{"format":"molecular-machines.cards","version":1,"updated":"2026-10-12","decks":[{"machine":"hemoglobin","name":"Hemoglobin","count":10,"url":"/learn/cards/hemoglobin.json"}],"cards":[{"id":"hemoglobin-job","machine":"hemoglobin","kind":"qa","prompt":"Every cell needs oxygen to make ATP, but water carries very little oxygen. So blood packs it into hemoglobin. What does hemoglobin carry, and from where to where?","answer":"Oxygen, from the lungs to the tissues.","explanation":"It grabs oxygen in the lungs and lets go in the tissues. It carries about 98.5 percent of the oxygen in blood.","section":"summary","topic":"purpose","sources":["machine:summary","story:summary","fact:N2"],"tags":["oxygen","transport"],"difficulty":1,"url":"/machines/hemoglobin#summary","cites":[{"source":"machine:summary","machine":"hemoglobin","label":"Summary","section":"summary"},{"source":"story:summary","machine":"hemoglobin","label":"Big picture","section":"story"},{"source":"fact:N2","machine":"hemoglobin","label":"Share of blood oxygen carried by hemoglobin: 98.5 %","section":"story"}]},{"id":"hemoglobin-no-fuel","machine":"hemoglobin","kind":"qa","prompt":"Most machines in the atlas burn ATP or ride an ion flow. Hemoglobin spends no chemical fuel at all. What does the work of loading and unloading oxygen instead?","answer":"The oxygen gradient and the molecule's own shape switch.","explanation":"Where oxygen is high it fills up; where oxygen is low it lets go.","section":"summary","topic":"purpose","sources":["machine:energy","link:serca"],"tags":["energy","no-fuel"],"difficulty":1,"url":"/machines/hemoglobin#summary","cites":[{"source":"machine:energy","machine":"hemoglobin","label":"Summary","section":"summary"},{"source":"link:serca","machine":"hemoglobin","label":"Link to SERCA calcium pump","section":"story"}]},{"id":"hemoglobin-sigmoid-why","machine":"hemoglobin","kind":"qa","prompt":"Hemoglobin has four oxygen seats, one on each heme. It fills up fully in the lungs and empties well in the tissues. How do the four seats work to make this possible?","answer":"As a team: the whole molecule flips from a weak-grip shape to a strong-grip shape.","explanation":"Once some seats fill, the rest fill more easily. This makes the loading curve S-shaped.","section":"summary","topic":"cycle","sources":["machine:summary","mechanism:The last oxygens bind","analogy:A delivery van with four seats that loads and unloads as a team"],"tags":["cooperativity","switch"],"difficulty":1,"url":"/machines/hemoglobin#summary","cites":[{"source":"machine:summary","machine":"hemoglobin","label":"Summary","section":"summary"},{"source":"mechanism:The last oxygens bind","machine":"hemoglobin","label":"Step: The last oxygens bind","section":"mechanism"},{"source":"analogy:A delivery van with four seats that loads and unloads as a team","machine":"hemoglobin","label":"Analogy: A delivery van with four seats that loads and unloads as a team","section":"story"}]},{"id":"hemoglobin-iron-into-plane","machine":"hemoglobin","kind":"qa","prompt":"Without oxygen, the iron in each heme sits a little out of its flat ring. What happens when the first oxygen binds that iron?","answer":"The iron moves into the ring and pulls a helix of the protein with it.","explanation":"This turns a tiny move of the iron into a move of a whole helix. The shift travels to the contact between the two halves and tips the switch.","section":"mechanism","topic":"cycle","sources":["mechanism:The first oxygen binds","step:The first oxygen binds an α heme","mechanism:The change reaches the dimer interface"],"tags":["iron","heme"],"difficulty":2,"url":"/machines/hemoglobin#mechanism","cites":[{"source":"mechanism:The first oxygen binds","machine":"hemoglobin","label":"Step: The first oxygen binds","section":"mechanism"},{"source":"step:The first oxygen binds an α heme","machine":"hemoglobin","label":"Step: The first oxygen binds an α heme","section":"mechanism"},{"source":"mechanism:The change reaches the dimer interface","machine":"hemoglobin","label":"Step: The change reaches the dimer interface","section":"mechanism"}]},{"id":"hemoglobin-acid-releases-oxygen","machine":"hemoglobin","kind":"qa","prompt":"Working muscle makes acid and carbon dioxide. Hemoglobin is like a sponge that squeezes itself in acid. What do acid and carbon dioxide do to hemoglobin there?","answer":"They push it back to its weak-grip shape, so it lets go of more oxygen.","explanation":"They bind the weak-grip shape better than the strong-grip one. 2,3-BPG in the red cell works the same way.","section":"mechanism","topic":"cycle","sources":["mechanism:Tissues push the switch back","step:Working tissue: acid and CO2","analogy:A sponge that squeezes itself in acid"],"tags":["bohr","release"],"difficulty":1,"url":"/machines/hemoglobin#mechanism","cites":[{"source":"mechanism:Tissues push the switch back","machine":"hemoglobin","label":"Step: Tissues push the switch back","section":"mechanism"},{"source":"step:Working tissue: acid and CO2","machine":"hemoglobin","label":"Step: Working tissue: acid and CO2","section":"mechanism"},{"source":"analogy:A sponge that squeezes itself in acid","machine":"hemoglobin","label":"Analogy: A sponge that squeezes itself in acid","section":"story"}]},{"id":"hemoglobin-per-red-cell","machine":"hemoglobin","kind":"qa","prompt":"A red blood cell is a tiny bag packed with hemoglobin, and your body has about 25 trillion of them. Roughly how many hemoglobin molecules does one red cell hold?","answer":"About 270 million.","explanation":"With four oxygen seats each, one full red cell carries about a billion oxygen molecules.","section":"story","topic":"numbers","sources":["fact:N8","story:summary","fact:N18"],"tags":["scale"],"difficulty":2,"url":"/machines/hemoglobin#story","cites":[{"source":"fact:N8","machine":"hemoglobin","label":"Hemoglobin molecules in one red cell: 2.7 × 10^8","section":"story"},{"source":"story:summary","machine":"hemoglobin","label":"Big picture","section":"story"},{"source":"fact:N18","machine":"hemoglobin","label":"Oxygen molecules in one full red cell: 1.1 × 10^9 O2","section":"story"}]},{"id":"hemoglobin-rbc-no-burn","machine":"hemoglobin","kind":"qa","prompt":"A red blood cell carries about a billion oxygen molecules on each trip. Why does it not burn that oxygen itself?","answer":"It has no mitochondria.","explanation":"A mature red cell has no nucleus and no mitochondria, so it delivers the oxygen instead of using it.","section":"story","topic":"purpose","sources":["fact:N9","stop:rbc"],"tags":["red-cell"],"difficulty":1,"url":"/machines/hemoglobin#story","cites":[{"source":"fact:N9","machine":"hemoglobin","label":"Nucleus and mitochondria in a mature red cell: None","section":"story"},{"source":"stop:rbc","machine":"hemoglobin","label":"Big picture: Red blood cell","section":"story"}]},{"id":"hemoglobin-cas9-fetal","machine":"hemoglobin","kind":"qa","prompt":"Fetal hemoglobin grips oxygen more tightly than adult hemoglobin, so a fetus can take oxygen from its mother's blood. How does an approved Cas9 therapy use this to treat sickle cell disease?","answer":"It edits a person's blood stem cells so their red cells make fetal hemoglobin again.","explanation":"Turning the fetal form back on treats sickle cell disease and beta-thalassaemia, without changing the hemoglobin protein itself.","section":"story","topic":"purpose","sources":["link:cas9","frontier:Re-induced fetal hemoglobin","step:2,3-BPG returns"],"tags":["disease","cas9"],"difficulty":2,"url":"/machines/hemoglobin#story","cites":[{"source":"link:cas9","machine":"hemoglobin","label":"Link to Cas9","section":"story"},{"source":"frontier:Re-induced fetal hemoglobin","machine":"hemoglobin","label":"Open question: Re-induced fetal hemoglobin","section":"summary"},{"source":"step:2,3-BPG returns","machine":"hemoglobin","label":"Step: 2,3-BPG returns","section":"mechanism"}]},{"id":"hemoglobin-myoglobin-no-switch","machine":"hemoglobin","kind":"qa","prompt":"Myoglobin in muscle has the same fold as one hemoglobin chain, but it is a single chain with a single heme. Why can it not switch as a team like hemoglobin?","answer":"It has no partner chains, so there is no contact that can pass the signal on.","explanation":"Teamwork needed a stable contact between chains, where a change in one chain could become a change of the whole molecule.","section":"summary","topic":"parts","sources":["evolution:Cooperativity after the monomer","species:Myoglobin"],"tags":["evolution","myoglobin"],"difficulty":2,"url":"/machines/hemoglobin#summary","cites":[{"source":"evolution:Cooperativity after the monomer","machine":"hemoglobin","label":"Cooperativity after the monomer","section":"summary"},{"source":"species:Myoglobin","machine":"hemoglobin","label":"Myoglobin","section":"summary"}]},{"id":"hemoglobin-beyond-two-states","machine":"hemoglobin","kind":"qa","prompt":"The classic picture says hemoglobin has just two shapes, a weak-grip one and a strong-grip one. What does newer work add to that picture?","answer":"Smaller changes inside each shape also change the grip, so two shapes are not the whole story.","explanation":"Effectors can lower oxygen grip even while the molecule stays in its strong-grip shape.","section":"evidence","topic":"debate","sources":["frontier:Beyond two states","evidence:effectors-act-inside-the-r-state"],"tags":["allostery","open-question"],"difficulty":2,"url":"/machines/hemoglobin#evidence","cites":[{"source":"frontier:Beyond two states","machine":"hemoglobin","label":"Open question: Beyond two states","section":"summary"},{"source":"evidence:effectors-act-inside-the-r-state","machine":"hemoglobin","label":"Change in R-state oxygen affinity caused by allosteric effectors, with the quaternary structure held fixed (Shibayama N 2025)","section":"evidence","anchor":"ev-effectors-act-inside-the-r-state"}]}]}