Topic 5a — Intracellular Compartments

§Introduction

The defining structural fact of a eukaryotic cell is internal membranes. A bacterium runs all its chemistry in one open cytoplasm; a eukaryotic cell partitions incompatible reactions into membrane-enclosed organelles, each a distinct chemical environment with its own enzymes, pH, and ion composition. This buys specialization — but it also creates a hard logistics problem: proteins made in the cytosol must be delivered to the right compartment, which is the subject of Topic 5b.

Two organelles get special attention because they are, in a real sense, former bacteria. Mitochondria and chloroplasts arose by endosymbiosis — an ancestral cell engulfed a bacterium and, instead of digesting it, kept it. The evidence is written into their structure: their own circular genomes, their own bacterial-type ribosomes, their double (or triple) membranes, and their inability to form de novo. This is not a hypothesis presented for balance — it is the consensus explanation, and the exam expects you to marshal the evidence for it.

This guide follows the lecture's two deep dives — the mitochondrion (structure, oxidative phosphorylation, its genome and maternal inheritance) and the nucleus (envelope, pore complex, import/export) — and restores three pieces of Ch. 12/14 the lecture skimmed or got wrong: the correct three-membrane architecture of chloroplasts, the peroxisome, and the topological logic that unifies the endomembrane system. Where the lecture's dates or membrane counts diverge from the record, it's flagged in amber.

◎Learning Objectives

1Why Compartments? Eukaryotic Organization

Bottom line: compartmentalization lets one cell run mutually incompatible chemistry at the same time. A prokaryote has genetic material but no membrane-enclosed organelles; a eukaryote partitions its interior into organelles, each an optimized micro-environment.

Why wall off chemistry — the trade-offs it buys

• Separate incompatible reactions. Degradative enzymes (lysosome) would destroy the cytosol if released; oxidative reactions (peroxisome) generate H₂O₂ that must be contained. Membranes keep these apart.
• Concentrate reactants and enzymes. Confining a pathway to a small volume raises local concentrations and reaction rates.
• Create specialized membrane surface. Many reactions (electron transport, protein synthesis) happen on membranes; internal membranes multiply the available area far beyond the plasma membrane alone.
• Enable independent regulation. Distinct pH and ion conditions per compartment (acidic lysosome, high-Ca²⁺ ER) allow processes to be tuned separately.

The major organelles (functional overview)

• Nucleus — largest organelle; holds the genome as chromatin; nucleolus assembles ribosomes (§7).
• Endoplasmic reticulum (ER) — membrane network continuous with the nuclear envelope. Rough ER (ribosome-studded) makes secretory/membrane proteins; smooth ER (no ribosomes) makes lipids and handles detox/Ca²⁺ storage.
• Golgi apparatus — receives ER products, modifies them (glycosylation, sorting), and dispatches them.
• Lysosomes — acidic degradative compartments ("garbage collectors"); enzymes break down worn components.
• Peroxisomes — oxidative reactions, detox, fatty-acid breakdown (§8).
• Mitochondria — ATP production by oxidative phosphorylation (§3–4).
• Chloroplasts (plants) — photosynthesis (§6).

2Endosymbiotic Theory

Bottom line: mitochondria and chloroplasts were once free-living bacteria, captured by an ancestral host cell and retained rather than digested. This is the consensus origin story, and the evidence is structural and genetic.

The scenario

A large anaerobic predatory cell engulfed an aerobic bacterium; the host couldn't digest it, and the two became mutually dependent — the bacterium became the mitochondrion. Separately, a host cell engulfed a photosynthetic bacterium (a cyanobacterium), which became the chloroplast. Each captured bacterium was surrounded by a host membrane, which contributes to the double-membrane arrangement.

The evidence — know all four

1. Own genome. Mitochondria and chloroplasts carry their own circular DNA, resembling a bacterial chromosome (no introns in mitochondrial genes; a single circular molecule).
2. Own ribosomes. They synthesize some of their own proteins on bacterial-type ribosomes — smaller than the eukaryotic cytosolic ribosomes and similar to bacterial ones.
3. Double membrane. Consistent with engulfment: the inner membrane derives from the original bacterium; the outer relates to the host.
4. No de novo formation. A cell that loses its mitochondria/chloroplasts cannot make new ones — they arise only by growth and division of existing ones, like bacteria. The nuclear genome lacks the full instruction set to build one from scratch.

Textbook note. The lecture attributes the endosymbiotic proposal to Lynn Margulis in 1981. Correction: Margulis first proposed the modern endosymbiotic theory in her 1967 paper ("On the Origin of Mitosing Cells," published under the name Lynn Sagan). 1981 is the year of her book Symbiosis in Cell Evolution, which expanded the argument — not the original proposal. The idea is 1967. (historical record; cf. Ch. 14 endosymbiosis) Flag this for the exam.

3Mitochondrion Structure

Bottom line: the mitochondrion's two membranes create two chemical compartments, and that two-compartment design is the entire physical basis for ATP synthesis. Structure here is function.

The two membranes and two spaces

• Outer membrane — smooth and relatively permeable: porin channels let small molecules pass freely, so the intermembrane space is chemically close to the cytosol.
• Inner membrane — highly folded into cristae (folds that dramatically increase surface area), and strictly selective (impermeable to most ions/molecules, including ATP/ADP). This is where the electron-transport chain and ATP synthase sit.
• Matrix — the space enclosed by the inner membrane; contains the mitochondrial DNA, ribosomes, and the citric-acid-cycle enzymes.
• Intermembrane space — between the two membranes; where protons are pumped and accumulate.

The point of the cristae is surface area: the more inner-membrane area, the more electron-transport chains and ATP synthases can operate, so cells with high energy demand pack in more cristae (and more mitochondria).

4Oxidative Phosphorylation & ATP Synthase

Bottom line: mitochondria make ATP in two coupled stages — (1) electron transport pumps H⁺ to build a gradient (active), and (2) H⁺ flows back through ATP synthase to make ATP (passive). This is the chemiosmosis you met in Topic 4, now localized to the inner membrane.

Stage 1 — build the proton-motive force

High-energy electrons (carried in from food breakdown as NADH) pass down the electron-transport chain (complexes I, III, IV). At each pumping complex, the energy released is used to pump H⁺ from the matrix into the intermembrane space. This builds an electrochemical H⁺ gradient across the inner membrane — the proton-motive force (the intermembrane space becomes more acidic/positive; the matrix more alkaline/negative).

Stage 2 — spend it through ATP synthase

H⁺ flows back into the matrix down its gradient through ATP synthase. The enzyme is a rotary machine: its membrane-embedded F₀ ring binds protons in the intermembrane space and releases them in the matrix, and this proton flow spins the rotor (~8000 rpm). The rotating shaft cycles the three catalytic sites of the F₁ head, altering their affinity for ATP/ADP and driving synthesis — ~3 ATP per turn (roughly 3 H⁺ per ATP). Because the inner membrane is impermeable to ATP and ADP, an ATP/ADP carrier exports the newly made ATP and imports ADP.

Textbook note. Topic 4's lecture labeled this same H⁺-gradient/ATP-synthase process "glycolysis in the mitochondria." Reminder: the process shown here is oxidative phosphorylation. Glycolysis is cytosolic — it happens in the cytoplasm, produces a small amount of ATP without any membrane gradient, and feeds the products that eventually supply the electrons (as NADH) used here. Don't conflate the two. (Ch. 14; cf. Topic 4 §4) Flag this for the exam.

5The Mitochondrial Genome & Maternal Inheritance

Bottom line: the mitochondrion keeps a small, bacterial-style genome, relies on the nucleus for most of its proteins, and is inherited only from the mother — which has consequences for disease and for tracing ancestry.

A bacterial-style genome — and a division of labor

• Circular and compact. The human mitochondrial genome is a single circular chromosome (~16,569 bp). Unlike the nuclear genome (where the vast majority is non-coding regulatory DNA), mtDNA is mostly coding with very little non-coding sequence.
• What it encodes. Its own rRNAs and tRNAs, plus a handful of proteins for oxidative phosphorylation/electron transport. Everything else the mitochondrion needs is encoded by the nuclear genome, made in the cytosol, and imported — the symbiotic bargain (the host supplies most proteins; the mitochondrion supplies energy).
• Own bacterial-type ribosomes. mtDNA genes are translated on ribosomes similar in size to bacterial ribosomes (smaller than eukaryotic cytosolic ribosomes).

Maternal inheritance and heteroplasmy

• Maternal only. mtDNA is transmitted through the egg, not the sperm. Sons receive their mother's mtDNA but cannot pass it on — they are a dead end for the mitochondrial line. mtDNA therefore traces the maternal lineage.
• Heteroplasmy and a threshold. A cell has many mitochondria and many mtDNA copies. In disease, cells can carry a mix of normal and mutant mtDNA; a phenotype appears only once mutant copies exceed a threshold — below it, normal mitochondria compensate.
• Disease example. Leber hereditary optic neuropathy (LHON) — sudden, irreversible vision loss from death of optic-nerve cells — is caused by mtDNA mutations, following maternal inheritance.

Why mtDNA is the tool for ancient/degraded samples

Because each cell has hundreds of mitochondria (and 5–10 mtDNA copies each), a cell holds far more mtDNA than nuclear DNA (only two copies of each nuclear gene). When sample DNA is scarce or degraded — e.g. sequencing Neanderthal remains — mtDNA's high copy number makes it the practical starting point, with the caveat that it reveals only the maternal line.

6Chloroplasts: Three Membrane Systems

Bottom line: a chloroplast is not just "a green mitochondrion with two membranes." It has three membrane systems, and the third one — the thylakoid — is where the light reactions happen.

The three membranes

• Outer membrane — permeable, like the mitochondrial outer membrane.
• Inner membrane — selective; encloses the stroma (the chloroplast's equivalent of the mitochondrial matrix, containing DNA, ribosomes, and carbon-fixation enzymes).
• Thylakoid membrane — a third, internal membrane system forming flattened sacs (thylakoids) stacked into grana. This is the key difference from mitochondria.

Why the third membrane exists — where photosynthesis is split

Chloroplasts run chemiosmosis like mitochondria, but on the thylakoid membrane: the light reactions pump H⁺ into the thylakoid space, and ATP synthase in the thylakoid membrane uses that gradient to make ATP. Carbon fixation (the Calvin cycle) then happens in the stroma. The thylakoid provides the dedicated membrane surface and enclosed space for the light-driven proton gradient — a job the two outer membranes can't do. Chloroplasts share the endosymbiotic origin story, descending from an engulfed cyanobacterium.

Textbook note. The lecture, drawing the parallel with mitochondria (two membranes, from a gram-negative bacterium), implies chloroplasts also have two membranes. Correction: chloroplasts have three membrane systems — outer, inner, and thylakoid. The thylakoid is not a fold of the inner membrane (the way cristae are); it's a separate internal membrane system enclosing its own space, and it's where the light reactions and their proton gradient operate. Counting only two membranes misses the compartment that does photosynthesis. (Ch. 14, Fig. 14–1 / 14–38) Flag this for the exam.

7The Nucleus & Nuclear Pore Complex

Bottom line: the nucleus separates transcription from translation, and that separation is only useful because a gated envelope controls exactly what crosses. The gate is the nuclear pore complex, and the tickets are signal sequences.

The nuclear envelope

• Double membrane. The envelope is two membranes (inner and outer), arranged as a flattened sac. The outer membrane is continuous with the rough ER — the nuclear envelope is effectively a specialized extension of the ER.
• Chromatin attachment. When the cell is not dividing, chromosomes are decondensed and associate with the inner nuclear membrane.
• Why separate at all. Enclosing the genome lets gene expression be regulated post-transcriptionally (e.g. RNA splicing) before the message ever reaches the cytosolic ribosomes.

The nuclear pore complex (NPC) and signal-directed traffic

• Structure. NPCs perforate the envelope, each built from ~30 different proteins (nucleoporins) and displaying eightfold rotational symmetry. They allow large molecules (RNAs, proteins) to cross selectively — small molecules pass freely.
• Import. A nuclear localization signal (NLS) — a stretch rich in the basic residues lysine and arginine — marks a protein for import. Nuclear import receptors bind the NLS and escort the cargo through the NPC.
• Export. A nuclear export signal (NES) — characteristically hydrophobic residues (e.g. leucine) — directs molecules out. Export works like import in reverse, with its own signal and receptors.
• Nucleolus. A sub-nuclear body where ribosomes are assembled (rRNA + ribosomal proteins → small and large subunits); the subunits then exit through the pores to the cytosol to do translation.

8Peroxisomes

Bottom line: peroxisomes are the cell's oxidation-and-detox compartment — a single-membrane organelle that runs reactions producing and consuming hydrogen peroxide. The lecture skipped them; the textbook (and the exam) does not.

What they do

• Single membrane. Unlike mitochondria/chloroplasts, a peroxisome has one bounding membrane — and no DNA of its own.
• Oxidative reactions. They use molecular O₂ to oxidize substrates, generating hydrogen peroxide (H₂O₂) as a byproduct.
• Catalase. The enzyme catalase then handles that H₂O₂ — decomposing it to water and O₂, or using it to oxidize other molecules (detoxification, e.g. of ethanol in the liver).
• Fatty-acid breakdown. Peroxisomes carry out β-oxidation of certain (notably very-long-chain) fatty acids.
• Biogenesis. They form by growth and division of existing peroxisomes and by import of components; proteins are imported after folding using a peroxisomal targeting signal — distinct from the unfolded import used by mitochondria and the ER.

9Topological Relationships of Compartments

Bottom line: the compartments of the endomembrane system aren't a random collection — their interiors are all topologically equivalent to the outside of the cell. This one idea explains why vesicle traffic works and why proteins end up where they do. The lecture didn't frame it this way; the textbook builds the whole sorting story on it.

What "topologically equivalent" means

Think of every membrane as having a "cytosolic" face and a "non-cytosolic" (lumenal/exterior) face. When a vesicle buds from one compartment and fuses with another, its lumenal contents are handed off without ever crossing a membrane. So a molecule in the ER lumen can travel ER → Golgi → cell surface and be secreted — and at every step it stays on the same (non-cytosolic) side. That means the lumens of the ER, Golgi, endosomes, lysosomes, and transport vesicles are all topologically continuous with the extracellular space.

The two "sides," and what belongs to each

• Topologically equivalent to the cell exterior (non-cytosolic): ER lumen, Golgi lumen, endosome/lysosome lumen, vesicle interiors. Cargo moves among them by budding/fusion, never crossing a membrane.
• Topologically equivalent to the cytosol: the nuclear interior — because nuclear pores make it continuous with the cytosol (proteins pass through the pore, not across a membrane).
• Neither — reached only by translocation across a membrane: the interiors of mitochondria, chloroplasts, and peroxisomes. These are not on the vesicular network; proteins must be threaded across their membranes by dedicated machinery (Topic 5b).

Why you care (two payoffs)

• It predicts protein topology. A domain facing the ER lumen will face the extracellular space once the protein reaches the plasma membrane — same side, throughout.
• It explains the glycocalyx. Sugars added in the ER/Golgi lumen end up on the outer (extracellular) face of the plasma membrane — exactly the outer-only glycocalyx from Topic 3. Same topological rule.

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