Most Things Biology

2.4 Lipids: Membranes, Steroids and the Lipoprotein Fleet

Case home  ·  Start at the beginning  ·  The Reveal

Time To Read

26–39 minutes

Date Last Modified

Just Enough Chemistry · Chapter 4 of 7

1986, age eleven, after Mom’s second surgery: “Now Mom can’t take the heat at all. She sits in front of the box fan with a wet washcloth on her neck, even in March.” The surgeon says the part of the brain that works like a thermostat sits right above where the tumor was. Mom sleeps in the afternoons, Dad works late, and when the fever comes Stina gets herself “the Tylenol and the bucket” and lies on the bathroom floor where the tile is cool. This chapter is about the molecules that won’t mix with water: the fat a body banks, the two-faced molecule every membrane is built from, the four-ringed steroids, the lipid that carries a fever’s last order to the thermostat, and the fleet of parcels that ships fat through watery blood — one of which an acute-phase protein climbs aboard.

The question you will answer

A pill on a bathroom floor, and a mother who can’t take the heat after surgery beside the brain’s thermostat: both stories run through molecules that don’t dissolve in water. Before you can read them you need fats and the shapes of their tails, the phospholipid that builds every membrane, the four rings of a steroid, the lipid messenger that resets a fever, and the protein-and-lipid parcels that carry fat through plasma. The question for the chapter: when a pill brings a fever down, which molecule did it stop — and which ones kept going?

Opener · Oil Won’t Join Water · about 1¼ minutes

The opener is the same narrated slide 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

1986 · age 11 · after Mom’s second surgery

1986 · 11
Mom’s tumor “came back and they had to go in again.” Afterward she can’t take the heat: the box fan, a wet washcloth on her neck even in March, never outside in August. The surgeon tells Dad it can happen “because the part of the brain that works like a thermostat sits right above where the tumor was.”

1986 · 11
Mom sleeps in the afternoons and Dad works late. With a fever, Stina gets herself “the Tylenol and the bucket” and lies on the bathroom floor “where the tile is cool.” No doctor unless it’s the ER, and no ER “unless I can’t stand up.” “Doctors cost money.”

1986 · 11
“I used to think grown-ups had a plan. Now I think there is a plan for Mom, sort of, and there is no plan for me, and the difference is that Mom has a scar they can point to.”

Told
“A stomach bug.” “Adjustment reaction to parental illness.”

Chart clues this module: 2 · this module (logged in Chapter 7) — this chapter adds no clue

Before you go on · your prediction

On the bathroom floor at eleven, Stina treats her own fever with Tylenol. When a pill like that brings a fever down, what has it done?

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

Treated the illness — if the fever falls, whatever caused it is over

Watch where in the chain the pill acts: does a fever coming down mean the molecule that started it has stopped?

Nothing worth noting — it was “a stomach bug” and would have passed anyway

Watch how many different molecules an attack involves, and which of them a fever pill can even reach.

Quieted one messenger near the end of a longer chain

Watch the fever relay: count the molecules between the first alarm and the hypothalamus, and see which ones keep running after the pill.

From Stina’s diary

Age 11 · “Nobody’s in charge”

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 · The Tylenol and the bucket

The tumor came back, and they had to go in again. After the second operation Mom can’t take the heat at all. She sits in front of the box fan with a wet washcloth on her neck, even in March, and she won’t go outside in August. The surgeon tells Dad it can happen when they operate in there, because the part of the brain that works like a thermostat sits right above where the tumor was. Stina is eleven. Since the operation Mom sleeps in the afternoons and Dad works late, so when the fever comes, Stina looks after it herself: “I get myself the Tylenol and the bucket and I lie on the bathroom floor where the tile is cool.” They don’t go to the doctor anymore unless it’s the emergency room, and they don’t go to the emergency room unless she can’t stand up. Doctors cost money, and they say the same thing every time anyway, and Mom can’t do the waiting room with her head. She used to think grown-ups had a plan. Now she thinks there is a plan for Mom, sort of, and there is no plan for her — “and the difference is that Mom has a scar they can point to.” The diary doesn’t say whether the Tylenol helped. It says only who fetched it.

See it move: Scene 1 · Oil Won’t Join Water · Scene 3 · Four Rings · Scene 4 · The Fever’s Last Messenger

II · Oil won’t join water

Lipids have the exact opposite personality from carbohydrates: they’re hydrophobic, they won’t join water, and that’s exactly why fat stores so much, so densely — nine kilocalories per gram, more than double a carb. If carbs are the cash in your pocket, lipids are the check you deposit in the fat bank. A triglyceride is one glycerol and three fatty-acid tails, joined by dehydration synthesis. Saturated tails are packed full of hydrogen and lie straight, so they stack — butter, solid on the counter. Unsaturated tails have C=C double bonds, and the cis ones kink the tail so it can’t pack — olive oil, liquid. A lot of nutrition riding on a few hydrogen atoms. Take one tail off and put a phosphate head on, and you have a phospholipid — a balloon with two strings, torn between two worlds: its head loves water and its tails fear it, so in water those molecules assemble themselves, in seconds, into a bilayer. Picture a sandwich, heads for the bread and tails for the filling: that’s the membrane around every cell you own. Cholesterol tucks into that membrane and keeps it from going too stiff or too runny, and it is also the parent of every steroid — four fused rings, three of six carbons and one of five. Hang different side groups on the rings and you get cortisol, aldosterone, estrogen, testosterone, bile salts and, with one ring opened by sunlight, vitamin D₃. Some lipids don’t store or build at all; they talk. Prostaglandins are cut from a 20-carbon fatty acid in your membranes, and one of them, PGE₂, tells your hypothalamus to turn up the thermostat. And because none of these can swim through watery plasma alone, they ship in parcels: the lipoproteins.

See it move: Scene 1 · Oil Won’t Join Water · Scene 2 · Two Faces and a Branch Point · Scene 3 · Four Rings · Scene 4 · The Fever’s Last Messenger · Scene 5 · The Lipoprotein Fleet

III · The fever’s last messenger

Here is what the student learns and the girl on the tile does not. In an attack of familial Mediterranean fever, the first alarm is a protein, interleukin-1 beta, released when the sensor pyrin trips inside her white cells. IL-1β reaches the blood vessels at the edge of the hypothalamus and orders an enzyme, COX-2, to turn a lipid — arachidonic acid, cut free from a membrane phospholipid — into prostaglandin E₂. PGE₂ is the last messenger: it raises the set point, and the chills and the heat follow. Acetaminophen works mainly inside the brain, at the end of that relay, so it can bring a fever down while IL-1β keeps pouring out and the inflamed lining stays inflamed. A steroid acts further upstream — cortisol, the brake from Module 1’s 1.5, is a four-ringed lipid built from cholesterol — yet FMF responds poorly to steroids; the treatment, when it finally comes, is a small plant alkaloid (Chapter 6). Two more lipid threads run through her story. Lipin-2, the enzyme Majeed syndrome lacks, is a fat-processing enzyme; without enough of it a second sensor, NLRP3, over-fires in macrophages. Module 1 told you she carries a changed copy of LPIN2 as well as MEFV; what one copy can do, and whether the two add up, is a hypothesis Module 4 takes up. And in every attack her liver pours out serum amyloid A, which rides the blood on HDL — the parcel this chapter ends on, and the protein Chapter 7 follows to amyloid. Honesty flag: Mom’s heat after an operation beside the hypothalamus has more than one possible explanation — damage near the thermostat itself, or pituitary hormones lost in surgery — and the diary can’t say which.

See it move: Scene 2 · Two Faces and a Branch Point · Scene 3 · Four Rings · Scene 4 · The Fever’s Last Messenger · Scene 6 · A Stowaway on HDL

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

Oil Won’t Join Water

Water-fearing, energy-dense, and a few hydrogen atoms between butter and olive oil.

Narrated slide · in production · 1 min 21 s

Oil Won’t Join Water

Two of Amy’s slides. LIPIDS (“Hydrophobic, energy-dense — and the source of the body’s fever signal”): the opening slide, which sets lipids up as “the exact opposite personality from carbohydrates,” built for the long haul, “tucked away for a rainy decade,” and names the prostaglandins Scene 4 returns to. THE STORAGE FAT: TRIGLYCERIDES (“One glycerol head, three fatty-acid tails”): Glycerol + 3 tails. Saturated — tails full of hydrogen. Unsaturated — some hydrogens removed. Major fat — dietary and storage; Amy’s lipid-types figure (a triglyceride, a phospholipid and a bilayer, steroid rings, a fat droplet, fat cells). Caption line under the first player: “Prostaglandins shape how an attack feels; the alarm that orders them, IL-1β, comes first (Scene 4).” Under the second: “Unsaturated means one or more C=C double bonds where hydrogens would be; cis double bonds put the kinks in the tails. Trans fats are unsaturated but straight.”

Prefer the full lecture? Lipids (original video on the site)

Explore · in production

Bend the tails

Six fatty-acid tails lie on a workbench — five 18-carbon zigzags and one short four-carbon tail — drawn as a bendable wire with its hydrogens as small beads (Alton Brown’s shoppers holding bags — every bead is a bag): stearic acid (saturated), oleic acid (one cis double bond — olive oil), linoleic acid (two), alpha-linolenic acid (three; an omega-3, its first double bond three carbons from the tail’s end, and essential — it has to come from food: flax, walnuts, oily fish), elaidic acid (one trans double bond — partially hydrogenated margarine and shortening) and short butyrate. Predict first: which of these will pack into a solid at room temperature? Click any carbon–carbon bond to make it a double bond: two hydrogen beads drop off, and a toggle sets the new bond cis (the tail kinks about 30 degrees) or trans (the tail stays straight). Then drag three tails onto a glycerol backbone — each attachment releases a water bead (dehydration synthesis) and the result is labeled TRIGLYCERIDE — and drop the triglycerides into a box: straight tails stack neatly and the thermometer reads SOLID AT 20 °C (butter, lard, the marbling on a steak); kinked tails jostle, leave gaps and read LIQUID (olive oil, the fat in an avocado); the trans tail stacks like a saturated one, which is why hydrogenated oil acts solid. A side shelf weighs the banks: the energy in 1 g of fat takes more than 2 g of carbohydrate (9 against 4 kcal per gram) — before counting the water glycogen holds — which is why a migrating bird fattens before it flies south for the winter and why adipose tissue is the body’s long-term store, its insulation and its cushion. A last card shows a wax (a long fatty acid joined to a long alcohol: beeswax, a leaf’s cuticle) for contrast. The reveal: a few hydrogen atoms and the shape of one bond decide whether a fat is solid or liquid — and every one of these molecules refuses to dissolve in water. Keyboard path: Tab between tails and bonds, Enter toggles a double bond, C or T sets cis or trans, arrow keys move a tail to the glycerol; the box’s state is read out in a live region. A details-block text version lists each tail with its double bonds, its shape and its melting behavior.

Think · not graded

Olive oil and butter are both mostly triglycerides — glycerol plus three fatty acids — and both are hydrophobic. Why is one a liquid and the other a solid on the same kitchen counter, and what does a single cis double bond do to the way tails pack together?

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

Check · Single Choice

Why don’t lipids dissolve in watery blood?

This self-scoring check is in production.

Evidence card 1

Add it to your evidence board

After the second surgery near the brain’s thermostat, Mom can’t take the heat — the box fan, the wet washcloth on her neck even in March, never outside in August.

Scene 2 of 6

Two Faces and a Branch Point

A balloon with two strings, a membrane that builds itself, and the enzyme at the fork: lipin-2.

Narrated slide · in production · 1 min 46 s

Two Faces and a Branch Point

Amy’s slide (“A balloon with two strings — and the bilayer it builds”): Phosphate head + 2 tails. Polar head — faces the water. Nonpolar tails — hide from water. Bilayer — every cell membrane. A BioRender bilayer sphere with a zoom of one phospholipid, its head and two tails. The page text under the player adds what the slide leaves out: phospholipids are amphipathic, they assemble into a bilayer by themselves in seconds, and on the way to being made they pass through a fork run by one enzyme — lipin-2 — that turns phosphatidic acid into diacylglycerol.

Prefer the full lecture? Lipids (original video on the site)

Explore · in production

Run the lipid line

A factory floor drawn left to right inside a cell. A conveyor starts with glycerol-3-phosphate; station 1 loads two fatty-acid tails and makes phosphatidic acid (a glycerol, two tails and a bare phosphate). The belt then reaches a gate labeled LIPIN-2 — a phosphatidic acid phosphatase that snips the phosphate off (its slot needs a Mg²⁺ token) — and out comes diacylglycerol. Past the gate the belt forks: one branch adds a third tail and drops triglycerides into a fat-droplet bin; the other adds a head group (choline or ethanolamine) and sends phospholipids up a chute to the membrane, where they lock into a bilayer. A side window shows a beaker: shaken into water, two-tailed phospholipids form a bilayer bubble and one-tailed ones a micelle, heads out (the shape bile salts use to carry fat in the gut), in seconds, not millennia. Predict first, from three options, what switching lipin-2 off will do: nothing, stop the belt completely, or pile one thing up and leave two others short. Switch it off: phosphatidic acid heaps up in front of the gate while the triglyceride bin and the phospholipid chute both slow. A dial sets lipin-2 to FULL, HALF (one working copy of the gene) and NONE (no working copy). A closing card opens only after the run and carries an honesty flag: “In macrophages, too little lipin-2 lets a different inflammasome sensor, NLRP3, over-fire — its trigger is potassium leaving the cell, not the one pyrin answers to — and more IL-1β comes out (Lordén et al., J Exp Med 2017). With no working copy, the result is Majeed syndrome, described in 1989 and reported in only about 35–40 people. What half a dose does in any one person is not known; this course treats it as a question, not an answer.” The reveal: the membrane, the fat store and an alarm’s restraint all run through one fork in a lipid assembly line. Keyboard path: Tab to the switch, the dial and the Run button; arrow keys set the dial; Space runs the belt; what piles up and what runs short is reported in a live region. A details-block text version describes each station and the result at every dial setting.

Think · not graded

A phospholipid has a head that loves water and two tails that fear it. Drop a handful into water and they build a bilayer by themselves, with no enzyme and no ATP. Why does the shape alone do the work — and what would form instead if every phospholipid had only one tail?

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

Check · Single Choice

Lipin-2 sits at a fork in fat-making. What does it do?

This self-scoring check is in production.

Evidence card 2

Add it to your evidence board

Family history, as Mom gave it to Dr. Gonzalez at eight — “Mom said no, just me” — and no doctor in these years is recorded asking again.

Scene 3 of 6

Four Rings

One cholesterol, four rings, a family of hormones — and why the body’s own brake doesn’t calm this fire.

Animation · in production · 2 min 15 s

Four Rings

A cholesterol molecule assembles ring by ring — A, B, C of six carbons, D of five — and a stamp reads YOU MAKE MOST OF IT: a liver lights up as its main factory. It slides into a membrane between phospholipid tails, steadying them. Then it lifts out onto a workbench: side groups snap on and off, and with each swap a gland lights on a body outline — adrenal cortex, fasciculata → CORTISOL; glomerulosa → ALDOSTERONE (“keep Na⁺, let K⁺ go”); testes → TESTOSTERONE; ovaries → ESTRADIOL; liver → BILE SALTS breaking a fat droplet into smaller ones. In the skin a sunbeam snaps ring B open: VITAMIN D₃, with an arrow to the calcium loop of Module 1’s 1.6. A cortisol molecule rides a carrier protein through a vessel, slips through a cell membrane and docks on a receptor inside the nucleus; inflammation genes dim. Across it a card: FMF — RESPONDS POORLY TO STEROIDS. Last, Mom at the box fan; a sagittal outline of the brain shows the hypothalamus sitting just above the pituitary and the sella, two dotted arrows labeled THERMOSTAT? and PITUITARY HORMONES? — and a question mark the diary leaves open.

Prefer the full lecture? Lipids (original video on the site)

Explore · in production

Swap the side groups

A large stencil of the steroid nucleus sits in the middle of the screen: four fused rings lettered A, B and C (six carbons each) and D (five carbons), with every carbon numbered. A tray of side groups lies below — a hydroxyl (–OH), a ketone (=O), a methyl, an aldehyde, a ring-A double bond, a short two-carbon side chain, and cholesterol’s eight-carbon tail. The stencil starts as cholesterol (an –OH at carbon 3, a double bond in ring B, the eight-carbon tail on ring D). Each target card shows a molecule’s sketch and the student swaps groups to match it: cortisol (=O at 3 with a double bond in ring A, –OH at 11, the two-carbon side chain carrying =O and –OH, –OH at 17), aldosterone (like cortisol but with no –OH at carbon 17, and an aldehyde on carbon 18), testosterone (=O at 3 with the ring-A double bond, no side chain, –OH at 17), estradiol (ring A made aromatic, –OH at 3 and at 17, one methyl gone). Before each build the student predicts which gland makes it, and a body map lights the answer: the adrenal cortex’s zona fasciculata for cortisol, its zona glomerulosa for aldosterone, the testes for testosterone (the zona reticularis adds weak androgens), the ovaries for estradiol, and the liver for bile salts, on a separate card. Then the scissors: cut the bond between carbons 9 and 10 of 7-dehydrocholesterol, a cholesterol cousin in the skin, and ring B springs open — vitamin D₃, a secosteroid made by UV light and finished by the liver and kidney into calcitriol (Module 1’s 1.6). A summary card asks what all of them share: four rings (or three and an opened one), mostly nonpolar, so in blood they ride carrier proteins (cortisol-binding globulin, sex hormone–binding globulin, albumin) and they cross membranes to act on receptors inside their target cells. A last card corrects two myths: most of the body’s cholesterol is made by the liver, not eaten; and a drug like prednisone suppresses the body’s own cortisol rather than raising it. The reveal: one parent ring system and a handful of side groups give a body its inflammation brake, its salt-keeper, its sex hormones, a detergent for fat and a calcium hormone. Keyboard path: Tab to a ring carbon, Enter to place the selected group, Delete to remove it, G to guess the gland; each change is read out in a live region. A details-block text version gives every target’s side groups and the gland that makes it.

Think · not graded

Cortisol is the body’s own inflammation brake (Module 1’s 1.5), a steroid built from cholesterol. Yet FMF attacks respond poorly to steroids, while a small plant alkaloid prevents them. What does that suggest about where in the chain of an attack cortisol acts — and why might a brake that works on one fire fail to reach another?

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

Check · Drag the Words

Drag each word into place. (Extra words on the bank: glucose, insulin.)

This self-scoring check is in production.

Evidence card 3

Add it to your evidence board

The surgeon’s explanation for Mom’s heat names a single structure — “the part of the brain that works like a thermostat” — and the diary records no test of her hormones.

Scene 4 of 6

The Fever’s Last Messenger

IL-1β gives the order, a lipid delivers it — and Tylenol on a bathroom floor quiets the messenger, not the alarm.

Narrated slide · in production · 1 min 55 s

The Fever’s Last Messenger

Amy’s slide (“From signal to treatment”): Block prostaglandins — and fever falls. Anti-inflammatories — do exactly that. NSAIDs — target their synthesis. The payoff — chemistry explains relief. Case box, THE TREATMENT LINK: drugs that block prostaglandin synthesis ease fever and pain, and knowing the molecule behind a symptom is what makes a targeted treatment possible. Caption line under the player: “NSAIDs (ibuprofen, aspirin) block COX, the enzyme that makes prostaglandins; acetaminophen (Tylenol) lowers the set point mainly inside the brain. Both can ease a fever and some of the pain. Neither stops IL-1β, the alarm upstream — and neither is the treatment for FMF, which is colchicine (Chapter 6).”

Prefer the full lecture? Lipids (original video on the site)

Explore · in production

Block the relay

A relay track runs across the screen with six stations, each a runner holding the next baton: 1 — white cells in an inflamed lining release IL-1β (the alarm); 2 — IL-1β reaches the blood vessels at the edge of the hypothalamus, where the blood–brain barrier is thin (the organum vasculosum); 3 — phospholipase A₂ cuts arachidonic acid, a 20-carbon fatty acid, out of a membrane phospholipid; 4 — COX-2, made to order because IL-1β asked for it, turns arachidonic acid into PGH₂ and then prostaglandin E₂; 5 — PGE₂ binds EP3 receptors on the thermostat neurons of the preoptic hypothalamus; 6 — the set point rises: shivering, cold hands, a fever. A side lane from station 1 keeps IL-1β’s other jobs visible — neutrophils called into the peritoneum, the liver ordered to make acute-phase proteins, a lining that hurts. A tray holds three blocker chips — NSAID (ibuprofen, aspirin), ACETAMINOPHEN (Tylenol) and STEROID (cortisol, prednisone) — and a fourth chip, locked, labeled ALARM BLOCKER (A&P II). Predict first: which chip will stop the most runners? Place a chip and press Run: an NSAID fits COX at station 4; acetaminophen fits at station 4, and mostly inside the brain; a steroid fits at stations 3 and 4, turning down phospholipase A₂ and the making of COX-2. Blocked runners gray out and the thermometer falls, but the side lane keeps running for every chip — the steroid slows it a little — and a card under the steroid chip reads: “FMF attacks respond poorly to steroids; the treatment is colchicine, which acts on the white cells at station 1 (Chapter 6).” The reveal is a sentence the student completes: the Tylenol on the bathroom floor brought the messenger down, not the alarm. Keyboard path: Tab to a chip, Enter to pick it up, arrow keys to choose a station, Enter to place it, R to run; which runners still move is announced in a live region. A details-block text version lists every station and what each chip leaves running.

Think · not graded

At eleven, Stina treats her own fevers with Tylenol on the bathroom floor. If acetaminophen lowers the set point inside the brain but leaves IL-1β pouring out, which parts of an attack would you expect it to ease, and which would carry on?

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

Check · Sort the Paragraphs

Put the fever relay in order, from the first alarm to the fever.

This self-scoring check is in production.

Evidence card 4

Add it to your evidence board

At eleven, with Mom asleep in the afternoons and Dad working late, she treats the fevers herself — “the Tylenol and the bucket” — attacks managed at home that the chart never saw.

Scene 5 of 6

The Lipoprotein Fleet

Fat can’t swim, so it ships: four parcels, two directions, and why “good” and “bad” are routes, not morals.

Animation · in production · 2 min 15 s

The Lipoprotein Fleet

A drop of oil lands in a river of plasma (92% WATER) and beads up, refusing to mix. A parcel builds around it: a shell of phospholipid heads facing out, free cholesterol and apolipoprotein bands, and an oily core of triglyceride and cholesteryl ester. Four parcels line up on a ruler from large and light to small and dense. CHYLOMICRON (ApoB-48) leaves a gut cell, enters a lacteal, rides the lymph to the subclavian vein, drops triglyceride at muscle and fat through lipoprotein lipase, and its shrunken remnant docks at the liver. VLDL (ApoB-100) leaves the liver, unloads triglyceride, shrinks through IDL into LDL, and LDL hands cholesterol to a cell through the LDL receptor. HDL (ApoA-I, about half protein) leaves nearly empty, collects cholesterol from tissues and an artery wall, and carries it home to the liver. A density meter rises as each parcel’s protein share rises. A crossed-out arrow — CHYLOMICRON → LDL → HDL — is stamped WRONG ROUTE. Two signs, GOOD and BAD, flip over to read BACK and OUT. In an artery wall, LDL left behind builds a small yellow plaque (BLOOD VESSELS →). A date card: 2026 · “The cholesterol is sky-high.”

Prefer the full lecture? Lipids (original video on the site)

Explore · in production

Load the parcel

A loading dock with four empty parcels on a conveyor, each a sphere with a SHELL slot (phospholipid, free cholesterol and an apolipoprotein tag) and a CORE slot (triglyceride and cholesteryl ester), beside a density meter reading from about 0.93 to 1.21 g/mL and a size ruler from about 1,000 nm down to about 10 nm. Crates on the floor: TRIGLYCERIDE, CHOLESTERYL ESTER, PHOSPHOLIPID, FREE CHOLESTEROL, and four apolipoprotein tags — ApoB-48, ApoB-100, ApoA-I and ApoE. Order cards ask for four builds, and the student predicts first which parcel will be the densest. Parcel 1, loaded almost entirely with triglyceride and tagged ApoB-48: the meter reads below 0.95 and the label CHYLOMICRON appears (made in gut cells from dietary fat). Parcel 2, mostly triglyceride with ApoB-100: VLDL (made in the liver). Parcel 3, mostly cholesteryl ester with ApoB-100: LDL. Parcel 4, about half protein, ApoA-I, little cargo: HDL, the smallest and densest. As the meter moves, the rule appears: lipid floats, protein sinks, so density rises with the protein share. Then the route map: drag each parcel along the body’s roads. The chylomicron leaves a gut cell into a lacteal, rides the lymph to the subclavian vein, unloads triglyceride to muscle and fat through lipoprotein lipase, and sends its remnant to the liver. VLDL leaves the liver, unloads triglyceride, and shrinks through IDL into LDL, which delivers cholesterol to cells through the LDL receptor. HDL collects extra cholesterol from tissues and artery walls and carries it back to the liver (reverse cholesterol transport). A trap card states the old chain — “chylomicrons become LDL, then HDL” — and the map shows the road it needs doesn’t exist: chylomicrons never turn into either. A last card asks: LDL bad, HDL good? Both are directions — out to the tissues and back home — and LDL left behind in an artery wall is where plaque begins (Blood Vessels). The reveal: four parcels, two directions, density set by protein. Keyboard path: Tab to a crate, Enter to pick it up, Tab to a parcel, Enter to load it; route steps are chosen from a list; the density readout is text in a live region. A details-block text version tabulates each parcel’s maker, cargo, apolipoprotein, density and destination.

Think · not graded

Plasma is about 92% water, and a triglyceride won’t dissolve in it. If you had to ship fat from the gut to muscle, and cholesterol out to every cell and back to the liver, what would your parcel need on its outside, what would go inside — and why would the parcel that’s mostly protein be the densest?

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

Check · Drag and Drop

Drag each label to the lipoprotein it describes.

This self-scoring check is in production.

Evidence card 5

Add it to your evidence board

In 2026 Stina’s two-column list for herself and Mom reads “High cholesterol. Both.” — and nothing in her childhood chart records a cholesterol, or any fat in the blood, being measured.

Scene 6 of 6

A Stowaway on HDL

In an attack the liver floods the blood with serum amyloid A — and it climbs aboard the parcel meant to take cholesterol home.

Animation · in production · 2 min 10 s

A Stowaway on HDL

An HDL particle rolls through plasma, its shell belted with ApoA-I. An IL-1β bell rings; IL-6 envelopes fly to a liver, and inside a hepatocyte a production board flips: ALBUMIN ↓, SAA ↑↑↑. Small serum amyloid A proteins (about 104 amino acids, 12 kDa) stream out into a vessel. A graph rises — up to about 1,000 times resting within a day or two (TEXTBOOK REFERENCE: RESTING UNDER ABOUT 6–10 mg/L). Each SAA swims to an HDL and takes a seat in its shell; ApoA-I belts slide off as the seats fill, until a label reads ACUTE-PHASE HDL. A small cholesterol counter on the particle slows, with the tag STILL BEING STUDIED. The graph’s day markers advance: the attack ends, the liver’s board flips back, SAA falls over several days and ApoA-I retakes its seats. A dashed line holds SAA a little high between attacks and a slow fuse beside the graph begins to glow, labeled → CHAPTER 7: AA AMYLOID. A closing card: NO SAA WAS MEASURED IN HER CHILDHOOD — IN THE 1980s IT WAS A RESEARCH TEST.

Prefer the full lecture? Blood Plasma (original video on the site)

Explore · in production

Read the manifest

An HDL particle is drawn from above as a small round bus, with a seat map around its edge — twelve seats, nine filled at the start by ApoA-I (belt-shaped, labeled) and three by smaller apolipoproteins — and a cargo hold of cholesteryl ester in the middle. Beneath it sit an ATTACK button, a DAY slider running seven days, and a graph with two lines: SAA in plasma and ApoA-I on HDL. Predict first: during an attack, will the seats fill with new passengers, empty out, or be swapped? Press ATTACK: an IL-1β bell rings, IL-6 icons fly to a liver, hepatocytes start shipping serum amyloid A (about 104 amino acids, about 12 kDa), and the SAA line climbs — by day two, up to about a thousand times its resting level (resting under about 6–10 mg/L: a textbook reference range, not Stina’s). Each SAA that arrives takes a seat; as they crowd in, ApoA-I passengers step off, until SAA is the main apolipoprotein on board and the bus is relabeled ACUTE-PHASE HDL. A HOMEWARD CHOLESTEROL counter on the bus slows, with a note that acute-phase HDL handles cholesterol differently and how much is still being studied. Drag the DAY slider forward: the attack ends, the liver stops shipping, SAA falls over several days and ApoA-I retakes its seats. A last toggle, WHAT IF IT NEVER FULLY FALLS?, keeps a low SAA line running between attacks week after week, and a slow fuse beside the graph begins to glow: SAA is the protein that, made for years, can be cut into fragments that stack into AA amyloid (Chapter 7). An honesty card closes the run: “Nobody measured SAA in Stina’s childhood; in the 1980s it was a research test. This graph is the textbook’s, not hers.” The reveal: the parcel that carries cholesterol home is where an acute-phase protein rides — the bridge from this chapter’s fats to Chapter 7’s amyloid. Keyboard path: Tab to ATTACK, the DAY slider and the toggle; arrow keys move the slider; seat counts and both graph values are read out in a live region. A details-block text version lists the seat counts and the SAA level for each day.

Think · not graded

SAA rises up to a thousand-fold within a day or two of an attack and should fall back within days after it. If you could measure it once a week for a year in someone whose attacks came about once a month, what would a normal pattern look like — and what would it mean if the line never came all the way down?

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

Check · Single Choice

During an attack, serum amyloid A pours out of the liver. How does most of it travel in the blood?

This self-scoring check is in production.

Evidence card 6

Add it to your evidence board

Her emergency rooms “did the X-ray and the blood” at nine and “took blood” at eleven; no childhood value of any acute-phase protein survives in her chart — only the undated row “Across these years.”

4

Your evidence board

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: A fever is chemistry: an alarm, then a lipid messenger, Neither on its own, or “Adjustment reaction to parental illness”. 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

Tylenol on the bathroom floor, a mother who can’t take the heat, and a fat-carrying parcel that SAA climbs aboard — read through the lipids, the reading that fits best is…

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

  • Treated the illness — if the fever falls, whatever caused it is over
  • Nothing worth noting — it was “a stomach bug” and would have passed anyway
  • Quieted one messenger near the end of a longer chain

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 · lipids

The Fever’s Last Messenger

1986, age eleven, after Mom’s second surgery: fevers handled alone with “the Tylenol and the bucket” on the cool bathroom tile, no doctor unless the ER, and a mother who can’t take the heat after an operation beside the brain’s thermostat. Chart entry: a fever is a chain, not a feeling — IL-1β gives the order and a lipid, PGE₂, carries it to the hypothalamus, so a pill can bring the fever down while the alarm runs on; cortisol, the body’s brake, is a steroid built from cholesterol but does little for this fire; and in every attack the liver’s serum amyloid A rides the blood on HDL. Held open: what SAA does when it is made for years (Chapter 7), what the changed LPIN2 copy Module 1 named can do (Module 4), and why Mom can’t take the heat — the diary allows more than one answer.

Next · Chapter 5

Proteins: Shape Is Everything

Nine, home from the worst ER: “Psychiatrist: a physician who treats diseases of the mind.” She read it three times in the big dictionary in the den and wondered about “a special medicine that you take with your brain.” Brain chemistry is amino-acid chemistry, and so was the belly’s: twenty letters, four levels of folding, and the proteins that fill a tube of blood.

Continue to Chapter 5 →

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.

Hop to:

← Familial Mediterranean Fever — case home

All pages for Module 2 — Just Enough Chemistry

2.0 Module Overview
2.1 Atoms, Ions, Electrolytes
2.2 Water, pH and the Buffer
2.3 Carbohydrates and Energy
2.4 Lipids: Membranes, Steroids and the Lipoprotein Fleet
2.5 Proteins: Shape Is Everything
2.6 The FMF Molecule Cast
2.7 Misfolding: From Pyrin to Amyloid
2.8 Conclusion and Assessment
2.9 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