmic reticulum content is high. (e.g., eye muscles).
7. Skeletal System
- Framework of bones and cartilages. Provides support, protection, shape, aids in movement (lever system), site of blood cell formation (hematopoiesis in bone marrow), and mineral storage (Calcium, Phosphate).
- Human adult skeleton: 206 bones.
- Divided into:
- Axial Skeleton (80 bones): Forms the main axis of the body.
- Skull (22 bones + 6 ear ossicles + 1 hyoid = 29 total):
- Cranial bones (8): Frontal, Parietal (2), Temporal (2), Occipital, Sphenoid, Ethmoid. Protect the brain.
- Facial bones (14): Form the front part of the skull. Include Mandible (lower jaw), Maxilla (2), Nasal (2), Zygomatic (2), Lacrimal (2), Palatine (2), Inferior nasal conchae (2), Vomer.
- Hyoid bone (1): U-shaped bone at the base of the buccal cavity. Not articulated with any other bone.
- Ear Ossicles (6): Malleus, Incus, Stapes (3 in each middle ear). Smallest bones.
- Vertebral Column (26 vertebrae): Protects spinal cord, supports head, attachment point for ribs and back muscles.
- Formula: Cervical (C7), Thoracic (T12), Lumbar (L5), Sacral (S1 - fused from 5), Coccygeal (Co1 - fused from 4). Total 33 in child, 26 in adult.
- Intervertebral discs (cartilaginous) between vertebrae act as shock absorbers.
- Atlas (C1) and Axis (C2) are modified first two cervical vertebrae. Atlas articulates with occipital condyles of skull. Axis has odontoid process.
- Sternum (1 bone): Flat bone on the ventral midline of the thorax.
- Ribs (12 pairs = 24 bones):
- True Ribs (Pairs 1-7): Attach directly to sternum via hyaline cartilage (vertebrosternal).
- False Ribs (Pairs 8-10): Attach indirectly to sternum via cartilage of the 7th rib (vertebrochondral).
- Floating Ribs (Pairs 11-12): Do not connect to the sternum ventrally (vertebral).
- Rib cage (Thoracic cage): Formed by thoracic vertebrae, ribs, and sternum. Protects heart and lungs.
- Skull (22 bones + 6 ear ossicles + 1 hyoid = 29 total):
- Appendicular Skeleton (126 bones): Bones of the limbs and their girdles.
- Girdles: Connect limbs to axial skeleton.
- Pectoral Girdle (Shoulder Girdle - 4 bones): Clavicle (collar bone - 2) and Scapula (shoulder blade - 2). Scapula has a shallow depression called the Glenoid cavity which articulates with the head of the humerus.
- Pelvic Girdle (Hip Girdle - 2 coxal bones): Each coxal bone is formed by the fusion of three bones: Ilium, Ischium, and Pubis. The two halves meet ventrally at the pubic symphysis (cartilaginous joint). Contains a deep socket called the Acetabulum which articulates with the head of the femur.
- Limbs:
- Forelimb (30 bones each = 60 total): Humerus (upper arm), Radius & Ulna (forearm), Carpals (wrist - 8), Metacarpals (palm - 5), Phalanges (digits/fingers - 14: 2 in thumb, 3 in each other finger).
- Hindlimb (30 bones each = 60 total): Femur (thigh bone - longest, strongest bone), Tibia & Fibula (lower leg), Patella (knee cap - sesamoid bone), Tarsals (ankle - 7), Metatarsals (sole - 5), Phalanges (digits/toes - 14: 2 in great toe, 3 in each other toe).
- Girdles: Connect limbs to axial skeleton.
- Axial Skeleton (80 bones): Forms the main axis of the body.
8. Joints
- Points of contact between bones, or between bones and cartilages. Essential for movement. Force generated by muscles uses joints as fulcrums.
- Classification: Based on structure and degree of movement.
- Fibrous Joints (Synarthroses):
- Bones joined by dense fibrous connective tissue.
- Immovable.
- Example: Sutures between skull bones.
- Cartilaginous Joints (Amphiarthroses):
- Bones joined by cartilage.
- Permit limited movement.
- Example: Joints between adjacent vertebrae (intervertebral discs), pubic symphysis.
- Synovial Joints (Diarthroses):
- Characterized by a fluid-filled synovial cavity between articulating bones. Allows considerable movement.
- Articular surfaces covered by articular cartilage (hyaline).
- Joint cavity enclosed by an articular capsule (outer fibrous layer, inner synovial membrane).
- Synovial membrane secretes synovial fluid (lubrication, nourishment).
- Often reinforced by ligaments.
- Types of Synovial Joints:
- Ball and Socket: Shoulder (humerus & glenoid cavity), Hip (femur & acetabulum). Allows movement in all planes.
- Hinge: Elbow, Knee, Interphalangeal joints. Allows movement in one plane (like a door hinge).
- Pivot: Between Atlas and Axis (atlanto-axial joint), between Radius and Ulna. Allows rotation.
- Gliding (Plane): Between carpals, between tarsals. Allows sliding/gliding movements.
- Saddle: Between carpal (trapezium) and metacarpal of thumb. Allows biaxial movement.
- Condyloid (Ellipsoidal): Between radius and carpals (wrist joint). Allows biaxial movement but not rotation.
- Fibrous Joints (Synarthroses):
9. Disorders of Muscular and Skeletal System
- Myasthenia Gravis: Autoimmune disorder affecting neuromuscular junction. Leads to fatigue, weakening, and paralysis of skeletal muscles. Antibodies block or destroy acetylcholine receptors.
- Muscular Dystrophy: Progressive degeneration of skeletal muscle, mostly due to genetic defects. Example: Duchenne Muscular Dystrophy (X-linked recessive).
- Tetany: Rapid muscle spasms (wild contractions) due to low Ca++ levels in body fluid (hypocalcemia).
- Arthritis: Inflammation of joints. Common types include:
- Osteoarthritis: Degeneration of articular cartilage.
- Rheumatoid Arthritis: Autoimmune disease attacking synovial membranes.
- Gouty Arthritis (Gout): Accumulation of uric acid crystals in joints, causing inflammation.
- Osteoporosis: Age-related disorder characterized by decreased bone mass and increased chances of fractures. Common cause is decreased estrogen levels (post-menopausal women).
- Gout: Inflammation of joints due to accumulation of uric acid crystals (related to purine metabolism).
Multiple Choice Questions (MCQs)
-
Which of the following structures represents the functional unit of muscle contraction?
a) Myofibril
b) Sarcolemma
c) Sarcomere
d) Fascicle -
The release of which ion from the sarcoplasmic reticulum initiates muscle contraction?
a) Sodium (Na+)
b) Potassium (K+)
c) Calcium (Ca++)
d) Magnesium (Mg++) -
During muscle contraction, which of the following does NOT shorten?
a) I-Band
b) H-Zone
c) A-Band
d) Sarcomere -
The protein that masks the active sites on actin filaments in a resting muscle fiber is:
a) Troponin
b) Myosin
c) Tropomyosin
d) Titin -
Which type of joint allows the maximum degree of movement?
a) Fibrous joint
b) Cartilaginous joint
c) Synovial joint
d) Suture -
The number of floating ribs in the human body is:
a) 7 pairs
b) 3 pairs
c) 2 pairs
d) 12 pairs -
The joint between the atlas and axis vertebrae is an example of a:
a) Hinge joint
b) Pivot joint
c) Saddle joint
d) Ball and socket joint -
Myasthenia gravis is an autoimmune disorder that affects:
a) Synovial membranes
b) Bone density
c) Neuromuscular junction
d) Muscle fiber structure -
Which of the following bones is part of the pectoral girdle?
a) Sternum
b) Ilium
c) Clavicle
d) Femur -
Accumulation of uric acid crystals in joints leads to a condition called:
a) Osteoporosis
b) Arthritis
c) Gout
d) Tetany
Answer Key:
- c) Sarcomere
- c) Calcium (Ca++)
- c) A-Band
- c) Tropomyosin
- c) Synovial joint
- c) 2 pairs (Pairs 11 and 12)
- b) Pivot joint
- c) Neuromuscular junction
- c) Clavicle
- c) Gout
Make sure you revise these points thoroughly. Understanding the sliding filament theory and the names/locations of bones and joints is particularly important for objective questions. Good luck with your preparation!