Locomotion and Movement ISC Class 11: Muscles, Skeleton and Joints

What Locomotion and Movement is about

Weightage: Locomotion and Movement belongs to the unit Human Physiology, which CISCE weights at 18 of the 70 marks in the ISC Class XI Biology Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Biology syllabus, examination year 2028, Class XI unit weightage

Key concepts

Types of movement: amoeboid, ciliary, flagellar and muscular

Amoeboid movement: some specialised cells, such as macrophages and leucocytes in blood, move by forming pseudopodia through streaming of cytoplasm, as in *Amoeba*; microfilaments (actin) take part. Ciliary movement: occurs in most internal tubular organs lined by ciliated epithelium. The coordinated beating of cilia in the trachea sweeps dust particles and foreign matter trapped in mucus towards the throat; cilia in the oviduct (fallopian tube) help move the ovum towards the uterus. Flagellar movement: a whip-like beating of flagella helps sperm swim, maintains the water current in the canal system of sponges, and moves protists such as *Euglena*. Muscular movement: contraction of muscles moves limbs, jaws, tongue and the body as a whole; together with the skeletal and nervous systems it brings about locomotion. Many organisms use the same structures for movement and locomotion: *Paramecium* uses cilia both to move food into its gullet and to swim.

Worked example

A smoker's cough is partly due to damaged cilia in the windpipe. Explain how normal cilia protect the lungs.

Try it first, then show the solutionSolution

1. The trachea is lined by ciliated epithelium with mucus-secreting cells
2. Mucus traps dust and microbes; cilia beat in waves towards the pharynx, carrying the mucus up to be swallowed or coughed out

Answer: Ciliary movement clears trapped particles from the airways

Properties and structure of skeletal muscle; contractile proteins

Muscle is a specialised tissue of mesodermal origin. There are three types: skeletal (striated, voluntary, attached to bones), visceral (smooth, involuntary, in walls of hollow organs) and cardiac (striated, involuntary, in the heart). Properties of muscle: excitability (responds to stimuli), contractility (shortens), extensibility (can be stretched) and elasticity (returns to original length). Structure of skeletal muscle: each muscle is made of bundles called fascicles, held together by collagenous connective tissue, the fascia. Each fascicle contains many muscle fibres (cells). Each fibre is covered by the plasma membrane, the sarcolemma, enclosing sarcoplasm with many nuclei (a syncytium). Its endoplasmic reticulum, the sarcoplasmic reticulum, stores calcium ions. The fibre is packed with parallel myofibrils, which show alternating dark and light bands, giving the striated appearance. The light I band (isotropic) contains actin (thin filaments); the dark A band (anisotropic) contains myosin (thick filaments). The I band is bisected by an elastic Z line, to which thin filaments are attached; the thick filaments are held together in the middle of the A band by the M line. The central part of the A band not overlapped by thin filaments is the H zone. The portion between two successive Z lines is a sarcomere, the functional unit of contraction. Contractile proteins: each actin (thin) filament is made of two F-actin strands (polymers of G-actin) twisted helically, along with tropomyosin strands and troponin at regular intervals; in resting muscle, troponin masks the active sites on actin that bind myosin. Each myosin (thick) filament is a polymer of meromyosin monomers; each monomer has a globular head with a short arm (heavy meromyosin, HMM) and a tail (light meromyosin, LMM). The head and arm project outwards as a cross arm. The head is an ATPase enzyme with binding sites for ATP and active sites for actin.

Worked example

Why is skeletal muscle called striated?

Try it first, then show the solutionSolution

1. Myofibrils have alternating dark A bands (myosin) and light I bands (actin)
2. These bands line up across neighbouring myofibrils, giving the fibre a striped appearance under the microscope

Answer: Because of the regular arrangement of dark and light bands

Sliding filament theory and chemical events in contraction

According to the sliding filament theory, a muscle fibre contracts because the thin filaments slide over the thick filaments; the filaments themselves do not shorten. Sequence: (1) A signal from the CNS travels down a motor neuron. A motor neuron and the fibres it supplies form a motor unit. (2) At the neuromuscular junction (motor end plate) the neuron releases the neurotransmitter acetylcholine, which produces an action potential in the sarcolemma. (3) The action potential spreads through the fibre and causes the sarcoplasmic reticulum to release Ca²⁺ into the sarcoplasm. (4) Ca²⁺ binds to troponin, which changes shape and unmasks the active sites on actin. (5) The myosin head, already energised by hydrolysing ATP into ADP + Pi, binds to the exposed active site, forming a cross bridge. (6) The head tilts, pulling the thin filament towards the centre of the A band; the Z lines attached to the actin are pulled inwards, so the sarcomere shortens: this is contraction. ADP and Pi are released. (7) A new ATP binds to the myosin head, the cross bridge breaks, and the ATP is hydrolysed to re-energise (re-cock) the head. (8) Cycles of cross-bridge formation and breaking continue, causing further sliding, as long as Ca²⁺ is present. (9) When stimulation stops, Ca²⁺ is pumped back into the sarcoplasmic reticulum, the actin sites are masked again, and the Z lines return to their original positions: relaxation. Band changes: during contraction the I bands shorten, the H zone narrows or disappears, but the A band keeps its length. Repeated activity can lead to lactic acid build-up from anaerobic breakdown of glycogen, causing fatigue.

Worked example

A sarcomere is 2.4 µm long at rest and 2.0 µm when contracted, and its A band is 1.6 µm. Find the total I band length in each state and the length of the A band after contraction.

Try it first, then show the solutionSolution

1. I band (both halves together) = sarcomere − A band: rest 2.4 − 1.6 = 0.8 µm; contracted 2.0 − 1.6 = 0.4 µm
2. The A band does not change because myosin filaments do not shorten: 1.6 µm

Answer: I band 0.8 µm at rest, 0.4 µm contracted; A band stays 1.6 µm

Summation, tetanus, rigor mortis, and red and white muscles

Muscle twitch: a single, brief contraction followed by relaxation in response to one stimulus. Summation: if a second stimulus arrives before the muscle has fully relaxed from the first, the second contraction adds to the first and produces a stronger contraction. Tetanus (physiological): when stimuli come so rapidly that the muscle has no time to relax between them, the contractions fuse into a smooth, sustained, maximal contraction; most normal voluntary movements are brief tetanic contractions. (Do not confuse this with tetany, a disorder, or the bacterial disease tetanus.) Rigor mortis: stiffening of the muscles a few hours after death. After death, ATP is no longer produced; since ATP is needed to detach myosin heads from actin, the cross bridges stay locked and the muscles become rigid. Red and white muscles: red muscle fibres contain plenty of the red, oxygen-storing pigment myoglobin, many mitochondria and a rich blood supply; they rely on aerobic respiration, contract more slowly and can work for long periods without fatigue (e.g. the extensor muscles along the back that maintain posture). White muscle fibres have little myoglobin and fewer mitochondria but a large amount of sarcoplasmic reticulum; they rely mainly on anaerobic respiration, contract fast but tire quickly (e.g. muscles that move the eyeball).

Worked example

Why can the back muscles keep you upright for hours, while your eye muscles tire if you keep moving your eyes rapidly?

Try it first, then show the solutionSolution

1. Back extensors are mainly red fibres: myoglobin and many mitochondria allow aerobic, fatigue-resistant activity
2. Eye muscles are rich in white fibres: fast contraction but mainly anaerobic, so they tire quickly

Answer: Red (aerobic, slow, enduring) versus white (anaerobic, fast, fatigable) fibres

Skeletal system: functions and the axial skeleton (80 bones)

Functions: (1) support and shape; (2) protection of soft organs (skull for the brain, ribs for heart and lungs, vertebral column for the spinal cord); (3) movement, by acting as levers to which muscles attach; (4) blood cell formation in red bone marrow; (5) storage of minerals such as calcium and phosphorus. The adult skeleton has 206 bones (plus some cartilages). Axial skeleton (80 bones): Skull (29): cranial bones (8), forming a hard protective box around the brain; facial bones (14), forming the front of the skull; a U-shaped hyoid bone (1) at the base of the buccal cavity; and the ear ossicles (6), three in each middle ear: malleus, incus and stapes. The skull joins the vertebral column through two occipital condyles (dicondylic skull). Vertebral column (26): 7 cervical, 12 thoracic, 5 lumbar, 1 sacrum (five fused) and 1 coccyx (four fused). Each vertebra has a central hollow neural canal through which the spinal cord passes; the first vertebra is the atlas, which articulates with the occipital condyles. The column protects the spinal cord, supports the head and is a point of attachment for ribs and back muscles. Sternum (1): a flat bone on the ventral midline of the thorax. Ribs (24, i.e. 12 pairs): each is a thin, flat bone joined dorsally to the vertebral column and ventrally to the sternum (bicephalic: two articulating surfaces dorsally). The first seven pairs are true ribs, joined directly to the sternum by hyaline cartilage; the 8th, 9th and 10th pairs are false (vertebrochondral) ribs, joined to the 7th rib's cartilage rather than directly to the sternum; the 11th and 12th pairs are floating ribs, not connected ventrally. The thoracic vertebrae, ribs and sternum form the rib cage.

Worked example

Show that the axial skeleton has 80 bones.

Try it first, then show the solutionSolution

1. Skull 29 (8 cranial + 14 facial + 1 hyoid + 6 ossicles); vertebral column 26 (7 + 12 + 5 + 1 + 1)
2. Sternum 1 + ribs 24: total 29 + 26 + 1 + 24 = 80

Answer: 80 bones

Appendicular skeleton (126 bones) and joints

Appendicular skeleton (126 bones): limbs and girdles. Each forelimb (30): humerus (1), radius and ulna (2), carpals (wrist, 8), metacarpals (palm, 5) and phalanges (digits, 14). Each hindlimb (30): femur (1, the longest bone), tibia and fibula (2), tarsals (ankle, 7), metatarsals (5), phalanges (14) and a cup-shaped patella (knee cap, 1). Pectoral girdle: on each side a clavicle (collar bone) and a scapula (shoulder blade), 4 bones in all; a depression in the scapula, the glenoid cavity, receives the head of the humerus. Pelvic girdle: two coxal bones, each formed by fusion of the ilium, ischium and pubis; where they meet is a cavity, the acetabulum, which receives the head of the femur. The two halves meet ventrally at the pubic symphysis (fibrous cartilage). Total: 60 + 60 + 4 + 2 = 126. Joints are points of contact between bones, or between bones and cartilages; muscle force is used to move bones about joints, which act as fulcrums. (1) Fibrous joints: no movement; bones joined by dense fibrous tissue, e.g. sutures between the flat skull bones. (2) Cartilaginous joints: limited movement; bones joined by cartilage, e.g. between adjacent vertebrae (intervertebral discs) and the pubic symphysis. (3) Synovial joints: considerable movement; a fluid-filled synovial cavity between the articulating surfaces of the bones, lined by a synovial membrane. Types: ball-and-socket (between humerus and pectoral girdle; femur and acetabulum), hinge (knee, elbow), pivot (between atlas and axis), gliding (between the carpals), saddle (between the carpal and metacarpal of the thumb).

Worked example

Name the type of joint and give the reason for its movement: (i) shoulder (ii) knee (iii) between atlas and axis.

Try it first, then show the solutionSolution

1. (i) Ball-and-socket: the rounded head of the humerus fits the glenoid cavity, allowing movement in all directions
2. (ii) Hinge: movement in one plane only; (iii) pivot: rotation of the atlas around a peg of the axis (turning the head)

Answer: Ball-and-socket; hinge; pivot

Disorders of the muscular and skeletal systems

Myasthenia gravis: the body's own antibodies attack the neuromuscular junction (an autoimmune condition), so nerve signals pass poorly to skeletal muscles, which tire easily, grow weak and may become paralysed. Muscular dystrophy: an inherited (genetic) condition in which skeletal muscle fibres break down progressively over the years and are not replaced. Tetany: when the Ca²⁺ level of body fluids falls too low, nerves and muscles become over-excitable and muscles go into sudden, uncontrolled spasms. Arthritis: the joints become inflamed, painful, swollen and stiff. Osteoporosis: with age, bone mass falls and bones become porous and brittle, so they fracture easily; reduced oestrogen after menopause is an important cause. Gout: uric acid crystals are deposited in joints, which become inflamed and very painful (often the big toe). Good diet (calcium, vitamin D), exercise and early treatment reduce many of these problems.

Worked example

A patient's blood calcium falls very low after damage to the parathyroid glands, and her muscles go into spasm. Name the disorder and explain.

Try it first, then show the solutionSolution

1. Low Ca²⁺ in body fluid makes nerves and muscles over-excitable
2. Muscles contract repeatedly and uncontrollably (spasms)

Answer: Tetany, caused by low Ca²⁺

Locomotion and Movement in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Muscle Structure and Sliding Filament Theory
• Skeletal System, Joints and Disorders

Locomotion and Movement: key facts

Bone count

$206=80(\text{axial})+126(\text{appendicular})$

Axial: skull 29 + vertebrae 26 + sternum 1 + ribs 24.

Limb formula

$\text{forelimb }1+2+8+5+14=30;\text{hindlimb }1+2+7+5+14+1=30$

Patella is the extra hindlimb bone.

Vertebral formula

$C_7{T}_{12}{L}_5{S}_{1(5)}C{o}_{1(4)}=26$

Numbers in brackets are fused vertebrae.

Sarcomere arithmetic

$\text{I band}=\text{sarcomere length}-\text{A band};\text{A band constant during contraction}$

H zone narrows; Z lines move closer.

Cross-bridge cycle

$C{a}^{2+}+\text{troponin}\to \text{sites exposed}\to \text{cross bridge}\to \text{power stroke}\stackrel{ATP}{\to }\text{detach}$

Relaxation when Ca²⁺ is pumped back.

Locomotion and Movement: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Locomotion and Movement

The question patterns to expect, and the method that works for each.

Tricks to remember Locomotion and Movement

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Breakfast at 7, Lunch at 12, Dinner at 5

  Cervical 7, thoracic 12, lumbar 5 vertebrae.

  Use it when: Vertebral column.

• I is Light, A is dArk

  I band (isotropic) is light with actin; A band (anisotropic) is dark with myosin.

  Use it when: Bands of a sarcomere.

• HI shrink, A stays

  During contraction the H zone and I band shorten; the A band stays the same.

  Use it when: Sliding filament theory.

• 7-3-2 ribs

  7 pairs true, 3 pairs false, 2 pairs floating.

  Use it when: Ribs.

• Ball, Hinge, Pivot, Glide, Saddle = shoulder, knee, neck, wrist, thumb

  Match each synovial joint with its classic example.

  Use it when: Types of joints.

Locomotion and Movement: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Locomotion and Movement

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying the filaments shorten

  Contraction is pictured as filaments getting shorter.

  Wrong working

  Actin and myosin shorten during contraction.

  Right working

  Thin filaments slide over thick ones; neither shortens; the sarcomere shortens.

  ✓Do this: Sliding, not shrinking.

  Usually costs: 1-2 marks.

• ✗Mistake: Confusing tetanus and tetany

  The words look alike.

  Wrong working

  Tetanus is muscle spasm due to low calcium.

  Right working

  Tetanus (physiology): sustained contraction from rapid stimuli; tetany: spasms due to low Ca²⁺.

  ✓Do this: Tetany = low calcium.

  Usually costs: 1 mark.

• ✗Mistake: Wrong role of ATP in rigor mortis

  ATP is linked only with contraction.

  Wrong working

  Without ATP, myosin cannot bind actin.

  Right working

  ATP is needed for detachment; without it cross bridges stay locked.

  ✓Do this: ATP = release.

  Usually costs: 1 mark.

• ✗Mistake: Miscounting ribs or vertebrae

  Pairs and single bones are confused.

  Wrong working

  There are 12 ribs; the vertebral column has 33 bones in adults.

  Right working

  12 pairs (24 ribs); adult vertebral column has 26 bones (sacrum and coccyx are fused).

  ✓Do this: Write 'pairs' and check fusion.

  Usually costs: 1 mark.

• ✗Mistake: Wrong example for a joint

  Examples are swapped.

  Wrong working

  The knee is a ball-and-socket joint.

  Right working

  Knee and elbow: hinge; shoulder and hip: ball-and-socket.

  ✓Do this: Test the movement on yourself.

  Usually costs: 1 mark.

Practice questions on Locomotion and Movement

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The sarcomere is the region between two successive:
   (a) M lines
   (b) H zones
   (c) Z lines
   (d) A bands

   Hint

   Unit of contraction.

   Answer

   (c)

2. Q2 · 1 mark · MCQExam level

   Which ion is released by the sarcoplasmic reticulum to start contraction?
   (a) Na⁺
   (b) K⁺
   (c) Ca²⁺
   (d) Mg²⁺

   Hint

   Binds troponin.

   Answer

   (c)

3. Q3 · 1 mark · Very short answerExam level

   Name the three ear ossicles.

   Hint

   Hammer, anvil, stirrup.

   Answer

   Malleus, incus and stapes

4. Q4 · 1 mark · Very short answerExam level

   Name the longest bone of the human body.

   Hint

   Thigh.

   Answer

   Femur

5. Q5 · 2 marks · Short answerExam level

   List four properties of muscle.

   Hint

   E, C, E, E.

   Answer

   Excitability, contractility, extensibility and elasticity.

6. Q6 · 2 marks · Short answerHarder exam level

   Distinguish between summation and tetanus.

   Hint

   Partial vs no relaxation.

   Answer

   Summation: a second stimulus before full relaxation adds to the first contraction, producing a stronger one. Tetanus: stimuli so rapid that no relaxation occurs, giving a smooth, sustained maximal contraction.

7. Q7 · 2 marks · Short answerHarder exam level

   Describe ciliary and flagellar movement with one example each in humans.

   Hint

   Trachea, sperm.

   Answer

   Ciliary: coordinated beating of many cilia moves materials over a surface, e.g. cilia in the trachea sweep mucus and dust upward (or move the ovum in the oviduct). Flagellar: whip-like beating of a flagellum propels a cell, e.g. swimming of sperm.

8. Q8 · 3 marks · Short answerHarder exam level

   Name the bones of the forelimb and give their numbers.

   Hint

   30.

   Answer

   Humerus (1), radius and ulna (2), carpals (8), metacarpals (5), phalanges (14): 30 bones.

9. Q9 · 3 marks · Short answerHarder exam level

   Describe myasthenia gravis, muscular dystrophy and tetany.

   Hint

   Autoimmune, genetic, calcium.

   Answer

   Myasthenia gravis: autoimmune disorder of the neuromuscular junction causing fatigue, weakness and paralysis. Muscular dystrophy: progressive degeneration of skeletal muscle, mostly genetic. Tetany: rapid muscle spasms due to low Ca²⁺ in body fluids.

10. Q10 · 2 marks · Short answerHarder exam level

    State four functions of the skeleton.

    Hint

    Support, protect, move, make.

    Answer

    Support and shape; protection of organs (skull, rib cage); movement by acting as levers for muscles; formation of blood cells in red marrow; storage of calcium and phosphorus.

11. Q11 · 5 marks · Long answerHarder exam level

    (a) Describe the axial skeleton with the number of bones in each part. [4]
    (b) What is a dicondylic skull? [1]

    Hint

    29 + 26 + 1 + 24.

    Answer

    (a) Skull 29 (8 cranial, 14 facial, 1 hyoid, 6 ear ossicles); vertebral column 26 (7 cervical, 12 thoracic, 5 lumbar, 1 sacrum, 1 coccyx); sternum 1; ribs 24 (7 pairs true, 3 pairs false, 2 pairs floating). Total 80. (b) A skull that articulates with the vertebral column through two occipital condyles, as in humans.

12. Q12 · 5 marks · Long answerHarder exam level

    (a) Describe the structure of a sarcomere with a labelled diagram. [3]
    (b) State the role of ATP at two different steps of the cross-bridge cycle. [2]

    Hint

    Z to Z; ATP both cocks and releases the myosin head.

    Answer

    (a) Region between two Z lines; thin actin filaments attached to Z lines form the I band (light) on either side; thick myosin filaments form the central A band (dark), held at the M line; central part of A band without overlap is the H zone. (b) (i) Hydrolysis of ATP to ADP + Pi by the ATPase of the myosin head energises (cocks) the head so it can bind the exposed active site on actin and make the power stroke. (ii) A fresh ATP binding to the myosin head breaks the cross bridge, detaching myosin from actin so the cycle can repeat (and the muscle can relax).

13. Q13 · 2 marks · Short answerHarder exam level

    A sarcomere with an A band of 1.6 µm measures 2.2 µm. What is the total I band length? If it contracts to 1.8 µm, what will the I band and A band measure?

    Hint

    Subtract A band.

    Answer

    I band at 2.2 µm = 2.2 − 1.6 = 0.6 µm. At 1.8 µm: I band = 0.2 µm; A band remains 1.6 µm.

14. Q14 · 3 marks · Short answerHOTS / competency

    Chicken breast meat is pale while duck breast meat is dark red. Relate this to the flying habits of the two birds.

    Hint

    Myoglobin and endurance.

    Answer

    Ducks fly long distances, so their flight muscles need sustained aerobic activity; they are rich in red fibres with much myoglobin and many mitochondria, making them dark. Chickens fly only in short bursts, so their breast muscles are mostly white fibres with little myoglobin, suited to fast anaerobic contraction but quick fatigue, hence pale.

15. Q15 · 3 marks · Short answerHOTS / competency

    A drug prevents Ca²⁺ from being pumped back into the sarcoplasmic reticulum. Predict the effect on the muscle and explain.

    Hint

    Relaxation needs Ca²⁺ removal.

    Answer

    Ca²⁺ would stay bound to troponin, keeping the active sites on actin exposed. As long as ATP is available, cross bridges would keep forming and the muscle would stay contracted and fail to relax (a sustained spasm), until ATP runs out.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Number of bones in adult human?Show answer

   206.

2. 2.Number of cranial bones?Show answer

   8.

3. 3.Joint between carpals?Show answer

   Gliding joint.

4. 4.Store of Ca²⁺ in muscle fibre?Show answer

   Sarcoplasmic reticulum.

5. 5.Protein masking actin's binding sites?Show answer

   Troponin (with tropomyosin).

6. 6.Neurotransmitter at neuromuscular junction?Show answer

   Acetylcholine.

7. 7.Oxygen-storing pigment of red muscle?Show answer

   Myoglobin.

8. 8.Disorder due to uric acid crystals?Show answer

   Gout.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give one example each of amoeboid, ciliary and flagellar movement?
• ☐Can you label a sarcomere?
• ☐Can you describe actin and myosin?
• ☐Can you list the steps of muscle contraction in order?
• ☐Can you explain rigor mortis?
• ☐Can you show that there are 206 bones?
• ☐Can you name the five synovial joints with examples?
• ☐Can you name the cause of each muscle and bone disorder?

Locomotion and Movement: frequently asked questions