Directions: In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true.
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Assertion: Visceral muscles are smooth in appearance.
Reason: Many muscle cells assemble in a branching pattern to form a visceral muscle.
(c) Visceral muscles are located in the inner walls of hollow visceral organs of the body like the alimentary canal, reproductive tract, etc. They do not exhibit any striation and are smooth in appearance. Hence, they are called smooth muscles (non-striated muscle). Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle.
Assertion: Muscle fibre is a syncitium.
Reason: Muscle fibre has a large number of parallelly arranged myofilaments in the sarcoplasm.
(b) Muscle fibre is a syncitium as the sarcoplasm contains many nuclei.
A characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils. Each myofibril has alternate dark and light bands on it.
Assertion: Biceps and triceps are antagonistic muscles.
Reason: The biceps flexes the arm and the triceps straightens the arm.
(a) Some muscles are arranged in sets of two and act in opposition to bring movement. Such muscles are called antagonistic muscles. For example, the biceps bends or flexes the arm and its antagonist, the triceps straightens or extends the arm.
Assertion: Mechanism of muscle contraction is explained by sliding-filament theory.
Reason: Contraction of muscle fibre takes place by the sliding of thick filaments over the thin filaments.
(c) Mechanism of muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
Assertion: On stimulation, a muscle cell releases calcium ions \(\left(\mathrm{Ca}^{2+}\right)\) from sarcoplasmic reticulum.
Reason: Calcium binds with a subunit of troponin on actin filament and thereby remove masking of active sites for myosin.
(b) A neural signal on reaching the neuromuscular junction releases a neurotransmitter (acetylcholine) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions from sarcoplasmic reticulum into the sarcoplasm. Increase in \(\mathrm{Ca}^{2+}\) level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby removing the mask of active sites for myosin.Â
Assertion: The portion of the myofibril between two successive Z-lines is considered as the functional unit of contraction called sarcomere.
Reason: During contraction, I-bands get reduced whereas A-bands retain the length, thereby causing shortening of the sarcomere.
(b) Each myofibril contains actin and myosin filaments which give it striated appearance due to dark and light bands. The light bands are made of actin and are called I-bands or isotropic band, whereas the dark bands are called ‘A’ or anisotropic bands and contain myosin. Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils. In the centre of each I-band is an elastic fibre called Z-line which bisects it. The portion of the myofibril between two successive Z-lines is considered as the functional unit of contraction and is called sarcomere. During contraction, the Z-lines attached to the actins are pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. Hence, during shortening of the muscle, i.e., contraction, the I-bands get reduced, whereas the A-bands retain the length.
Assertion: A person undergoes fatigue very soon during exercise.
Reason: Muscle fibres undergo oxygen debt during exercise.
(b) During strenuous exercise, the muscle does not get sufficient oxygen to meet its energy requirement immediately. So, muscle contracts in the absence of oxygen. But it gets fatigued sooner in the absence of oxygen as the metabolic products of cellular respiration mainly lactic acid accumulates. The accumulation of lactic acid leads to muscle fatigue. After strenuous exercise, the oxygen consumption of muscle exceeds. The extra oxygen consumed during recovery is called oxygen debt of the muscle. It is used in oxidising the accumulated lactic acid aerobically and in restoring the depleted creatine phosphate and ATP in the muscle fibre. A small part of oxygen debt also goes to myoglobin which binds and stores oxygen for future use.
Assertion: Red muscles depend on anaerobic process for energy.
Reason: Red muscles have large number of mitochondria in them.
(d) Muscle contains a red coloured oxygen storing pigment called myoglobin. Myoglobin content is high in some of the muscles which gives a reddish appearance to them. Such muscles are called the red fibres. These muscles also contain plenty of mitochondria which can utilise the large amount of oxygen stored in them for ATP production. These muscles, therefore, can also be called aerobic muscles.
Assertion: Bone has very hard matrix whereas cartilage has pliable matrix.
Reason: Bone has calcium salts in its matrix whereas cartilage has chondroitin salts in its matrix.
(a) Bone is hard as its matrix has an inflexible material, the ossein. Matrix of the bone also contains calcium and magnesium salts which also add in building rigidity of the tissue. Cartilage is soft and pliable as its matrix has a flexible material, the chondrin. Matrix of the cartilage chiefly consists of chondroitin sulphate, that is capable of withstanding considerable pressure.
Assertion: Human has dicondylic skull.
Reason: Skull articulates with superior region of the vertebral column with the help of two occipital condyles.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
Assertion: First seven pairs of ribs are called true ribs.
Reason: True ribs are not connected ventrally to the sternum.
(c) First seven pairs of ribs are called true ribs. Dorsally, they are attached to the thoracic vertebrae and ventrally connected to the sternum with the help of hyaline cartilage.
Assertion: Ulna is longer than radius.
Reason: Ulna has large olecranon process.
(b) Ulna is longer than radius. Its upper end has a larger olecranon process that forms the eminence of our elbow. The head of the radius articulate with the humerus.
Assertion: Fibrous joints play a significant role in locomotion.
Reason: Fibrous joints are present in bones of skull.
(d) Fibrous joints do not allow any movement. This type of joint is found in flat skull bones which fuse end-to-end with the help of dense fibrous connective tissues in the form of sutures, to form the cranium.
Assertion: The joint between the atlas and axis is an example of gliding joint.
Reason: Gliding joint is a monoaxial joint.
(d) A gliding joint permits back-and-forth and side-to-side movements. No rotation or twisting is possible because the bones are packed closely together or held in place by ligaments. Gliding joints are found between the carpal bones and between the tarsal bones. The joint between the atlas and axis is an example of pivot joint.
Assertion: Tetany is rapid spasm in muscle.
Reason: Tetany is usually caused by an increase in the blood calcium level.
(c) Tetany is a spasm and twitching of the muscles, particularly of face, hands and feet. Rapid spasms (wild contractions) in muscle is due to low \(\mathrm{Ca}^{2+}\) in body fluid which may be due to an underactive parathyroid gland.
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