Q.Write a short note on: Actin filament.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Sliding Filament Theory
The Big Picture: What Happens When You Flex?
Imagine a crowd of people standing arm-in-arm in a long line. If everyone pulls their elbows in toward their chest, the whole line gets shorter — but nobody's arms actually shrink. The bones just slide closer together. That's the core idea of muscle contraction: the parts slide, they don't crumple.
Your muscles are made of thousands of tiny repeating units called sarcomeres. Each sarcomere is like a miniature engine with two sets of filaments — one thick, one thin — arranged in parallel. When you contract a muscle, these filaments don't shorten themselves. Instead, the thin filaments are pulled inward, sliding past the thick filaments, making the whole sarcomere shorter. This is the Sliding Filament Theory.
The sarcomere is the functional unit of a muscle — the smallest part that can contract. It runs from one Z-line to the next Z-line.
The Cast: Thick and Thin Filaments
Two types of protein filaments do all the work:
- Thick filaments are made of myosin. Each myosin molecule has a long tail and a globular head that can grab onto the thin filament and pull.
- Thin filaments are made of actin, along with two regulatory proteins: tropomyosin (a long rope-like protein that covers binding sites) and troponin (a calcium-sensitive switch).
In a relaxed muscle, tropomyosin blocks the myosin-binding sites on actin. Nothing can happen until calcium arrives.
The Mechanism: Step by Step
Here is the sequence that actually shortens the sarcomere:
- Calcium is released from the sarcoplasmic reticulum (a storage tank inside the muscle cell) when a nerve signal arrives.
- Calcium binds to troponin, which shifts tropomyosin out of the way, exposing the binding sites on actin.
- Myosin heads attach to actin, forming a cross-bridge.
- The power stroke — the myosin head pivots, pulling the thin filament toward the center of the sarcomere. This is the actual sliding motion.
- ATP binds to myosin, causing it to release actin.
- ATP is hydrolyzed (split into ADP + phosphate), which cocks the myosin head back into its high-energy position.
- If calcium is still present, the cycle repeats. If not, the cross-bridge cycle stops and the muscle relaxes.
ATP is required for detachment of myosin from actin, not for the power stroke itself. Without ATP, myosin stays locked onto actin — this is what causes rigor mortis after death.
What Changes and What Doesn't
During contraction:
| Structure | What happens |
|---|---|
| A-band (where thick filaments are) | Stays the same length |
| I-band (thin filaments only) | Gets shorter |
| H-zone (thick filaments only, center) | Gets shorter or disappears |
| Sarcomere length | Decreases |
| Filament length | Does not change |
Actin is a thin filament made of an F-actin backbone plus tropomyosin and troponin, which together control access to actin's myosin-binding sites. …
Step 1. The actin (thin) filament is a complex structure made of three components. Its backbone, F-actin, is a double-stranded structure formed by polymerised globular G-actin molecules, each carrying one bound ADP molecule.
Step 2. Two additional strands of the protein tropomyosin run loosely alongside the F-actin backbone. In the resting muscle, tropomyosin physically covers the sites on actin where myosin heads would otherwise bind (the 'active sites'). …
Describe the actin filament's three components in order, and note the functional role of each ( …
- Naming only F-actin and forgetting tropomyosin and troponin. …
- CBSE 2026Set ANNUAL1 markMCQQ.The functional unit of a muscle fibre is :(a) Myosin(b) Sarcomere(c) Actin(d) Sarcoplasm
›Reveal solutionSolution
The sarcomere — the segment of a myofibril between two successive Z-lines, containing overlapping actin and myosin filaments — is the smallest structural and functional unit of contraction in a muscle fibre.
Within a skeletal muscle fibre, many thread-like myofibrils run the length of the cell. Each myofibril is divided along its length into repeating segments called sarcomeres, marked off by thin, dense boundary lines called Z-lines (or Z-discs). Within one sarcomere, thin actin filaments and thick myosin filaments overlap in an ordered arrangement. During contraction, myosin heads on the thick filaments pull the thin actin filaments inward (the sliding filament mechanism), shortening the sarcomere; because sarcomeres are arranged end to end along …
- CBSE 2024Set ANNUAL1 markMCQQ.In each of the following questions (Q. No. 15, 16, 17 & 18), a statement of Assertion (A) is given followed by a corresponding statement of Reason (R) just below it. Of the statement, choose the correct answer using the codes given below. Assertion (A) : In muscle contraction, length of both A-bands and I-band decreased. Reason (R) : Both myosin of A-band and actin of I-band are contractile proteins and decrease in size during muscle contraction.(a) Both Assertion (A) and Reason (R) are true and Reason (R) is correct explanation of Assertion (A).(b) Both Assertion (A) and Reason (R) are true but Reason (R) is not correct explanation of Assertion (A).(c) Assertion (A) is true but Reason (R) is false.(d) Both Assertion (A) and Reason (R) are false.
›Reveal solutionSolution
During muscle contraction the actin (thin) filaments slide over the myosin (thick) filaments; the A-band (length of the myosin filament) stays constant, only the I-band and H-zone shorten, and neither actin nor myosin filaments themselves shorten.
According to the sliding filament theory of muscle contraction:
- The A-band (the region occupied by myosin, or thick, filaments) corresponds to the length of the myosin filaments and does not change during contraction.
- The I-band (the region containing only actin, or thin, filaments) decreases in width as the actin filaments slide further into the A-band.
- The H-zone also decreases as the actin filaments approach each other in the centre of the sarcomere.
So the Assertion — that both the A-band and I-band decrease — is factually false, since the A-band length remains unchanged; only the I-band decreases.
…
- CBSE 2023Set ANNUAL1 markMCQQ.The functional unit of a muscle fibre is :(a) Myosin(b) Sarcomere(c) Actin(d) Sarcoplasm
›Reveal solutionSolution
The sarcomere — the region of a myofibril between two Z-lines, containing thin (actin) and thick (myosin) filaments — is the basic functional unit of muscle contraction.
Each skeletal muscle fibre contains many myofibrils, and each myofibril is divided lengthwise into repeating units called sarcomeres, bounded by Z-lines.
- Actin and myosin are individual contractile proteins that make up the thin and thick filaments respectively, but neither alone is 'the functional unit'.
- Sarcoplasm is simply the cytoplasm of the muscle fibre, containing myofibrils, mitochondria and stored glycogen — a component, not the functional unit. …
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