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Q3:
Describe the important steps in muscle contraction.
Solution :
Theoretical Foundation: The Sliding Filament Theory
Muscle contraction is governed by the Sliding Filament Theory, which posits that contraction occurs as the thin actin filaments slide over the thick myosin filaments, thereby shortening the sarcomere without changing the length of the individual filaments [Per Huxley and Hanson, 1954].
Step 1: Signal Transduction and Neuromuscular Activation
The process initiates when a neural signal reaches the Neuromuscular Junction (NMJ). A motor neuron releases the neurotransmitter Acetylcholine (ACh) into the synaptic cleft [Mechanism of Synaptic Transmission].
- ACh binds to receptors on the sarcolemma, generating an Action Potential.
- This electrical impulse propagates throughout the muscle fiber via the T-tubules.
Step 2: Calcium Ion Release and Exposure of Active Sites
The action potential triggers the sarcoplasmic reticulum to release $Ca^{2+}$ ions into the sarcoplasm [By the Calcium-Induced Calcium Release principle].
- $Ca^{2+}$ binds to the protein Troponin on the thin filament.
- This binding induces a conformational shift in the Tropomyosin complex, physically moving it away from the active binding sites on the F-actin filaments [Structural Configuration of Thin Myofilaments].
Step 3: Cross-Bridge Formation
With the actin active sites now exposed, the myosin head (which has already hydrolyzed $ATP$ into $ADP + Pi$) binds to the actin to form a Cross-Bridge.
Step 4: The Power Stroke and Filament Sliding
The release of the inorganic phosphate ($Pi$) and $ADP$ triggers the Power Stroke. The myosin head pivots, pulling the attached actin filament toward the center of the A-band (M-line).
Step 5: Detachment and Reactivation
A new molecule of $ATP$ binds to the myosin head, causing the cross-bridge to detach [ATP-dependent dissociation]. The myosin head hydrolyzes the $ATP$, returning to its "cocked" high-energy state to repeat the cycle as long as $Ca^{2+}$ levels remain elevated.
Summary of Sarcomere Dynamics
| Component | During Contraction |
|---|---|
| I-Band | Shortens |
| H-Zone | Shortens/Disappears |
| A-Band | Remains Constant [Law of Conservation of Filament Length] |
| Z-Lines | Move closer together |
Final Solution: Muscle contraction proceeds through the neural excitation of the sarcolemma, the release of $Ca^{2+}$ from the sarcoplasmic reticulum, the formation of actin-myosin cross-bridges, and the cyclic power strokes fueled by $ATP$ hydrolysis, which collectively slide the actin filaments inward to shorten the sarcomere.
More Questions from Class 11 Biology Locomotion and Movement EXERCISES
- Q1: Draw the diagram of a sarcomere of skeletal muscle showing different regions.
- Q2: Define sliding filament theory of muscle contraction.
- Q4(a): Write true or false. If false change the statement so that it is true. (a) Actin is present in thin filament
- Q4(b): Write true or false. If false change the statement so that it is true. (b) H-zone of striated muscle fibre represents both thick and thin filaments.
- Q4(c): Write true or false. If false change the statement so that it is true. (c) Human skeleton has 206 bones.
- Q4(d): Write true or false. If false change the statement so that it is true. (d) There are 11 pairs of ribs in man.
- Q4(e): Write true or false. If false change the statement so that it is true. (e) Sternum is present on the ventral side of the body.
- Q5(a): Write the difference between : (a) Actin and Myosin
- Q5(b): Write the difference between : (b) Red and White muscles
- Q5(c): Write the difference between : (c) Pectoral and Pelvic girdle
- Q6: Match Column I with Column II : Column I Column II (a) Smooth muscle (i) Myoglobin (b) Tropomyosin (ii) Thin filament (c) Red muscle (iii) Sutures (d) Skull (iv) Involuntary
- Q7: What are the different types of movements exhibited by the cells of human body?
- Q8: How do you distinguish between a skeletal muscle and a cardiac muscle?
- Q9(a): Name the type of joint between the following:- (a) atlas/axis
- Q9(b): Name the type of joint between the following:- (b) carpal/metacarpal of thumb
- Q9(c): Name the type of joint between the following:- (c) between phalanges
- Q9(d): Name the type of joint between the following:- (d) femur/acetabulum
- Q9(e): Name the type of joint between the following:- (e) between cranial bones
- Q9(f): Name the type of joint between the following:- (f) between pubic bones in the pelvic girdle
- Q10(a): Fill in the blank spaces: (a) All mammals (except a few) have __________ cervical vertebra.
- Q10(b): Fill in the blank spaces: (b) The number of phalanges in each limb of human is __________
- Q10(c): Fill in the blank spaces: (c) Thin filament of myofibril contains 2 ‘F’ actins and two other proteins namely __________ and __________.
- Q10(d): Fill in the blank spaces: (d) In a muscle fibre $Ca^{++}$ is stored in __________
- Q10(e): Fill in the blank spaces: (e) __________ and __________ pairs of ribs are called floating ribs.
- Q10(f): Fill in the blank spaces: (f) The human cranium is made of __________ bones.
CBSE Solutions for Class 11 Biology Locomotion and Movement
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Neural Control and Coordination
- Chemical Coordination and Integration
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