The Cross Bridge Cycle

Q. Consider the following statements and choose the incorrect option
A. Large and more numerous nucleoli are present in cells actively carrying out protein synthesis.
B. Nuclear pores allow bidirectional movement of molecules.
C. Cytoskeleton is a glycolipid structure for mechanical support, motility and maintenance of the shape of the cell.
D. Steroidal hormones are synthesized by Golgi complex.

1. B and C
2. C and D
3. A and D
4. A and B

Q. Arrange the order of events in the cross-bridge cycle:
(i) The myosin head pulls the actin filament towards the M-line.
(ii) The myosin head releases ADP and phosphate.
(iii) The myosin head hydrolyses ATP.
(iv) The myosin head binds actin.

1. i, ii, iii, iv
2. i, iii, ii, iv
3. ii, iii, i, iv
4. iii, iv, i, ii

Q. The globular head of mysosin has:
I. ATP binding site
II. Actin binding site
III. Myosin binding site
IV. Troponin binding site
V. Calcium binding

1. I and II
2. I and III
3. I, III, and V
4. I, III and IV

Q. In a resting muscle fibre, troponin partially covers

1. Ca binding sites on actin
2. Ca binding sites on troponin
3. actin binding site on myosin
4. myosin binding site on actin

Q. Which of the following statements is/are correct?
i. Contraction of muscle is an active process.
ii. Relaxation of muscle is an active process.
iii. Calcium is stored in sarcoplasmic reticulum.

1. Both i and ii
2. Both i and iii
3. Only iii
4. Only i

Q. Assertion: Calcium ions play a crucial role in muscle contraction.
Reason: Calcium ions bind troponin causing the displacement of tropomyosin, allowing the myosin head to bind actin.
Select the appropriate option:

1. Both assertion and reason are correct statements and reason is the correct explanation of the assertion
2. Both assertion and reason are correct statements and reason is an incorrect explanation of the assertion
3. Only assertion is correct
4. Both assertion and reason are incorrect

Q.

What is treadmilling?

Q. During muscle contraction, which of the following shortens?
(i) A band
(ii) I band
(iii) Sarcomere

1. i and ii
2. ii and iii
3. i and iii
4. i, ii and iii

Q.

During contraction of muscles

1. actin filament slide

2. actin filament slide over myosin

3. myosin filament slide over actin

4. myosin filament slide over myosin.

Q.

How does a muscle shorten during its contraction and return to its original form during relaxation?

Q. During the resting state, the myosin binding sites on actin filaments are blocked by

1. troponin
2. tropomyosin
3. troponin-tropomyosin complex
4. none of these

Q. What happens when the calcium ion level increases in the sarcoplasm?

1. Filament sliding occurs
2. Filament sliding stops
3. Muscle relaxes
4. No change in muscle state

Q. During the muscle contraction, which zone decreases?

1. H-zone
2. M-zone
3. I-zone
4. Z-zone

Q. With which of the following molecules does $C{a}^{2+}$ released from the sarcoplasmic reticulum bind?

1. Actin
2. Troponin
3. Tropomyosin
4. Myosin

Q.

According to the sliding filament mechanism of skeletal-muscle contraction, during contraction:

1. Both thick and thin filaments shorten.

2. The thick filaments stay the same size but the thin filaments shorten.

3. The sarcomeres shorten.

4. The thin filaments stay the same size but the thick filaments shorten.

Q. The immediate source of energy for muscle contraction is

1. ATP
2. phosphocreatine
3. glycogen
4. phosphocreatine kinase

Q. During the power stroke phase of the cross-bridge cycle

1. The myosin head pivots toward the middle of the sarcomere and pulls the attached actin filament with it.
2. The actin head pivots toward the middle of the sarcomere and pulls the attached myosin filament with it.
3. Ca++ ions are removed from the sarcoplasm
4. Crossbridges rotate so that the myosin filaments fold in half

Q. When is ATP formed in glycolysis?

Q. Write all tautomeric structures of CH3COCH2COCH3 and arrange them in order of their increasing stability

Q. Extremely low levels of ${\mathrm{Ca}}^{2+}$ may result in

1. loss of action potential reaching the muscle fibre
2. loss of muscle contraction
3. failure of dissociation of troponin from tropomyosin
4. both b and c

Q. The globular head of mysosin has:
I. ATP binding site
II. Actin binding site
III. Myosin binding site
IV. Troponin binding site
V. Calcium binding

1. I and II
2. I and III
3. I, III and IV
4. I, III, and V

Q. As per the sliding filament theory, during muscle contraction the ___________ filament slides over the thick filament.

1. actin
2. myosin
3. collagen
4. creatine

Q. Chemical ions responsible for muscle contraction are

1. $C{a}^{++}$ and $K^{+}$
2. $N{a}^{+}$ and $K^{+}$
3. $N{a}^{+}$ and $C{a}^{++}$
4. $C{a}^{++}$ and $M{g}^{++}$ ions

Q. Arrange the order of events in the cross-bridge cycle:
(i) The myosin head pulls the actin filament towards the M-line.
(ii) The myosin head releases ADP and phosphate.
(iii) The myosin head hydrolyses ATP.
(iv) The myosin head binds actin.

1. i, ii, iii, iv
2. i, iii, ii, iv
3. ii, iii, i, iv
4. iii, iv, i, ii

Q. In a resting muscle fibre, troponin partially covers

1. Ca binding sites on actin
2. Ca binding sites on troponin
3. actin binding site on myosin
4. myosin binding site on actin

Q. Muscle band that remains unchanged during contraction and relaxation of skeletal muscle is

1. I
2. H
3. A
4. Z-line
5. H and Z lines

Q. The globular head of mysosin has
I. ATP binding site
II. Actin binding site
III. Myosin binding site
IV. Troponin binding site
V. Calcium binding

1. I and II
2. I and III
3. I, III and IV
4. I, III, and V

Q.

What is the correct sequence of events in the cross bridge formation?

i. The power stroke

ii. Cross bridge formation

iii. Myosin head is activated by ATP

iv. Calcium released binds to the troponin unblocks the actin sites

v. Cross bridge detachment

vi. Reactivation of the myosin head

1. iv, iii, ii, i, v, vi.

2. i, v, ii, iii, iv, vi.

3. iii, iv, i, v, ii, vi.

4. ii, i, iv, iii, v, vi.

Q. Match column I with column, option from. the codes given below:

Column I	Column II
A. Structural and functional -unit of a myofibril	(i) H-zone
B. Protein of thin filament	(ii) Myosin
C. Protein of thick filament	(iii) Sarcomere
D. The central part of thick filament not overlapped by thin filament	(iv) Actin

1. A-(i), B-(ii), C, (iii), D-(iv)
2. A-(i), B-(iii), C-(ii), D-(iv)
3. A-(i), B-(iv), C-(iii), D-(ii)
4. A-(iii), B-(iv), C-(ii), D-(i)

Q. The role of calcium in muscle contraction is

1. To break the cross bridges as a cofactor in the hydrolysis of ATP
2. To bind with troponin, changing its shape so that the actin filament is exposed
3. To transmit the action potential across the neuromuscular junction
4. To re-establish the polarization of the plasma membrane following an action potential