SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Locomotion and Movement

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Exercises 17.1–17.10

  1. Exercise 17.1

    Draw the diagram of a sarcomere of skeletal muscle showing different regions.

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    Draw a sarcomere: the portion of a myofibril between two successive 'Z' lines, which is the functional unit of contraction.
    Draw two vertical 'Z' lines as the two ends of the diagram, and show thin filaments (actin) firmly attached to each 'Z' line and running inwards towards the centre.
    Draw thick filaments (myosin) as stouter rods lying in the middle, parallel to the thin filaments and to the long axis of the myofibril.
    Label the light region at each end as a half 'I' band (Isotropic band) — it contains only actin; the 'Z' line bisects a full 'I' band.
    Label the dark central region as the 'A' band (Anisotropic band) — it contains the myosin filaments, with the free ends of the thin filaments overlapping the ends of the thick filaments.
    Mark the 'M' line as a vertical line at the exact centre of the 'A' band — the thin fibrous membrane that holds the thick filaments together.
    Mark the 'H' zone as the central part of the 'A' band that is not overlapped by thin filaments, i.e., thick filaments only.
    Required labels: 'Z' line, 'I' band, 'A' band, 'H' zone, 'M' line, actin (thin) filament, myosin (thick) filament, and the arrow spanning one 'Z' line to the next marked 'sarcomere'.
    The book prints this as Figure $\displaystyle 17.2$(b).
    NCERT_Solution_Class11_Biology_Ch17_Q17-1

  2. Exercise 17.2

    Define sliding filament theory of muscle contraction.

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    The sliding filament theory states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
    The myosin heads form cross bridges with the actin filaments and pull them towards the centre of the 'A' band.
    The 'Z' lines attached to these actin filaments are pulled inwards, so the sarcomere shortens — that shortening is the contraction.
    Hence during contraction the 'I' bands get reduced while the 'A' bands retain their length: the filaments themselves do not shorten, they only slide past each other.

  3. Exercise 17.3

    Describe the important steps in muscle contraction.

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    Signal from the CNS: contraction is initiated by a signal sent by the central nervous system through a motor neuron. A motor neuron together with the muscle fibres connected to it forms a motor unit.
    Neurotransmitter release: the signal reaches the neuromuscular junction (motor-end plate) — the junction between the motor neuron and the sarcolemma — and releases the neurotransmitter acetyl choline.
    Action potential: this generates an action potential in the sarcolemma, which spreads through the muscle fibre.
    Calcium release: the action potential causes the release of \(\displaystyle Ca^{++}\) from the sarcoplasmic reticulum into the sarcoplasm.
    Unmasking of active sites: the rise in \(\displaystyle Ca^{++}\) makes calcium bind to a subunit of troponin on the actin filament, which removes the masking of the active sites for myosin.
    Cross bridge formation: using energy from ATP hydrolysis, the myosin head binds to the exposed active site on actin to form a cross bridge.
    Sliding and shortening: the head rotates and pulls the attached actin filaments towards the centre of the 'A' band. The 'Z' lines are pulled inwards, shortening the sarcomere — this is contraction. The 'I' bands get reduced; the 'A' bands retain their length.
    Breaking of the cross bridge: the myosin releases ADP and \(\displaystyle P_i\) and returns to its relaxed state; a new ATP binds and the cross bridge is broken. The ATP is hydrolysed again by the myosin head and the cycle of cross bridge formation and breakage repeats, causing further sliding.
    Relaxation: the process continues until \(\displaystyle Ca^{++}\) is pumped back into the sarcoplasmic cisternae, which masks the actin filaments again; the 'Z' lines return to their original position and the muscle relaxes.
    Fatigue: repeated activation leads to accumulation of lactic acid from anaerobic breakdown of glycogen, causing fatigue.

  4. Exercise 17.4

    Write true or false. If false change the statement so that it is true.
    (a)
    Actin is present in thin filament
    (b)
    H-zone of striated muscle fibre represents both thick and thin filaments.
    (c)
    Human skeleton has 206\displaystyle 206 bones.
    (d)
    There are 11\displaystyle 11 pairs of ribs in man.
    (e)
    Sternum is present on the ventral side of the body.

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    (a) True. Actin is the protein of the thin filament, which forms the light 'I' (Isotropic) band.
    (b) False. Corrected: *The H-zone of a striated muscle fibre represents only the thick filaments.* It is the central part of the thick filament that is not overlapped by the thin filaments.
    (c) True. The human skeletal system is made up of $\displaystyle 206$ bones and a few cartilages ($\displaystyle 80$ axial + the appendicular bones).
    (d) False. Corrected: *There are $\displaystyle 12$ pairs of ribs in man* — $\displaystyle 7$ pairs true, $\displaystyle 3$ pairs vertebrochondral (false) and the last $\displaystyle 2$ pairs floating.
    (e) True. The sternum is a flat bone lying on the ventral midline of the thorax.

  5. Exercise 17.5

    Write the difference between :
    (a)
    Actin and Myosin
    (b)
    Red and White muscles
    (c)
    Pectoral and Pelvic girdle

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    (a) Actin and Myosin
    Actin is the thin filament; myosin is the thick filament.
    Actin forms the light 'I' band; myosin forms the dark 'A' band.
    Each actin filament is made of two 'F' (filamentous) actins helically wound together, each 'F' actin being a polymer of monomeric 'G' (globular) actin; two tropomyosin filaments and troponin run along it.
    Each myosin filament is a polymer of monomers called meromyosin, each having a globular head with a short arm (heavy meromyosin, HMM) and a tail (light meromyosin, LMM).
    Actin carries the active binding sites for myosin, masked at rest by a subunit of troponin; the myosin globular head is an active ATPase with binding sites for ATP and active sites for actin.
    Thin filaments are firmly attached to the 'Z' line; thick filaments are held together in the middle of the 'A' band by the 'M' line.
    (b) Red and White muscles
    Red fibres contain a high amount of the red oxygen-storing pigment myoglobin, giving them a reddish appearance; white fibres possess very little myoglobin and appear pale or whitish.
    Red fibres contain plenty of mitochondria; white fibres have only a few.
    White fibres have a high amount of sarcoplasmic reticulum; red fibres have less.
    Red fibres use the large amount of stored oxygen for ATP production and are therefore called aerobic muscles; white fibres depend on the anaerobic process for energy.
    (c) Pectoral and Pelvic girdle
    The pectoral girdle helps in the articulation of the upper limbs with the axial skeleton; the pelvic girdle articulates the lower limbs.
    Each half of the pectoral girdle consists of a clavicle and a scapula; each half of the pelvic girdle is a single coxal bone formed by the fusion of three bones — ilium, ischium and pubis.
    The scapula is a large triangular flat bone in the dorsal part of the thorax between the second and seventh ribs, bearing a spine that projects as the acromion, with which the clavicle articulates.
    The pectoral girdle bears the glenoid cavity (below the acromion), which articulates with the head of the humerus to form the shoulder joint; the pelvic girdle bears the acetabulum, at the point of fusion of the three bones, to which the thigh bone articulates.
    The two halves of the pelvic girdle meet ventrally to form the pubic symphysis, containing fibrous cartilage.

  6. Exercise 17.6

    Match Column I with Column II : Column I
    (a)
    Smooth muscle
    (b)
    Tropomyosin
    (c)
    Red muscle
    (d)
    Skull Column II
    (i)
    Myoglobin
    (ii)
    Thin filament
    (iii)
    Sutures
    (iv)
    Involuntary

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    (a) - (iv) — Smooth muscle is involuntary: visceral (smooth) muscle is not under the voluntary control of the nervous system.
    (b) - (ii) — Tropomyosin runs along the actin filament, i.e., it is a protein of the thin filament.
    (c) - (i) — Red muscle owes its reddish colour to its high content of the oxygen-storing pigment myoglobin.
    (d) - (iii) — The flat bones of the skull are joined end-to-end by fibrous joints in the form of sutures, forming the cranium.

  7. Exercise 17.7

    What are the different types of movements exhibited by the cells of human body?

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    Cells of the human body exhibit three main types of movement: amoeboid, ciliary and muscular.
    Amoeboid movement — shown by specialised cells such as macrophages and leucocytes; it is effected by pseudopodia formed by the streaming of protoplasm (as in Amoeba), and cytoskeletal elements like microfilaments are also involved.
    Ciliary movement — occurs in most internal tubular organs lined by ciliated epithelium; coordinated cilia in the trachea remove dust particles and foreign substances inhaled with air, and ciliary movement also facilitates the passage of ova through the female reproductive tract.
    Muscular movement — movement of the limbs, jaws, tongue, etc.; it uses the contractile property of muscles and is the basis of locomotion and most other movements.

  8. Exercise 17.8

    How do you distinguish between a skeletal muscle and a cardiac muscle?

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    Location: skeletal muscles are closely associated with the skeletal components of the body; cardiac muscles are the muscles of the heart.
    Organisation: a skeletal muscle is made of a number of muscle bundles (fascicles) held together by a common collagenous connective tissue layer, the fascia; many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle.
    Control: skeletal muscles are under the voluntary control of the nervous system and are hence voluntary muscles; cardiac muscles are involuntary, as the nervous system does not control their activities directly.
    Function: skeletal muscles are primarily involved in locomotory actions and changes of body posture; cardiac muscle brings about the working of the heart.
    Note: appearance does not separate them — both are striated, so the distinction rests on location, branching and nature of regulation.

  9. Exercise 17.9

    Name the type of joint between the following:-
    (a)
    atlas/axis
    (b)
    carpal/metacarpal of thumb
    (c)
    between phalanges
    (d)
    femur/acetabulum
    (e)
    between cranial bones
    (f)
    between pubic bones in the pelvic girdle

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    (a) atlas/axis — pivot joint (a synovial joint; the book names this pair as its example of a pivot joint).
    (b) carpal/metacarpal of thumb — saddle joint (synovial).
    (c) between phalanges — hinge joint (synovial, allowing movement in one plane; the book's named hinge example is the knee joint).
    (d) femur/acetabulum — ball and socket joint (synovial; the head of the thigh bone sits in the acetabulum of the coxal bone).
    (e) between cranial bones — fibrous joint, in the form of sutures; these flat skull bones fuse end-to-end and allow no movement.
    (f) between pubic bones in the pelvic girdle — cartilaginous joint, the pubic symphysis, which contains fibrous cartilage.

  10. Exercise 17.10

    Fill in the blank spaces:
    (a)
    All mammals (except a few) have __________ cervical vertebra.
    (b)
    The number of phalanges in each limb of human is __________
    (c)
    Thin filament of myofibril contains 2\displaystyle 2 ‘F’ actins and two other proteins namely __________ and __________.
    (d)
    In a muscle fibre Ca++\displaystyle Ca^{++} is stored in __________
    (e)
    __________ and __________ pairs of ribs are called floating ribs.
    (f)
    The human cranium is made of __________ bones.

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    (a) seven ($\displaystyle 7$) — the number of cervical vertebrae is seven in almost all mammals, including human beings.
    (b) $\displaystyle 14$ — the phalanges (digits) number $\displaystyle 14$ in each limb.
    (c) tropomyosin and troponin — two filaments of tropomyosin run along the 'F' actins, and the complex protein troponin is distributed at regular intervals on the tropomyosin.
    (d) the sarcoplasmic reticulum — the endoplasmic reticulum of the muscle fibre is the store house of calcium ions.
    (e) 11th and 12th — these last two pairs of ribs are not connected ventrally and are called floating ribs.
    (f) $\displaystyle 8$ — the cranial bones are $\displaystyle 8$ in number and form the hard protective covering for the brain.