What is the role of calcium in muscle contraction?
Calcium binds to tropomyosin to expose the myosin-binding sites on actin.
Calcium is released from the sarcoplasmic reticulum to initiate the sliding of actin and myosin filaments.
Calcium activates the motor neurons to stimulate muscle contraction.
Calcium is required for the relaxation of muscles after contraction.
Correct Answer : B
Muscle contraction is a complex process that involves the interaction between actin and myosin filaments in the muscle fibers. The sliding of these filaments is initiated by the release of calcium ions from the sarcoplasmic reticulum, a specialized organelle in muscle cells. The calcium ions bind to the protein troponin, which causes a conformational change in the troponin-tropomyosin complex, exposing the myosin-binding sites on actin. This allows the myosin heads to bind to actin, forming cross-bridges that pull the actin filaments towards the center of the sarcomere, resulting in muscle contraction.
Option a) is incorrect because calcium does not bind to tropomyosin directly, but rather binds to the protein troponin, causing a conformational change in the troponin-tropomyosin complex. Option c) is incorrect because calcium does not activate motor neurons, but rather is released from the sarcoplasmic reticulum in response to an action potential that travels down the motor neuron to the neuromuscular junction. Option d) is incorrect because calcium is required for muscle contraction, not relaxation. The relaxation of muscles after contraction is due to the active transport of calcium ions back into the sarcoplasmic reticulum, which allows the troponin-tropomyosin complex to return to its resting conformation, blocking the myosin-binding sites on actin and ending the cross-bridge cycle.
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Correct Answer is A
Explanation
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Adaptive immunity, on the other hand, is developed over time and provides specific protection against particular pathogens. It involves the recognition of antigens, which are specific components of pathogens, by immune cells called lymphocytes. The lymphocytes then produce antibodies that are specific to the antigens, allowing for a targeted response to the pathogen. This process takes time to develop, as the immune system needs to encounter the pathogen and mount a response.
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Correct Answer is D
Explanation
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Here’s why the other options are not suitable treatments:
- A. Methanol oxidase, which would increase the rate of the reaction: This would not be a treatment; it would worsen the situation by promoting the conversion of methanol to toxic formaldehyde.
- B. Methanol, which would saturate the methanol oxidase: This option would also be harmful, as adding more methanol would only lead to more formaldehyde production.
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Thus, administering ethanol is an effective treatment to prevent the toxic effects of methanol metabolism.
Correct Answer is C
Explanation
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The other options are not related to acne:
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- B. Sudoriferous glands: These are sweat glands, which are involved in perspiration, not typically linked to acne.
- D. Ceruminous glands: These are found in the ear canal and produce earwax, not involved in facial skin health.
Correct Answer is B
Explanation
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- A. It decreases the pH of the solution: This is incorrect because a buffer does not always decrease pH; it resists changes in both directions.
- C. It causes the pH of a solution to become neutral: Incorrect because buffers do not necessarily make a solution neutral; they stabilize pH around a certain value.
- D. It permanently binds hydrogen ions: Incorrect as the binding is reversible, which is essential for maintaining pH balance.
Correct Answer is D
Explanation
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- Neutralization Reaction: Acid + Base → Water + Salt
- In this case:
- Acid: HCl (hydrochloric acid)
- Base: Mg(OH)₂ (magnesium hydroxide)
- Products: H₂O (water) and MgCl₂ (magnesium chloride)
The other options do not apply:
- A. Decomposition: A single compound breaks down into two or more substances. Not the case here.
- B. Combustion: A substance reacts with oxygen, often producing heat and light (usually with organic compounds). Not the case here.
- C. Synthesis: Two or more substances combine to form a single product. Not applicable to this reaction.
Correct Answer is B
Explanation
Muscle contraction is a complex process that involves the interaction between actin and myosin filaments in the muscle fibers. The sliding of these filaments is initiated by the release of calcium ions from the sarcoplasmic reticulum, a specialized organelle in muscle cells. The calcium ions bind to the protein troponin, which causes a conformational change in the troponin-tropomyosin complex, exposing the myosin-binding sites on actin. This allows the myosin heads to bind to actin, forming cross-bridges that pull the actin filaments towards the center of the sarcomere, resulting in muscle contraction.
Option a) is incorrect because calcium does not bind to tropomyosin directly, but rather binds to the protein troponin, causing a conformational change in the troponin-tropomyosin complex. Option c) is incorrect because calcium does not activate motor neurons, but rather is released from the sarcoplasmic reticulum in response to an action potential that travels down the motor neuron to the neuromuscular junction. Option d) is incorrect because calcium is required for muscle contraction, not relaxation. The relaxation of muscles after contraction is due to the active transport of calcium ions back into the sarcoplasmic reticulum, which allows the troponin-tropomyosin complex to return to its resting conformation, blocking the myosin-binding sites on actin and ending the cross-bridge cycle.
Correct Answer is A
Explanation
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Correct Answer is C
Explanation
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Here’s why the other options are not correct in the context of water transport:
- A. Gated channels: While aquaporins can be gated, this term generally refers to channels that open and close in response to specific signals, which is not the primary mechanism for water transport in most cells.
- B. Electrochemical gradients: This term relates to the combined effect of electrical and chemical gradients across a membrane, typically for ions rather than water molecules directly. Water movement can be influenced by osmotic gradients but is not solely dependent on electrochemical gradients.
- D. Proton pumps: These are involved in transporting protons (H⁺ ions) across membranes, primarily for establishing an electrochemical gradient, not for the transport of water.
Thus, water molecules enter cells mainly by facilitated diffusion through aquaporins.
Correct Answer is C
Explanation
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