Organelles and Cellular Trafficking

Nucleus and Nucleolus

The nucleus is the largest organelle and houses the cell's genetic material. It is enclosed by a double membrane called the nuclear envelope, which is continuous with the rough ER and perforated by nuclear pore complexes (NPCs) that regulate macromolecular traffic between the nucleus and cytoplasm. mRNA exits through NPCs after processing; proteins destined for the nucleus (e.g., transcription factors, histones) enter via nuclear localization signals (NLS) recognized by importin proteins. Inside the nucleus, chromatin exists as euchromatin (loosely packed, transcriptionally active) or heterochromatin (tightly packed, transcriptionally silent). The nucleolus is a dense, non-membrane-bound subcompartment within the nucleus where rRNA is transcribed, processed, and assembled with ribosomal proteins to form ribosomal subunits.

Ribosomes

Ribosomes are the protein synthesis machinery of the cell, composed of a small and a large subunit, each built from rRNA and ribosomal proteins. Eukaryotic ribosomes are 80S (comprising 40S and 60S subunits), while prokaryotic ribosomes are 70S — a difference exploited by antibiotics such as tetracyclines and macrolides. Ribosomes exist in two cellular locations: free ribosomes in the cytosol synthesize proteins destined for the cytosol, nucleus, mitochondria, and peroxisomes; membrane-bound ribosomes on the rough ER synthesize secretory proteins, membrane proteins, and lysosomal enzymes. The decision of where a ribosome translates is determined early in translation by the presence or absence of a signal peptide on the nascent polypeptide.

Rough and Smooth ER

The endoplasmic reticulum (ER) is a network of membrane-enclosed tubules and cisternae continuous with the nuclear envelope. The rough ER is studded with ribosomes and is the site of synthesis for proteins destined for secretion, incorporation into membranes, or delivery to lysosomes. As the polypeptide is translated, it is co-translationally translocated into the ER lumen, where chaperones (e.g., BiP) assist folding and enzymes catalyze initial glycosylation (N-linked glycosylation). The smooth ER lacks ribosomes and performs diverse functions: lipid synthesis (phospholipids, steroids), detoxification of drugs and toxins (via cytochrome P450 enzymes, particularly in hepatocytes), calcium storage (especially in muscle cells as the sarcoplasmic reticulum for excitation-contraction coupling), and carbohydrate metabolism (glucose-6-phosphatase in gluconeogenesis).

Golgi Apparatus

The Golgi apparatus is a stack of flattened, membrane-enclosed cisternae that functions as the central sorting and modification hub of the cell. It has polarity: the cis face receives vesicles from the ER, and the trans face dispatches vesicles to the plasma membrane, lysosomes, or secretory granules. As proteins pass through the cis, medial, and trans cisternae (via cisternal maturation, in which cisternae themselves migrate and change identity), they undergo further modifications: trimming and addition of N-linked oligosaccharides, O-linked glycosylation, and phosphorylation of lysosomal enzymes (mannose-6-phosphate tagging). The Golgi also synthesizes sphingomyelin and other complex lipids.

Lysosomes and Peroxisomes

Lysosomes are membrane-bound organelles containing acid hydrolases — degradative enzymes that function optimally at pH ~5.0. They digest material delivered via endocytosis, phagocytosis, and autophagy. The lysosomal membrane contains a V-type H⁺-ATPase that maintains the acidic interior, and its proteins are heavily glycosylated on the lumenal face to protect against degradation. Lysosomal storage diseases (e.g., Tay-Sachs, Gaucher disease) result from deficiencies in specific lysosomal enzymes, leading to substrate accumulation. Peroxisomes are single-membrane organelles that carry out oxidative reactions. They contain enzymes that transfer hydrogen atoms to O₂, producing hydrogen peroxide (H₂O₂), which is then decomposed by catalase to water and O₂. Key functions include β-oxidation of very-long-chain fatty acids, synthesis of plasmalogens (a class of phospholipids critical for myelin), and detoxification. Unlike lysosomes, peroxisomes are not part of the endomembrane system — they are self-replicating organelles that import proteins from the cytosol via peroxisomal targeting signals (PTS1 and PTS2).

Mitochondria

Mitochondria are double-membrane organelles that generate most of the cell's ATP through oxidative phosphorylation. The outer membrane is porous due to porin channels and permits passage of small molecules and ions. The inner membrane is highly folded into cristae to increase surface area and is impermeable to ions — it houses the electron transport chain complexes (I–IV), ATP synthase (Complex V), and metabolite transporters. The intermembrane space accumulates protons pumped by the ETC, creating the proton-motive force. The matrix, enclosed by the inner membrane, contains the enzymes for the citric acid cycle, fatty acid β-oxidation, mitochondrial DNA (a circular chromosome encoding 13 ETC proteins, 22 tRNAs, and 2 rRNAs), and mitochondrial ribosomes. Mitochondria are maternally inherited and replicate by binary fission. They are not part of the endomembrane system.

Vesicles and Protein Targeting

Cargo moves between organelles in membrane-bound vesicles. Three coat proteins drive vesicle budding: COPII coats vesicles moving from ER to Golgi (anterograde), COPI coats vesicles returning from Golgi to ER (retrograde), and clathrin coats vesicles budding from the Golgi or plasma membrane for endocytosis and lysosomal delivery. Coat assembly is regulated by small GTPases (Sar1 for COPII, ARF for COPI and clathrin). Vesicle targeting and fusion are mediated by complementary pairs of SNARE proteins: v-SNAREs (on the vesicle) and t-SNAREs (on the target membrane). When the correct v-SNARE and t-SNARE interact, they form a stable four-helix bundle that brings the membranes into close proximity, driving fusion. Rab GTPases provide an additional layer of specificity by tethering vesicles to the correct target membrane. Protein targeting is directed by intrinsic signal sequences. Secretory and membrane proteins carry an N-terminal ER signal sequence that directs the ribosome to the ER (co-translational translocation via the SRP and translocon). Lysosomal enzymes receive a mannose-6-phosphate tag in the Golgi and are sorted to lysosomes via mannose-6-phosphate receptors. Mitochondrial proteins carry N-terminal presequences recognized by TOM/TIM translocases. Nuclear proteins carry an NLS recognized by importin. Proteins lacking a signal sequence default to the cytosol.

How it works

The endomembrane system operates as a directional manufacturing pipeline. A secretory protein begins with translation on a free ribosome. The emerging N-terminal signal peptide is recognized by the signal recognition particle (SRP), which pauses translation and escorts the ribosome-nascent chain complex to the SRP receptor on the rough ER. Translation resumes as the polypeptide is threaded into the ER lumen through the Sec61 translocon. Inside the ER, the signal peptide is cleaved, chaperones assist folding, and N-linked glycans are attached. Correctly folded proteins are packaged into COPII-coated vesicles at ER exit sites and delivered to the cis-Golgi. As the cisterna matures into medial and trans cisternae, glycan modifications occur. At the trans-Golgi network, proteins are sorted by destination: those destined for secretion are packaged into secretory vesicles (constitutive or regulated); lysosomal enzymes are tagged with mannose-6-phosphate and routed to lysosomes via clathrin-coated vesicles. For mitochondrial proteins, targeting is post-translational. Cytosolic chaperones (Hsp70) maintain the precursor protein in an unfolded state. The N-terminal presequence is recognized by receptors of the TOM complex on the outer membrane, which feeds the protein to the TIM23 complex on the inner membrane. The protein is pulled into the matrix by the PAM complex (mitochondrial Hsp70), and the presequence is cleaved by matrix processing peptidase (MPP).

How it works

The endomembrane system operates as a directional manufacturing pipeline. A secretory protein begins with translation on a free ribosome. The emerging N-terminal signal peptide is recognized by the signal recognition particle (SRP), which pauses translation and escorts the ribosome-nascent chain complex to the SRP receptor on the rough ER. Translation resumes as the polypeptide is threaded into the ER lumen through the Sec61 translocon. Inside the ER, the signal peptide is cleaved, chaperones assist folding, and N-linked glycans are attached. Correctly folded proteins are packaged into COPII-coated vesicles at ER exit sites and delivered to the cis-Golgi. As the cisterna matures into medial and trans cisternae, glycan modifications occur. At the trans-Golgi network, proteins are sorted by destination: those destined for secretion are packaged into secretory vesicles (constitutive or regulated); lysosomal enzymes are tagged with mannose-6-phosphate and routed to lysosomes via clathrin-coated vesicles. For mitochondrial proteins, targeting is post-translational. Cytosolic chaperones (Hsp70) maintain the precursor protein in an unfolded state. The N-terminal presequence is recognized by receptors of the TOM complex on the outer membrane, which feeds the protein to the TIM23 complex on the inner membrane. The protein is pulled into the matrix by the PAM complex (mitochondrial Hsp70), and the presequence is cleaved by matrix processing peptidase (MPP).

Comparisons

• Biochemistry: Co-translational translocation, chaperone-mediated folding, and post-translational modifications (glycosylation, disulfide bond formation) are directly tested. Understanding the logic of signal sequences bridges molecular biology and cell biology.
• Genetics and Molecular Biology: Mitochondrial inheritance is maternal, and mutations in mtDNA cause diseases (e.g., Leber hereditary optic neuropathy). Nuclear import/export is central to gene regulation — transcription factors shuttle between cytoplasm and nucleus in response to signals.
• Human Physiology and Disease: I-cell disease results from failure to add mannose-6-phosphate to lysosomal enzymes, causing them to be secreted instead of targeted to lysosomes. Cystic fibrosis involves defective trafficking of CFTR to the plasma membrane.

Common confusions

• ER vs. Golgi functions: The ER adds the core N-linked glycan; the Golgi modifies it. Do NOT say the Golgi performs N-linked glycosylation — only trimming and further additions. Confusing which organelle adds what modification is a classic trap.
• Lysosomes vs. peroxisomes: Both degrade material, but lysosomes use acid hydrolases at low pH and are part of the endomembrane system. Peroxisomes use oxidative enzymes and are NOT part of the endomembrane system. Peroxisomes handle very-long-chain fatty acids; mitochondria handle medium- and short-chain fatty acids.
• Ribosome location confusion: Free ribosomes make cytosolic, nuclear, mitochondrial, and peroxisomal proteins. RER-bound ribosomes make secreted, membrane, and lysosomal proteins. A protein ending up in the lysosome is first made on the RER, not free ribosomes.
• Mitochondria are NOT endomembrane: They have a double membrane derived from an ancient endosymbiotic event, their own circular DNA, and maternal inheritance. They replicate by fission, not by budding from the ER or Golgi.
• COPII vs. COPI direction: COPII = ER → Golgi (anterograde). COPI = Golgi → ER (retrograde). The MCAT will ask you to predict the effect of inhibiting one or the other on protein secretion.
• Signal recognition particle (SRP): The SRP pauses translation and targets the ribosome to the ER. If translation completes before SRP binds, the protein remains in the cytosol — this is how cells produce a cytosolic isoform of an otherwise secreted protein.

Quick review

• Nucleus: double membrane, nuclear pores, euchromatin (active) vs. heterochromatin (silent). Nucleolus: rRNA synthesis and ribosome assembly.
• 80S eukaryotic ribosomes (40S + 60S subunits). Free ribosomes: cytosolic/nuclear/mitochondrial/peroxisomal proteins. RER-bound: secreted/membrane/lysosomal proteins.
• Rough ER: protein synthesis and N-linked glycosylation. Smooth ER: lipid synthesis, detoxification (CYP450), Ca²⁺ storage (sarcoplasmic reticulum).
• Golgi: cis (receiving) → medial → trans (shipping). Cisternal maturation model. Mannose-6-phosphate tags lysosomal enzymes.
• Lysosomes: acid hydrolases, pH ~5.0 (V-ATPase), endomembrane system. Peroxisomes: oxidative reactions, catalase, NOT endomembrane.
• Mitochondria: double membrane, cristae, matrix. mtDNA, maternal inheritance, binary fission. NOT endomembrane.
• COPII: ER → Golgi. COPI: Golgi → ER. Clathrin: Golgi/PM budding. v-SNARE + t-SNARE = fusion.
• SRP binds signal peptide → targets ribosome to ER translocon (Sec61). Co-translational translocation.
• Mitochondrial import: TOM (outer) + TIM23 (inner) + Hsp70 chaperones. Post-translational.
• NLS = nuclear import. No signal sequence = cytosolic default.