{"format":"molecular-machines.cards","version":1,"updated":"2026-10-01","decks":[{"machine":"flagellar-motor","name":"Bacterial flagellar motor","count":27,"url":"/learn/cards/flagellar-motor.json"}],"cards":[{"id":"flagellar-motor-what-it-turns","machine":"flagellar-motor","kind":"qa","prompt":"What does the bacterial flagellar motor turn?","answer":"A long helical filament (the flagellum).","explanation":"The motor sits in the cell envelope; the turning filament works as the cell's propeller.","section":"summary","topic":"purpose","sources":["machine:summary","component:FliC (flagellin)"],"tags":["rotation","swimming"],"difficulty":1,"url":"/machines/flagellar-motor#summary","cites":[{"source":"machine:summary","machine":"flagellar-motor","label":"Summary","section":"summary"},{"source":"component:FliC (flagellin)","machine":"flagellar-motor","label":"Part: FliC (flagellin)","section":"summary"}]},{"id":"flagellar-motor-why-reverse","machine":"flagellar-motor","kind":"qa","prompt":"Why does a bacterium need its flagellar motor to reverse?","answer":"Switching between runs and tumbles steers the cell toward food.","explanation":"Counterclockwise rotation gives runs; a short spell of clockwise rotation gives a tumble and a new direction.","section":"summary","topic":"purpose","sources":["machine:summary","story:question"],"tags":["chemotaxis","switch"],"difficulty":1,"url":"/machines/flagellar-motor#summary","cites":[{"source":"machine:summary","machine":"flagellar-motor","label":"Summary","section":"summary"},{"source":"story:question","machine":"flagellar-motor","label":"Big picture","section":"story"}]},{"id":"flagellar-motor-fuel-by-species","machine":"flagellar-motor","kind":"cloze","prompt":"The E. coli flagellar motor runs on an inward flow of protons; the Vibrio polar flagellar motor runs on an inward flow of {{sodium ions}}.","answer":"sodium ions","explanation":"Both ions flow into the cell across the inner membrane. Vibrio uses PomA/PomB stator units instead of MotA/MotB.","section":"summary","topic":"purpose","sources":["machine:energy","species:Vibrio polar motor","component:PomA"],"tags":["ions","energy"],"difficulty":1,"url":"/machines/flagellar-motor#summary","cites":[{"source":"machine:energy","machine":"flagellar-motor","label":"Summary","section":"summary"},{"source":"species:Vibrio polar motor","machine":"flagellar-motor","label":"Vibrio polar motor","section":"summary"},{"source":"component:PomA","machine":"flagellar-motor","label":"Part: PomA","section":"summary"}]},{"id":"flagellar-motor-stator-build","machine":"flagellar-motor","kind":"qa","prompt":"What subunits make one stator unit of the flagellar motor, and how are they arranged?","answer":"A ring of five MotA around two MotB (MotA5B2).","explanation":"Deme and colleagues found the same 5:2 build in stator units from Vibrio, Clostridium and Bacillus.","section":"summary","topic":"parts","sources":["evidence:stator-mota5b2","mechanism:Ions enter the stator","ref:deme2020"],"tags":["stator","structure"],"difficulty":1,"url":"/machines/flagellar-motor#summary","cites":[{"source":"evidence:stator-mota5b2","machine":"flagellar-motor","label":"Subunit stoichiometry of the stator unit (Deme JC 2020)","section":"evidence","anchor":"ev-stator-mota5b2"},{"source":"mechanism:Ions enter the stator","machine":"flagellar-motor","label":"Step: Ions enter the stator","section":"mechanism"},{"source":"ref:deme2020","machine":"flagellar-motor","label":"Deme et al.","section":"sources","anchor":"ref-deme2020","href":"https://doi.org/10.1038/s41564-020-0788-8"}]},{"id":"flagellar-motor-motb-role","machine":"flagellar-motor","kind":"qa","prompt":"In the flagellar motor stator unit, what two jobs does MotB do?","answer":"It anchors the stator unit to the cell wall (peptidoglycan) and carries the aspartate that binds the ion.","explanation":"Because MotB is held fixed by its anchor, the MotA ring can turn around it.","section":"summary","topic":"parts","sources":["component:MotB","mechanism:Ions enter the stator"],"tags":["stator","parts"],"difficulty":2,"url":"/machines/flagellar-motor#summary","cites":[{"source":"component:MotB","machine":"flagellar-motor","label":"Part: MotB","section":"summary"},{"source":"mechanism:Ions enter the stator","machine":"flagellar-motor","label":"Step: Ions enter the stator","section":"mechanism"}]},{"id":"flagellar-motor-chey-site","machine":"flagellar-motor","kind":"cloze","prompt":"Phosphorylated CheY reverses the flagellar motor by binding the N-terminus of {{FliM}} on the C-ring.","answer":"FliM","explanation":"FliM is the middle ring of the C-ring; in the clockwise structure all 34 FliM carry a CheY.","section":"summary","topic":"parts","sources":["component:FliM","step:CheY binds the switch","ref:lee2001"],"tags":["switch","chemotaxis"],"difficulty":1,"url":"/machines/flagellar-motor#summary","cites":[{"source":"component:FliM","machine":"flagellar-motor","label":"Part: FliM","section":"summary"},{"source":"step:CheY binds the switch","machine":"flagellar-motor","label":"Step: CheY binds the switch","section":"mechanism"},{"source":"ref:lee2001","machine":"flagellar-motor","label":"Lee et al.","section":"sources","anchor":"ref-lee2001","href":"https://doi.org/10.1038/83053"}]},{"id":"flagellar-motor-stall-torque","machine":"flagellar-motor","kind":"qa","prompt":"About how much torque does a fully induced E. coli flagellar motor give at high load?","answer":"About 1,260 pN·nm.","explanation":"Measured with 1 µm beads at about 63 Hz, close to stall. One stator unit gives about 146 pN·nm.","section":"summary","topic":"numbers","sources":["stat:Stall torque, E. coli (fully induced)","evidence:stall-torque-fully-induced","ref:reid2006"],"tags":["torque"],"difficulty":2,"url":"/machines/flagellar-motor#summary","cites":[{"source":"stat:Stall torque, E. coli (fully induced)","machine":"flagellar-motor","label":"Key number: Stall torque, E. coli (fully induced)","section":"summary"},{"source":"evidence:stall-torque-fully-induced","machine":"flagellar-motor","label":"Torque of a fully induced E. coli motor at high load (Reid SW 2006)","section":"evidence","anchor":"ev-stall-torque-fully-induced"},{"source":"ref:reid2006","machine":"flagellar-motor","label":"Reid et al.","section":"sources","anchor":"ref-reid2006","href":"https://doi.org/10.1073/pnas.0509932103"}]},{"id":"flagellar-motor-steps-vs-flig","machine":"flagellar-motor","kind":"qa","prompt":"The flagellar rotor takes 26 steps per turn at low driving force. Why can those steps no longer be read as one step per FliG?","answer":"A Salmonella C-ring has 34 FliG subunits, not 26.","explanation":"Sowa and colleagues linked 26 to the FliG ring; later cryo-EM (Tan et al. 2024) counted 34 FliG, so the two counts differ.","section":"summary","topic":"debate","sources":["stat:Steps per turn","evidence:steps-per-revolution","evidence:c-ring-composition","ref:sowa2005","ref:tan2024"],"tags":["steps","c-ring"],"difficulty":2,"url":"/machines/flagellar-motor#summary","cites":[{"source":"stat:Steps per turn","machine":"flagellar-motor","label":"Key number: Steps per turn","section":"summary"},{"source":"evidence:steps-per-revolution","machine":"flagellar-motor","label":"Steps per revolution of the rotor (Sowa Y 2005)","section":"evidence","anchor":"ev-steps-per-revolution"},{"source":"evidence:c-ring-composition","machine":"flagellar-motor","label":"Subunit counts and size of the C ring-containing motor (Tan J 2024)","section":"evidence","anchor":"ev-c-ring-composition"},{"source":"ref:sowa2005","machine":"flagellar-motor","label":"Sowa et al.","section":"sources","anchor":"ref-sowa2005","href":"https://doi.org/10.1038/nature04003"},{"source":"ref:tan2024","machine":"flagellar-motor","label":"Tan et al.","section":"sources","anchor":"ref-tan2024","href":"https://doi.org/10.1038/s41422-024-01017-z"}]},{"id":"flagellar-motor-wider-ring-proposal","machine":"flagellar-motor","kind":"qa","prompt":"What proposed (not yet done) way could build a stronger flagellar motor, copying how nature raises torque?","answer":"Transplant scaffold rings that hold more stator units at a wider radius.","explanation":"High-torque motors such as those of Vibrio and Campylobacter use scaffolds to place more stator units further out. The idea's status is \"proposed\".","section":"summary","topic":"debate","sources":["frontier:Torque from stator-unit placement","evolution:Wider rings for more torque","ref:beeby2016"],"tags":["torque","frontier"],"difficulty":2,"url":"/machines/flagellar-motor#summary","cites":[{"source":"frontier:Torque from stator-unit placement","machine":"flagellar-motor","label":"Open question: Torque from stator-unit placement","section":"summary"},{"source":"evolution:Wider rings for more torque","machine":"flagellar-motor","label":"Wider rings for more torque","section":"summary"},{"source":"ref:beeby2016","machine":"flagellar-motor","label":"Beeby et al.","section":"sources","anchor":"ref-beeby2016","href":"https://doi.org/10.1073/pnas.1518952113"}]},{"id":"flagellar-motor-proton-site","machine":"flagellar-motor","kind":"qa","prompt":"In the E. coli flagellar stator unit, which residue takes up the incoming proton?","answer":"MotB Asp32 (a conserved aspartate on MotB).","explanation":"The proton enters a channel between MotA and MotB; Asp32 sits inside a ring of five MotA Thr209.","section":"mechanism","topic":"cycle","sources":["step:Ions flow through the stators","mechanism:Ions enter the stator","ref:santiveri2020"],"tags":["stator","ions"],"difficulty":2,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"step:Ions flow through the stators","machine":"flagellar-motor","label":"Step: Ions flow through the stators","section":"mechanism"},{"source":"mechanism:Ions enter the stator","machine":"flagellar-motor","label":"Step: Ions enter the stator","section":"mechanism"},{"source":"ref:santiveri2020","machine":"flagellar-motor","label":"Santiveri et al.","section":"sources","anchor":"ref-santiveri2020","href":"https://doi.org/10.1016/j.cell.2020.08.016"}]},{"id":"flagellar-motor-36-degrees","machine":"flagellar-motor","kind":"cloze","prompt":"In the proposed model of the flagellar stator, each ion turns the MotA ring about 36°, so {{ten}} ions turn MotA once.","answer":"ten","explanation":"This is a model built from the lopsided 5:2 structures; the MotA step has not been seen directly.","section":"mechanism","topic":"cycle","sources":["step:MotA turns around MotB","evidence:stator-36-degree-model","ref:deme2020"],"tags":["stator","rotation"],"difficulty":2,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"step:MotA turns around MotB","machine":"flagellar-motor","label":"Step: MotA turns around MotB","section":"mechanism"},{"source":"evidence:stator-36-degree-model","machine":"flagellar-motor","label":"Proposed rotation of the MotA ring per ion (Deme JC 2020)","section":"evidence","anchor":"ev-stator-36-degree-model"},{"source":"ref:deme2020","machine":"flagellar-motor","label":"Deme et al.","section":"sources","anchor":"ref-deme2020","href":"https://doi.org/10.1038/s41564-020-0788-8"}]},{"id":"flagellar-motor-ratchet-direction","machine":"flagellar-motor","kind":"qa","prompt":"In the Deme model of the flagellar stator, why do ions always push the MotA ring the same way?","answer":"The 5:2 ring is lopsided, so the two MotB aspartates sit in different surroundings; binding at one while the other lets go acts like a ratchet.","explanation":"Ions bind and leave the two MotB helices in turn, and MotB stays fixed by its cell-wall anchor.","section":"mechanism","topic":"cycle","sources":["step:MotA turns around MotB","ref:deme2020"],"tags":["stator","ratchet"],"difficulty":3,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"step:MotA turns around MotB","machine":"flagellar-motor","label":"Step: MotA turns around MotB","section":"mechanism"},{"source":"ref:deme2020","machine":"flagellar-motor","label":"Deme et al.","section":"sources","anchor":"ref-deme2020","href":"https://doi.org/10.1038/s41564-020-0788-8"}]},{"id":"flagellar-motor-torque-per-unit","machine":"flagellar-motor","kind":"qa","prompt":"Torque is force times lever. What force and lever radius give each enteric flagellar stator unit its torque of about 146 pN·nm?","answer":"About 7 pN at about 20 nm from the axis.","explanation":"The 7.3 pN is derived (146 pN·nm ÷ 20 nm), not measured directly. A full set of units gives the 1,260 pN·nm of an E. coli motor.","section":"mechanism","topic":"cycle","sources":["step:MotA drives FliG like a gear","evidence:force-per-stator-model","ref:beeby2016"],"tags":["torque","lever"],"difficulty":2,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"step:MotA drives FliG like a gear","machine":"flagellar-motor","label":"Step: MotA drives FliG like a gear","section":"mechanism"},{"source":"evidence:force-per-stator-model","machine":"flagellar-motor","label":"Force per stator unit and predicted motor torques (Beeby M 2016)","section":"evidence","anchor":"ev-force-per-stator-model"},{"source":"ref:beeby2016","machine":"flagellar-motor","label":"Beeby et al.","section":"sources","anchor":"ref-beeby2016","href":"https://doi.org/10.1073/pnas.1518952113"}]},{"id":"flagellar-motor-drive-train","machine":"flagellar-motor","kind":"qa","prompt":"In the flagellar motor, what part does the rod spin inside as it passes through the cell wall and outer membrane?","answer":"The LP-ring, a bushing that does not turn.","explanation":"Torque goes C-ring → MS-ring → rod → hook → filament; the LP-ring (FlgH, FlgI) is anchored and lets the rod spin.","section":"mechanism","topic":"cycle","sources":["step:The rotor turns counterclockwise","mechanism:Torque travels up the drive shaft","ref:johnson2021"],"tags":["rotor","bearing"],"difficulty":2,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"step:The rotor turns counterclockwise","machine":"flagellar-motor","label":"Step: The rotor turns counterclockwise","section":"mechanism"},{"source":"mechanism:Torque travels up the drive shaft","machine":"flagellar-motor","label":"Step: Torque travels up the drive shaft","section":"mechanism"},{"source":"ref:johnson2021","machine":"flagellar-motor","label":"Johnson et al.","section":"sources","anchor":"ref-johnson2021","href":"https://doi.org/10.1038/s41564-021-00895-y"}]},{"id":"flagellar-motor-switch-flig-flip","machine":"flagellar-motor","kind":"qa","prompt":"When CheY-P binds the flagellar C-ring, how do the FliG domains move, and what does that change for the stator units?","answer":"They turn by 180°, so the stator units meet FliG from the inside of the ring instead of the outside.","explanation":"The ring keeps its 34-fold symmetry; only the contact side changes.","section":"mechanism","topic":"cycle","sources":["mechanism:CheY-P flips the switch","evidence:switch-fligc-180-johnson","ref:johnson2024","ref:tan2024"],"tags":["switch","c-ring"],"difficulty":2,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"mechanism:CheY-P flips the switch","machine":"flagellar-motor","label":"Step: CheY-P flips the switch","section":"mechanism"},{"source":"evidence:switch-fligc-180-johnson","machine":"flagellar-motor","label":"FliG movement and stator position in the two rotation states (Johnson S 2024)","section":"evidence","anchor":"ev-switch-fligc-180-johnson"},{"source":"ref:johnson2024","machine":"flagellar-motor","label":"Johnson et al.","section":"sources","anchor":"ref-johnson2024","href":"https://doi.org/10.1038/s41564-024-01630-z"},{"source":"ref:tan2024","machine":"flagellar-motor","label":"Tan et al.","section":"sources","anchor":"ref-tan2024","href":"https://doi.org/10.1038/s41422-024-01017-z"}]},{"id":"flagellar-motor-gear-reversal","machine":"flagellar-motor","kind":"qa","prompt":"MotA keeps turning clockwise in both states of the flagellar motor. Why does moving its contact from the outer to the inner face of the FliG ring reverse the rotor?","answer":"Gears that touch on their outer faces turn opposite ways; a gear driving the inside of a ring turns it the same way.","explanation":"So the rotor turns counterclockwise with outside contact and clockwise with inside contact, without reversing the ion flow.","section":"mechanism","topic":"cycle","sources":["step:The rotor turns clockwise","mechanism:CheY-P flips the switch","ref:johnson2024"],"tags":["switch","gear"],"difficulty":3,"url":"/machines/flagellar-motor#mechanism","cites":[{"source":"step:The rotor turns clockwise","machine":"flagellar-motor","label":"Step: The rotor turns clockwise","section":"mechanism"},{"source":"mechanism:CheY-P flips the switch","machine":"flagellar-motor","label":"Step: CheY-P flips the switch","section":"mechanism"},{"source":"ref:johnson2024","machine":"flagellar-motor","label":"Johnson et al.","section":"sources","anchor":"ref-johnson2024","href":"https://doi.org/10.1038/s41564-024-01630-z"}]},{"id":"flagellar-motor-catch-bond","machine":"flagellar-motor","kind":"qa","prompt":"Why do more stator units bind the flagellar motor when the load rises?","answer":"The MotB anchor forms a catch bond that holds longer under force.","explanation":"Stator units bind and leave all the time; under high load each bound unit stays longer, so more gather and torque rises.","section":"mechanism","topic":"cycle","sources":["mechanism:Load sets the number of stator units","evidence:stator-catch-bond","ref:nord2017"],"tags":["stator","load"],"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"},{"source":"ref:nord2017","machine":"flagellar-motor","label":"Nord et al.","section":"sources","anchor":"ref-nord2017","href":"https://doi.org/10.1073/pnas.1716002114"}]},{"id":"flagellar-motor-coast-distance","machine":"flagellar-motor","kind":"qa","prompt":"About how far does a swimming bacterium coast after its flagellar motors stop?","answer":"About 0.1 Å, less than an atom's width.","explanation":"At this size water feels like thick syrup (Reynolds number about 3 × 10^-5), so drag stops the cell almost at once. Purcell's order-of-magnitude estimate.","section":"story","topic":"numbers","sources":["fact:F6","stop:cell","ref:purcell1977"],"tags":["low-reynolds","swimming"],"difficulty":2,"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"},{"source":"ref:purcell1977","machine":"flagellar-motor","label":"Purcell 1977","section":"sources","href":"https://doi.org/10.1119/1.10903"}]},{"id":"flagellar-motor-reverse-gear-analogy","machine":"flagellar-motor","kind":"qa","prompt":"The flagellar motor is like a car with a reverse gear. Where does that analogy break?","answer":"Reversing does not drive the cell backward; it breaks up the bundle and the cell tumbles to a new heading.","section":"story","topic":"purpose","sources":["analogy:A car with a reverse gear"],"tags":["analogy","switch"],"difficulty":2,"url":"/machines/flagellar-motor#story","cites":[{"source":"analogy:A car with a reverse gear","machine":"flagellar-motor","label":"Analogy: A car with a reverse gear","section":"story"}]},{"id":"flagellar-motor-protons-per-turn","machine":"flagellar-motor","kind":"qa","prompt":"About how many protons pass through the flagellar motor of Streptococcus per rotor turn?","answer":"About 1,240 (± 240).","explanation":"Measured in Streptococcus, not E. coli, so it should not be multiplied by E. coli motor speeds. Proton flux rises in direct proportion to speed.","section":"story","topic":"numbers","sources":["fact:F10","evidence:proton-flux-speed","ref:bnid109759"],"tags":["protons","energy"],"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":"evidence:proton-flux-speed","machine":"flagellar-motor","label":"Proton flux through the motor against speed (Meister M 1987)","section":"evidence","anchor":"ev-proton-flux-speed"},{"source":"ref:bnid109759","machine":"flagellar-motor","label":"BNID 109759","section":"sources","href":"https://bionumbers.hms.harvard.edu/bionumber.aspx?id=109759"}]},{"id":"flagellar-motor-atp-synthase-link","machine":"flagellar-motor","kind":"qa","prompt":"Which part of the flagellar motor resembles the F1 α and β subunits of ATP synthase?","answer":"The export ATPase FliI.","explanation":"FliJ also looks like the γ coiled-coil, so the flagellar export machine and the rotary ATPases likely share an ancestor.","section":"story","topic":"parts","sources":["link:atp-synthase","evolution:Shared parts with ATP synthase","ref:imada2007","ref:ibuki2011"],"tags":["evolution","atp-synthase"],"difficulty":2,"url":"/machines/flagellar-motor#story","cites":[{"source":"link:atp-synthase","machine":"flagellar-motor","label":"Link to ATP synthase","section":"story"},{"source":"evolution:Shared parts with ATP synthase","machine":"flagellar-motor","label":"Shared parts with ATP synthase","section":"summary"},{"source":"ref:imada2007","machine":"flagellar-motor","label":"Imada et al.","section":"sources","anchor":"ref-imada2007","href":"https://doi.org/10.1073/pnas.0608090104"},{"source":"ref:ibuki2011","machine":"flagellar-motor","label":"Ibuki et al.","section":"sources","anchor":"ref-ibuki2011","href":"https://doi.org/10.1038/nsmb.1977"}]},{"id":"flagellar-motor-how-torque-measured","machine":"flagellar-motor","kind":"qa","prompt":"How did Reid and colleagues measure the torque of single E. coli flagellar motors?","answer":"They tracked a 1 µm bead on a flagellar stub by laser interferometry and took torque = speed × bead drag coefficient.","explanation":"At these low speeds torque sits near its stall value, so a fully induced motor gives the motor's maximum, about 1,260 pN·nm.","section":"evidence","topic":"numbers","sources":["evidence:stall-torque-fully-induced","ref:reid2006"],"tags":["bead-assay","torque"],"difficulty":2,"url":"/machines/flagellar-motor#evidence","cites":[{"source":"evidence:stall-torque-fully-induced","machine":"flagellar-motor","label":"Torque of a fully induced E. coli motor at high load (Reid SW 2006)","section":"evidence","anchor":"ev-stall-torque-fully-induced"},{"source":"ref:reid2006","machine":"flagellar-motor","label":"Reid et al.","section":"sources","anchor":"ref-reid2006","href":"https://doi.org/10.1073/pnas.0509932103"}]},{"id":"flagellar-motor-resurrection-count","machine":"flagellar-motor","kind":"qa","prompt":"In \"resurrection\" experiments on paralysed E. coli flagellar motors, how was the number of stator units counted?","answer":"By counting the discrete jumps in bead speed as new stator units joined the motor.","explanation":"Reid and colleagues counted at least 11 levels, beating the older estimate of eight; the jumps got smaller at high numbers.","section":"evidence","topic":"numbers","sources":["evidence:stator-number-resurrection","evidence:resurrection-eight-steps","ref:reid2006","ref:blair1988"],"tags":["stator","resurrection"],"difficulty":2,"url":"/machines/flagellar-motor#evidence","cites":[{"source":"evidence:stator-number-resurrection","machine":"flagellar-motor","label":"Maximum number of torque-generating units (Reid SW 2006)","section":"evidence","anchor":"ev-stator-number-resurrection"},{"source":"evidence:resurrection-eight-steps","machine":"flagellar-motor","label":"Torque restoration in repaired motors (Blair DF 1988)","section":"evidence","anchor":"ev-resurrection-eight-steps"},{"source":"ref:reid2006","machine":"flagellar-motor","label":"Reid et al.","section":"sources","anchor":"ref-reid2006","href":"https://doi.org/10.1073/pnas.0509932103"},{"source":"ref:blair1988","machine":"flagellar-motor","label":"Blair & Berg, Science 1988","section":"sources","anchor":"ref-blair1988","href":"https://doi.org/10.1126/science.2849208"}]},{"id":"flagellar-motor-zero-load-one-unit","machine":"flagellar-motor","kind":"qa","prompt":"Near zero load, how does the speed of the E. coli flagellar motor depend on the number of stator units?","answer":"It does not: one unit gives the same speed as many (about 300 Hz at 23 °C).","explanation":"Yuan and Berg used 60 nm gold spheres on hooks with no filament; a sphere jumped from rest to full speed when the first unit arrived.","section":"evidence","topic":"numbers","sources":["evidence:zero-load-one-stator","ref:yuan2008"],"tags":["speed","load"],"difficulty":3,"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":"ref:yuan2008","machine":"flagellar-motor","label":"Yuan & Berg, PNAS 2008","section":"sources","anchor":"ref-yuan2008","href":"https://doi.org/10.1073/pnas.0711539105"}]},{"id":"flagellar-motor-torque-speed-knee","machine":"flagellar-motor","kind":"cloze","prompt":"At 23 °C the torque of the E. coli flagellar motor stays roughly constant up to a knee near {{200 Hz}}, then falls to zero near 350 Hz.","answer":"200 Hz","explanation":"Chen and Berg varied the load with Ficoll. Later work puts the knee at 170–175 Hz.","section":"evidence","topic":"numbers","sources":["evidence:knee-speed","stat:Knee speed, E. coli (23 °C)","evidence:zero-torque-speed","ref:chen2000"],"tags":["torque-speed"],"difficulty":2,"url":"/machines/flagellar-motor#evidence","cites":[{"source":"evidence:knee-speed","machine":"flagellar-motor","label":"Knee of the torque–speed curve (Chen X 2000)","section":"evidence","anchor":"ev-knee-speed"},{"source":"stat:Knee speed, E. coli (23 °C)","machine":"flagellar-motor","label":"Key number: Knee speed, E. coli (23 °C)","section":"summary"},{"source":"evidence:zero-torque-speed","machine":"flagellar-motor","label":"Speed at which torque reaches zero (Chen X 2000)","section":"evidence","anchor":"ev-zero-torque-speed"},{"source":"ref:chen2000","machine":"flagellar-motor","label":"Chen & Berg, Biophys J 2000","section":"sources","anchor":"ref-chen2000","href":"https://doi.org/10.1016/s0006-3495(00)76662-8"}]},{"id":"flagellar-motor-backward-steps","machine":"flagellar-motor","kind":"qa","prompt":"What did the occasional backward steps of the slowed flagellar motor (Sowa et al. 2005) show about each step?","answer":"Each step uses little energy, close to that of one ion crossing.","explanation":"They slowed a Na+-driven chimeric motor in E. coli with low sodium-motive force and few stator units to see 26 steps per turn.","section":"evidence","topic":"cycle","sources":["evidence:steps-per-revolution","mechanism:The motor moves in steps","ref:sowa2005"],"tags":["steps","energy"],"difficulty":3,"url":"/machines/flagellar-motor#evidence","cites":[{"source":"evidence:steps-per-revolution","machine":"flagellar-motor","label":"Steps per revolution of the rotor (Sowa Y 2005)","section":"evidence","anchor":"ev-steps-per-revolution"},{"source":"mechanism:The motor moves in steps","machine":"flagellar-motor","label":"Step: The motor moves in steps","section":"mechanism"},{"source":"ref:sowa2005","machine":"flagellar-motor","label":"Sowa et al.","section":"sources","anchor":"ref-sowa2005","href":"https://doi.org/10.1038/nature04003"}]},{"id":"flagellar-motor-hill-disagreement","machine":"flagellar-motor","kind":"qa","prompt":"Cluzel et al. found a Hill coefficient of about 10 for the flagellar switch; Yuan and Berg found 16.5–20.7. Why the difference?","answer":"Yuan and Berg changed CheY-P faster than motors could change their FliM content; the earlier curve pooled motors that had adapted their FliM.","explanation":"With a fixed make-up the switch is about twice as steep.","section":"evidence","topic":"debate","sources":["evidence:switch-hill-fixed-flim","evidence:switch-ultrasensitivity","stat:Switch sensitivity to CheY-P","ref:yuan2013","ref:cluzel2000"],"tags":["switch","ultrasensitivity"],"difficulty":3,"url":"/machines/flagellar-motor#evidence","cites":[{"source":"evidence:switch-hill-fixed-flim","machine":"flagellar-motor","label":"Hill coefficient of the switch for motors with fixed FliM content (Yuan J 2013)","section":"evidence","anchor":"ev-switch-hill-fixed-flim"},{"source":"evidence:switch-ultrasensitivity","machine":"flagellar-motor","label":"Steepness of the CW bias response to CheY-P (Cluzel P 2000)","section":"evidence","anchor":"ev-switch-ultrasensitivity"},{"source":"stat:Switch sensitivity to CheY-P","machine":"flagellar-motor","label":"Key number: Switch sensitivity to CheY-P","section":"summary"},{"source":"ref:yuan2013","machine":"flagellar-motor","label":"Yuan & Berg, J Mol Biol 2013","section":"sources","anchor":"ref-yuan2013","href":"https://doi.org/10.1016/j.jmb.2013.02.016"},{"source":"ref:cluzel2000","machine":"flagellar-motor","label":"Cluzel et al.","section":"sources","anchor":"ref-cluzel2000","href":"https://doi.org/10.1126/science.287.5458.1652"}]}]}