{"format":"molecular-machines.cards","version":1,"updated":"2026-10-12","decks":[{"machine":"serca","name":"SERCA calcium pump","count":10,"url":"/learn/cards/serca.json"}],"cards":[{"id":"serca-job","machine":"serca","kind":"qa","prompt":"A muscle contracts when calcium floods out of a store inside each fibre. What job does SERCA do after each contraction?","answer":"It pumps the calcium back into the store, so the muscle relaxes.","explanation":"The store is the sarcoplasmic reticulum, a membrane sleeve around every muscle fibre. SERCA sits in its membrane.","section":"summary","topic":"purpose","sources":["machine:summary","stop:twitch","machine:where"],"difficulty":1,"url":"/machines/serca#summary","cites":[{"source":"machine:summary","machine":"serca","label":"Summary","section":"summary"},{"source":"stop:twitch","machine":"serca","label":"Big picture: One twitch","section":"story"},{"source":"machine:where","machine":"serca","label":"Summary","section":"summary"}]},{"id":"serca-ca-per-atp","machine":"serca","kind":"cloze","prompt":"SERCA pays for its work with ATP: each ATP it spends pushes {{two}} calcium ions into the store.","answer":"two","section":"summary","topic":"numbers","sources":["stat:Calcium ions per ATP","machine:tagline","stop:machine"],"difficulty":1,"url":"/machines/serca#summary","cites":[{"source":"stat:Calcium ions per ATP","machine":"serca","label":"Key number: Calcium ions per ATP","section":"summary"},{"source":"machine:tagline","machine":"serca","label":"Summary","section":"summary"},{"source":"stop:machine","machine":"serca","label":"Big picture: The pump","section":"story"}]},{"id":"serca-gradient-size","machine":"serca","kind":"qa","prompt":"SERCA pushes calcium uphill, into a store that already holds a lot. Roughly how much more free calcium is inside the store than in the resting muscle fibre?","answer":"About 4,000 times more.","explanation":"One ATP gives enough energy to push two calcium ions up this gradient.","section":"story","topic":"numbers","sources":["fact:S4","story:summary"],"difficulty":2,"url":"/machines/serca#story","cites":[{"source":"fact:S4","machine":"serca","label":"Calcium gradient across the SR membrane: 4,000 times","section":"story"},{"source":"story:summary","machine":"serca","label":"Big picture","section":"story"}]},{"id":"serca-airlock-doors","machine":"serca","kind":"qa","prompt":"SERCA works like an airlock: one door opens to the fibre, it shuts, then a door opens to the store. Why must the two doors never be open at the same time?","answer":"So the calcium cannot leak back out of the store.","explanation":"Unlike an airlock, SERCA's doors are driven by the energy of ATP.","section":"mechanism","topic":"cycle","sources":["analogy:An airlock","stop:machine","mechanism:The pump phosphorylates itself"],"difficulty":1,"url":"/machines/serca#mechanism","cites":[{"source":"analogy:An airlock","machine":"serca","label":"Analogy: An airlock","section":"story"},{"source":"stop:machine","machine":"serca","label":"Big picture: The pump","section":"story"},{"source":"mechanism:The pump phosphorylates itself","machine":"serca","label":"Step: The pump phosphorylates itself","section":"mechanism"}]},{"id":"serca-phosphorylates-itself","machine":"serca","kind":"qa","prompt":"SERCA does not simply burn ATP like fuel in an engine. What does it do with the phosphate group of the ATP?","answer":"It puts the phosphate on itself, and that drives its change of shape.","explanation":"The shape change closes the door to the fibre and opens the way to the store. Later the pump removes the phosphate and resets.","section":"mechanism","topic":"cycle","sources":["mechanism:The pump phosphorylates itself","step:The pump phosphorylates itself","analogy:A pump lifting water back into a tank"],"difficulty":2,"url":"/machines/serca#mechanism","cites":[{"source":"mechanism:The pump phosphorylates itself","machine":"serca","label":"Step: The pump phosphorylates itself","section":"mechanism"},{"source":"step:The pump phosphorylates itself","machine":"serca","label":"Step: The pump phosphorylates itself","section":"mechanism"},{"source":"analogy:A pump lifting water back into a tank","machine":"serca","label":"Analogy: A pump lifting water back into a tank","section":"story"}]},{"id":"serca-protons-empty-sites","machine":"serca","kind":"qa","prompt":"After SERCA lets its two calcium ions go inside the store, its binding sites are empty. What fills the empty seats for the trip back?","answer":"Protons, which move the other way.","section":"mechanism","topic":"cycle","sources":["mechanism:Protons take the empty seats","stat:Proton counter-transport"],"difficulty":2,"url":"/machines/serca#mechanism","cites":[{"source":"mechanism:Protons take the empty seats","machine":"serca","label":"Step: Protons take the empty seats","section":"mechanism"},{"source":"stat:Proton counter-transport","machine":"serca","label":"Key number: Proton counter-transport","section":"summary"}]},{"id":"serca-resting-oxygen","machine":"serca","kind":"qa","prompt":"Even at rest, a muscle keeps its pumps busy to hold calcium in the store. About how much of resting muscle's oxygen use goes to SERCA?","answer":"Close to half.","explanation":"Measured at 40 to 50 percent in resting mouse muscle.","section":"story","topic":"numbers","sources":["fact:S9"],"difficulty":2,"url":"/machines/serca#story","cites":[{"source":"fact:S9","machine":"serca","label":"Share of resting muscle oxygen use spent on SERCA: 40–50 %","section":"story"}]},{"id":"serca-thapsigargin-tool","machine":"serca","kind":"qa","prompt":"A plant toxin, thapsigargin, jams SERCA. Why did it become a standard lab tool for studying calcium?","answer":"It blocks SERCA at a tiny dose and leaves other calcium pumps and channels alone.","section":"evidence","topic":"debate","sources":["evidence:thapsigargin-subnanomolar","stat:Thapsigargin inhibition","stop:site"],"difficulty":2,"url":"/machines/serca#evidence","cites":[{"source":"evidence:thapsigargin-subnanomolar","machine":"serca","label":"Lowest free thapsigargin concentration that inhibits Ca2+ uptake (Sagara Y 1991)","section":"evidence","anchor":"ev-thapsigargin-subnanomolar"},{"source":"stat:Thapsigargin inhibition","machine":"serca","label":"Key number: Thapsigargin inhibition","section":"summary"},{"source":"stop:site","machine":"serca","label":"Big picture: The calcium sites","section":"story"}]},{"id":"serca-heart-failure-gene-transfer","machine":"serca","kind":"qa","prompt":"In heart failure, the heart's calcium cycling falls, and SERCA activity falls with it. What treatment idea tries to fix this, and how well has it worked?","answer":"Adding more SERCA by gene transfer; it helped failing heart muscle, but results in patients have been mixed.","section":"summary","topic":"debate","sources":["frontier:Gene transfer for heart failure"],"difficulty":2,"url":"/machines/serca#summary","cites":[{"source":"frontier:Gene transfer for heart failure","machine":"serca","label":"Open question: Gene transfer for heart failure","section":"summary"}]},{"id":"serca-myosin-link","machine":"serca","kind":"qa","prompt":"Calcium switches on myosin, the motor that makes muscle contract. How does SERCA switch myosin off at the end of a contraction?","answer":"It takes the calcium away.","explanation":"SERCA also spends the ATP that ATP synthase makes, running the opposite trade: ATP in, ion gradient out.","section":"story","topic":"purpose","sources":["link:myosin","link:atp-synthase"],"difficulty":1,"url":"/machines/serca#story","cites":[{"source":"link:myosin","machine":"serca","label":"Link to Myosin","section":"story"},{"source":"link:atp-synthase","machine":"serca","label":"Link to ATP synthase","section":"story"}]}]}