Cell Structure & Organelles

Organelles for the MCAT

Part 1 of 4 — The Compartment Map & How We Isolate It

Why Compartments Exist

Eukaryotic organelles create dedicated chemical environments: the lysosome runs acid hydrolysis at pH ~4.5-5 that would be fatal in the cytosol (pH ~7.2); the mitochondrial inner membrane maintains the proton gradient oxidative phosphorylation spends; the ER lumen is oxidizing enough to form disulfide bonds that the reducing cytosol forbids.

The Inventory (Membranes Count!)

Not membrane-bound: ribosomes, nucleolus, centrioles, proteasomes. A passage calling the nucleolus an organelle "compartment" is testing exactly this.

Cell Fractionation — Taking the Cell Apart

1. Homogenize cells gently (osmotic or mechanical) → a slurry of organelles
2. Differential centrifugation — spin at increasing speeds; bigger/denser things pellet first:

1. Density-gradient (equilibrium) centrifugation separates the mixed medium-speed pellet by buoyant density
2. Marker enzymes identify fractions: catalase = peroxisome; acid phosphatase = lysosome; succinate dehydrogenase/cytochrome oxidase = mitochondria

• More marker enzymes: glucose-6-phosphatase = ER; lactate dehydrogenase = cytosol.

Compartments & Fractionation 🎯

The Cytoskeleton — Three Systems, Three Jobs

• Actin research drugs: cytochalasin blocks actin polymerization; phalloidin stabilizes filaments (and stains them in microscopy).
• Pathologists type tumors by their intermediate filaments: keratin marks an epithelial origin.

• Motors: kinesin walks toward microtubule (+) ends (generally outward/anterograde); dynein toward (−) ends (inward/retrograde; also powers ciliary bending); myosin moves on actin
• Cilia/flagella: 9+2 microtubule axoneme with dynein arms; basal body = 9×3 like a centriole. Kartagener syndrome (dynein arm defect): immotile cilia → chronic sinusitis/bronchiectasis, infertility, and situs inversus (embryonic nodal cilia set left-right asymmetry)

Reading Electron Micrographs (Passage Skill)

• Studded parallel membranes = rough ER; smooth tubular network = smooth ER
• Stacked flattened sacs with budding vesicles = Golgi
• Double membrane + internal folds (cristae) = mitochondrion; double membrane + pores = nuclear envelope
• Dense, uniform granular body inside the nucleus = nucleolus
• Very dense small dots free or on ER = ribosomes

First Fractionation Story: De Duve

Lysosomes were discovered by fractionation, not microscopy: acid phosphatase activity in a fraction increased after aging or detergent — because intact lysosomes hide their enzymes from substrate (enzyme latency). Increased marker activity after membrane disruption is evidence the enzyme lives inside a membrane-bound compartment — a logic MCAT passages reuse constantly.

Cytoskeleton & EM Logic 🎯

Key Takeaways — Part 1

• Compartments buy incompatible chemistry: acidic lysosome, oxidizing ER lumen (disulfides), proton-tight mitochondrial inner membrane
• Membrane count: double = nucleus, mitochondria; single = ER, Golgi, lysosome, peroxisome; none = ribosome, nucleolus, centriole, proteasome
• Cell specialization is organelle census: plasma cell = rough ER; hepatocyte/steroid gland = smooth ER; steroid cells secrete by diffusion, not vesicles
• Differential centrifugation orders by size/density (nuclei → mitochondria/lysosomes/peroxisomes → microsomes → ribosomes); marker enzymes name the fraction (catalase, acid phosphatase, SDH)
• Enzyme latency (activity unmasked by detergent) = the enzyme is inside a membrane compartment — De Duve's lysosome logic
• Cytoskeleton: actin (7 nm, contraction/crawling), intermediate filaments (10 nm, strength, cell-type markers), microtubules (25 nm, transport highways, spindle, 9+2 cilia)
• Motors: kinesin (+, anterograde), dynein (−, retrograde and ciliary beat — Kartagener), myosin (actin)

• Low-yield extras: glucose-6-phosphatase marks the ER fraction and lactate dehydrogenase the cytosol; cytochalasin blocks actin polymerization and phalloidin stabilizes actin filaments; tumors are typed by their intermediate filaments (keratin = epithelial origin)

Organelles for the MCAT

Part 2 of 4 — Protein Sorting & the Secretory Pathway

Every Protein Carries a Zip Code

Translation always starts on free cytosolic ribosomes. The protein's own sequence then decides its destination:

• The classic peroxisomal tripeptide is SKL (Ser-Lys-Leu).

The SRP Cycle — Getting Into the ER

1. Ribosome begins translating; a hydrophobic signal sequence emerges
2. SRP (signal recognition particle) binds the signal AND pauses translation
3. SRP docks on the SRP receptor at the ER; translation resumes through the translocon channel into the lumen
4. Signal peptidase clips the signal sequence; the protein folds in the lumen
5. For membrane proteins, hydrophobic stop-transfer sequences exit the translocon sideways and become transmembrane segments — their number and order fix the protein's final topology

In the ER lumen: chaperones assist folding; disulfide bonds form (oxidizing lumen only); N-linked glycosylation adds a preassembled sugar block to asparagine. Misfolded proteins are retro-translocated to the cytosol and destroyed by the proteasome (ERAD); overload triggers the unfolded protein response (expand ER capacity or die).

• Machinery names: the translocon is the Sec61 channel; the main luminal chaperone is BiP; protein disulfide isomerase forms and reshuffles disulfides; the N-linked block is a 14-sugar core.

ER → Golgi → Sorting

• Vesicles carry cargo ER → Golgi (anterograde); other vesicles return escaped ER residents (retrograde), recognized by a C-terminal retrieval tag

• Coat names: COPII coats anterograde ER → Golgi vesicles; COPI coats retrograde vesicles, whose receptors read the KDEL retrieval tag.

• Golgi processes N-glycans, adds O-linked sugars, and at the cis-Golgi phosphorylates mannose on lysosomal hydrolases → M6P
• Trans-Golgi network sorts: M6P receptors divert hydrolases to endosomes/lysosomes (acidic endosome releases cargo; receptor recycles); regulated secretory cargo condenses into granules awaiting Ca$^{2+}$; everything else defaults to constitutive secretion

I-Cell Disease — The Experiment Nature Ran

Loss of the phosphotransferase that creates M6P → hydrolases are secreted into the blood instead of reaching lysosomes → lysosomes fill with undigested substrates ("inclusion cells"). Diagnostic pattern: high lysosomal enzymes in serum, enzyme-poor lysosomes swollen with substrate — the definitive proof that M6P is the lysosomal address label.

Sorting Signals 🎯

Topology — The Rule That Answers Whole Passages

A compartment's lumen is topologically "outside." Once a protein's domain faces the ER lumen, that domain faces the Golgi lumen, the vesicle lumen, and finally the extracellular space — without ever crossing another membrane.

• N-glycosylation happens only on luminal domains → a glycosylated domain of a plasma-membrane protein is the extracellular one
• Disulfide bonds form in the ER lumen → found in extracellular domains and secreted proteins, rare in cytosolic ones
• A receptor's kinase domain must be cytosolic — it stays cytosolic through every trafficking step

Glycosylation Contrast Table

• N-glycosylation details: the acceptor sequence is Asn-X-Ser/Thr; the core is 14 sugars; the lectin chaperone calnexin holds glycoproteins until they fold.

Experimental Probes of the Pathway

• Named drug and enzyme probes: tunicamycin is the drug that blocks N-glycosylation; brefeldin A collapses the Golgi into the ER, blocking ER → Golgi transport; Endo H cleaves only high-mannose (pre-medial-Golgi) N-glycans, so Endo H resistance means the protein reached the medial Golgi — a molecular odometer.

Protease-protection logic: if a domain survives protease treatment of intact microsomes but is digested after detergent, it was inside the vesicle — the biochemical mirror of the topology rule.

Topology & Pathway Probes 🎯

Key Takeaways — Part 2

• Zip codes: signal sequence → ER (co-translational, SRP pauses translation); NLS → nucleus (folded, importins/Ran); presequence → mitochondria (unfolded, TOM/TIM, post-translational); C-terminal tripeptide → peroxisome; M6P sugar tag → lysosome
• Fusion experiments prove signals are sufficient; deletion proves necessity
• ER lumen work: signal peptidase, chaperone-assisted folding, disulfides, N-glycosylation on Asn; failures → ERAD (proteasome) or UPR
• Vesicles carry cargo forward to the Golgi and retrieve escaped ER residents backward; trans-Golgi sorts M6P cargo to lysosomes, granules for regulated secretion, default = constitutive secretion
• I-cell disease: no M6P tag → hydrolases secreted to serum, lysosomes stuffed — nature's proof of the pathway
• Topology rule: luminal = future extracellular; glycans and disulfides mark luminal/extracellular domains; kinase domains stay cytosolic forever
• Probes: protease protection (protected = luminal, maps topology), microsomes (reconstitute co-translational import), glycan processing (how far along the pathway a protein got), blocking N-glycosylation (lighter protein, misfolding → UPR), blocking ER → Golgi transport (cargo piles up with ER-type glycans)

• Low-yield extras: SKL is the peroxisomal tripeptide; Sec61 = translocon, BiP = luminal chaperone, PDI makes disulfides, the N-linked core has 14 sugars on Asn-X-Ser/Thr and calnexin runs folding quality control; COPII = anterograde coat, COPI = retrograde coat reading KDEL; tunicamycin blocks N-glycosylation, brefeldin A blocks ER → Golgi transport, Endo H resistance = reached the medial Golgi

Organelles for the MCAT

Part 3 of 4 — Mitochondria, Lysosomes & Peroxisomes in Depth

Mitochondria — Architecture Maps to Function

• The inner membrane's impermeability IS the point: the ETC pumps H$^+$ out of the matrix, and the only easy way back is ATP synthase — chemiosmosis
• Key inner-membrane transporters: ADP/ATP translocase, pyruvate carrier, carnitine shuttle (fatty acids), and the shuttles (malate-aspartate, glycerol-3-phosphate) that move cytosolic NADH equivalents in
• Tissues scale mitochondria to demand: cardiac muscle, brown fat (uncoupling protein UCP1 makes heat by short-circuiting the gradient), proximal tubule

The Endosymbiotic Theory — Evidence List

1. Own circular DNA, no histones — like a bacterial chromosome
2. Own ribosomes, bacterial-type (70S-like) and sensitive to some antibacterial antibiotics — a source of drug side effects
3. Double membrane — the inner membrane's lipids resemble bacterial membranes (cardiolipin)
4. Divide by fission, independent of the cell cycle
5. Sequence phylogeny places mtDNA among alpha-proteobacteria

• The antibiotics with mitochondrial side effects include aminoglycosides and linezolid; the outer-membrane porin is VDAC.

Genetics consequences: mtDNA is maternally inherited (sperm mitochondria are eliminated); each cell carries many genome copies → heteroplasmy (mixtures of mutant and normal mtDNA), and disease severity tracks the mutant fraction and the tissue's energy demand (nerve, muscle first). Most mitochondrial proteins are nuclear-encoded and imported (Part 2), so "mitochondrial disease" can be Mendelian too.

• Named mtDNA diseases: MELAS (mitochondrial encephalomyopathy, lactic acidosis, stroke-like episodes) and LHON (Leber hereditary optic neuropathy).

Lysosomes — Degradation Central

Substrates arrive by three routes: endocytosis (extracellular material), phagocytosis (particles; the phagosome fuses with lysosomes), and autophagy (a double membrane engulfs worn organelles → autophagosome → fuses with lysosome). Starvation strongly induces autophagy (recycling amino acids; mTOR inhibition is the trigger).

Storage diseases — a missing hydrolase means its substrate accumulates:

Peroxisomes — The Other Oxidizers

• Very-long-chain fatty acids (VLCFA, >22C) and branched-chain lipids are oxidized here FIRST, then handed to mitochondria; peroxisomal beta-oxidation transfers electrons to O$_2$ making H$_2$O$_2$ (no ATP directly)
• Catalase disposes of H$_2$O$_2$; peroxisomes also synthesize plasmalogens (myelin lipids) and contribute to bile acid synthesis
• Zellweger syndrome: defective peroxisome assembly (empty "ghosts") → VLCFA accumulate, plasmalogens missing → severe neurologic disease; adrenoleukodystrophy: defective VLCFA transporter into peroxisomes

• The classic branched-chain substrate is phytanic acid (from dietary chlorophyll), which peroxisomes shorten by alpha-oxidation before beta-oxidation can proceed.

Organelle Pathophysiology 🎯

Mitochondria as the Apoptosis Switchboard

The organelle that powers life also licenses death (integrates with the cell-cycle lesson):

• Cytochrome c normally shuttles electrons between complexes III and IV
• Intrinsic apoptotic signals let BAX/BAK permeabilize the outer membrane → cytochrome c enters the cytosol → apoptosome → caspase-9
• Severe injury (Ca$^{2+}$ overload, ROS) can instead swell and rupture mitochondria → necrosis rather than apoptosis
• One protein, two jobs, two compartments: location determines function — an MCAT-favorite principle

• The injury-driven opening of a large inner-membrane pore is called the mitochondrial permeability transition.

Reactive Oxygen Species — Made and Managed

Connect across topics: the pentose phosphate pathway's NADPH maintains reduced glutathione — why G6PD deficiency causes oxidative hemolysis, and why rapidly respiring tissues need robust antioxidant systems.

Organelle Cooperation Case Study — Fatty Acid Handling

1. Dietary VLCFA enter the peroxisome through a dedicated transporter, shortened by peroxisomal beta-oxidation (electrons → H$_2$O$_2$ → catalase)
2. Medium-length products transfer to the mitochondrion via the carnitine shuttle for standard beta-oxidation → acetyl-CoA → TCA
3. Excess acetyl-CoA in liver → ketone bodies (mitochondrial matrix) or → cytosolic fatty acid synthesis (citrate shuttle out)
4. New phospholipids are assembled on the smooth ER, distributed by vesicles and lipid-transfer proteins

A single metabolic thread crossing four organelles — passages that name an accumulating intermediate are asking which hand-off broke.

• The peroxisomal VLCFA transporter is ABCD1, the gene mutated in X-linked adrenoleukodystrophy.

Mitochondrial Integration 🎯

Key Takeaways — Part 3

• Mitochondrial map: porous outer membrane (porins), proton-tight inner membrane (ETC, ATP synthase, cristae), matrix (TCA, beta-oxidation, mtDNA); uncouplers dissipate the gradient as heat (UCP1, DNP) while O$_2$ use continues
• Endosymbiosis evidence: circular histone-free DNA, 70S-like ribosomes (antibiotic side effects), double membrane, fission; mtDNA = maternal inheritance + heteroplasmy → variable severity by mutant load and tissue demand
• Lysosome inputs: endocytosis, phagocytosis, autophagy (starvation-induced, mTOR-gated); storage diseases = substrate of the missing hydrolase accumulates (Tay-Sachs/GM2, Gaucher/glucocerebroside, Pompe/lysosomal glycogen)
• Compartment logic: normal cytosolic enzymes cannot rescue substrate trapped in a lysosome (Pompe), and normal enzymes fail without delivery (I-cell, ALD)
• Peroxisomes: VLCFA and branched-lipid oxidation → H$_2$O$_2$ → catalase; plasmalogen synthesis; Zellweger = assembly failure, ALD = import failure
• ROS chain: ETC leak → superoxide → SOD → H$_2$O$_2$ → catalase/glutathione peroxidase (NADPH from PPP)
• Cytochrome c: electron carrier in place, apoptosis trigger displaced — location is function

• Low-yield extras: VDAC is the outer-membrane porin; aminoglycosides and linezolid hit mitochondrial ribosomes; MELAS and LHON are named mtDNA diseases; phytanic acid is the classic branched-chain peroxisomal substrate (alpha-oxidation); the mitochondrial permeability transition drives necrosis; ABCD1 is the VLCFA transporter mutated in ALD

Organelles for the MCAT

Part 4 of 4 — The Nucleus, Organelle Dynamics & MCAT Integration

The Nucleus — Gatekeeping the Genome

• Nuclear envelope: two membranes; the outer one is continuous with the rough ER and can carry ribosomes. The envelope is perforated by nuclear pore complexes (NPCs).
• Traffic rules: ions and proteins under ~40 kDa diffuse through pores freely; larger proteins need a nuclear localization signal (NLS) recognized by importins, and export needs a nuclear export signal (NES) with exportins. The Ran-GTP gradient (high GTP-bound Ran inside the nucleus) gives transport its direction — cargo drops off where Ran-GTP binds importin.
• Nuclear lamina: a mesh of lamin intermediate filaments under the inner membrane. Phosphorylation of lamins by mitotic CDKs disassembles the envelope in prometaphase; dephosphorylation reassembles it in telophase.

• Lamin mutations cause progeria (Hutchinson-Gilford premature aging).

• Nucleolus: not membrane-bound; the site of rRNA transcription (RNA Pol I) and ribosomal subunit assembly. Cells ramping up protein synthesis grow larger nucleoli.
• Key consequence of the envelope: transcription and translation are uncoupled in eukaryotes — splicing happens before export, so introns never meet a ribosome. Prokaryotes, lacking a nucleus, translate mRNA while it is still being made.

Organelles Are Not Static — Dynamics & Contact Sites

• Machinery names: Drp1 drives fission; mitofusins (outer membrane) and OPA1 (inner membrane) drive fusion; ER-mitochondria contacts are called MAMs (mitochondria-associated membranes).

• Inheritance at division: organelles are not built from scratch — ER, Golgi fragments, and mitochondria are partitioned to daughters, and mitochondria arise only from existing mitochondria (fission), echoing endosymbiosis.
• Vesicle identity refresher (from Part 2): ER-to-Golgi vesicles run anterograde, Golgi-to-ER vesicles retrieve escaped ER residents; clathrin coats vesicles from the trans-Golgi to lysosomes and in receptor-mediated endocytosis.

• Coat names: COPII = ER to Golgi (anterograde); COPI = Golgi to ER (retrograde, KDEL receptors).

The Passage-Strategy Layer

Organelle passages almost never ask "what does the Golgi do?" They give you an experiment and expect you to run four moves:

1. Localize: use fractionation speed, marker enzymes, or microscopy tags to decide which compartment the data describe.
2. Trace the route: any secreted or membrane protein must run rough ER to Golgi to vesicle; any break in the chain strands cargo one station upstream of the lesion.
3. Classify the lesion: enzyme broken, delivery broken (I-cell, ALD), or compartment missing (Zellweger) — Part 3's taxonomy.
4. Check the gradient: pH, Ca$^{2+}$, redox, or Ran-GTP — many answers reduce to "the gradient that powers this step collapsed."

Worked Example — A Nuclear Transport Experiment

Passage-style problem. Researchers microinject three fluorescent proteins into the cytosol of cultured fibroblasts and image nuclear fluorescence 30 minutes later:

• Protein A (25 kDa, no targeting sequences): fluorescence equilibrates between cytosol and nucleus.
• Protein B (90 kDa, carries an NLS): fluorescence concentrates strongly in the nucleus.
• Protein C (90 kDa, same NLS, cells pretreated with a drug that locks Ran in its GDP-bound state everywhere): fluorescence stays cytosolic.

Question 1 — Why does Protein A enter without an NLS? At 25 kDa it is below the ~40 kDa passive-diffusion limit of the nuclear pore. It equilibrates (equal brightness both sides) rather than accumulates — passive diffusion cannot concentrate anything.

Question 2 — What does Protein B demonstrate? Signal-dependent, receptor-mediated import: importins bind the NLS and ferry cargo through the pore. The nucleus becomes BRIGHTER than the cytosol — movement against a concentration gradient, which requires energy. The energy source is the Ran-GTP gradient, not ATP hydrolysis at the pore itself.

Question 3 — Why does the Ran-GDP drug block import of Protein C? Cargo release inside the nucleus requires Ran-GTP binding to importin. With Ran trapped as Ran-GDP, import complexes cannot be dissociated and importins cannot be recycled, so signal-dependent transport stalls; passive diffusion of small proteins is unaffected. Note the experimental logic: the drug separates the recognition step (NLS binding, intact) from the directionality step (Ran gradient, broken).

Extension the MCAT loves: during mitosis this whole system is moot — nuclear envelope breakdown (lamin phosphorylation) makes compartmentalization vanish until telophase. A protein "suddenly gaining access to chromatin only in M phase" needs no transport explanation at all.

Nucleus, Dynamics & Integration 🎯

Key Takeaways — Part 4

• Nuclear pores: passive diffusion below ~40 kDa; larger cargo needs an NLS + importin, with direction set by the Ran-GTP gradient (high inside); export uses NES + exportin
• Accumulation against a gradient = signal + receptor + energy; equilibration = passive diffusion — brightness patterns in injection experiments tell you which
• Nuclear lamina = lamin intermediate filaments; CDK phosphorylation dissolves the envelope in mitosis, dephosphorylation rebuilds it; nucleolus (no membrane) = rRNA synthesis and subunit assembly
• The envelope uncouples transcription from translation: splicing precedes export, so eukaryotes never translate introns; prokaryotes co-transcriptionally translate
• Mitochondria remodel constantly: fission (segregation, mitophagy) vs fusion (content mixing buffers heteroplasmy); ER-mitochondria contacts pass Ca$^{2+}$ and lipids directly
• Passage strategy: localize (markers, fractionation), trace the secretory route and stall one station upstream of the lesion, classify enzyme-vs-delivery-vs-compartment defects, and ask which gradient (pH, Ca$^{2+}$, redox, Ran-GTP) powers the step
• Specific-cargo defects point to specific sorting signals/receptors; everything-affected defects point to shared machinery

• Low-yield extras: lamin mutations cause progeria; Drp1 drives fission, mitofusins (outer) and OPA1 (inner) drive fusion, ER-mitochondria contacts are MAMs; COPII = anterograde coat, COPI = retrograde coat (KDEL)