Question

1. Which of the following is the correct sequence that describes the excitation and contraction of a skeletal muscle fiber?

   1. Tropomyosin shifts and unblocks the cross-bridge binding sites.

   2. Calcium is released and binds to the troponin complex.

   3. The sarcoplasmic reticulum is depolarized when a wave of depolarization moves from the neuromuscular junction into the cell's interior by way of the transverse tubules.

   4. The thin filaments are ratcheted across the thick filaments by the heads of the myosin molecules using energy from ATP.

   5. An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane at the neuromuscular junction.

   	a.	1 → 2 → 3 → 4 → 5
   	b.	2 → 1 → 3 → 5 → 4
   	c.	3 → 4 → 2 → 1 → 5
   	d.	5 → 3 → 1 → 2 → 4
   	e.	5 → 3 → 2 → 1 → 4

2. All of the following statements about a true hormone are correct except:

   	a.	They can be modified amino acids, peptides or steroid molecules
   	b.	They are used to communicate between different organisms.
   	c.	They are carried by the circulatory system
   	d.	They elicit specific biological responses from target cells.
   	e.	All of the above are correct statements about hormones.

Answer 1

1. The correct sequence that describes the excitation and contraction of a skeletal muscle fibre is, e.

5------->3------>2------>1-------->4

Steps involved in muscle contraction
a. Neurotransmitter acetylcholine is released by the axon terminals of a motor neuron at neuromuscular junction. Acetylcholine binds to the receptors present on the membrane of the muscle fibre and causes depolarisation by opening the sodium channels.

b. Transverse tubules transmit the wave of depolarisation into the interior of the muscle fibers causing depolarisation of the sarcoplasmic reticulum.

c. Depolarisation of the sarcoplasmic reticulum causes release of the calcium ions into the sarcoplasm. Calcium ions binds to the troponin protein.

d. Troponin binding of calcium causes displacement of the troponin tropomyosin complex revealing the myosin binding sites present on the actin.

e. Myosin heads bind to the actin and perform power stroke using ATP energy. This helps in the muscle contraction.

2. Statement b is incorrect

Hormones are not used to communicate between different organisms. They are used as a medium of communication amongst body cells within a single organism.

statements a, c, d are correct

a. Hormones can be modified amino acid, example: Thyroxine   

Hormones can be a peptide, example: Oxytocin, ADH.

Hormones can be steroid molecules, example: Testosterone, estrogen.

c. Hormones travel from the endocrine cells to the target site via blood.

d. Target cells generate specific response after recognising the hormone.