Cellular Ultrastructure

Cell Membrane & Transport - The Dynamic Barrier

• Structure: Fluid mosaic model - phospholipid bilayer with cholesterol (fluidity buffer). Proteins: integral (transmembrane) and peripheral. Glycocalyx (cell coat) for recognition & adhesion.
• Passive Transport (No ATP):
  • Simple Diffusion: Solutes move down concentration gradient.
  • Facilitated Diffusion: Protein-mediated (channels/carriers) down gradient.
  • Osmosis: Water movement across semipermeable membrane (low to high solute concentration).
• Active Transport (Requires ATP):
  • Primary: Direct ATP hydrolysis. E.g., Na+/K+ pump (📌 PUMPKIN: 3 Na+ out, 2 K+ in).
  • Secondary: Uses electrochemical gradient established by primary active transport.
• Bulk Transport:
  • Endocytosis: Internalization. Phagocytosis (

Nucleus & Protein Synthesis - The Cell's Brain & Builders

• Nucleus: Control center.
  • Nuclear Envelope: Double membrane, perinuclear cisterna; Nuclear Pore Complexes (NPCs) for transport.
  • Nucleolus: rRNA synthesis, ribosome biogenesis.
  • Chromatin: DNA & histones.
    • Euchromatin: Active, dispersed. 📌 Euchromatin = 'Expressed/Extended'.
    • Heterochromatin: Inactive, condensed. 📌 Heterochromatin = 'Condensed/Closed'.
  • Nucleoplasm: Nuclear matrix.
• Protein Synthesis:
  • Ribosomes: Translation. Eukaryotic: 80S ($60S + 40S$ subunits).
    • Free: For cytosolic/nuclear proteins.
    • ER-bound: For secreted/membrane proteins.
  • mRNA: Carries genetic code.
  • tRNA: Brings amino acids.
  • Translation Overview: mRNA decoded to proteins by ribosomes.

⭐ The Barr body, an inactive X chromosome visible in female somatic cells, is a classic example of facultative heterochromatin.

Endomembrane System - The Cellular Assembly Line

• Endoplasmic Reticulum (ER): Network of interconnected membranes.
  • Rough ER (RER): Studded with ribosomes; synthesizes proteins for secretion, insertion into membranes, or delivery to organelles (e.g., lysosomal enzymes). Site of N-linked glycosylation.
  • Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis (e.g., steroids), detoxification (especially in liver), and Ca²⁺ storage (sarcoplasmic reticulum in muscle).
• Golgi Apparatus: Stack of flattened sacs (cisternae) - cis (entry), medial, and trans (exit) faces. Modifies (e.g., further glycosylation, phosphorylation), sorts, and packages proteins & lipids received from ER.
  • 📌 Mnemonic: 'Cis' face is for incoming vesicles (from ER); 'Trans' face for outgoing vesicles.
• Vesicular Transport: Mediates movement between organelles.
  • COPII-coated vesicles: Anterograde transport (ER → cis-Golgi).
  • COPI-coated vesicles: Retrograde transport (cis-Golgi → ER; also within Golgi).
  • Clathrin-coated vesicles: Trans-Golgi → lysosomes; Plasma membrane → endosomes (receptor-mediated endocytosis).
  • 📌 Mnemonic: 'COPII' goes forward (ER to Golgi), 'COPI' comes back (Golgi to ER).
• Lysosomes: Membrane-bound organelles containing diverse acid hydrolases (optimal pH ~5).
  • Functions: Digest macromolecules, worn-out organelles (autophagy), cellular debris, and pathogens.
  • Primary lysosomes (inactive enzymes) fuse with endosomes/phagosomes to become secondary lysosomes (active digestion).
  • Lysosomal Storage Disease example: Tay-Sachs disease (deficient Hexosaminidase A, leading to GM2 ganglioside accumulation, neurodegeneration).

⭐ I-cell disease (Mucolipidosis II) is caused by a deficiency in N-acetylglucosaminyl-1-phosphotransferase. This enzyme normally adds mannose-6-phosphate tags to lysosomal enzymes in the Golgi, targeting them for delivery to lysosomes. Without the tag, these enzymes are mistakenly secreted from the cell, and substrates accumulate within lysosomes, leading to severe clinical features.

Mitochondria, Peroxisomes & Cytoskeleton - Power, Purity, & Poise

• Mitochondria: "Powerhouse"
  • Double membrane (cristae), matrix. ATP via oxidative phosphorylation (ETC, chemiosmosis). Apoptosis initiation. Maternal mtDNA.
• Peroxisomes: "Purity"
  • Single membrane. Catalase, oxidases. β-oxidation (VLCFAs), detoxification, plasmalogen synthesis. Zellweger syndrome.
• Cytoskeleton: "Poise" - Shape, motility, organization.

  Element	Subunit	Diameter	Functions
  Microfilaments	Actin	~7 nm	Cell shape/motility, cytokinesis, muscle contraction.
  Intermediate Fil.	Various (keratins, vimentin, desmin, etc.)	~8-12 nm	Structural support, mechanical strength. 📌 'VICK LAGS'.
  Microtubules	α- & β-tubulin	~25 nm	Cell shape, transport (kinesin/dynein), cilia, flagella, mitotic spindle. 📌 'Microtubules Get Constructed Very Poorly'.

  • Centrioles & Centrosome: MTOC.

⭐ Kartagener syndrome (Primary Ciliary Dyskinesia): Autosomal recessive, dynein arm defects → situs inversus, sinusitis, bronchiectasis.

High‑Yield Points - ⚡ Biggest Takeaways

• Plasma membrane: Fluid mosaic model; lipid rafts rich in cholesterol.
• Mitochondria: Double membrane, cristae for ↑ATP synthesis; maternal inheritance.
• RER: Studded with ribosomes for protein synthesis & modification.
• SER: Lipid synthesis, detoxification, Ca2+ storage.
• Golgi apparatus: Modifies, sorts, packages proteins; cis face (receiving), trans face (shipping).
• Lysosomes: Contain acid hydrolases; defects cause lysosomal storage diseases (e.g., Tay-Sachs).
• Peroxisomes: Catalase & oxidase enzymes; VLCFA metabolism; Zellweger syndrome.