Locomotion and Movement Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1A sarcomere is the structural and functional unit of a striated muscle fibre, defined as the segment between two successive Z lines.
2. 2It measures about 2.0–2.5 µm from one Z line to the next, and a single muscle fibre contains many sarcomeres in series.
3. 3Label the parts from left to right: Z line, I band (half), A band, H zone, A band, I band (half), Z line.
4. 4The I band is light and contains only thin actin filaments; the A band is dark and contains the full length of the thick myosin filaments.
5. 5The H zone is the paler central region inside the A band and contains thick filaments only, with no actin overlap.
6. 6The Z line is the boundary of the sarcomere and is the anchoring protein on which the thin filaments are fixed.
7. 7Schematic of one sarcomere, with the A band and the H zone marked as nested regions:
8. 8
   $$\underset{\text{one sarcomere, }2.0\text{–}2.5\mu \mathrm{m}}{\underbrace{Z|\underset{\text{I band: thin (actin) only}}{\underbrace{I}}|\underset{\text{A band: full length of myosin}}{\underbrace{A\supset \underset{\text{H zone: thick only}}{\underbrace{H}}}}|\underset{\text{I band}}{\underbrace{I}}|Z}}$$
9. 9The test of a correct diagram: the A band spans the entire length of the thick filament, and the I band lies outside it at each end.

Step-by-step solution

1. 1The theory was proposed by A. F. Huxley and his colleagues on the basis of electron micrographs of contracting muscle.
2. 2Its central claim is that a muscle shortens because the thin filaments slide past the thick filaments, and not because the filaments themselves become shorter.
3. 3The length of each individual actin and myosin filament therefore stays constant throughout contraction; only the degree of overlap between them changes.
4. 4The proof is that the I band, which contains actin only, narrows, and the H zone, which contains myosin only, also narrows or disappears.
5. 5The A band, which corresponds to the full length of the myosin filament, stays the same width during contraction — this is the observation the theory rests on.
6. 6As the thin filaments are pulled toward the centre of the sarcomere, the Z lines are drawn closer together and the sarcomere shortens, so the whole muscle shortens.

Step-by-step solution

1. 1A nerve impulse reaches the neuromuscular junction, depolarising the sarcolemma; the action potential then spreads over the muscle fibre and down the T tubules.
2. 2The sarcoplasmic reticulum releases Ca²⁺ into the sarcoplasm, raising the cytosolic Ca²⁺ concentration.
3. 3Ca²⁺ binds the troponin subunit of the troponin–tropomyosin complex on the thin filament, which shifts tropomyosin away from the myosin-binding sites on actin.
4. 4An energised myosin head carrying ADP and inorganic phosphate now forms a cross-bridge with the exposed active site on actin; the first cross-bridge is the axial attachment.
5. 5ATP is hydrolysed by myosin ATPase to ADP and inorganic phosphate, and the released energy energises the myosin head — this is called energisation.
6. 6The myosin head pivots and pulls the actin filament toward the centre of the sarcomere; this is the power stroke, and ADP and Pi are released.
7. 7A new ATP binds the myosin head and dissociates the cross-bridge; the head returns to its original position, ready to attach further along the actin.
8. 8Fresh cross-bridges form repeatedly and the thin filaments are pulled past one another toward the centre, so the sarcomere shortens.
9. 9Consistently, the I band and the H zone narrow while the A band stays the same width — this is the sliding filament evidence.
10. 10When the nerve stimulation stops, Ca²⁺ is pumped actively back into the sarcoplasmic reticulum at the cost of ATP, tropomyosin re-covers the binding sites on actin, and the cross-bridges break, so the muscle relaxes.
11. 11Note that the energy for each power stroke is provided by ATP, and that the myosin head stays attached in rigor mortis because no ATP is available to break the cross-bridge.

Step-by-step solution

1. 1(a) TRUE. The thin filament of a myofibril is built from two twisted F-actin filaments together with tropomyosin and troponin, so actin is indeed present in it.
2. 2(b) FALSE. The H zone is the central part of the A band and contains the thick (myosin) filaments only. The zone in which both thick and thin filaments lie together is the peripheral region of the A band, outside the H zone.
3. 3Corrected (b): The H-zone of a striated muscle fibre represents the thick filaments only.
4. 4(c) TRUE. The adult human skeleton has 206 bones, made up of 80 skeletal and 80 dental bones of the axial and appendicular skeleton combined.
5. 5(d) FALSE. Man has 12 pairs of ribs, not 11. The 1st to 7th pairs are true ribs, the 8th to 10th pairs are false or vertebrochondral ribs, and the 11th and 12th pairs are floating ribs.
6. 6Corrected (d): There are 12 pairs of ribs in man.
7. 7(e) TRUE. The sternum forms the ventral part of the rib cage, lying on the ventral or anterolateral aspect of the thorax.

Step-by-step solution

1. 1(a) Actin and Myosin — position: actin forms the thin filament, lying in the I band and the peripheral region of the A band; myosin forms the thick filament, lying in the A band and the H zone.
2. 2(a) Composition: a thin filament contains two F-actin helices wound helically, plus tropomyosin and troponin; a thick filament contains many myosin II molecules, each with a globular head.
3. 3(a) Function: actin is the track that myosin walks along and provides the binding sites; myosin is the motor protein, since only myosin has ATPase activity and its head forms the cross-bridge that produces the power stroke.
4. 4(b) Red and White muscles — colour and pigment: red muscle is red because it is rich in myoglobin, while white muscle has little or no myoglobin and so is pale.
5. 5(b) Fuel and pathway: red muscle respires aerobically and relies mainly on fatty acids, whereas white muscle respires anaerobically by lactic acid fermentation and uses glycogen.
6. 6(b) Speed, fatigue and structure: red muscle contracts slowly and is resistant to fatigue with many mitochondria and a rich blood supply; white muscle contracts rapidly, fatigues easily, has fewer mitochondria, more glycogen and a poorer blood supply.
7. 7(b) Both are striated, and both are under voluntary control.
8. 8(c) Pectoral girdle — it is made of two scapulae and two clavicles, it articulates with the forelimbs, and it is present in all vertebrates including fishes.
9. 9(c) Pelvic girdle — it is made of two innominate (coxal) bones, each of which is fused from an ilium, an ischium and a pubis; it articulates with the hind limbs.
10. 10(c) Presence and position: the pelvic girdle is absent in snakes and is present from amphibians onwards, and it lies on the ventral side, whereas the pectoral girdle lies dorsally.

Step-by-step solution

1. 1(a) Smooth muscle is not under voluntary control, so it is involuntary — a match for (iv).
2. 2(b) Tropomyosin is one of the two accessory proteins built into the thin filament, the other being troponin, so it matches (ii).
3. 3(c) Red muscle gets its red colour from the large quantity of myoglobin it stores, so it matches (i).
4. 4(d) The bones of the skull are joined to one another by sutures, which are immovable fibrous joints, so it matches (iii).
5. 5The unused item, (iv) Involuntary, is used by (a), and the unused head, Myoglobin, is used by (c); every item in both columns is used exactly once.

Step-by-step solution

1. 1There are three types of cell movement in the human body, named for the mechanism that produces them.
2. 2Amoeboid movement is produced by cytoplasmic streaming into pseudopodia. It is shown by macrophages and other white blood cells as they move out of the blood vessels to reach an infection site, and by Amoeba.
3. 3Ciliary movement is produced by the coordinated beating of cilia. It moves the ovum along the fallopian tube toward the uterus, moves mucus and dust out of the trachea, and helps move the sperm in the seminiferous tubules.
4. 4Muscular movement is produced by the contraction of muscle cells, and it accounts for the movement of limbs, the tongue, the jaws, and the beating of the heart.

Step-by-step solution

1. 1Both are striated and both use the same actin–myosin sliding filament machinery, so striation itself is not a distinguishing feature.
2. 2Control: a skeletal muscle is under voluntary control, whereas a cardiac muscle is involuntary and cannot be willed to act.
3. 3Position: a skeletal muscle is attached to bones, while a cardiac muscle forms the myocardium of the heart wall only.
4. 4Nuclei: a skeletal fibre is multinucleate with many peripheral nuclei, whereas a cardiac cell usually has one centrally placed nucleus, occasionally two.
5. 5Intercalated discs: a cardiac muscle has intercalated discs joining the ends of adjacent cells, which a skeletal muscle lacks.
6. 6Energy supply: a cardiac muscle has a very rich blood supply and many mitochondria and depends on aerobic respiration continuously, whereas a skeletal muscle has fewer mitochondria and can use anaerobic glycolysis during a sprint.
7. 7Fatigue: a cardiac muscle is non-fatigable and must contract throughout life without rest, whereas a skeletal muscle is fatigable.
8. 8Glycogen: a skeletal muscle stores abundant glycogen, while a cardiac muscle stores very little.

Step-by-step solution

1. 1A synovial joint is characterised by the presence of a joint cavity, articular cartilage covering the ends and a synovial membrane; a fibrous joint has no cavity and does not move, and a cartilaginous joint moves only slightly.
2. 2(a) The atlas–axis joint is a pivot (trochoid) synovial joint, which allows the head to rotate from side to side, as in saying no.
3. 3(b) The joint between a carpal bone and the metacarpal of the thumb is a saddle joint, which allows movement in two planes and gives the thumb its grip — this joint is absent from the other fingers.
4. 4(c) The joint between phalanges is a hinge synovial joint, which allows movement in only one plane, as in the fingers and the knee.
5. 5(d) The femur–acetabulum joint is a ball and socket joint, the most freely movable joint of the body, allowing movement in all directions and giving stability at the hip.
6. 6(e) The joint between the bones of the cranium is a suture, a fibrous immovable joint, which is what makes the skull a rigid box protecting the brain.
7. 7(f) The two pubic bones are joined at the anterior end by the pubic symphysis, a secondary cartilaginous joint, which allows only slight movement and is what permits the pelvis to widen slightly during childbirth.

Step-by-step solution

1. 1(a) One. All mammals except a few, such as the sloth and the manatee, have a single cervical vertebra, the atlas, which allows great mobility of the head; a few mammals such as the giraffe instead have seven.
2. 2(b) 14. Each limb carries fourteen phalanges, arranged as two in the thumb or great toe and three in each of the remaining four digits.
3. 3(c) Troponin and tropomyosin. The two F-actin filaments are accompanied by tropomyosin, which lies in the groove of the actin helix, and by troponin, which is distributed at intervals on the thin filament.
4. 4(d) The sarcoplasmic reticulum. This is a specialised endoplasmic reticulum that stores and releases Ca²⁺, and the release of Ca²⁺ from it is the trigger for contraction.
5. 5(e) The 11th and 12th. These two pairs have no connection to the sternum in front and are therefore called floating ribs; pairs 1 to 7 are true ribs, 8 to 10 are false ribs, and 11 to 12 are floating ribs.
6. 6(f) 8. The cranium is formed of eight bones — frontal, paired parietal, paired temporal, occipital, ethmoid and sphenoid — and it encloses and protects the brain.