Most Things Biology

3.1 The Membrane

Case home  ·  Start at the beginning  ·  The Reveal

Time To Read

31–47 minutes

Date Last Modified

The Cell & Its Transport · Chapter 1 of 7

1990, age fifteen: she has worked out the pattern herself — “Day one of my period: the fever. Then the vomiting, the kind that doesn’t stop when there’s nothing left. Then the legs” — and she has been to the emergency room too many times to be believed. “On the first visit they run tests. By the third they ask whether there’s a lot of stress at home. By the fifth they read the chart before they look at me.” This chapter starts at the wall every one of those tests stopped at: the plasma membrane — its oily bilayer, the fluids on either side of it, the proteins that move and sense — and the messenger that turns a lock on the outside of a cell without ever coming in.

The question you will answer

“They run tests” — but tests of what? Before you can weigh a run of emergency-room visits that found nothing, you need to know which side of a cell’s membrane each test could actually reach, and where an inflammatory alarm is raised and where it is received. The question for the chapter: when every test samples the outside of cells, what could still be happening behind the wall — and what message could still cross it?

Opener · Five Visits, One Wall · about 2½ minutes

The opener is the same narrated animation you will watch in Scene 1.

Watch it in Scene 1 ↓

Watch the opener first. It sets the question; the chapter answers it.

1

Read the chart

Patient chart · Stina

1990 · age 15 · the cycle, and the visits that ran out of credibility

1986–87 · 11–12
Module 2’s years: the move, Mom’s second surgery, the third ER of the year at eleven (“they took blood”), period-day fevers and hot knees at twelve, a truant officer, a second psychiatrist after “blood and X-ray and nothing.”

1986–90 · 11–15
Her feet, shins, back and legs hurt – standing on the school’s concrete floors, sitting in desks too tall for her, carrying a twenty-pound backpack. Gym leaves her soaked, shaking and wrung out in a way the other kids are not.

1990 · 15
She works out the pattern on her own calendar: day one of her period, the fever; then vomiting, “the kind that doesn’t stop when there’s nothing left”; then the legs; three or four days, then fine, and again four weeks later. Too many ER visits to be believed — “On the first visit they run tests.” No test or result is recorded.

Told
“Growing pains.” “Deconditioned.” A school nurse asks whether she has her period. In her own account of the ER visits, by the third “they ask whether there’s a lot of stress at home.”

Chart clues this module: 3 · this module (logged in Chapter 7) — undated, “Every workup”; no lab values or cultures are recorded for 1986–90

Before you go on · your prediction

Fifteen, the cycle worked out on her calendar, and emergency rooms that ran tests on the first visit and asked about stress at home by the third. Before you learn what a membrane does, what is the best reading of tests that kept finding nothing?

Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.

Nothing was there to find — “a lot of stress at home”

Watch which side of the membrane each test samples, and whether stress can put a fever on the same square of a calendar every month.

Tests of the fluid outside cells, while the trouble sits inside them

Watch which molecules never leave the inside of a cell, and which ones leave and work on another cell without ever entering it.

“An infection we cannot find” — a germ hiding where the tests kept missing it

Watch what a culture can grow and what a blood count can count — and what a germ would leave on the outside of cells for a test to catch.

From Stina’s diary

Age 15 · “The cycle”

2

The story

Three short movements: the case, the anatomy and physiology, then the tie back to Stina. Each links to the scene where you’ll see it move.

I · It’s on my calendar

Stina was fifteen, and she had worked out the pattern, and she wished she hadn’t, because now she could see it coming. Day one of her period: the fever. Then the vomiting, “the kind that doesn’t stop when there’s nothing left.” Then the legs. Three or four days of it, then it lifted and she was fine, and four weeks later it happened again. She had been to the emergency room so many times that she had a name for what it does to a person: “you use up your credibility.” She could chart it by visit. On the first visit they ran tests. By the third they asked whether there was a lot of stress at home. By the fifth they read the chart before they looked at her, and their faces changed before anyone had said a word. “You can feel that kind of shame through a curtain,” she wrote. The diary doesn’t say which tests that first visit ran. The ones she had named before were the ordinary kind — “they took blood” at eleven, “blood and X-ray and nothing” at twelve — and none of them, then or now, had put a name to the fevers. Two years earlier her mother had said she was growing. A year earlier she had refused to see the doctor at all, because she already knew they would say normal. Now she had something nobody in the hospital had asked to see, and she had made it herself, one circled square at a time. “Nobody has ever asked what all the visits have in common,” she wrote. “I know what they have in common. It’s on my calendar.”

See it move: Scene 1 · Five Visits, One Wall · Scene 6 · The Message at the Door

II · The wall, the water and the doors

A membrane is not a brick wall; it’s a busy doorway, and doorways are where the drama happens. Every cell is wrapped in a plasma membrane — the property line between the living inside and everything else — and it separates without isolating, the way the doors and windows of your house let you control heat and air without letting the outside pour in. Its fabric is the phospholipid bilayer: each phospholipid a water-loving phosphate head on a glycerol backbone with two water-hating fatty-acid tails. Drop millions in water and the water does it for free — heads out, tails hidden, a sheet about 7 to 10 nanometers thick. That oily core is the whole point. Oxygen and carbon dioxide slip through, water creeps through slowly (aquaporins are the express lane), and ions, glucose and proteins are stopped. Cholesterol tucked between the tails buffers fluidity, stiffening the bilayer when warm and keeping it fluid when cold. The membrane also draws the body’s map. About two-thirds of your water is intracellular fluid; the rest is extracellular — mostly interstitial fluid bathing every cell, plus plasma in the vessels — and the two sides differ on purpose: sodium about 145 mM outside and 12–15 inside, potassium about 4 outside and 140 inside. The work is done by proteins. Integral proteins span the bilayer; peripheral ones cling to one face. They channel, carry, receive, catalyze, identify and anchor, and the sugar chains on glycoproteins and glycolipids crust the outside like sugar on a candy — the glycocalyx, a name tag that says “hello, my name is Amy.” Channels come with three kinds of lock: none at all (leak), a chemical key — a ligand, which just means “thing” — and a change in voltage, like a hotel keycard. And receptors read messengers that never come inside.

See it move: Scene 2 · The Water Does It for Free · Scene 3 · Inside, Outside, In Between · Scene 4 · Sugar on the Outside · Scene 5 · Doors With Three Kinds of Lock

III · A message delivered, on schedule

Read the cycle with the membrane in mind and something comes into view that nobody behind those curtains was placed to see. The routine tests a fifteen-year-old was likely to get in an emergency room in 1990 all sampled the outside of cells. A chemistry panel reads plasma — sodium, potassium, glucose, albumin — the extracellular side of the line. A blood count counts whole cells: it can say there are more white cells than usual, not what any one of them is doing inside. An X-ray is a shadow of tissue. None of them can open a white blood cell sitting in an inflamed lining and look in. The disease you are learning, familial Mediterranean fever, begins exactly there. Its alarm sensor, a protein called pyrin, is not in the membrane at all; it sits in the cytosol of neutrophils and monocytes, behind the property line. When it fires, the cell releases interleukin-1β, and that messenger works on other cells — among them the cells lining blood vessels at the edge of the hypothalamus, where it turns a receptor’s lock from the outside and the thermostat goes up. So “Day one of my period: the fever” reads as a message delivered on schedule to a door that exists in every one of us. This is not autoimmunity, either: no antibody is misreading her cells’ name tags. It is an alarm with no intruder, raised inside her own cells. Honesty flag: the diary records no tests by name for these visits, no values and no culture; this is a reading of what such tests could and could not see, not a report of what hers showed. The chart’s word for the pattern — “An infection we cannot find” — waits in Chapter 7.

See it move: Scene 1 · Five Visits, One Wall · Scene 4 · Sugar on the Outside · Scene 6 · The Message at the Door

3

Watch · explore · think · check

6 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

How to read the scenes

  • Animation (dark teal) — a short animated explainer, one idea each
  • Slide (dark teal) — a still to study: a micrograph, a diagram or a map
  • Explore (purple) — something to push on while you watch
  • Game (purple) — a quick challenge
  • Think (gold) — reason it out, then spot-check yourself with a few true/false questions; not graded
  • Check (gold) — a quick H5P that scores itself

Scene 1 of 6

Five Visits, One Wall

“On the first visit they run tests” — and every tube, film and plate an emergency room takes holds what lies outside cells.

Animation · in production · 2 min 25 s

Five Visits, One Wall

An ER curtain in 1990; behind it, a fifteen-year-old on a cart. At home, a wall calendar with day one of each period circled and the next three or four days shaded. A visit counter ticks 1 … 3 … 5: at 1 a tag reads “they run tests,” at 3 “stress at home?”, at 5 a chart is opened before anyone looks at the patient. Back to visit 1: a needle at the elbow fills a tube, and a centrifuge splits it into straw-colored plasma above and packed cells below. A second card shows the kinds of tests an ER of 1990 could order — a chemistry panel, a blood count, an X-ray film, a culture plate — stamped “the diary names none at fifteen.” The screen becomes a body-water map: one cell drawn with a bright plasma membrane, INTRACELLULAR FLUID inside (about two-thirds of body water), and outside it EXTRACELLULAR FLUID, split into INTERSTITIAL FLUID (most of it) and PLASMA (the rest, inside a vessel). Each test drops onto the map and lands outside the bright line: the chemistry panel in plasma; the blood count on whole cells, counted but not opened (a white cell with a padlock over its cytosol); the X-ray as a gray shadow; the culture plate lands outside the line too, stamped GROWS ONLY GERMS FROM OUTSIDE CELLS · NONE RECORDED AT 15. Inside the padlocked white cell a small sensor glows, labeled PYRIN — CYTOSOL, and a paper airplane marked 1β leaves the cell toward the edge of the frame. Last frame: the bright line labeled THE PROPERTY LINE, and Amy’s line, “A membrane is not a brick wall — it’s a busy doorway.”

Prefer the full lecture? Cell Membranes (original video on the site)

Explore · in production

Sort the sample tubes

A lab bench split by a bold horizontal line labeled PLASMA MEMBRANE — INSIDE below the line, OUTSIDE above it. Below the line sits one rack: INTRACELLULAR FLUID (about two-thirds of body water). Above it sit two racks: INTERSTITIAL FLUID (the larger share of the outside, bathing every cell) and PLASMA (the fluid part of blood, in a vessel drawn along the top edge, separated from the interstitial rack by a dashed capillary wall). A tray holds ten capped sample tubes, each labeled with one molecule: Na⁺ · K⁺ · glucose · albumin · CRP · hemoglobin · ATP · pyrin · pro-IL-1β · IL-1β. Round 1, where does it live? Before each drop the student predicts its main home; a tube can go in more than one rack, and a MOSTLY toggle marks the main one. Each placement opens a card with the reason and a textbook value, labeled as such: Na⁺ — mostly outside, about 145 mM in plasma and interstitial fluid against 12–15 mM inside; K⁺ — mostly inside, about 140 mM against about 4 mM outside; glucose — outside, in plasma and interstitial fluid (fasting plasma about 70–99 mg/dL), and low as free glucose inside because cells tag it with a phosphate the moment it enters; albumin — plasma (about 3.5–5 g/dL), with much less in interstitial fluid because the capillary wall holds most of it back (Chapter 5’s colloid pressure); CRP — made inside liver cells and shipped out into plasma, where it rises with inflammation; hemoglobin — inside red blood cells, so intracellular even though it rides in the blood; ATP — inside every cell at millimolar levels, almost none outside; pyrin — inside, in the cytosol of neutrophils and monocytes, never outside; pro-IL-1β — inside, in the cytosol, built locked (Module 2.6); IL-1β — made inside, released into the interstitial fluid around the cells that made it, a little reaching plasma. Misplacements explain themselves rather than just turning red (“hemoglobin rides in the blood but lives inside the red cell — a blood draw catches it only because it catches whole cells”). Round 2, which would a 1990 blood draw see? Two tubes appear: a CHEMISTRY tube (spun down; the lab reads its plasma or serum) and a COUNT tube (whole blood; the lab counts the cells and breaks the red cells open to read hemoglobin). The student ticks every molecule either tube would report. The reveal sorts the ten: routine — Na⁺, K⁺, glucose and albumin from the chemistry tube, hemoglobin and the white count from the count tube; orderable but not recorded for her — CRP (the test existed in 1990; canon keeps no lab values for these years); on no clinical slip — ATP, pyrin, pro-IL-1β; research assays only — IL-1β. A twist card: “A sample that sits too long, or is shaken, reads a falsely high potassium. Why?” The red cells break, and their 140 mM inside spills into a 4 mM outside: the membrane is the property line, and the lab is built to read one side of it. Closing line on the bench: every number a 1990 emergency room could print came from outside the line or from counting whole cells; the molecules that start an attack live inside it. Keyboard path: Tab moves through the tubes, Enter picks one up, arrow keys choose a rack, Space places it, M toggles MOSTLY; each placement is announced in a polite live region (“K⁺ placed: intracellular fluid, mostly — about 140 mM inside”). A <details> text version gives the ten molecules as a table (molecule · main home · textbook value · on a 1990 blood slip?).

Think · not graded

“On the first visit they run tests.” Pick one test an emergency room might have run — a chemistry panel, a blood count, an X-ray — and write down which side of a cell’s membrane it actually samples. Then name one molecule that test could never have seen, and say exactly where in the body that molecule lives.

The true/false spot-check for this prompt is in production.

Check · Single Choice

What does it mean that the plasma membrane is selectively permeable?

This self-scoring check is in production.

Evidence card 1

Add it to your evidence board

“On the first visit they run tests”: at fifteen she can count the ER visits, and the diary records no result from any of them.

Scene 2 of 6

The Water Does It for Free

Drop phospholipids in water and nobody has to arrange them — and the oily sheet they make is the whole point of a membrane.

Narrated slide · in production

The Water Does It for Free

Two of Amy’s narrated slides, each trimmed before its closing case lines. Slide 2, The Plasma Membrane: “the barrier between living and non-living things… but separation is not the same as isolation”; “I want you to think about the doors and windows of your house”; fluid mosaic — “fluid because it’s always rearranging itself, and mosaic because it is built from many different pieces, phospholipids, cholesterol, a whole bunch of different proteins”; the aliases sarcolemma (muscle) and axolemma (neurons): “same structure, fancier names, which just depends on the neighborhood.” Slide 3, The Phospholipid Bilayer: a polar head of glycerol and phosphate and two nonpolar tails; “Nobody arranges these — water does this for free”; “That oily core… is the entire point of the membrane”; “The membrane’s whole personality — picky, selective, gate-kept — comes from this one chemical fact.” Caption lines under the player: (1) While Slide 2 plays, the FMF box on its screen is out of date: colchicine is not part of Stina’s story at these ages, and potassium crossing a membrane trips a different alarm (NLRP3, Chapter 6) — what an attack sends across a white cell’s membrane is IL-1β, going out (Chapter 4). (2) Timed cue, Slide 3 at about 0:12, “a polar water-loving head made of glycerol and phosphate”: the head is a phosphate group on a glycerol backbone. (3) Timed cue, Slide 3 at about 1:13, “Water and glucose and ions… need a protein doorway”: water does cross the bilayer on its own, slowly; aquaporins are the express lane.

Prefer the full lecture? Cell Membranes (original video on the site)

Explore · in production

Squeeze the bilayer

A clear acrylic box, seen side-on, holds a slab of phospholipid bilayer — two rows of round heads facing water above and below, tangled tails meeting in the middle — with a scale bar reading about 7–10 nm. A warm-up before anything else: tap SHAKE and a scatter of loose phospholipids in water snaps into a bilayer by itself (“nobody arranges them — the water does it for free”; one line back to Module 2.4’s self-assembly). Two controls sit on the box: a TEMPERATURE DIAL (10 °C · 25 °C · 37 °C body · 40 °C fever · 45 °C) and a CHOLESTEROL SLIDER (none · low · typical animal plasma membrane · high), with small four-ring cholesterol molecules sliding in between the tails as it rises. A squeeze handle on the side presses the box, and two gauges respond. FLUIDITY: one tagged lipid blinks, and its wandering trail grows longer or shorter. LEAKINESS: a stream of test particles hits the slab from above — O₂ and CO₂ (small, nonpolar), water and urea (small, polar), glucose, Na⁺ and K⁺ — and a counter tallies what gets through each second. Before every change the student predicts MORE FLUID or LESS, MORE LEAKY or LESS. The model’s rules sit beside it in plain words: cold packs the tails tight (gel-like, stiff, it cracks under the squeeze); heat shakes them loose (sloppy, and leakier to small polar particles); cholesterol’s rigid rings get in the way both times — they break up the packing in the cold and restrain the tails in the heat — so cholesterol narrows the swing: a fluidity buffer. O₂ and CO₂ cross at every setting; water trickles; glucose and the ions never cross the bare bilayer at any setting. The student logs three findings: (1) no cholesterol at 10 °C — stiff; add cholesterol — fluid again; (2) no cholesterol at 45 °C — loose and leaky; add cholesterol — steadier; (3) 37 °C to 40 °C — a small change, labeled honestly: a fever of a few degrees nudges fluidity, it does not melt membranes, and a fever’s purpose lies elsewhere (Module 1.5). Last control, an AQUAPORIN toggle: drop water channels into the slab and the water counter jumps by orders of magnitude — one aquaporin can pass up to about three billion water molecules a second — while glucose and the ions still bounce off: water crosses slowly on its own, aquaporins are the express lane, and the barrier to everything charged is the whole point. A side card offers Amy’s hand version: hold your own hands up, heads out and tails in, and feel the bilayer arrange itself. Keyboard path: the dial and the slider are native range inputs with labeled steps; Space squeezes; every gauge change is announced in a live region in words (“Fluidity lower. Leak: water slower, ions none”). A <details> text version gives the model’s rules and a table of eight settings and their results.

Think · not graded

Your body runs at about 37 °C, and Stina’s fevers climb several degrees above that. Cholesterol stiffens a membrane when it’s warm and keeps it fluid when it’s cold. Predict what a membrane with no cholesterol would do at 40 °C and at 10 °C, and explain how one molecule can push fluidity in opposite directions depending on the temperature.

The true/false spot-check for this prompt is in production.

Check · Drag the Words

Build the membrane from the molecule up. Two distractors are on the bank.

This self-scoring check is in production.

Evidence card 2

Add it to your evidence board

“Three or four days of it, then it lifts and I’m fine”: each attack ends on its own; the entry names no treatment.

Scene 3 of 6

Inside, Outside, In Between

Two-thirds of your water is on the inside of a wall about eight nanometers thick — and every transport story starts with naming the two sides.

Narrated slide · in production

Inside, Outside, In Between

Amy’s narrated slide, Fluid Compartments, trimmed before its case close: “This slide looks almost too simple to be anatomy, but I promise you it is — we’re just labeling fluids instead of bones”; “imagine pouring all the cytoplasm from all your cells into one enormous glass, and that total volume is your intracellular fluid”; “transport is always a story about two sides of the membrane… here and there”; the compartment terms are “the coordinate system… the map we’re going to navigate by every time an ion or a water molecule moves.” Caption lines under the player: (1) Timed cue at about 0:26, where the audio says “we can call it also the interstitial fluid” and then “one particular cluster of cells”: extracellular fluid is interstitial fluid plus plasma (plus smaller pools — lymph, cerebrospinal, serous and synovial fluid), and interstitial fluid bathes the cells of every tissue, not one cluster — read “most of which is interstitial fluid.” (2) Intracellular fluid is about two-thirds of the body’s water.

Prefer the full lecture? Cell Membranes (original video on the site)

Explore · in production

Paint the map

A large line-drawn stencil fills the screen: one cell cut open (a plasma membrane drawn as a double line, cytosol, a few organelles, a nucleus inside its envelope), set in a strip of tissue with a capillary running past it (its wall, the plasma inside, two red cells floating in the plasma), and the spaces between cells left blank. At the edges sit two neighbors drawn in the same style — a muscle fiber and a neuron’s axon — each with its own outline unpainted. A palette of ten paint pots, each labeled with a clue rather than a name: “a bilayer about 7–10 nm thick” · “K⁺ about 140 mM, fixed negative proteins” · “Na⁺ about 145 mM, glucose, very little protein” · “Na⁺ about 145 mM and albumin, about 3.5–5 g/dL” · “about two-thirds of the body’s water” · “where the DNA is kept” · “the cytoplasm minus the organelles: the broth” · “the same membrane, in muscle” · “the same membrane, in a neuron” · “what a chemistry tube reads”. The student picks a pot, predicts its region, and paints. Correct fills lock with the region’s name (plasma membrane · cytosol / intracellular fluid · interstitial fluid · plasma · nucleus · sarcolemma · axolemma). Misfills explain themselves instead of just turning red: paint the albumin pot into the space between cells and the card says the capillary wall holds most albumin back, so interstitial fluid is roughly plasma with most of its protein removed; paint “two-thirds of the body’s water” into one cell and the color spreads to every cell at once — Amy’s one enormous glass of everyone’s cytoplasm; paint “the same membrane, in muscle” onto the capillary and it slides over to the muscle fiber — same structure, fancier names, depending on the neighborhood; paint “what a chemistry tube reads” anywhere inside the cell and it drains back out into the plasma. When the map is done, a water-budget bar fills in beneath it: intracellular about two-thirds; extracellular about one-third, split roughly four-fifths interstitial fluid and one-fifth plasma (a textbook model, labeled as one). One last pot appears: “fluid in a serous space, where it doesn’t normally gather.” It fits nowhere on the stencil until a thin pocket opens between two lining layers at the edge — third-spacing, named and locked with CHAPTER 5 — and a note says no such pocket is recorded in Stina’s diary at thirteen to fifteen. Keyboard path: Tab moves through the pots, Enter picks one, arrow keys move between regions, Space paints; each region’s name and fill are announced. A <details> text version gives every clue, its region and the reason.

Think · not graded

Potassium is about 140 mM inside cells and about 4 mM in plasma. If a blood sample sits too long, or is shaken so its red cells break, the lab’s potassium comes back falsely high. Using only the words intracellular, extracellular and membrane, explain why — and what that tells you about which compartment a routine blood test is built to read.

The true/false spot-check for this prompt is in production.

Check · Fill in the Blanks

Before the machinery, the map. Fill in each blank.

This self-scoring check is in production.

Evidence card 3

Add it to your evidence board

“Then the vomiting, the kind that doesn’t stop when there’s nothing left”: every cycle costs her body water — a fluid story Chapter 5 takes up.

Scene 4 of 6

Sugar on the Outside

Every cell wears a sugar-crusted name tag — and Stina’s disease is not a name tag being misread.

Animation · in production · 2 min 25 s

Sugar on the Outside

A boiled candy rolled in sugar; the camera dives to a cell surface crusted the same way — sugar chains on glycoproteins and glycolipids, the glycocalyx, all on the outside face. A name tag pops up: HELLO, MY NAME IS AMY. Zoom to the membrane in cross-section, built piece by piece from a labeled kit: phospholipids, cholesterol rings between the tails, an integral (transmembrane) protein spanning the bilayer, a peripheral protein resting on the inner face. Six job icons light along the strip — CHANNEL and CARRIER (transport), RECEPTOR, ENZYME, IDENTITY, ANCHOR / JOIN. On a nucleated cell an MHC class I tag holds up a fragment of what the cell is making; on a red cell, which has no nucleus, the tag is the ABO sugars instead. A T cell walks past, reads a tag, moves on. Split screen: AUTOIMMUNE on the left — a T cell and antibodies stamping a self tag as an enemy (type 1 diabetes and lupus named as textbook examples); AUTOINFLAMMATORY on the right — no tag misread, no antibody, and inside a neutrophil a cytosolic sensor labeled PYRIN sparks on its own. Last frame: NOT A MISREAD NAME TAG — AN ALARM WITH NO INTRUDER.

Prefer the full lecture? Cell Membranes (original video on the site)

Explore · in production

Front-desk check-in

The cell’s front desk: a counter set into the membrane with a scanner light, a guest book and three stamps — SELF · NOT SELF · SELF, BUT ALTERED. A queue of eight visitors arrives one at a time, each drawn with its surface sugars and tags visible: (1) one of your own neutrophils — MHC class I on its surface, self sugars; (2) one of your own red cells — no MHC class I (no nucleus), its name tag the ABO blood-group sugars; (3) a bacterium — no MHC at all, a cell wall studded with molecular patterns no human cell carries; (4) a cell from a donated kidney — MHC class I, but someone else’s; (5) one of your own cells infected by a virus — your own MHC class I, holding up a viral fragment; (6) a speck of dust — no tags, just debris; (7) one of your own insulin-making cells, which the queue card says a misguided T cell has started stamping NOT SELF — the autoimmune card, with type 1 diabetes named as the textbook example; (8) a blank card that reads only “a cell alarm with no intruder at all.” For each visitor the student predicts a stamp, then scans; the light reads the glycocalyx and the tags, and a card explains the call: patterns on a bacterium are flagged by innate sensors that need no name tag at all; someone else’s MHC is why transplants are matched; a viral fragment on your own MHC is how killer T cells find an infected cell; dust is swept up by a macrophage; the insulin-cell card shows what autoimmunity is — the adaptive immune system misreading a self tag, with autoantibodies or autoreactive T cells as the evidence a doctor can test for. The twist is visitor 8. The scanner finds nothing at the desk — no germ, no foreign tag, no altered self tag — yet a thermometer beside the guest book climbs and a white cell’s alarm lights. The question: which tag would explain it? Every stamp is offered, and none fits. The reveal turns the camera around: the alarm is not at the desk at all. It is inside the cell, in the cytosol, where a sensor named pyrin fires — no antibody, no misread name tag, an innate alarm with no intruder. The card names it autoinflammation, a word proposed in 1999 for exactly this family of diseases, and sets it beside autoimmunity: same immune system, different arm, different place, different evidence. Keyboard path: Tab to the visitor, Enter to scan, arrow keys choose a stamp, Space stamps; each scan result is announced in a polite live region. A <details> text version lists all eight visitors, their tags, the correct stamp and the reason.

Think · not graded

Two girls each have recurring fevers. In one, a blood test finds antibodies against her own thyroid. In the other, nothing on any cell’s surface is being misread, yet her white cells sound an alarm on schedule every month. Which one is autoimmune and which is autoinflammatory? Which part of the membrane — or which part of the cell — would you have to look at to tell them apart, and why would a test of the outside of cells help with only one of them?

The true/false spot-check for this prompt is in production.

Check · Single Choice

Which statement describes autoinflammation rather than autoimmunity?

This self-scoring check is in production.

Evidence card 4

Add it to your evidence board

“By the third they ask whether there’s a lot of stress at home”: the visits turn from tests to her household, and no germ is named at any of them.

Scene 5 of 6

Doors With Three Kinds of Lock

Some doors never close, some need a key, some open for a change in charge — and an open door still moves nothing without a gradient.

Narrated slide · in production

Doors With Three Kinds of Lock

Four of Amy’s narrated slides, each trimmed before its closing case lines. Slide 1, Cell Membrane Proteins: “Last time we built the wall and today we meet the gatekeepers”; “How does anything cross an oily barrier that polar molecules can’t slip through on their own?… channels, carriers, pumps, receptors” (Amy’s rule for this topic: everything here applies only to proteins embedded in a membrane — keep that fence around it). Slide 3, Leaky Channels: “Being open all the time is not the same as letting everything through”; “Open and moving are different things… An open channel with no gradient is a door onto a flat hallway, nobody’s going anywhere.” Slide 4, Chemically-Gated Channels: “The molecule that acts as the key is usually not the molecule that moves through the pore”; a ligand “is just an old chemistry term that essentially means thing… half of feeling competent in this physiology is refusing to be bullied by the vocabulary.” Slide 5, Voltage-Gated Channels: “I really do hate electricity… I just, I have a hard time understanding it”; the hotel keycard; “Each of these channels has a threshold”; the action potential “is like the basis” of this slide, scaled up. Caption lines under the player: (1) Timed cue, Slide 5 at about 0:39, “remember what voltage is. It’s just, just moving ions”: voltage is a separation of charge across the membrane, measured in millivolts; current is ions moving. (2) Timed cue, Slide 5 at about 0:20, the hotel keycard: the card is read by the lock’s stripe or chip reader; keep the picture — change the field, open the door — not the idea that the door senses a voltage. (3) Timed cue, Slide 4 at about 1:50, “gated channels are precisely how a cell listens”: true of signals at the surface, but pyrin is not a membrane protein; it is a sensor in the cytosol. The FMF doors at the membrane are the IL-1 receptor on the cells IL-1β commands (Scene 6) and the pores IL-1β leaves through (Chapter 4).

Prefer the full lecture? Cell Membrane Proteins (original video on the site)

Explore · in production

Three doors, one key ring

A long hallway seen from above, its wall drawn as a membrane with three doors set into it — outside on the left, inside on the right. DOOR 1, LEAK: always open, sized for potassium. DOOR 2, LIGAND: shut, with a binding site on its outer face shaped for acetylcholine; behind it, a pore sized for Na⁺. DOOR 3, VOLTAGE: shut, with a keycard reader and a threshold meter wired to a voltage dial (−90 to +30 mV, threshold marked near −55 mV, a textbook neuron value). On each side of each door stands a crowd of ions the student can thin or thicken with a GRADIENT slider, and over every door hang two lamps: OPEN? and MOVING? A key ring hangs on a hook with four keys: acetylcholine · glucose · IL-1β · a keycard labeled CHANGE THE CHARGE. Before every try the student predicts both lamps and the direction of any flow. Round 1, the flat hallway: the leak door with equal crowds on both sides — OPEN lights, MOVING stays dark (“a door onto a flat hallway — nobody’s going anywhere”); thicken one side and K⁺ moves, always toward the thinner crowd. Round 2, the ligand door: a steep Na⁺ gradient shoves against the closed door and nothing moves; try glucose — it doesn’t fit; try IL-1β — it doesn’t fit either (a card: IL-1β is a ligand for a receptor that is not a channel at all — Scene 6); try acetylcholine — it binds, the door changes shape, the key lets go and leaves, and Na⁺ pours through: the key is not the cargo. Round 3, the voltage door: turn the dial up slowly; the door stays shut until the meter crosses threshold, then opens, and ions flow down their gradient; turn it back and the door shuts. A side panel separates two words students mix up: the dial shows VOLTAGE — plus and minus charges lined up on opposite faces of the wall, a separation of charge measured in millivolts; the moving dots are CURRENT — ions actually crossing. Final card, three locks: none (leak), a chemical key (ligand), a change in charge (voltage) — and in every case the gradient, not the door, sets the direction. Keyboard path: Tab moves door to door, arrow keys set the gradient slider and the voltage dial, Enter tries the selected key; both lamps are announced in a polite live region (“Door 1: open, not moving”). A <details> text version gives every door, key and setting with its result.

Think · not graded

An open channel with no gradient is “a door onto a flat hallway.” For each door — leak, ligand-gated, voltage-gated — describe one situation in which the door is open but nothing moves, and one in which the door stays shut however steep the gradient. In each case, say what the gradient is doing and what the door is doing.

The true/false spot-check for this prompt is in production.

Check · Drag the Words

Three doors, three kinds of lock. Two distractors are on the bank.

This self-scoring check is in production.

Evidence card 5

Add it to your evidence board

“By the fifth they read the chart before they look at me”: by the fifth visit, the chart — not the patient — is what each new doctor reads first.

Scene 6 of 6

The Message at the Door

IL-1β never enters the cell it commands: it turns a lock on the outside, and the door’s inside end does the talking.

Animation · in production · 2 min 20 s

The Message at the Door

A wall calendar with day one circled and FEVER stamped on the square. Off-screen left, a white cell — labeled “where the alarm is raised: Chapter 7” — releases paper airplanes marked 1β into the blood. Follow one to a small vessel at the base of the brain, beside a thermostat dial labeled HYPOTHALAMUS. Zoom to a lining cell’s membrane: the IL-1 receptor, IL-1R1, drawn with three loops outside (the lock) and a tail inside. The airplane lands in the lock without crossing the membrane; a co-receptor, IL-1RAcP, swings in beside it; their two inside tails touch, and a relay lights inside the cell — MyD88, then NF-κB, then COX-2 — and PGE₂ drifts toward the thermostat, which ticks up (label: MODULE 1.5 BUILT THIS LOOP). A doorbell card: the visitor stays on the porch; the bell rings inside. Second pass: a pale key labeled IL-1Ra sits in the same lock; the co-receptor never joins; the inside tails never touch; the thermostat stays put. Last frame: the calendar square again, re-captioned A MESSAGE DELIVERED, ON SCHEDULE.

Prefer the full lecture? Cell Membrane Proteins (original video on the site)

Explore · in production

Whose key turns?

A workbench holds a slab of membrane from a cell lining a blood vessel at the edge of the hypothalamus — outside above, inside below. Mounted in it is the IL-1 receptor, IL-1R1, drawn with three loops on the outside (the lock) and a tail below (its inside end); beside it, unattached, waits the co-receptor IL-1RAcP. Under the slab, inside the cell, a relay strip of four beads (MyD88 → kinases → NF-κB → COX-2) ends at a small workshop that makes prostaglandin E₂ and a thermostat dial labeled SET POINT. At the far end of the bench sit two more fixtures: a second, different lock — the IL-6 receptor and its partner — with its own short relay; and a decoy receptor, IL-1R2, with an outside lock and no inside tail at all. A key tray holds six cards: IL-1β · IL-1α · IL-1Ra · IL-6 · TNF · pyrin. Before each try the student predicts TURNS · FITS BUT DOESN’T TURN · DOESN’T FIT. Round 1, one key at a time. IL-1β fits; the co-receptor swings in; the two inside tails touch and the relay lights bead by bead; the dial climbs (a one-line label: Module 1.5 owns the loop). IL-1α is a second key to the same lock and turns it too. IL-1Ra fits the same lock and sits there — the co-receptor never joins, the tails never touch, the dial stays flat: the body’s own brake. IL-6 doesn’t fit IL-1R1 but slides down the bench to its own lock and lights its own relay — a reminder that a fever has more than one messenger, and that each needs its own door. TNF doesn’t fit this lock either. The pyrin card can’t be placed on the bench at all: a note explains that pyrin is a sensor in the cytosol of a different cell, a white blood cell, upstream of the key (Chapter 7). Round 2, competition: a crowd slider sets how many IL-1β and how many IL-1Ra molecules are in the fluid above the slab, and the student predicts how much IL-1Ra it takes to keep the dial flat; the reveal shows the blocker needs a large excess, because every receptor it misses is a door IL-1β can still ring. Round 3, the decoy: drop IL-1β on IL-1R2 and the key is caught with nothing inside to talk to — a sink that soaks up messenger. The reveal card: the messenger never crosses the membrane; a receptor is a transmembrane protein whose outside end reads and whose inside end speaks; and a fever on day one of a period is a message delivered to a door the cells lining those vessels carry in every one of us. Keyboard path: Tab to a key, Enter picks it up, Tab to a lock, Enter drops it; the crowd slider is a native range input; each result is announced in a polite live region (“IL-1Ra: fits, does not turn — no relay”). A <details> text version lists every key, every lock and the outcome.

Think · not graded

Day one of her period, the fever comes first. If its messenger, IL-1β, never enters the hypothalamic vessel cells it commands, how does the message get inside them? Name the two receptor proteins and the job of each end. Then explain why a blocker that fits the same lock, IL-1Ra, can sit there for hours and nothing happens — and why a body that keeps such a blocker in its fluids still runs fevers.

The true/false spot-check for this prompt is in production.

Check · Single Choice

IL-1β reaches a blood vessel at the edge of the hypothalamus. How does it raise the thermostat?

This self-scoring check is in production.

Evidence card 6

Add it to your evidence board

“Day one of my period: the fever”: every cycle the fever arrives first, on a date she can predict.

4

Your evidence board

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: Tests of the outside, an alarm inside, Neither on its own, or A lot of stress at home. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Evidence board · drag and drop · in production

The sorting board for this chapter’s cards is being built. Until it arrives, sort your evidence cards on paper into the three columns above.

5

Your verdict

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.

Look back at your prediction

A fever on day one of every period, emergency-room visits that ran tests and then asked about stress at home — read through the membrane, the reading that fits best is…

Find the one you chose at the top. Does the evidence support it, refute it, or revise it?

  • Nothing was there to find — “a lot of stress at home”
  • Tests of the fluid outside cells, while the trouble sits inside them
  • “An infection we cannot find” — a germ hiding where the tests kept missing it

Build your verdict · no writing

Choose the statement that holds in each set. The ones you keep become your argument.

This activity is in production.

6

The finding

Finding · the membrane

Every Test Stopped at the Wall

1990, age fifteen: the cycle on her own calendar — day one the fever, then the vomiting, then the legs, three or four days, then fine — and emergency rooms that ran tests on the first visit and asked about stress at home by the third. Chart entry: the plasma membrane is a selectively permeable bilayer about 7–10 nm thick that keeps two-thirds of the body’s water on the inside and holds the two sides different (K⁺ about 140 mM in and 4 out; Na⁺ about 145 out and 12–15 in); the routine tests of 1990 read the extracellular side or counted whole cells, so “found nothing” means “found nothing outside”; the fever itself is a message, IL-1β, read by a receptor on the outside of the hypothalamus’s vessel cells; and autoinflammation is an innate alarm raised inside cells, not an autoimmune misreading of their name tags. Held open: which cells carry that alarm (Chapter 2), where its messenger is built and how it leaves (Chapter 4), and Chart Clue #3, “Every workup” (Chapter 7).

Next · Chapter 2

Meet the Cells

1988, age thirteen: “Things that hurt today, from the ground up” — feet, shins, the backs of her knees, her back, and a twenty-pound backpack weighed twice on the bathroom scale. One school day, seven cells, each measured against a ten-micrometer bar.

Continue to Chapter 2 →

Reliable information on rare diseases

NORD · NIH GARD · Orphanet · MedlinePlus Genetics: FMF · FMF & AID Global Association · Autoinflammatory Alliance · Global Genes · Amyloidosis Foundation

Stina has one rare disease. In the United States, about 1 in 10 people live with one of the more than 7,000 known rare diseases. These organizations are where patients, families and clinicians go for trustworthy information.

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← Familial Mediterranean Fever — case home

All pages for Module 3 — The Cell & Its Transport

3.0 Module Overview
3.1 The Membrane
3.2 Meet the Cells
3.3 Organelles and the Scaffold
3.4 The Protein Factory and Its Two Exits
3.5 Osmosis, Tonicity, Effusion
3.6 Pumps, Channels and Voltage
3.7 Inside the Neutrophil
3.8 Conclusion and Assessment
3.9 The Evidence Behind the Case

All Modules
  1. Anatomical Language, Membranes & Homeostasis
  2. Just Enough Chemistry
  3. The Cell & Its Transport
  4. Making Cells & Proteins
  5. The Integumentary System
  6. The Skeletal System
  7. The Muscular System
  8. Nervous Tissue & the Senses
  9. The Spinal Cord
  10. The Brain & the Blood–Brain Barrier
  11. The Autonomic Nervous System
  12. Special Senses (in development)
  13. The Endocrine System
  14. Blood
  15. The Heart
  16. Blood Vessels
  17. The Digestive System
  18. The Respiratory System
  19. The Urinary System
  20. Fluids, Electrolytes & Acid–Base Balance
  21. The Reproductive System
  22. The Immune System

List of terms