Most Things Biology

3.4 The Protein Factory and Its Two Exits

Case home  ·  Start at the beginning  ·  The Reveal

Time To Read

32–48 minutes

Date Last Modified

The Cell & Its Transport · Chapter 4 of 7

1990, age fifteen: “They took my appendix out on Tuesday.” The surgeon called it textbook — fever, pain low on the right, white count up — and at the follow-up the pathology came back normal. “‘Unremarkable,’ he said, like that was good news.” This chapter walks the cell’s protein factory — the nucleus that keeps the recipes, the ribosomes that read them, the rough and smooth ER, the pita-bread Golgi and the vesicles that ship — and then finds the second way out: a door with no label, which the alarm protein of an inflammatory attack uses to leave a white cell in minutes.

The question you will answer

A surgeon saw a textbook appendicitis; a pathologist saw a normal appendix. Both statements can be true if the trouble was a protein that had already left its cells. Every protein a cell exports has to cross the plasma membrane somehow. The question for the chapter: which proteins leave by the factory’s front door, which leave by a door with no label — and what can a slice of fixed tissue still show once they have gone?

Opener · Tuesday, Two Doors · about 2¼ 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 Tuesday of the appendectomy, and the follow-up

1986 · 11
Module 2’s ER for “a sensitive tummy”: pain that “starts under my belly button and slides down low on the right”; “they said it wasn’t my appendix, it was a stomach bug.” “By Sunday I was fine.”

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
“The cycle”: “Day one of my period: the fever. Then the vomiting… Then the legs. Three or four days of it.” “I know what they have in common. It’s on my calendar.”

1990 · 15 · Tuesday
Fever, stabbing pain low on the right, white count up: textbook appendicitis. The appendix comes out. (No value is recorded for the white count.)

1990 · 15 · follow-up
Pathology reads it as completely normal. The surgeon: “Unremarkable”; “sometimes these things are hard to explain”; “probably an early appendicitis that settled down or a virus.”

1990 · 15 · three weeks later
The same pain returns in the same place. She tells no one.

Told
“Appendectomy; appendix unremarkable.” “Fifteen is right on schedule for an appendix.” “It’s out now, so I’ll never have to worry about it again.”

Chart clues this module: 3 · this module (logged in Chapter 7) — undated, “Every workup”; the only lab-like line for 1990 is “white count up”, and it has no number

Before you go on · your prediction

Fifteen: “textbook: fever, pain low on the right, white count up,” and an appendix that came back normal. Before you walk the protein factory, what does “Unremarkable” most likely mean?

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

A normal organ, with the trouble somewhere the slide didn’t include

Watch which proteins were leaving cells that week, by which door, and whether a pathologist’s section includes the lining around an organ.

Nothing was inflamed — “Appendectomy; appendix unremarkable”

Watch what a slice of fixed tissue can hold, and what kind of cell a white count going up is counting.

It was “probably an early appendicitis that settled down or a virus”

Watch whether either guess would leave a trace a pathologist could see, and how long a messenger protein stays where it was made.

From Stina’s diary

Age 15 · “Unremarkable”

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 · Textbook

They took her appendix out on a Tuesday in 1990, when she was fifteen. The surgeon called it textbook: fever, pain low on the right, white count up. He told her she was lucky they had caught it in time, and that fifteen is right on schedule for an appendix — the way a grown-up tells you a train came on time. It was not the first time the pain had gone there. At eleven it had started under her belly button and slid down low on the right, and the emergency room had said it wasn’t her appendix, it was a stomach bug, and by Sunday she was fine. By fifteen she had worked something out on her own. Day one of her period: the fever, then the vomiting, then the legs, three or four days of it, and four weeks later the same again. “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.” The diary doesn’t say where that Tuesday fell on the calendar, and nobody at the hospital asked. At the follow-up the surgeon said the pathology report was back and the appendix was normal. “Unremarkable,” he said, like that was good news. She asked what had caused the pain, then. He said sometimes these things are hard to explain — it was probably an early appendicitis that settled down, or a virus — and anyway, it was out now, so she would never have to worry about it again. She was fifteen, and the explanation she went home with was two guesses joined by an “or.” What came back three weeks later is Chapter 7’s to tell.

See it move: Scene 1 · Tuesday, Two Doors · Scene 6 · The Door With No Label

II · From recipe to door

Inside a cell there are two kinds of organelles: the ones that help make proteins and the ones that don’t. The protein makers, together the endomembrane system, run like an assembly line as disciplined as any factory floor — storage, interpretation, assembly, folding, shipping — and the cart between stations is a vesicle. Storage is the nucleus, wrapped in a double envelope pierced by pores. The pores are protein gateways, not sieves: histones and polymerases go in, and mRNA, tRNA and ribosome subunits (built in the nucleolus) come out. Picture a library’s reserve room: you may copy the encyclopedia, you may not carry the original out the door. The recipe book itself is DNA, about two meters of it in every nucleus, wrapped almost twice around eight-histone spools (about 147 base pairs per nucleosome), coiled into supercoils and — only when a cell divides — condensed into 46 visible chromosomes. The rest of the time it is loose, spaghetti-like chromatin: same exact molecule, different outfit for the occasion, recipes open on the counter. The copy, mRNA, is read by ribosomes. Free ribosomes make proteins that stay in the cytosol; ribosomes docked on the rough ER, docked because the new chain begins with a signal peptide, make proteins to be secreted or set in a membrane. Endo, plasmic, reticulum: an inside network, nothing to fear. The rough ER folds and checks those chains; the smooth ER makes lipids and steroids, stores Ca²⁺ and detoxifies with cytochrome P450 enzymes. Last stop, the Golgi, a stack of pita bread: in on the cis face, a shipping label, out on the trans face in a fresh vesicle — to the membrane, to a lysosome, or outside by exocytosis. Recipe, read, built, folded, labeled, shipped. The factory builds; transport delivers.

See it move: Scene 2 · The Reserve Room · Scene 3 · Free or Docked · Scene 4 · Rough and Smooth · Scene 5 · Stacked Like Pita Bread

III · Two doors, one slide

Here is that week as a cell biologist would read it — labeled for what it is, a textbook expectation, because nothing was measured that Tuesday but a white count, and the chart gives it no number. The disease you are learning, familial Mediterranean fever, is inflammation started from inside white cells. In an abdominal attack, neutrophils and monocytes crowd the peritoneum, the lining that wraps the gut and every loop of it, an appendix included. Inside them pyrin, a sensor made on free ribosomes that never enters the ER, fires; the textbook FMF variants make it fire too easily. Its alarm, interleukin-1β, leaves by the door with no label: no signal peptide, a pro-form waiting in the cytosol, a caspase-1 cut, and out through gasdermin-D pores, in minutes. Hours later the liver answers through the front door — serum amyloid A and C-reactive protein, the classical route, the proteins Module 2 named. Afterward a fibroblast’s procollagen, shipped the same way, knits the incision into a scar. Two kinds of protein, two exits. A pathologist examines a fixed slice of the organ: its cells and its architecture as they were at the moment of fixing. That slide cannot hold a messenger that had already left its cell and been carried off in the blood, and it shows little of a lining that may barely be in the section. So “Unremarkable” can be a true sentence about the appendix and say nothing about the pain. Honesty flag: the gasdermin-D pore is the best-established exit for IL-1β, not the only one, and how much each route matters in a human neutrophil in an inflamed lining is still being worked out. Her own changed copy, and what one copy can do, is the Chart Reveal’s question (Module 4).

See it move: Scene 1 · Tuesday, Two Doors · Scene 6 · The Door With No Label

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

Tuesday, Two Doors

Two kinds of protein were leaving cells that Tuesday, and they used two different doors.

Animation · in production · 2 min 20 s

Tuesday, Two Doors

A hospital corridor in 1990 and a chart card in typewriter type: FEVER · PAIN LOW ON THE RIGHT · WHITE COUNT UP — TEXTBOOK. No number appears on it. The camera dives through skin into three cells working the same week, each drawn as a small factory with two doors and a label: A TEXTBOOK EXPECTATION, NOT A MEASUREMENT. In a hepatocyte (A5875), albumin and the emergency proteins SAA and CRP each carry a small ticket at their front end, SIGNAL PEPTIDE: the ribosome parks on the rough ER, the chain folds, the Golgi stamps a label, and a vesicle fuses with the plasma membrane — FRONT DOOR · CLASSICAL ROUTE. In a fibroblast at the incision (A0047), procollagen takes the same route and becomes a scar. In a neutrophil in the belly lining (A0468), a granule spills elastase out the front door too — but a paper airplane tagged 1β (case icon, folded) has no ticket. It sits in the cytosol, is cut loose (MODULE 2.6, one beat), and flies out through a ring of pores drawn in the membrane — SIDE DOOR · NO LABEL. A clock strip runs beside the two doors: MINUTES at the side door once the cut is made; HOURS for the liver’s new orders, because they start by copying genes. Last frame: a pathologist’s slide of an appendix, cells and layers in pink and purple, stamped UNREMARKABLE — and the paper airplane already gone off the top of the screen.

Prefer the full lecture? Protein-Making Organelles (original video on the site)

Explore · in production

The loading bay

A factory loading bay drawn in cross-section at the edge of a cell. On the left, a wide FRONT DOOR with a vesicle dock (the classical route: signal peptide → rough ER → Golgi → vesicle → exocytosis); on the right, a small SIDE DOOR with no sign on it, drawn as a ring of pores; at the back, a shelf labeled STAYS HOME (the cytosol). A conveyor brings ten sealed crates, each stenciled with its maker: albumin (hepatocyte) · serum amyloid A (hepatocyte) · C-reactive protein (hepatocyte) · fibrinogen (hepatocyte) · procollagen (fibroblast) · elastase (a neutrophil granule) · interleukin-1β (neutrophil or monocyte) · pyrin (neutrophil) · hemoglobin (red blood cell) · gasdermin D (neutrophil). Round 1, PREDICT: before opening a crate the student picks FRONT, SIDE or STAYS HOME. Round 2, READ THE TICKET: opening a crate shows the first stretch of the chain as a printed ticket — SIGNAL PEPTIDE (a run of about 15–30 mostly oily amino acids at the very front) or NO TICKET — and the student drags the crate to a door. A rules card sits on the wall in plain words: a ticket means the chain was built on a ribosome docked at the rough ER and leaves by the front door; no ticket means it was built on a free ribosome and either stays home or needs another way out. Reveals, one per crate: albumin — made as preproalbumin, ticket clipped in the ER, about 10–15 g leaves the liver every day; SAA — a 122-amino-acid chain whose first 18 are the ticket; CRP — five identical pieces snapped into a ring inside the ER before shipping; fibrinogen — six chains assembled in the ER; procollagen — wound into a triple helix in the rough ER, its ends trimmed off outside the cell, and the scar of the incision is built from it; elastase — front door too, but made back in the marrow while the neutrophil was young and stored in granules (specialized lysosomes), released by exocytosis only when the cell is triggered; interleukin-1β — no ticket: built as a 31 kDa pro-form in the cytosol, cut by caspase-1 (Module 2.6), out the SIDE door; pyrin — no ticket, STAYS HOME: a sensor that lives in the cytosol and never leaves; hemoglobin — no ticket, STAYS HOME, in a cell with no nucleus left to make more. Gasdermin D is the twist card: no ticket, and it doesn’t leave at all — once caspase-1 cuts it, its front half becomes the side door (Scene 6). Round 3, THE TUESDAY BOARD: the student drags the three makers that were most likely busy in an attack (liver, belly-lining white cells, and — after the operation — fibroblasts at the incision) onto a timeline of minutes, hours and days, labeled as a textbook expectation with no 1990 measurements. The reveal card: a pathologist’s slide of the removed organ — fixed, stained, cells and layers — sits beside the empty side door: “A slice of fixed tissue shows cells and structure. It does not hold a messenger that already left through a side door and was carried away in the blood.” A caveat line underneath: what the 1990 slide showed beyond “normal” is not recorded. Keyboard path: Tab moves crate to crate, Enter opens one, arrow keys choose a door, Space sends it; each placement is announced in a polite live region (“Albumin: signal peptide, front door”). A <details> text version below gives a table of all ten proteins: maker · ticket · door · one-line note.

Think · not graded

Pick one protein from each cell working that Tuesday — a hepatocyte, a fibroblast at the incision, a neutrophil in the belly lining — and say which door it used and how you would know from the first stretch of its chain. Then explain how a messenger could have been leaving cells in the lining all morning and still leave nothing for a pathologist to see on a slide of the organ.

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

Check · Single Choice

The surgeon’s “textbook” Tuesday. Which of these proteins leaves its cell without ever passing through the Golgi?

This self-scoring check is in production.

Evidence card 1

Add it to your evidence board

Tuesday, 1990: “The surgeon called it textbook: fever, pain low on the right, white count up.”

Scene 2 of 6

The Reserve Room

You may copy the encyclopedia; you may not carry the original out the door.

Narrated slide · in production · 2 min 00 s

The Reserve Room

Two of Amy’s narrated slides. Slide 2, The Assembly Line: “there are basically two types of organelles — ones that help make proteins and ones that don’t”; together the protein makers are the endomembrane system and behave “exactly like a factory assembly line”: the recipe out of storage, interpreted, assembled, folded into working shape, shipped; a vesicle is “the factory’s internal delivery cart”; “memorize that sequence — storage, interpretation, assembly, folding, shipping”; “the last verb is shipping, and shipping means out — the hinge of this module”; “one sentence with a comma in it: the factory builds, the transport delivers.” Slide 4, Genetic Material: DNA “is the recipe book; you are the collection of dishes it knows how to make”; one molecule with different names “the way we call one human an infant and another geriatric”; 46 chromosomes, DNA wound on histones, “eight histones wrapped in DNA make a nucleosome,” nucleosomes coil into supercoils (“an official term”) and condense into a chromosome visible under a microscope only while a cell divides; the rest of the time it is “a spaghetti-like tangle we call chromatin — same exact molecule, different outfit.” Caption lines under the player: (1) The nucleus and ribosomes feed the endomembrane line but aren’t counted as membrane parts of it in the strict sense, and this chapter adds a second exit the line never uses (Scene 6). (2) Nuclear pores are protein complexes: proteins such as histones and polymerases go in; mRNA, tRNA and ribosome subunits come out. (3) The nucleolus builds ribosome subunits. (4) A nucleosome is about 147 base pairs of DNA wrapped almost twice around a histone octamer; one nucleus holds about 2 m of DNA in all.

Prefer the full lecture? Protein-Making Organelles (original video on the site)

Explore · in production

The copy desk

The nucleus drawn as a library reserve room behind a double wall (two membranes, labeled — “the most exclusive door in the cell”) with eight numbered gates — the nuclear pores — and a copy desk at the front. Inside, shelves of bound volumes stand open or shut. Round 1, WHO MAY PASS?: nine visitors queue at a gate, and before each steps up the student predicts IN, OUT, BLOCKED or WALKS RIGHT THROUGH. A histone (in — DNA needs its spools), an RNA polymerase (in), a transcription factor (in), an mRNA copy (out), a tRNA (out), a small ribosome subunit and a large one, built in the nucleolus at the back of the room (out, separately — a whole ribosome is never assembled inside), an ATP molecule or a K⁺ ion (walks right through; small things pass freely), and the original DNA volume itself (BLOCKED: “you may copy the encyclopedia; you may not carry the original out the door”). A correction card pins itself over the old slide’s line “only nucleotides pass”: the pores are protein gateways, and the big traffic is proteins in and RNA out. Round 2, REQUEST A RECIPE: the student picks a reader cell — hepatocyte, neutrophil, fibroblast or mature red blood cell — and a recipe slip (albumin, pyrin, pro-IL-1β, procollagen, hemoglobin). The librarian (RNA polymerase) copies a recipe into mRNA only if that volume lies open as chromatin in that cell type: the hepatocyte copies albumin all day; the fibroblast, procollagen; the neutrophil reads pyrin and, once primed by a danger signal, pro-IL-1β. Every nucleated cell owns every volume — “the blueprint is filed right here, in each cell’s own control center” — but each keeps different ones open. The red blood cell’s desk is empty: it pushed its nucleus out as a young erythroblast, so it cannot copy anything new for the rest of its roughly 120 days (Chapter 2). A side card corrects the old idea of a white-cell nucleus that grows as it remembers invaders: immune memory lives in memory lymphocytes; a neutrophil’s nucleus is 2–5 lobes because it is mature, not because it is full. Round 3, OUTFIT CHANGE: a slider from “busy making proteins” to “dividing” winds the volumes up — loose chromatin (spaghetti, or a curly telephone cord from the 1980s) → nucleosomes (eight histones, about 147 base pairs wrapped almost twice) → supercoils (“an official term”) → 46 X-shaped chromosomes. At the chromosome end the copy desk closes: condensed DNA is packed for moving, not for reading. “Same exact molecule, different outfit for the occasion” — the infant and the geriatric, both people. A size card: about 2 m of DNA in one nucleus a few micrometers across; the longest chromosome’s DNA is about 8.5 cm (the old page’s “nine-foot” chromosomes corrected). A locked drawer at the end of the shelf reads: MEFV — every nucleus has one; her own changed copy, and what one copy can do, is the Chart Reveal’s question (Module 4). It cannot be opened. Amy’s joke card for the last screen: “Your DNA is not your nuclei.” Keyboard path: Tab to a visitor or a slip, arrow keys choose an answer, Enter submits; the outfit slider is a native range input with five labeled steps; a live region narrates each pass or block. A <details> text version gives the gate rules, the reader-cell table and the five packing levels as lists.

Think · not graded

Amy’s reserve room: you may copy the encyclopedia, never carry the original out. Why would a cell keep its DNA behind a double wall and send out copies instead? Compare a neutrophil that needs pyrin, pro-IL-1β and new membrane proteins today with a mature red blood cell that has no nucleus at all — what can each one still make tomorrow?

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

Check · Drag the Words

The reserve room and the recipe book. Drag each word into place. Two distractors are on the bank.

This self-scoring check is in production.

Evidence card 2

Add it to your evidence board

At eleven the pain “slides down low on the right,” and the ER said “it wasn’t my appendix, it was a stomach bug” — four years before the appendectomy.

Scene 3 of 6

Free or Docked

The same machine, two addresses: the first stretch of a new chain decides which door it will use.

Narrated slide · in production · 1 min 50 s

Free or Docked

Two of Amy’s narrated slides. Slide 5, Ribosomes: “think of the nucleus like the library’s reserve room — you may copy the encyclopedia, but you may not carry the original book out the door. That copy… that’s what mRNA is,” and a ribosome builds the chain “one amino acid at a time”; some ribosomes “float free in the cytoplasm” and make proteins for the cell’s own use, others are bound to the endoplasmic reticulum and make proteins “shipped elsewhere, out of the cell”; and a quick reasoning check — a cell “absolutely packed with ribosomes” makes a great deal of protein. Slide 6, Endoplasmic Reticulum: “let’s defuse it… by dissecting the name. Endo means inside. Plasmic points to something inside the plasma membrane. Reticulum is simply an old word meaning network” — “a webbed maze of membranous tubules wrapping around the nucleus… just an inside network. Nothing to fear”; rough and smooth ER are physically attached but count as separate organelles because they do different jobs; root-word dissection as “a small act of rebellion against the way anatomy is usually taught, as a wall of memorization — you’re allowed to take them apart.” Caption lines under the player: (1) Busy-ribosome cells to picture: plasma cells (antibodies), pancreatic acinar cells (digestive enzymes), hepatocytes (albumin, SAA, CRP). (2) What decides free or bound is the new chain itself: a signal peptide at its front end docks the ribosome on the rough ER. Free ribosomes make cytosolic proteins — pyrin, pro-IL-1β, tubulin, hemoglobin; bound ribosomes make secreted, membrane and lysosomal proteins. (3) In a flaring white cell the overtime is pro-IL-1β, built on free ribosomes (the Explore’s OVERTIME switch).

Prefer the full lecture? Protein-Making Organelles (original video on the site)

Explore · in production

Ticket check

A slice of cytosol with a tray of loose ribosomes on the left and, on the right, a dock built into the rough ER membrane: a gate (the translocon) and a uniformed ticket inspector labeled SRP (signal recognition particle). The student threads an mRNA onto a free ribosome and presses BUILD; the chain grows one bead per amino acid. Once the front of the chain — where a signal peptide would be — pokes out of the ribosome’s exit tunnel (roughly 50–70 beads in), the inspector reads it. If it starts with a SIGNAL PEPTIDE (a run of oily beads), the inspector pauses the ribosome, walks it to the dock and plugs it into the gate; the chain threads into the ER as it is built, and a small pair of scissors (signal peptidase) clips the ticket off. If there is no ticket, the ribosome stays free and the finished protein drops into the cytosol. Nine mRNAs wait in the queue, and before each the student predicts FREE or DOCKED: albumin (docked → secreted), serum amyloid A (docked → secreted), procollagen (docked → secreted), the IL-1 receptor, IL-1R1 (docked, with a second oily stretch that stays stuck in the membrane as an anchor → the receptor on a target cell, Chapter 1), a lysosomal digestive enzyme (docked → lysosome, Scene 5), pyrin (free), pro-IL-1β (free), tubulin (free — the rails of Chapter 3) and hemoglobin β in a young red cell (free). A forced-entry test: drag the pro-IL-1β chain to the dock anyway — the inspector shakes its head (“no ticket”) and the chain stays in the cytosol, which is why its exit has to be a different door (Scene 6). Round 2, RIBOSOME CENSUS: four cells drawn in cutaway — a hepatocyte, a plasma cell, a pancreatic acinar cell and a mature neutrophil — with a counter for free and docked ribosomes. The first three are crowded with rough ER (secretors); the neutrophil has few ribosomes, mostly free (its granule proteins were made back in the marrow). A melanocyte card waits in the tray as the trap: melanin is not a protein — a ribosome-built enzyme, tyrosinase, makes it. An OVERTIME switch then primes the neutrophil with a danger signal: its free ribosomes speed up and pro-IL-1β airplanes stack in the cytosol — Amy’s factory on overtime — while the dock stays quiet. The reveal: the ribosome is the same machine wherever it sits — “blameless and precise”; the first words of a protein decide its address and its door. A correction note pins over the 3139 page’s “free ribosomes make membrane proteins”: it is the other way round. Keyboard path: Tab to an mRNA, Enter loads it, B builds, F or D records the prediction; the census counters are buttons; a live region reads each result (“Albumin: signal peptide found, docked at the rough ER”). A <details> text version lists the nine proteins with ticket, address and destination, and the census as a table.

Think · not graded

A ribosome is the same machine whether it floats free or sits on the rough ER — so what decides where it ends up? Use albumin and pro-IL-1β to explain how the first stretch of a new chain chooses its address, and then say what that one fact rules out about the way interleukin-1β can leave a cell.

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

Check · Mark the Words

Mark the six words that carry the ribosome story — what ribosomes do, where they sit, why they sit there, and what a crowd of them means. Leave the rest unmarked.

This self-scoring check is in production.

Evidence card 3

Add it to your evidence board

“White count up” — the only lab-like line recorded anywhere in 1986–1990, and the chart gives it no number.

Scene 4 of 6

Rough and Smooth

One network, two departments: the rough ER folds and checks; the smooth ER makes lipids, stores calcium and detoxifies.

Slide · in production

Rough and Smooth

Left, a nucleus (A4023) with the rough ER running straight out of its envelope, studded with ribosome dots (A3913) and labeled ROUGH ER · FOLD · ADD SUGARS · CHECK; an inset shows a new chain threading in through its gate and the signal-peptide ticket clipped off at the door. Right, the smooth ER’s branching tubules (A4028), labeled SMOOTH ER · LIPIDS AND STEROIDS · Ca²⁺ STORE · DETOX (CYTOCHROME P450s, INCLUDING CYP3A4). Below, a hepatocyte (A5875) with both departments drawn in and a tag: A LONG-TOLERANT DRINKER’S LIVER GROWS MORE OF THIS. A strip across the bottom: PYRIN · PRO-IL-1β · HEMOGLOBIN — NEVER ENTER THE ER (FREE RIBOSOMES, SCENE 3). Lecture line: the rough and smooth ER are physically one network but count as two organelles because they do different jobs. The rough ER folds the chains that will be secreted or set in membranes, starts adding their sugars and checks the fold — a protein’s shape is its function, and a chain that folds wrong, like the CFTR of cystic fibrosis in Matilda’s case, is held back and destroyed before it reaches the Golgi. The smooth ER builds membrane lipids and steroid hormones, stores and releases calcium (in muscle it is the sarcoplasmic reticulum, Module 7), and detoxifies by changing drugs and poisons chemically with cytochrome P450 enzymes. That is why hepatocytes are crammed with it, and why a long-tolerant drinker’s liver physically expands its detox department. One of those enzymes, CYP3A4, clears colchicine, the drug used for FMF — Chapter 6 shows why that matters. Shape equals function.

Explore · in production

Work orders

Two departments drawn side by side and joined in the middle, because they are one network: ROUGH (curved sacs dotted with ribosomes) and SMOOTH (branching tubules), with a front-office inbox above them and a third tray marked NOT THIS BUILDING. Ten work orders drop into the inbox one at a time, each written as a job a cell needs done; before routing each, the student predicts ROUGH, SMOOTH or NOT THIS BUILDING. The orders: (1) fold a new albumin chain and start its sugars — ROUGH; (2) check a new CFTR chain, and hold back and destroy a copy that folded wrong — ROUGH (ER quality control; the cystic fibrosis variant ΔF508 is caught here, before the Golgi — Matilda’s case, page 11630); (3) add hydroxyl groups to procollagen with vitamin C’s help so its triple helix holds — ROUGH (a scurvy card flips: no vitamin C, weak collagen, bleeding gums, wounds that won’t knit); (4) build phospholipids for new membrane — SMOOTH; (5) make cortisol in an adrenal cell or testosterone in a testis — SMOOTH (steroid hormones); (6) store Ca²⁺ and release it on a signal — SMOOTH (in muscle, the sarcoplasmic reticulum — Module 7); (7) release glucose from a liver cell’s stores — SMOOTH (the last enzyme, glucose-6-phosphatase, sits in the ER membrane); (8) inactivate a drug molecule with a cytochrome P450 — SMOOTH (CYP3A4 clears colchicine and some statins; Chapter 6); (9) break down alcohol in a hepatocyte — SMOOTH; (10) build pyrin — NOT THIS BUILDING (free ribosomes in the cytosol; Scene 3). A score card keeps predictions against answers. Round 2, THE LONG-TOLERANT DRINKER: a slider runs from “never drinks” to “years of heavy drinking” and the smooth department visibly grows — more tubules, more P450 enzymes drawn as tools on the walls. A dose meter shows the same drink cleared faster (tolerance), and a second meter shows another drug handled by the same enzymes cleared faster too. A BLOCK THE ENZYME toggle then does the opposite: with the enzyme jammed by a second drug, the first piles up past a red line — the preview of Chapter 6’s pump and enzyme collision. Two correction cards close the round: “The smooth ER changes toxins chemically; it doesn’t wrap them in lipid” and “Cirrhosis is scarring after years of injury, not too much smooth ER.” The reveal: one network, two departments — rough for proteins with a ticket, smooth for lipids, steroids, calcium and detox — and the cytosol’s own proteins never come in at all. Keyboard path: Tab to the inbox, arrow keys choose a department, Enter routes the order; the drinker slider and the block toggle are native controls; a live region announces each routing and meter change. A <details> text version gives the ten orders, their departments and the two meter scenarios as a list.

Prefer the full lecture? Protein-Making Organelles (original video on the site)

Think · not graded

A long-tolerant drinker’s hepatocytes grow more smooth ER. Explain what that department does to alcohol and to many drugs, and predict what happens to a second drug cleared by the same enzymes — first in a liver that has expanded the department, then in one where another drug has jammed the enzyme. Why does none of this touch pyrin?

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

Check · Dialog Cards

RER and SER — one network, two departments. Read each front, answer out loud, then flip to check.

This self-scoring check is in production.

Evidence card 4

Add it to your evidence board

Tuesday, 1990: “He said I was lucky we caught it in time, and that fifteen is right on schedule for an appendix.”

Scene 5 of 6

Stacked Like Pita Bread

A stack of pita bread with a label printer: recipe, read, built, folded, labeled, shipped.

Narrated slide · in production

Stacked Like Pita Bread

Amy’s narrated slide, The Golgi: “our last station is the Golgi something — it has every name after it… I don’t care, let’s just go with the Golgi,” one organelle wearing three names (Golgi body, Golgi complex, Golgi apparatus); “it really does look like a stack of pita bread — you kind of cut it open and look at the side,” flattened sacs layered one on another. Its job as a conveyor belt: a protein arrives from the rough ER inside a vesicle, enters on the receiving face, gets a shipping label that marks its final destination, “and then it’s pooped out the far side” in a brand-new transport vesicle, addressed and ready to go. “It does more”: it pinches off vesicles that patch the plasma membrane, and some of its vesicles mature into lysosomes. “Recipe, read, built, folded, labeled, shipped.” A labeled vesicle heads for the membrane to release its cargo outside the cell — exocytosis, “literally the first topic of the module’s second half” — so the factory tour “hands off directly into the transport story.” Caption lines under the player: (1) The receiving face is the cis face, nearest the ER; the shipping face is the trans face. (2) The Golgi finishes a protein’s sugars, adds tags (a mannose-6-phosphate tag sends digestive enzymes to lysosomes) and sorts cargo into secretory vesicles, membrane vesicles and lysosomes. (3) Its classical exports include SAA, CRP, albumin and procollagen; interleukin-1β never passes through the Golgi (Scene 6).

Prefer the full lecture? Protein-Making Organelles (original video on the site)

Explore · in production

Label and ship

A Golgi drawn as five curved sacs stacked like pita bread and turned into a conveyor: the cis face at the left, facing the ER, the trans face at the right. Vesicles from the ER dock at the cis face one at a time, and a label printer sits at the trans face with five label rolls: SECRETE NOW (continuous secretion) · STORE UNTIL SIGNALED (regulated secretion) · MEMBRANE · LYSOSOME · RETURN TO ER. Nine cargo cards arrive, each describing what its protein does rather than naming its destination: albumin (“carries fatty acids and holds water in the blood”), serum amyloid A (“rides on HDL particles in the blood in an inflammatory week”), procollagen (“becomes the cable of a scar”), the IL-1 receptor, IL-1R1 (“catches interleukin-1β on a target cell’s surface”), a glucose carrier, GLUT1 (“lets glucose through the membrane”), an acid hydrolase (“digests worn-out parts at pH about 4.5–5”), elastase in a young neutrophil in the marrow (“kept loaded for a fight”), and an ER folding helper that slipped out by mistake (“belongs back in the ER”). Before printing, the student predicts the label. As a card rides from cis to trans its sugar chains are trimmed and finished, drawn as a little bead-string changing shape. At the trans face the vesicle buds and travels to where its label sends it: SECRETE NOW fuses with the plasma membrane and empties outside (exocytosis), and the vesicle’s own membrane becomes part of the plasma membrane — a patch; STORE UNTIL SIGNALED sits in a granule shelf until a signal releases it; MEMBRANE leaves its protein standing in the plasma membrane facing out; LYSOSOME (read off a mannose-6-phosphate tag) buds a vesicle that becomes a lysosome — “some of its vesicles mature into the lysosomes we met earlier”; RETURN TO ER sends the helper back along a return track. Mismatched labels play out honestly: an acid hydrolase labeled SECRETE NOW drifts off into the blood and does nothing useful there. The procollagen card adds a last step outside the cell: enzymes trim its ends and it assembles into collagen fibers — the scar. A tally board fills in Amy’s six verbs with the organelle under each: recipe (nucleus) · read (ribosome) · built (on the rough ER) · folded (rough ER) · labeled (Golgi) · shipped (vesicle, exocytosis). Last card: an interleukin-1β crate never arrives at the cis dock at all — the conveyor waits, and a note reads “not on this conveyor: no ticket, no ER, no Golgi — Scene 6.” Keyboard path: Tab to the waiting cargo, arrow keys choose a label, Enter prints, Space runs the conveyor; each destination is announced in a live region (“Albumin: secrete now — exocytosis”). A <details> text version gives the nine cargoes, their labels and destinations, and the six-verb tally as a table.

Think · not graded

Recipe, read, built, folded, labeled, shipped. Walk one albumin molecule through all six verbs and name the organelle at each. Then try to walk interleukin-1β through the same six — at which verb does it fall off the line, and what does that tell you about the door it must use?

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

Check · Single Choice

A vesicle of new albumin arrives from the rough ER. What does the Golgi do with it?

This self-scoring check is in production.

Evidence card 5

Add it to your evidence board

At the follow-up: “the pathology report was back and the appendix was normal. ‘Unremarkable,’ he said, like that was good news.”

Scene 6 of 6

The Door With No Label

No ticket, no Golgi, no vesicle: the alarm leaves through a hole the cell punches in itself.

Animation · in production · 2 min 20 s

The Door With No Label

A neutrophil drawn large and cut open. In the cytosol, free ribosomes turn out paper airplanes still attached to their sheets — PRO-IL-1β · NO SIGNAL PEPTIDE — while the rough ER and the Golgi sit dimmed, a sign on their door: NOT THIS ROUTE. A small ring lights (AN INFLAMMASOME — CHAPTER 7) and a pair of scissors labeled CASPASE-1 makes two cuts. The first frees the airplane (MODULE 2.6, one beat). The second splits a long protein, GASDERMIN D, in two, and a word-card takes the name apart: GASTRO + DERMIS — first found in the gut and the skin. The freed front halves drift to the inner face of the plasma membrane, gather into a ring of about thirty and punch a hole roughly 20 nanometers across. Airplanes fly out through it: SIDE DOOR · NO LABEL · MINUTES. A one-line card: PYROPTOSIS — A CELL CAN DIE BY ITS OWN PORES (PYRO = FIRE, PTOSIS = FALLING). A footnote card slides in: OTHER UNCONVENTIONAL EXITS EXIST — VESICLES THAT SKIP THE GOLGI, A DYING CELL’S SPILL — HOW MUCH EACH MATTERS IS STILL BEING WORKED OUT. The messenger rides a blood vessel away toward an outline of the brain (THE RECEPTOR — CHAPTER 1), and the last frame returns to the scene: a slide of fixed tissue with the cells still on it and no airplane anywhere.

Prefer the full lecture? Protein-Making Organelles (original video on the site)

Explore · in production

Two lanes

A race track with two parallel lanes and one shared clock along the top, drawn on a stretched scale so seconds, minutes and hours all fit (tick marks: 1 s · 1 min · 10 min · 1 h · 6 h · 24 h), labeled “illustrative textbook orders of magnitude, not measurements.” Lane 1, CLASSICAL, carries SAA from a hepatocyte through eight checkpoints: an IL-6 message arrives → the SAA gene is copied (transcription) → the mRNA leaves through a nuclear pore → a ribosome docks on the rough ER (signal peptide) → the chain is folded → the Golgi labels it → a vesicle carries it → exocytosis into the blood. Lane 2, PORE, carries interleukin-1β from a primed neutrophil through five: pro-IL-1β already built on free ribosomes and waiting in the cytosol → the sensor fires (Chapter 7) → caspase-1 cuts pro-IL-1β (Module 2.6) and gasdermin D → gasdermin-D pieces gather into pores in the plasma membrane → IL-1β out. Before the start the student predicts which car finishes first and roughly when, on the clock. Run: the pore car is out within minutes to about an hour after the sensor fires; the classical car needs hours, because the liver’s answer begins by copying genes — the acute-phase proteins climb over 6–8 hours and more (Module 2’s curves). Three toggles test why. UNPRIMED: the neutrophil has no pro-IL-1β in stock, so Lane 2 must first copy and build it — and the pore car drops back into hours. The point is printed on the finish line: fast because it was stocked, not because the door is better. STORED CARGO: a third car, elastase, starts in a neutrophil granule — built on the classical line in the marrow long ago and stored — and on a signal it is out in seconds to minutes by exocytosis. Pre-made cargo is fast by either door. PORE COUNT: a slider from few to many. With few pores the cell stays alive and keeps releasing; with many, water rushes in, the cell swells and bursts — pyroptosis — and its contents spill. A fourth figure, pyrin, stands at the start of Lane 2 and never runs: a sensor stays in the cytosol. The last card freezes the track hours later and drops in a pathologist’s slide: tissue and cells are on it; Lane 2 is empty. A hedge card sits beside the slide: “Other unconventional exits are known — vesicles that bypass the Golgi, a dying cell’s spill — and how much each matters in a human neutrophil in an inflamed lining is still being worked out (the 3.9 evidence page).” Keyboard path: Tab to the prediction controls, then RUN (Enter); the toggles and the pore slider are native controls; each checkpoint is announced in a live region with its clock time (“SAA: Golgi, about 40 minutes after the chain was built”). A <details> text version lists both lanes’ checkpoints and the outcome of every toggle.

Think · not graded

Interleukin-1β has no shipping label, yet once it is cut it is out of the cell within minutes. Explain how the same enzyme that cuts pro-IL-1β also opens the door, and why a cell that opens too many of those doors dies. Then say what that means for a pathologist who looks, days later, at a fixed slice of tissue from the place where it happened.

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

Check · Single Choice

Pro-IL-1β has no signal peptide. Once caspase-1 has cut it, how does mature interleukin-1β mainly leave a neutrophil?

This self-scoring check is in production.

Evidence card 6

Add it to your evidence board

“I asked what caused the pain, then. He said sometimes these things are hard to explain.”

4

Your evidence board

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: Two exits, one normal slide, Neither on its own, or An early appendicitis that settled down, or a virus. 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

Fever, pain low on the right, white count up — and an appendix that came back “Unremarkable.” Read through the protein factory, the reading that fits best is…

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

  • A normal organ, with the trouble somewhere the slide didn’t include
  • Nothing was inflamed — “Appendectomy; appendix unremarkable”
  • It was “probably an early appendicitis that settled down or a virus”

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 two exits

Two Doors and a Normal Slide

1990, age fifteen: “textbook: fever, pain low on the right, white count up,” an appendix removed, and a pathology report that read “Unremarkable” — explained as “probably an early appendicitis that settled down or a virus.” Chart entry: proteins leave cells by two doors. Those that begin with a signal peptide — the liver’s SAA, CRP and albumin, a fibroblast’s procollagen, a neutrophil’s granule enzymes — are built on ribosomes docked at the rough ER, folded and checked there, labeled in the Golgi and shipped by exocytosis. Interleukin-1β has no signal peptide: it is built as a pro-form on free ribosomes in the cytosol and, once caspase-1 cuts it, leaves through gasdermin-D pores within minutes. A slide of fixed tissue shows cells and structure, not a messenger already gone, so “Unremarkable” can be true of the organ and silent about the lining. Held open: where the white cells that raised the count actually went, what came back three weeks later, and how much of IL-1β’s exit runs through routes other than the pore — Chapter 7 logs Chart Clue #3.

Next · Chapter 5

Osmosis, Tonicity, Effusion

Day one of the cycle at fifteen: the fever, then “the vomiting, the kind that doesn’t stop when there’s nothing left” — three or four days of water and salt leaving her, what that does to every cell, and why an inflamed lining leaks.

Continue to Chapter 5 →

Reliable information on rare diseases

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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