Sliding filament theory: does ATP attach or detach the myosin head? I keep writing the opposite in tests
Mehak AroraDropper Sample
21 Sept 2026 · 1 min read
It's 3 am again and I'm awake because of this. I've done the muscle contraction chapter three times now (second attempt, so I should be better at this) and I still get the cross-bridge cycle question wrong in tests.
What I understand: the sarcomere is between two Z lines. Thin filaments (actin) are attached to the Z line, thick filaments (myosin) are in the A band. During contraction the actin filaments slide over the myosin and the Z lines come closer. The I band and H zone become shorter, A band stays the same.
What I can't fix is the cycle. NCERT says the myosin head has ATPase activity, ATP is hydrolysed to ADP + Pi, and the head binds to the exposed active site on actin, forming the cross-bridge. Then the head pulls the actin, ADP and Pi are released, a new ATP binds and the cross-bridge breaks. In my head the order keeps getting scrambled, and I always write "ATP binds so the head attaches". In the last test the question was "which step needs ATP?" and I marked the attaching one and lost marks.
Can someone give me the sequence in a way that sticks? Also what exactly "unmasks" the active sites on actin? I know calcium, but I mix up troponin and tropomyosin. 😞
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[+][–]Nisha Verma12thSample· 29d ago
I think the way to put it is: ATP binds to the myosin head, which gives it the energy to grab onto actin, and then the pull (power stroke) happens as the ATP gets used up. So ATP is needed for attachment, and the energy is released during the pull. That's how I answered it in my boards.
[+][–]Siddharth RaoDropperSample· 29d ago
Nisha, this is what Mehak has been writing and it loses marks. Here's the sequence, one line per step, check against the NCERT text:
- The myosin head (already holding ATP) hydrolyses it to ADP + Pi, which "cocks" the head. It stays attached to the ADP and Pi.
- The cocked head binds to the exposed active site on actin → cross-bridge formed.
- The head pulls the actin toward the centre of the sarcomere (power stroke) and ADP + Pi are released.
- A new ATP binds to the myosin head → the cross-bridge breaks (detachment).
- ATP is hydrolysed again → head cocked again → repeat.
So ATP binding = detachment. Hydrolysis = re-cocking. Release of ADP + Pi = the pull.
[+][–]Nisha Verma12thSample· 29d ago
oh. the rigor mortis point too. I had it backwards then, glad this came up before the boards review. thanks Sid
[+][–]Tanvi Deshmukh12thSample· 29d ago
Your sarcomere description is correct!! Here's my usual table on what changes:
Part During contraction Z lines come closer Sarcomere shortens I band (only actin) shortens H zone (only myosin, centre of A band) shortens / disappears A band (full length of myosin) stays same A is the survivor "A for Always the same" ✨. PYQ-style questions love "which band does not change in length?", answer A band.
[+][–]Zoya Khan12thSample· 29d ago
About what unmasks the sites, a gentle way to hold it: tropomyosin covers, troponin senses calcium. In a resting muscle, the active sites on actin are masked (covered) by tropomyosin, so myosin can't bind. The signal from a motor neuron at the neuromuscular junction releases acetylcholine, which generates an action potential in the sarcolemma. That makes the sarcoplasmic reticulum release Ca²⁺. Ca²⁺ binds to a subunit of troponin on the actin filaments, and this shifts things so the active sites are no longer masked. Now myosin heads can bind.
When stimulation stops, Ca²⁺ goes back into the sarcoplasmic reticulum, the sites get masked again, and the muscle relaxes.
[+][–]Rahul DubeyDropperSample· 29d ago
Mehak, a trick I used in my third attempt: think of ATP as the key to unlock, not the energy to grab. This explains rigor mortis too. After death, no new ATP is made. The myosin heads that are attached can't detach without a new ATP, so the muscle stays stiff until the proteins break down later. If ATP were needed to attach, the muscles would go limp without ATP, which is the opposite of what we see.
For the test question "which step needs ATP", the answer is that hydrolysis of ATP energises the head, and binding of a fresh ATP is what lets it release the actin. Read the options carefully, because the question wording decides which one they want.
[+][–]Mehak AroraDropperSample· 29d agoAuthor
The rigor mortis logic is the first thing that made it stick, thank you Rahul bhaiya. I can finally sleep 🙏
