Q.ATPase of the muscle is located in
🔒You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.
🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Actin And Myosin Filaments
Actin and Myosin Filaments
Imagine trying to close a heavy curtain by pulling on a rope. Your hand grips the rope, pulls it toward you, then lets go and grips again further along. Each pull shortens the rope a little. That is essentially what happens inside a muscle cell — except the "rope" is made of actin filaments, and the "hand" is myosin.
Muscle contraction is a coordinated, microscopic tug-of-war. Billions of these tiny pulls happen simultaneously, and their sum is what lets you lift a cup, sprint, or blink.
The Two Filaments
Actin (The Thin Filament)
Actin is a long, twisted chain of globular protein subunits, like beads on a double-stranded necklace. It is the track that myosin pulls on. Actin doesn't work alone — it has two regulatory partners wrapped around it:
- Tropomyosin — a long, thin protein that lies in the groove of the actin helix. In a relaxed muscle, it physically blocks the myosin-binding sites on actin.
- Troponin — a smaller, three-part protein complex attached to tropomyosin. One part binds calcium ions (Ca²⁺), and when it does, it pulls tropomyosin out of the way, exposing the binding sites.
Think of tropomyosin as a blinder covering the actin track, and troponin as the switch that removes the blinder when calcium arrives.
Myosin (The Thick Filament)
Myosin looks like a bundle of golf clubs — each "club" actually has TWO globular heads (the clubface) joined to one long tail (the shaft) -- myosin II is a double-headed motor protein. The tails bundle together to form the thick filament, while the paired heads stick out at intervals along it, ready to grip actin.
Each myosin head has two critical features:
- An ATP-binding site, where energy comes from.
- An actin-binding site, where it grips the thin filament.
The head can pivot, like a hinge, and that pivoting motion is what generates force.
How They Work Together: The Sliding Filament Model
Step 1 — Resting state. ATP is bound to the myosin head, keeping it detached from actin. Tropomyosin blocks the binding sites. The muscle is relaxed.
Step 2 — Calcium arrives. A nerve signal triggers calcium release inside the muscle cell. Calcium binds to troponin, which shifts tropomyosin off the actin-binding sites. The track is now open.
Step 3 — The power stroke. The myosin head splits ATP into ADP and an inorganic phosphate group. This "cocks" the head into a high-energy, ready position. The head then binds to actin, releases the phosphate, and snaps forward — the power stroke. It pulls the actin filament toward the centre of the sarcomere, the basic contractile unit, and the muscle shortens.
Step 4 — Release. A fresh ATP molecule binds to the myosin head, causing it to let go of actin. The cycle can then repeat.
Without ATP, myosin cannot release actin. That is why rigor mortis occurs — after death, ATP runs out, and myosin heads stay locked onto actin, making the muscles stiff.
The Key Insight: Filaments Don't Shorten
A common first mistake is to think the filaments themselves shrink. They don't. Actin and myosin filaments stay the same length — they just slide past each other. The overlap zone increases, and the overall sarcomere shortens. That is why this is called the sliding filament model. …
The globular head of the myosin (thick filament) molecule is the working, energy-using part of the contractile apparatus. It functions as an active ATPase enzyme, meaning it can hydrolyse ATP to release the energy used for the cross-bridge power stroke, and it also carries the site that binds actin. Troponin and actin belong to the thin filament and have regulatory/structural roles, not e …
ATPase activity belongs to the globular head of myosin, the thick filament protein - option (C).
The two contractile proteins of a sarcomere, actin and myosin, play different roles. Actin, the thin filament, is a helical pair of F-actin strands built from G-actin monomers, running alongside two tropomyosin filaments with troponin sitting at intervals; in the resting state troponin masks actin's myosin-binding sites, but nothing in this thin-filament system carries enzymatic activity.
Myosin, the thick filament, is built from repeating units called meromyosins, each with a globular head plus short arm (heavy meromyosin) and a tail (light meromyosin). It is specifically the globular head that:
- functions as an active ATPase enzyme, hydrolysing ATP for energy, …
- AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQQ.Study the following statements regarding contractile proteins of muscle: I. Actin is a protein present in thin filament present in I-band of a sarcomere. II. Myosin forms the thick filament and contains ATPase activity in its tail region. III. Troponin binds calcium ions and helps in exposing the active sites on thin filament. IV. Tropomyosin actively hydrolyses ATP during muscle contraction. Identify the correct statements (A) I, II and III (B) I and IV (C) I and III (D) I, III and IV
›Reveal solutionSolution
Of the four statements on sarcomere contractile proteins, only I (actin/thin filament in the I-band) and III (troponin binds Ca2+ to expose binding sites) are correct; myosin's ATPase is in the head (not the tail), and tropomyosin itself never hydrolyses ATP.
Concept and Intuition
The sarcomere, the contractile unit of skeletal muscle, is built from overlapping thick (myosin) and thin (actin) filaments. The I-band is, by definition, the region containing only thin (actin) filaments, with no myosin overlap — appearing lighter under polarised light. The thick filament's myosin molecules each have a globular head (with actin-binding and ATP-binding/ATPase sites) and a tail (structural, forming the backbone of the thick filament) — the head, not the tail, is what hydrolyses ATP to power the cross-bridge cycle. Regulation of contraction is achieved by the troponin-tropomyosin complex on the thin filament: in the resting state tropomyosin covers the myosin-binding sites on actin; when Ca2+ binds to troponin (specifically the troponin-C subunit), it causes a conformational shift that moves tropomyosin aside, exposing the binding sites so cross-bridge cycling (and ATP hydrolysis by myosin) can proceed. Tropomyosin itself is purely a structural/regulatory protein with no enzymatic (ATPase) activity of its own.
Step-by-Step Solution
- Statement I: I-band = thin filament (actin) only region of the sarcomere. True. …
- AP EAPCET 2022Set ap-2022-07-11-FN1 markMCQQ.Which of the following protein can convert chemical energy in the ATP molecules in to mechanical energy? (A) Actin (B) Myosin (C) Troponin (D) Tropo myosin
›Reveal solutionSolution
This tests the molecular motor of muscle contraction; the answer is Myosin.
Concept and Intuition
Muscle contraction follows the sliding filament theory: thin actin filaments slide past thick myosin filaments. The myosin head is an ATPase — it binds ATP, hydrolyses it to ADP + Pi, and uses the energy released to swing (the 'power stroke'), pulling the attached actin filament along. This head-swinging action is literally the conversion of ATP's chemical bond energy into the mechanical force/movement of contraction.
Step-by-Step Solution
- Actin (A) is the thin filament protein that myosin heads pull on — it provides the track/binding sites but does not itself hydrolyse ATP or generate the mechanical power stroke.
- Troponin (C) is a regulatory protein complex that, along with tropomyosin, controls the exposure of myosin-binding sites on actin in response to Ca2+ — a regulatory role, not the energy-converting motor. …
- AP EAPCET 2021Set ap-2021-09-06-AN1 markMCQQ.Assertion (A): Light meromyosin and Heavy meromyosin are often called regulatory proteins. Reason (R): The above said molecules have a role in masking and unmasking the active sites on actin. (A) Both A and R are correct and R is the correct explanation of A (B) Both A and R are correct and R is not the correct explanation of A (C) A is correct but R is wrong (D) Both A and R are wrong
›Reveal solutionSolution
This tests knowledge of muscle contraction proteins; LMM/HMM are structural fragments of myosin, not regulatory proteins, and the masking/unmasking of actin's active sites is done by troponin-tropomyosin, not by LMM/HMM — so both statements are false.
Concept and Intuition
Muscle contraction depends on two categories of proteins: contractile proteins (actin and myosin, which physically slide past each other) and regulatory proteins (troponin and tropomyosin, which control when actin's myosin-binding sites are exposed). Light meromyosin (LMM) and heavy meromyosin (HMM) are simply the two fragments obtained when the myosin molecule is proteolytically cleaved for biochemical study — LMM corresponds to the rod-shaped tail that bundles into the backbone of the thick filament, while HMM contains the globular head and neck region bearing ATPase activity and the actin-binding site. Neither fragment is a distinct regulatory protein; they are both parts of the motor protein myosin itself. The actual regulatory function — masking the myosin-binding sites on actin in the resting state (tropomyosin lying across the groove) and unmasking them when Ca2+ binds troponin C during excitation — is carried out entirely by the troponin-tropomyosin complex, not by any part of the myosin molecule.
Step-by-Step Solution …
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.