{"format":"molecular-machines.cards","version":1,"updated":"2026-10-12","decks":[{"machine":"flagellar-motor","name":"Bacterial flagellar motor","count":10,"url":"/learn/cards/flagellar-motor.json"}],"cards":[{"id":"flagellar-motor-what-it-turns","machine":"flagellar-motor","kind":"qa","prompt":"Many bacteria swim with a few long corkscrew tails. The flagellar motor sits at the base of each tail, built into the cell wall. What does the motor do to the tail?","answer":"It spins it, like a rotary engine turning a corkscrew.","explanation":"Several tails turn together as one bundle and push the cell.","section":"summary","topic":"purpose","sources":["machine:summary","stop:cell","analogy:An outboard motor and propeller"],"difficulty":1,"url":"/machines/flagellar-motor#summary","cites":[{"source":"machine:summary","machine":"flagellar-motor","label":"Summary","section":"summary"},{"source":"stop:cell","machine":"flagellar-motor","label":"Big picture: One cell","section":"story"},{"source":"analogy:An outboard motor and propeller","machine":"flagellar-motor","label":"Analogy: An outboard motor and propeller","section":"story"}]},{"id":"flagellar-motor-fuel","machine":"flagellar-motor","kind":"qa","prompt":"Unlike most motors in a cell, the flagellar motor burns no ATP. What powers it in E. coli?","answer":"Protons that flow into the cell across the inner membrane.","explanation":"Some bacteria, such as Vibrio, use sodium ions instead of protons.","section":"summary","topic":"purpose","sources":["machine:energy","stop:envelope"],"difficulty":1,"url":"/machines/flagellar-motor#summary","cites":[{"source":"machine:energy","machine":"flagellar-motor","label":"Summary","section":"summary"},{"source":"stop:envelope","machine":"flagellar-motor","label":"Big picture: Cell envelope","section":"story"}]},{"id":"flagellar-motor-stators-push","machine":"flagellar-motor","kind":"qa","prompt":"A ring of stators grips the cell wall around the motor's rotor. Each stator lets ions into the cell one at a time. What does that ion flow do to the rotor?","answer":"It pushes the rotor round, one small step per ion.","explanation":"Each ion nudges a small ring in the stator, and that ring nudges the rotor like a gear.","section":"mechanism","topic":"parts","sources":["stop:stator","mechanism:Ions enter the stator","step:Ions flow through the stators"],"difficulty":1,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"stop:stator","machine":"flagellar-motor","label":"Big picture: One stator","section":"story"},{"source":"mechanism:Ions enter the stator","machine":"flagellar-motor","label":"Step: Ions enter the stator","section":"mechanism"},{"source":"step:Ions flow through the stators","machine":"flagellar-motor","label":"Step: Ions flow through the stators","section":"mechanism"}]},{"id":"flagellar-motor-why-reverse","machine":"flagellar-motor","kind":"qa","prompt":"A bacterium cannot see food. It swims straight for about a second, then tumbles and picks a new direction at random. Why does its motor need to reverse?","answer":"Reversing makes the cell tumble, and tumbling less when things get better steers it toward food.","explanation":"When the motor reverses, the bundle of tails flies apart, so the cell tumbles in place instead of swimming backward.","section":"summary","topic":"purpose","sources":["machine:summary","stop:tracks","story:question"],"difficulty":1,"url":"/machines/flagellar-motor#summary","cites":[{"source":"machine:summary","machine":"flagellar-motor","label":"Summary","section":"summary"},{"source":"stop:tracks","machine":"flagellar-motor","label":"Big picture: Swimming tracks","section":"story"},{"source":"story:question","machine":"flagellar-motor","label":"Big picture","section":"story"}]},{"id":"flagellar-motor-signal-reverses","machine":"flagellar-motor","kind":"qa","prompt":"A signal protein carries news from the cell's sensors to the motor. When enough of it binds the rotor, what does the motor do?","answer":"It reverses its direction of spin.","explanation":"The switch is very steep, almost all or nothing. The ions still flow the same way; only the contact with the rotor changes.","section":"mechanism","topic":"cycle","sources":["stop:switch","mechanism:CheY-P flips the switch","step:CheY binds the switch"],"difficulty":1,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"stop:switch","machine":"flagellar-motor","label":"Big picture: Reverse switch","section":"story"},{"source":"mechanism:CheY-P flips the switch","machine":"flagellar-motor","label":"Step: CheY-P flips the switch","section":"mechanism"},{"source":"step:CheY binds the switch","machine":"flagellar-motor","label":"Step: CheY binds the switch","section":"mechanism"}]},{"id":"flagellar-motor-coast-distance","machine":"flagellar-motor","kind":"qa","prompt":"At the size of a bacterium, water feels like thick syrup. How far does a swimming cell coast after its motors stop?","answer":"Less than the width of an atom.","explanation":"Drag stops the cell almost at once, so there is no gliding.","section":"story","topic":"numbers","sources":["fact:F6","stop:cell"],"difficulty":1,"url":"/machines/flagellar-motor#story","cites":[{"source":"fact:F6","machine":"flagellar-motor","label":"How far the cell coasts when its motors stop: 0.1 Å","section":"story"},{"source":"stop:cell","machine":"flagellar-motor","label":"Big picture: One cell","section":"story"}]},{"id":"flagellar-motor-protons-per-turn","machine":"flagellar-motor","kind":"qa","prompt":"Each proton that passes through a stator moves the rotor only a tiny step. Roughly how many protons does one full turn of the rotor take?","answer":"About a thousand.","explanation":"The measured value, from Streptococcus, is about 1,240 per turn.","section":"story","topic":"numbers","sources":["fact:F10","stop:stator"],"difficulty":2,"url":"/machines/flagellar-motor#story","cites":[{"source":"fact:F10","machine":"flagellar-motor","label":"Protons that pass per rotor turn: 1,240 protons per turn","section":"story"},{"source":"stop:stator","machine":"flagellar-motor","label":"Big picture: One stator","section":"story"}]},{"id":"flagellar-motor-catch-bond","machine":"flagellar-motor","kind":"qa","prompt":"Stator units join and leave the motor all the time, like workers who come and go. What makes more of them stay on the motor?","answer":"A higher load: they hold on longer when the motor must push hard.","explanation":"The stator's anchor to the cell wall grips tighter under force.","section":"mechanism","topic":"cycle","sources":["mechanism:Load sets the number of stator units","evidence:stator-catch-bond"],"difficulty":2,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"mechanism:Load sets the number of stator units","machine":"flagellar-motor","label":"Step: Load sets the number of stator units","section":"mechanism"},{"source":"evidence:stator-catch-bond","machine":"flagellar-motor","label":"Stator lifetime against applied force (Nord AL 2017)","section":"evidence","anchor":"ev-stator-catch-bond"}]},{"id":"flagellar-motor-zero-load-one-unit","machine":"flagellar-motor","kind":"qa","prompt":"Each extra stator unit adds push to the motor. But when the motor turns with almost no load, how fast does one stator unit spin it compared with many?","answer":"Just as fast as many.","explanation":"Extra units only help when the load is high.","section":"evidence","topic":"numbers","sources":["evidence:zero-load-one-stator","step:The rotor turns counterclockwise"],"difficulty":2,"url":"/machines/flagellar-motor#evidence","cites":[{"source":"evidence:zero-load-one-stator","machine":"flagellar-motor","label":"Speed near zero load against number of stator units (Yuan J 2008)","section":"evidence","anchor":"ev-zero-load-one-stator"},{"source":"step:The rotor turns counterclockwise","machine":"flagellar-motor","label":"Step: The rotor turns counterclockwise","section":"mechanism"}]},{"id":"flagellar-motor-dynein-link","machine":"flagellar-motor","kind":"qa","prompt":"Sperm tails and the cilia in your airways are also called flagella, but no rotary motor turns them. How do they move instead?","answer":"Dynein motors inside them slide filaments, so the tail bends.","explanation":"Bacteria rotate a stiff corkscrew; our own flagella beat by bending.","section":"story","topic":"purpose","sources":["link:dynein"],"difficulty":1,"url":"/machines/flagellar-motor#story","cites":[{"source":"link:dynein","machine":"flagellar-motor","label":"Link to Dynein","section":"story"}]}]}