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Q2:
Define sliding filament theory of muscle contraction.
Solution :
The Sliding Filament Theory: Theoretical Foundation
The Sliding Filament Theory, first proposed by Andrew Huxley and Hugh Huxley in 1954, serves as the fundamental mechanism explaining how skeletal muscles generate tension and shorten during contraction. The theory postulates that contraction is not caused by the shortening of individual filaments, but rather by the sliding of actin (thin) filaments past myosin (thick) filaments, which increases the degree of overlap between them [Per the sliding filament model of muscular contraction].
Step 1: Structural Components of the Sarcomere
The functional unit of muscle contraction is the sarcomere, defined as the region between two successive $Z$-lines. The arrangement is as follows:
- Thick Filaments: Composed of polymerized myosin molecules with cross-bridge heads.
- Thin Filaments: Composed of $F$-actin, troponin, and tropomyosin proteins.
- $A$-Band: The dark region containing the entire length of the myosin filament.
- $I$-Band: The light region containing only actin filaments.
Step 2: The Mechanism of Contraction (Cross-Bridge Cycle)
The process initiates via electrical signaling and follows a strictly regulated biochemical pathway:
- Neural Stimulation: An action potential reaches the neuromuscular junction, triggering the release of $Ca^{2+}$ ions from the sarcoplasmic reticulum into the sarcoplasm [Excitatory-Contraction Coupling].
- Binding of Calcium: $Ca^{2+}$ binds to the troponin subunit, causing a conformational change that moves tropomyosin away from the active binding sites on the actin filament.
- Cross-Bridge Formation: Exposed sites on actin bind to the energized myosin heads, forming a cross-bridge.
- Power Stroke: Utilizing energy derived from $ATP$ hydrolysis ($ATP \rightarrow ADP + P_i$), the myosin head pivots, pulling the actin filament toward the center of the $A$-band ($M$-line).
- Detachment: A new $ATP$ molecule binds to the myosin head, causing it to detach from the actin filament, resetting the cycle.
Step 3: Analytical Observation of Morphological Changes
During the contraction process, specific zones of the sarcomere exhibit distinct dimensional modifications:
| Component | Behavior During Contraction |
|---|---|
| $I$-band | Shortens / Decreases in width |
| $H$-zone | Disappears or narrows significantly |
| $A$-band | Remains constant in length |
| Sarcomere length | Decreases [Mechanical shortening] |
Final Summary/Result
Final Solution: The Sliding Filament Theory asserts that muscle contraction is achieved when myosin heads bind to actin, utilizing ATP-driven power strokes to slide thin actin filaments over thick myosin filaments, effectively shortening the sarcomere while the filaments themselves maintain a constant length.
More Questions from Class 11 Biology Locomotion and Movement EXERCISES
- Q1: Draw the diagram of a sarcomere of skeletal muscle showing different regions.
- Q3: Describe the important steps in 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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