NCERT Solutions for Class 11 Biology Chapter 17: Locomotion and Movement

Concept. A skeletal muscle is made of muscle bundles (fascicles) containing muscle fibres. Each fibre has many parallel myofibrils which show alternate dark and light bands. This striated appearance is due to the distribution pattern of two proteins, actin (thin filaments) and myosin (thick filaments). The portion of a myofibril between two successive Z lines is the sarcomere, the functional unit of contraction.

Regions of a sarcomere (as shown in the diagram)

1. Z line: an elastic fibre in the centre of each I band, bisecting it. Thin filaments are firmly attached to it. Two successive Z lines mark the ends of a sarcomere.
2. A band (anisotropic band): the dark band containing the thick (myosin) filaments; it also includes the overlapping ends of thin filaments. Its length remains constant during contraction.
3. I band (isotropic band): the light band containing only thin (actin) filaments; each sarcomere has two half I bands, one on either side of the A band. It shortens during contraction.
4. M line: a thin fibrous membrane in the middle of the A band which holds the thick filaments together.
5. H zone: the central part of the thick filaments, in the A band, not overlapped by the thin filaments in the resting state; it narrows or disappears in contraction.
6. Thin filaments (actin) are attached to Z lines and project towards the centre; thick filaments (myosin) are in the middle with cross arms projecting outwards.

Extra depth. The sarcomere pattern is: Z line – half I band – A band (with H zone and M line in the middle) – half I band – Z line. In a resting state the edges of the thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments, leaving the H zone.

Conclusion: A sarcomere, the region between two successive Z lines, has a central dark A band (thick myosin filaments with the M line and H zone) and two half I bands (thin actin filaments attached to the Z lines); it is the functional unit of contraction of a skeletal muscle.

Concept. The mechanism of muscle contraction is best explained by the sliding filament theory. A muscle fibre is made of myofibrils whose sarcomeres contain two sets of protein filaments: thin filaments of actin and thick filaments of myosin, arranged parallel to each other and to the long axis of the myofibril.

Definition. The sliding filament theory states that contraction of a muscle fibre takes place by the sliding of the thin filaments (actin) over the thick filaments (myosin).

1. Filaments do not shorten. The lengths of the thin and thick filaments remain unchanged; only their overlap changes.
2. Role of the myosin heads. Using energy from ATP hydrolysis, myosin heads bind to exposed active sites on actin, form cross bridges and pull the thin filaments towards the centre of the A band.
3. Z lines move inward. The Z lines attached to the thin filaments are pulled inward, so the sarcomere shortens; the I band and the H zone are reduced, but the A band retains its length.
4. Relaxation. When Ca⁺⁺ returns to the sarcoplasmic reticulum, the active sites are masked again, the cross bridges break and the Z lines return to their original positions.

Extra depth. The theory (also called the sliding filament model) explains how a muscle can shorten without any filament shortening and why the A band does not change while the I band does (Fig. 17.5).

Conclusion: According to the sliding filament theory, a muscle fibre contracts when thin actin filaments slide over thick myosin filaments through myosin cross bridges, shortening the sarcomere without changing the length of the filaments.

Concept. Muscle contraction is a highly regulated process that links the nervous system (signal), the sarcoplasmic reticulum (calcium) and the contractile proteins (actin and myosin) with ATP as the energy source. The steps are:

1. Stimulus from the CNS. Contraction is initiated by a signal sent by the central nervous system via a motor neuron. A motor neuron with the muscle fibres connected to it forms a motor unit.
2. Neuromuscular junction. The junction between the motor neuron and the sarcolemma is the neuromuscular junction (motor-end plate). The neural signal releases the neurotransmitter acetylcholine, which generates an action potential in the sarcolemma.
3. Release of calcium. The action potential spreads through the muscle fibre and causes the release of Ca⁺⁺ from the sarcoplasmic reticulum into the sarcoplasm.
4. Unmasking of actin sites. Ca⁺⁺ binds with a subunit of troponin on the actin filaments and removes the masking of the active sites for myosin.
5. Cross-bridge formation. Utilising the energy of ATP hydrolysis, the myosin head binds to the exposed active sites on actin, forming a cross bridge.
6. Sliding and shortening. The myosin heads rotate and pull the attached actin filaments towards the centre of the A band; the Z lines are pulled inward and the sarcomere shortens — contraction. I bands are reduced, A bands retain their length.
7. Breaking of the bridge. Myosin releases ADP and P_i and returns to its relaxed state; a new ATP binds and the cross bridge is broken. ATP is hydrolysed again by the myosin head and the cycle of cross-bridge formation and breakage repeats, causing further sliding.
8. Relaxation. The process continues until Ca⁺⁺ ions are pumped back into the sarcoplasmic cisternae, masking the actin filaments again; the Z lines return to their original positions and the muscle relaxes.

Fatigue and fibre types. Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen, causing fatigue. Red fibres (rich in myoglobin and mitochondria) are aerobic; white fibres (little myoglobin, much sarcoplasmic reticulum) depend on anaerobic processes.

Conclusion: Muscle contraction begins with a motor-neuron signal that releases acetylcholine, causes Ca⁺⁺ release from the sarcoplasmic reticulum, unmasks actin sites by troponin, and uses ATP for repeated cross-bridge formation and sliding of actin over myosin; pumping back of Ca⁺⁺ brings relaxation.

Concept. Each statement tests a basic fact from the sections on muscle structure and the skeletal system. For false statements the corrected form must be written.

1. (a) True. The thin filaments are the actin filaments, which are attached to the Z line; the thick filaments are myosin. Actin is lighter and thinner, hence called thin filament.
2. (b) False — corrected: “The H-zone of a striated muscle fibre represents only the thick filaments (the central part of the A band not overlapped by thin filaments).” Where thick and thin filaments overlap there is no H zone, and the H zone narrows during contraction.
3. (c) True. The human skeleton is made up of 206 bones and a few cartilages, grouped into the axial skeleton (80 bones) and appendicular skeleton.
4. (d) False — corrected: “There are 12 pairs of ribs in man”: 7 pairs of true ribs, 3 pairs of false (vertebrochondral) ribs and 2 pairs of floating ribs.
5. (e) True. The sternum lies on the ventral midline of the thorax and is joined to the first seven pairs of ribs by hyaline cartilage; dorsally the vertebral column lies opposite it.

Conclusion: Statements (a), (c) and (e) are true; (b) is false because the H-zone represents only thick filaments, and (d) is false because man has 12 pairs of ribs.

Concept. The three pairs compare the two contractile proteins, the two types of skeletal muscle fibres and the two skeletal girdles. A good answer gives definitions and at least four contrasting points for each pair in tabular form.

(a) Actin and myosin

(b) Red and white muscles

(c) Pectoral and pelvic girdle

Extra depth. Each girdle is formed of two halves. The pectoral girdle gives a wide range of movement, while the pelvic girdle is firmer for bearing the body weight.

Conclusion: Actin forms the thin, Z-line-attached filaments while myosin forms the thick filaments with ATPase heads; red muscles are myoglobin-rich aerobic fibres and white muscles are myoglobin-poor anaerobic fibres; the pectoral girdle (clavicle + scapula) joins the fore limbs and the pelvic girdle (two coxal bones) joins the hind limbs to the axial skeleton.

Concept. Each item of Column I has a key association explained in the chapter. The correct matches are shown below.

1. (a) Smooth muscle → (iv) Involuntary. Visceral muscles are located in the inner walls of hollow organs like the alimentary canal and reproductive tract; they have no striations and their activities are not under voluntary control, so they are involuntary muscles.
2. (b) Tropomyosin → (ii) Thin filament. The thin filament is composed of two F actins, two filaments of tropomyosin and the complex protein troponin.
3. (c) Red muscle → (i) Myoglobin. Muscles with high myoglobin content appear reddish and are red fibres; they also have many mitochondria and are aerobic.
4. (d) Skull → (iii) Sutures. Fibrous joints allow no movement; the flat bones of the skull are fused end-to-end by dense fibrous connective tissue in the form of sutures.

Conclusion: (a)–(iv), (b)–(ii), (c)–(i), (d)–(iii).

Concept. Movement is an essential feature of living beings. Cells of the human body exhibit three main types of movements: amoeboid, ciliary and muscular.

1. Amoeboid movement. Some specialised cells in the body, like macrophages and leucocytes in blood, show amoeboid movement. It is effected by pseudopodia formed by the streaming of protoplasm (as in Amoeba). Cytoskeletal elements like microfilaments are also involved. It helps in engulfing germs (phagocytosis) and in moving through tissues.
2. Ciliary movement. It occurs in most of our internal tubular organs lined by ciliated epithelium. Coordinated movement of cilia in the trachea removes dust particles and foreign substances inhaled with air; the passage of ova through the female reproductive tract is also facilitated by ciliary movement.
3. Muscular movement. Movement of limbs, jaws, tongue, etc. require muscular movement. The contractile property of muscles is used for locomotion and other movements by humans and the majority of multicellular organisms. Locomotion requires perfectly coordinated activity of the muscular, skeletal and neural systems.

Extra depth. Flagellar movement is another cellular movement (sperm swimming in humans; Euglena, sponges in other organisms). Movement and locomotion cannot be studied separately: all locomotions are movements but not all movements are locomotions.

Conclusion: The cells of the human body show amoeboid movement (macrophages and leucocytes), ciliary movement (ciliated epithelium of the trachea and reproductive tract) and muscular movement (limbs, jaws, tongue and other organs).

Concept. Based on location, muscles are of three types: skeletal, visceral and cardiac. Skeletal and cardiac muscles are both striated, but they differ in structure, control and function.

Common feature and key difference. Both are striated because of the regular arrangement of actin and myosin; the key difference is that skeletal muscle is voluntary and unbranched whereas cardiac muscle is involuntary and branched. Visceral (smooth) muscle is non-striated and involuntary and lines the hollow visceral organs.

Extra depth. Skeletal muscle works in association with bones and nerves for locomotion, which is why it is controlled by the somatic motor neurons; cardiac muscle is auto-regulated by the nodal tissue and moderated by the autonomic nervous system.

Conclusion: Skeletal muscle is attached to bones, striated, unbranched and voluntary, whereas cardiac muscle forms the heart wall, is striated, branched and involuntary.

Concept. Joints are points of contact between bones, or between bones and cartilages. The force generated by the muscles is used to carry out movement through joints, where the joint acts as a fulcrum. Joints are classified into three structural forms: fibrous, cartilaginous and synovial; synovial joints are further of several kinds.

1. (a) Pivot joint. NCERT lists the joint between the atlas and axis as a pivot joint: the atlas rotates around the axis; the head turns left and right.
2. (b) Saddle joint. The joint between the carpal and metacarpal of the thumb is a saddle joint, giving the thumb its wide range of movement.
3. (c) Hinge joint. The finger joints between phalanges are hinge joints, like the knee joint listed in NCERT, permitting movement in a single plane.
4. (d) Ball and socket joint. The rounded head of the femur fits into the cup-like acetabulum of the pelvic girdle and moves in all directions (as at the shoulder between humerus and pectoral girdle).
5. (e) Fibrous joint. The flat skull bones fuse end-to-end with dense fibrous connective tissue in the form of sutures; no movement is allowed.
6. (f) Cartilaginous joint. The two halves of the pelvic girdle meet ventrally at the pubic symphysis containing fibrous cartilage; movement is limited.

Extra depth. Synovial joints are characterised by a fluid-filled synovial cavity between the articulating surfaces, which allows considerable movement; hence these joints help in locomotion. Fibrous joints allow no movement, cartilaginous joints (like those between adjacent vertebrae) permit limited movement.

Conclusion: (a) Pivot, (b) saddle, (c) hinge, (d) ball and socket, (e) fibrous (sutures), (f) cartilaginous (pubic symphysis).

Concept. The six blanks test facts about the vertebral column, limb bones, contractile proteins, muscle physiology, ribs and the skull.

1. (a) Seven. Seven cervical vertebrae occur even in the giraffe; the exceptions are a few mammals such as sloths and manatees (general knowledge).
2. (b) 14. Hand: 14 phalanges; foot: 14 phalanges, so each limb of a human has 14 phalanges.
3. (c) Tropomyosin and troponin. Tropomyosin runs along the F actins and troponin, a complex protein, is distributed on it at regular intervals; in the resting state a troponin subunit masks the myosin-binding sites.
4. (d) Sarcoplasmic reticulum. Release of Ca⁺⁺ from it triggers contraction and pumping back relaxes the muscle.
5. (e) 11th and 12th. The first seven pairs are true ribs, the 8th–10th are false (vertebrochondral) ribs and the last two are floating ribs.
6. (f) Eight. The cranium, which forms the hard protective covering for the brain, has 8 bones (frontal, 2 parietal, 2 temporal, occipital, sphenoid, ethmoid).

Conclusion: (a) seven, (b) 14, (c) tropomyosin and troponin, (d) sarcoplasmic reticulum, (e) 11th and 12th, (f) 8.