Cell Biology and Organelles

Cell Membrane & Transport - Border Patrol Gates

• Structure: Fluid mosaic model. Amphipathic phospholipid bilayer; cholesterol (fluidity buffer). Proteins: integral (transmembrane channels, carriers, pumps), peripheral (enzymes, receptors). Glycocalyx (carbohydrate coat; cell ID, protection, adhesion).

• Transport Mechanisms:

• Key Players:

  • Na+/K+ Pump: 3 Na+ out, 2 K+ in per ATP. Vital for gradients.
  • GLUTs: Facilitated glucose entry (e.g., GLUT2 in liver, pancreas; GLUT4 insulin-dependent).

  ⭐ SGLT1 (kidney/intestine) & SGLT2 (kidney) are Na+-glucose cotransporters targeted by SGLT2 inhibitors (gliflozins).

• Bulk Transport: Endocytosis (phagocytosis, pinocytosis, receptor-mediated), Exocytosis (secretion).

Cytoskeleton & Junctions - Cell's Inner Framework

Cytosol: Fluid matrix; Inclusions: Glycogen, lipids.

• Microfilaments (Actin): Cell shape, motility, muscle contraction.
• Intermediate Filaments (IFs): Support. Types: Keratin (epith), Vimentin (mesench), Desmin (musc), GFAP (glia), Lamins (nucl). Diagnostic.
• Microtubules (Tubulin): Cilia, flagella, mitotic spindle. Drug targets: Colchicine, Vinca alkaloids, Taxanes.

Cell Junctions:

Endomembrane System - Protein Factory Mail

• Nucleus: Double envelope with pores; nucleolus (rRNA synthesis); chromatin (euchromatin-active, heterochromatin-condensed).
• Rough ER (RER): Studded with ribosomes; synthesizes proteins for secretion/organelles, N-glycosylation.
• Smooth ER (SER): Lipid/steroid synthesis, detoxification, Ca²⁺ storage (sarcoplasmic reticulum in muscle).
• Golgi Apparatus: Cis→Medial→Trans. Modifies (e.g., O-glycosylation), sorts, packages proteins. Vesicles: COP-II (ER→Golgi), COP-I (Golgi→ER).
• Lysosomes: Contain acid hydrolases (pH ~5). Functions: autophagy, crinophagy. Storage diseases (e.g., Tay-Sachs, Gaucher).

⭐ Mannose-6-phosphate (M6P) targets enzymes to lysosomes. Defect causes I-cell disease (Mucolipidosis II).

Mitochondria & Peroxisomes - Power Plant Cleaners

• Mitochondria (Cell's Powerhouse)
  • Structure: Double membrane; inner folded into cristae (↑ surface area for ATP synthesis). Matrix: Krebs cycle enzymes.
  • Energy: ATP synthesis via oxidative phosphorylation (ETC on inner membrane).
  • Genetics: Own circular mtDNA, maternally inherited.
• Peroxisomes (Detox & Lipid Hub)
  • Functions:
    • Oxidases (produce $H_2O_2$) & catalase (degrades $H_2O_2$).
    • β-oxidation of Very Long Chain Fatty Acids (VLCFA).
    • Plasmalogen synthesis (ether phospholipids for myelin).
    • Detoxification.
  • Disorders:

    ⭐ Zellweger Syndrome: Autosomal recessive; absent/empty peroxisomes cause severe neurological issues.

Cell Cycle & Division - Duplication Dance Drama

• Phases: Interphase (G1-growth, S-DNA replication, G2-prep for M); M-phase (Mitosis/Meiosis). 📌 G1→S→G2→M.
• Regulation: Cyclins & CDKs drive progression. Tumor suppressors: p53 (genome guardian), Rb (G1/S checkpoint).
• Checkpoints: G1/S (restriction), G2/M (DNA integrity), Spindle assembly (metaphase).
• Mitosis: PMAT stages → 2 identical diploid (2n) cells. For somatic cell proliferation.
• Meiosis: Meiosis I & II → 4 unique haploid (n) gametes; genetic recombination (Prophase I).
• Cell Death: Apoptosis (programmed, caspases via intrinsic/extrinsic pathways, non-inflammatory) vs. Necrosis (pathological, inflammatory).

⭐ p53 gene mutations are found in over 50% of human cancers, highlighting its critical role.

High‑Yield Points - ⚡ Biggest Takeaways

• Mitochondria: ATP synthesis (oxidative phosphorylation), double membrane, maternal inheritance.
• RER: Ribosome-studded; synthesis & modification of secretory/membrane proteins.
• SER: Lipid & steroid synthesis, detoxification, Ca2+ storage.
• Golgi Apparatus: Protein/lipid modification, sorting, packaging for delivery.
• Lysosomes: Acid hydrolases for degradation; defects lead to storage diseases (e.g., Tay-Sachs).
• Peroxisomes: VLCFA beta-oxidation, detoxification (H2O2 metabolism).
• Nucleolus: Site of rRNA synthesis and ribosome assembly.