Q4. Assertion: during contraction the I bands get shorter. Reason: the thin filaments, attached to the Z lines, slide over the thick filaments towards the centre of the A band. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the sliding pulls the Z lines inwards, so the I bands shorten while the A bands keep their length (NCERT pages 222–223).
Q5. What is the neuromuscular junction also called, and which neurotransmitter is released there?
The motor-end plate; acetylcholine (NCERT page 222).
Q6. In NCERT’s labelled myosin model, which region is called heavy meromyosin?
The head with its short arm; NCERT labels the tail light meromyosin (page 221). Beyond the NCERT text, HMM and LMM are fragments obtained by cutting myosin with an enzyme, as explained in the lesson.
Q7. A muscle holds a weight still without changing its overall length. Is it inactive, and does it need ATP?
It is active. Beyond the NCERT text (Wikipedia, “Muscle contraction”, facts in our words): this is an isometric contraction, in which tension is produced without overall shortening. ATP is still needed for cross-bridge cycling and for the calcium pump (SERCA) that takes Ca back during relaxation. Contraction does not always mean a visible change in length (see Mechanism above).