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 Assertion is true but Reason is false.
(d) If both Assertion and Reason are false.
Q.1. Assertion : There are similarities between the locomotion of unicellular organisms and multicellular animals.
Reason : Ciliary, flagellar and amoeboid movements occur in unicellular organisms.
AnswerAnswer: (b) Main movement found in unicellular organisms are ciliary, flagellar and amoeboid movements. In multicellular animals also, phagocytes migrate through tissues by amoeboid movements. Ciliary movement of cells lining the upper respiratory tract, fallopian tubes and vasa efferentia of testes transport, respectively dust particles, ova and sperms in specific direction in those organs. Mammalian sperms move into the female reproductive tract by flagellar movements.
Q.2. Assertion: Muscle fibre is a syncitium.
Reason: Muscle fibre has a large number of parallelly arranged myofilaments in the sarcoplasm.
AnswerAnswer: (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.
Q.3. Assertion: Visceral muscles are smooth in appearance.
Reason: Many muscle cells assemble in a branching pattern to form a visceral muscle.
AnswerAnswer: (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 (nonstriated muscle). Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle.
Q.4. Assertion: Locomotion in Hydra is carried out by two types of contractile cells.
Reason: Muscle fibres are lacking in Hydra.
AnswerAnswer: (a) As muscle fibres are lacking in Hydra, the animal uses two types of contractile cells for this purpose. Processes of these cells run in the body wall both along the long axis of the body and arround the central body cavity. Contraction and relaxation of these cells respectively, shorten and elongate these processes. They consequently cause all types of movement of Hydra including shortening, elongation and also bending of body and tentacular movement. Locomotion is carried out by somersaults looping.
Q.5. 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.
AnswerAnswer: (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 Ibands or isotropic band, whereas the dark bands are called ‘A’ or anisotropic bands and contain myosin. Both the proteins are arranged as rod-like structres, 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 a 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.
Q.6. Assertion : Extra oxygen consumption in human body is known as oxygen debt.
Reason : The extra oxygen is required by the body to oxidise the accumulated lactic acid produced during strenous exercise.
AnswerAnswer: (b) During strenous exercise, the muscle does not get sufficient oxygen to meet its energy needs immediately. So, it contracts anaerobically and accumulates lactic acid. During recovery, the oxygen consumption of the muscle by far exceeds than in the resting state. The extra oxygen consumed during recovery is called oxygen debt of the muscle.
Q.7. Assertion : Biceps and triceps are called antagonistic muscles.
Reason : This is due to the fact that they contract and relax together.
AnswerAnswer: (c) Antagonistic muscles are those muscles which contract to produce opposite movements at the same joint. When a muscle contracts to produce a movement, its antagonist must relax to allow that movement to take place. Biceps and triceps are called antagonistic muscles. Because during flexion at the elbow, biceps contracts and triceps relaxes; during extension at the same joint, triceps contract and biceps relax.
Q.8. Assertion : Muscle contraction force increases with rise in strength of stimulus.
Reason : This is due to increased contraction of individual muscle fibres with increase in stimulus strength.
AnswerAnswer: (c) The muscle fibre always contracts with the maximum force and this force doesn’t rise on increasing the strength of the stimulus. If the stimulus is of strength below the threshold, then the muscle fibre doesn’t contract at all. This is known as all or none law. But the entire muscle doesn’t obey this law, it means that force of contraction of muscle increase with rise in strength of the stimulus. This is due to the fact that the strength of the threshold stimulus varies from muscle fibre to muscle fibre in a muscle.
Q.9. Assertion: Fatigue is inability of muscle to relax.
Reason: It is due to lactic acid accumulation by repeated contractions. [AIIMS 1998]
AnswerAnswer: (a) Due to the accumulation of lactic acid, muscles do not respond to a stimuli after a prolonged previous activity
Q.10. Assertion : The phase of muscle contraction occurs when myosin binds and releases actin.
Reason : Muscle contraction is initiated by a signal sent by the peripheral nervous system via motor neuron. [AIIMS 2016]
AnswerAnswer: (c) The phase of muscle contraction occurs when myosin binds and releases actin. Muscle contraction is initiated by a signal sent by the central nervous system via a motor neuron. A motor neuron along with the muscle fibres connected to it constitutes a motor unit.
Q.11. 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.
AnswerAnswer: (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 actin filaments over the thick myosin filaments.
Q.12. Assertion: On stimulation, a muscle cell releases calcium ions (Ca2+) from sarcoplasmic reticulum.
Reason: By reacting with a protein complex, Ca2+ uncover active sites on the actin filaments.
AnswerAnswer: (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 Ca2+ 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.
Q.13. Assertion: A person becomes fatigue very soon during exercise.
Reason: Muscle fibres undergo oxygen debt during exercise.
Q.14. Assertion: Red muscles depend on anaerobic process for energy.
Reason: Red muscles have few number of mitochondria in them.
AnswerAnswer: (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.
Q.15. Assertion: Rapid spasm in muscle is termed as tetany.
Reason: Tetany is usually caused by an increase in the blood calcium level.
AnswerAnswer: (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 Ca2+ in body fluid which may be due to underactive parathyroid gland.
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