{"format":"molecular-machines.cards","version":1,"updated":"2026-10-12","decks":[{"machine":"myosin","name":"Myosin","count":10,"url":"/learn/cards/myosin.json"}],"cards":[{"id":"myosin-track","machine":"myosin","kind":"qa","prompt":"Myosin is the motor in your muscles, and in other cells it also hauls cargo. Along which kind of filament does it pull?","answer":"Actin filaments.","explanation":"Kinesin and dynein walk on microtubules instead.","section":"summary","topic":"purpose","sources":["machine:summary","machine:where","link:kinesin"],"tags":["actin","track"],"difficulty":1,"url":"/machines/myosin#summary","cites":[{"source":"machine:summary","machine":"myosin","label":"Summary","section":"summary"},{"source":"machine:where","machine":"myosin","label":"Summary","section":"summary"},{"source":"link:kinesin","machine":"myosin","label":"Link to Kinesin","section":"story"}]},{"id":"myosin-lever-swing","machine":"myosin","kind":"qa","prompt":"A myosin head binds actin and then lets its phosphate go. What does the head do next that pulls the filament along?","answer":"It swings its lever arm: the power stroke.","explanation":"The lever swings through a large angle, mostly along the filament, and its end moves the load.","section":"mechanism","topic":"cycle","sources":["mechanism:The power stroke","machine:summary","mechanism:The cleft closes and phosphate leaves"],"tags":["power-stroke","lever"],"difficulty":1,"url":"/machines/myosin#mechanism","cites":[{"source":"mechanism:The power stroke","machine":"myosin","label":"Step: The power stroke","section":"mechanism"},{"source":"machine:summary","machine":"myosin","label":"Summary","section":"summary"},{"source":"mechanism:The cleft closes and phosphate leaves","machine":"myosin","label":"Step: The cleft closes and phosphate leaves","section":"mechanism"}]},{"id":"myosin-atp-frees-head","machine":"myosin","kind":"qa","prompt":"After the power stroke, the myosin head stays stuck tight to actin with an empty fuel pocket. What must bind to make the head let go?","answer":"A new ATP.","explanation":"A head cannot hold both ATP and actin tightly, so gripping ATP frees it.","section":"mechanism","topic":"cycle","sources":["mechanism:ATP frees the head","step:The rear head lets go","mechanism:ADP leaves: rigor"],"tags":["atp","release"],"difficulty":1,"url":"/machines/myosin#mechanism","cites":[{"source":"mechanism:ATP frees the head","machine":"myosin","label":"Step: ATP frees the head","section":"mechanism"},{"source":"step:The rear head lets go","machine":"myosin","label":"Step: The rear head lets go","section":"mechanism"},{"source":"mechanism:ADP leaves: rigor","machine":"myosin","label":"Step: ADP leaves: rigor","section":"mechanism"}]},{"id":"myosin-recovery-stroke","machine":"myosin","kind":"qa","prompt":"Once free of actin, the myosin head closes around its ATP and splits it. What does this do to the lever?","answer":"It swings the lever back and cocks it for the next stroke.","explanation":"The head holds ADP and phosphate in this primed state until it binds actin again.","section":"mechanism","topic":"cycle","sources":["mechanism:Recovery stroke off actin","machine:summary"],"tags":["recovery","priming"],"difficulty":1,"url":"/machines/myosin#mechanism","cites":[{"source":"mechanism:Recovery stroke off actin","machine":"myosin","label":"Step: Recovery stroke off actin","section":"mechanism"},{"source":"machine:summary","machine":"myosin","label":"Summary","section":"summary"}]},{"id":"myosin-uyeda-lever-test","machine":"myosin","kind":"qa","prompt":"Inside the head only a small part turns, and a long lever arm turns that into a big stroke. When engineers made the lever longer, what happened to the speed of the motor?","answer":"It went up, in step with the lever length.","explanation":"Longer levers give longer strokes, so the filaments glide faster. Shorter levers glide slower.","section":"evidence","topic":"parts","sources":["stat:Velocity versus lever length","evidence:uyeda1996-velocity-vs-lever","step:The lead head swings its lever"],"tags":["lever","speed"],"difficulty":2,"url":"/machines/myosin#evidence","cites":[{"source":"stat:Velocity versus lever length","machine":"myosin","label":"Key number: Velocity versus lever length","section":"summary"},{"source":"evidence:uyeda1996-velocity-vs-lever","machine":"myosin","label":"Actin sliding velocity against lever-arm length (Uyeda TQ 1996)","section":"evidence","anchor":"ev-uyeda1996-velocity-vs-lever"},{"source":"step:The lead head swings its lever","machine":"myosin","label":"Step: The lead head swings its lever","section":"mechanism"}]},{"id":"myosin-muscle-layout","machine":"myosin","kind":"qa","prompt":"Each myosin stroke moves only a few nanometres, yet a muscle shortens by centimetres. How does the layout of muscle add these strokes up?","answer":"Myosins sit side by side, so their forces add; sarcomeres sit end to end, so their shortenings add.","explanation":"Like train carriages, units in a row add their travel, and units side by side add their force.","section":"story","topic":"purpose","sources":["story:summary","analogy:Train carriages coupled in a row","stop:sarcomere"],"tags":["muscle","sarcomere"],"difficulty":2,"url":"/machines/myosin#story","cites":[{"source":"story:summary","machine":"myosin","label":"Big picture","section":"story"},{"source":"analogy:Train carriages coupled in a row","machine":"myosin","label":"Analogy: Train carriages coupled in a row","section":"story"},{"source":"stop:sarcomere","machine":"myosin","label":"Big picture: The sarcomere","section":"story"}]},{"id":"myosin-gain-to-hand","machine":"myosin","kind":"qa","prompt":"A myosin stroke starts with a movement of about half a nanometre near one phosphate in the head. By the time it reaches your hand, roughly how many times larger has that movement grown?","answer":"About 30 million times.","explanation":"The head's lever, the heads taking turns, thousands of sarcomeres in a row and the lever of the arm each multiply it.","section":"story","topic":"numbers","sources":["fact:M9","story:summary"],"tags":["scale","gain"],"difficulty":2,"url":"/machines/myosin#story","cites":[{"source":"fact:M9","machine":"myosin","label":"Gain from the phosphate to the hand: 3 × 10^7 times","section":"story"},{"source":"story:summary","machine":"myosin","label":"Big picture","section":"story"}]},{"id":"myosin-muscle-low-duty","machine":"myosin","kind":"qa","prompt":"Muscle myosin heads let go of actin for most of their cycle, and each head works on its own clock. How does muscle still pull steadily?","answer":"Many heads work together, so at any moment some are pulling.","explanation":"It is like a rowing crew, except the rowers do not pull in time.","section":"summary","topic":"cycle","sources":["stat:Duty ratio, myosin-5","analogy:A rowing crew","evolution:Duty ratio and function"],"tags":["ensemble","duty-ratio"],"difficulty":2,"url":"/machines/myosin#summary","cites":[{"source":"stat:Duty ratio, myosin-5","machine":"myosin","label":"Key number: Duty ratio, myosin-5","section":"summary"},{"source":"analogy:A rowing crew","machine":"myosin","label":"Analogy: A rowing crew","section":"story"},{"source":"evolution:Duty ratio and function","machine":"myosin","label":"Duty ratio and function","section":"summary"}]},{"id":"myosin-cardiac-drugs","machine":"myosin","kind":"qa","prompt":"Cardiac myosin drives every heartbeat, and it is now a drug target. What do the new cardiac myosin drugs do to this motor?","answer":"One turns the motor up and one turns it down.","explanation":"The activator and the inhibitor bind the same pocket in the head. Regulators have approved the inhibitor, mavacamten.","section":"summary","topic":"purpose","sources":["frontier:Cardiac myosin drugs","species:Beta-cardiac myosin"],"tags":["drugs","heart"],"difficulty":1,"url":"/machines/myosin#summary","cites":[{"source":"frontier:Cardiac myosin drugs","machine":"myosin","label":"Open question: Cardiac myosin drugs","section":"summary"},{"source":"species:Beta-cardiac myosin","machine":"myosin","label":"Beta-cardiac myosin","section":"summary"}]},{"id":"myosin-link-serca","machine":"myosin","kind":"qa","prompt":"A calcium signal switches muscle myosin on. Which machine switches it off again, and how?","answer":"SERCA, by pumping the calcium away.","section":"story","topic":"purpose","sources":["link:serca"],"tags":["serca","link","calcium"],"difficulty":1,"url":"/machines/myosin#story","cites":[{"source":"link:serca","machine":"myosin","label":"Link to SERCA calcium pump","section":"story"}]}]}