Cell Biology – Plasma Membrane, Transport & Cytoplasmic Organelles

Plasma Membrane (Cell/Plasmalemma)

  • Limiting membrane of the cell; synonymous with “cell membrane”.
  • Acts as a selective, semi-permeable barrier that maintains intracellular ion composition and separates cytoplasm from the extracellular environment.
  • Thickness: 7.5–10 nm7.5\text{–}10\,\text{nm}.
Molecular Composition
  • Lipids (≈ 50%50\% by mass)
    • Phospholipids (outer leaflet rich in phosphatidylcholine & sphingomyelin; inner leaflet rich in phosphatidylserine, phosphatidylethanolamine & phosphatidylinositol).
    • Cholesterol (≈ 1:11{:}1 molar ratio with phospholipids; restricts phospholipid motion & modulates fluidity).
    • Glycolipids (extend only from outer leaflet → produces membrane asymmetry).
  • Proteins (≈ 40%40\%)
    • Integral (strongly embedded; removable only with detergents).
    • Peripheral (loosely attached to one surface; removable with salt solutions).
    • Transmembrane (span both leaflets; all transmembrane proteins are integral, not vice-versa).
  • Carbohydrates (≈ 10%10\%) – attach to lipids (glycolipids) or proteins (glycoproteins) → form cell-surface “glycocalyx” for recognition, adhesion & hormone response.
Amphipathic Bilayer Architecture
  • Each phospholipid: two non-polar, hydrophobic fatty-acid “tails” + one polar, hydrophilic “head”.
  • Bilayer orientation:
    • Polar heads face aqueous cytosol & extracellular fluid.
    • Hydrophobic tails face inward, forming a water-excluding core.
Functional Summary
  • Communication – membrane receptors bind extracellular ligands → signal transduction.
  • Intercellular connection – junctional complexes, glycocalyx.
  • Physical barrier – phospholipid bilayer separates compartments.
  • Selective permeability – regulates entry/exit of ions, nutrients, wastes.
    • Passive transport: diffusion & ion channels (no ATP).
    • Active transport: pumps (e.g.
      Na+ / K+\text{Na}^+\,/\,\text{K}^+-ATPase) require ATP.

Vesicular Transport

Endocytosis ("cell taking-in")
  • General mechanism: membrane folds → pinches off as a vesicle enclosing extracellular material.
  • Types:
    1. Phagocytosis ("cell eating")
    • Large particles (bacteria, debris).
    • Extensions of cytoplasm (pseudopodia) surround target → fuse → phagosome.
    • Typical in neutrophils & macrophages.
    1. Pinocytosis / Fluid-phase ("cell drinking")
    • Small invaginations trap soluble extracellular fluid.
    • Produces ≈50–80 nm\approx50\text{–}80\,\text{nm} pinocytic vesicles.
    1. Receptor-mediated endocytosis
    • Ligand binds specific membrane receptor → coated pit → vesicle (endosome) containing ligand–receptor complex.
Exocytosis ("cell secretion")
  • Membrane-bounded vesicle fuses with plasma membrane → releases contents without loss of membrane integrity.
  • Common trigger: rise in cytosolic Ca2+\text{Ca}^{2+}.

Cytoplasm & Cytosol

  • Cytoplasm = cytosol (fluid) + organelles (solid structures) + inclusions.
  • Cytosol: aqueous solution of enzymes, O<em>2<em>2, CO</em>2</em>2, electrolytes, metabolites & wastes.

Membrane-Bound Organelles

Mitochondria ("powerhouse")
  • Name origin: mitos (thread) + chondros (granule).
  • Site of aerobic respiration & ATP generation; number correlates with cell energy demand (e.g.
    abundant in cardiac myocytes).
  • Structure:
    • Outer membrane with porin channels.
    • Inner membrane folded into cristae (larger cristae ≈ higher energy need) ↔ houses respiratory chain & ATP synthase\text{ATP}\,\text{synthase}.
    • Matrix contains enzymes, mtDNA, rRNA, mRNA & tRNA.
    • Compartments: intermembrane space, intracristal space, matrix.
  • Clinical: mtDNA mutations → mitochondrial myopathies (e.g.
    eyelid ptosis, muscle weakness).
Endoplasmic Reticulum (ER)
  • Continuous membrane network of intercommunicating cisternae.
  • Rough ER (rER)
    • Flattened sacs studded with polyribosomes (gives “rough” look).
    • Functions: segregate nascent proteins destined for secretion/lysosomes/membrane; synthesize phospholipids; post-translational modification; N-glycosylation.
    • Prominent in secretory cells (pancreatic acinar, plasma cells, fibroblasts).
  • Smooth ER (sER)
    • Lacks ribosomes → smooth tubules.
    • Functions: lipid & steroid biosynthesis; detoxification (liver); Ca$^{2+}$ sequestration (skeletal muscle sarcoplasmic reticulum).
Golgi Complex
  • Stack of flattened, non-interconnected cisternae.
  • Polarity: cis-Golgi network (CGN) → medial cisternae → trans-Golgi network (TGN).
  • Functions: modification (glycosylation, sulfation), packaging & sorting of proteins to lysosomes, secretory vesicles or plasma membrane.
Lysosomes
  • Membrane-limited vesicles containing >4040 hydrolytic enzymes (proteases, nucleases, phosphatases, lipases, sulfatases).
  • Optimal at acidic pH ≈5\approx5 (maintained by proton pumps).
  • Types
    • Heterophagosome = phagosome + lysosome (digests external material).
    • Autophagosome = old/defective organelle enveloped → fuses with lysosome for self-digestion.
  • Residual body: indigestible material left after lysosomal action → may accumulate as lipofuscin.
Peroxisomes (Microbodies)
  • Membrane-bound, spherical; rich in oxidative enzymes.
  • Use O<em>2<em>2 to remove H atoms → form H</em>2O<em>2\text{H}</em>2\text{O}<em>2; catalase then degrades H</em>2O2\text{H}</em>2\text{O}_2.
  • Detoxify ethanol & drugs (abundant in liver & kidney); not linked to ATP production.
Proteasomes
  • Barrel-shaped protein complexes (non-membranous).
  • Degrade ubiquitin-tagged, denatured or short-lived proteins → regulate protein activity.

Non-Membranous Organelles

Ribosomes
  • 60S60\text{S} large + 40S40\text{S} small subunit → 80S80\text{S} complete.
  • Composed of 4 rRNAs + >8080 proteins.
  • Free ribosomes → proteins for cytosol, nucleus, mitochondria.
  • rER-bound ribosomes → proteins for secretion, lysosomes, membrane.
Cytoskeleton
  • Provides shape, structural support, intracellular transport & motility.
Microtubules
  • Largest filament: ≈25 nm\approx25\,\text{nm} diameter; 13 protofilaments of α\alpha- & β\beta-tubulin heterodimers.
  • Polar (+ end polymerizes faster).
  • Core of cilia & flagella (axoneme 9+29+2 pattern: nine peripheral doublets + two central microtubules).
    • Nexin bridges, radial spokes, dynein arms (ATPase) → bending motion.
    • Basal body anchors axoneme to cytoplasm.
Microfilaments (Actin filaments)
  • Smallest: 5–7 nm5\text{–}7\,\text{nm}.
  • G-actin monomers polymerize into F-actin double helix.
  • Thread-milling: addition at + end, removal at – end.
  • Functions: cell shape changes, cytokinesis, cytoplasmic streaming, core of microvilli & stereocilia.
Intermediate Filaments
  • Diameter 10–12 nm10\text{–}12\,\text{nm}; tensile strength.
    1. Keratins – epithelial cells, hair, nails; protect vs abrasion & water loss.
    2. Vimentin – mesenchymal cells.
    3. Neurofilaments – neurons (three molecular-weight variants).
    4. Lamins – form nuclear lamina under inner nuclear membrane.

Cytoplasmic Inclusions (Storage/Residue, Non-metabolic)

  • Fat droplets – adipocytes, adrenal cortex, hepatocytes.
  • Glycogen granules – polymerized glucose; visible as dense clusters (e.g.
    liver, striated muscle).
  • Lipofuscin – golden-brown residual bodies accumulating with age in long-lived, non-dividing cells (cardiac muscle, neurons).

Nucleus

  • Usually central, spherical/ovoid; uniform size & morphology in normal tissue (variation → atypia/cancer).
Nuclear Envelope
  • Double membrane + perinuclear space (30–50 nm30\text{–}50\,\text{nm}).
  • Outer membrane continuous with rER; studded with ribosomes.
  • Inner membrane supported by nuclear lamina (intermediate filament proteins – lamins) for structural stability.
  • Nuclear pore complex (octagonal ring) – bidirectional transport nucleus ↔ cytoplasm.
Chromatin
  • DNA + histones.
    • Euchromatin: lightly stained, dispersed, transcriptionally active.
    • Heterochromatin: dark, condensed, transcriptionally inactive.
  • Nucleosome = DNA wrapped ~2×2\times around octamer of small histones (H2A, H2B, H3, H4); linker histone H1 binds entry/exit site.
Nucleolus
  • Dense, basophilic, non-membranous body where rRNA is synthesized & ribosomal subunits assemble.
  • Intensity due to high rRNA concentration, not heterochromatin.
  • Regions:
    • Pars fibrosa (5–10 nm fibrils) – rRNA transcripts.
    • Pars granulosa (15–20 nm granules) – maturing ribosomal particles.
    • Nucleolar organizer DNA (NOR) – rRNA gene sequences.
    • Nucleolus-associated chromatin (function unclear).

Key Integrations & Clinical Notes

  • Mitochondrial cristae number correlates with energy demand (cardiac vs epidermal cells).
  • sER abundance in hepatocytes underlies drug-detoxifying capacity.
  • Lipofuscin identifies aging/non-dividing cells in histology.
  • Nuclear atypia (variation in size/shape/staining) is a diagnostic hallmark of malignancy.
  • Structural proteins (lamins, tubulin, actin, keratin) illustrate cytoskeleton diversity & disease links (e.g.
    laminopathies, keratin mutations causing epidermolysis bullosa).

Numerical / Statistical Highlights

  • Plasma membrane thickness: 7.5–10 nm7.5\text{–}10\,\text{nm}.
  • Glycocalyx carbohydrate layer thickness: up to 20 nm20\,\text{nm} (context-dependent).
  • Pinocytic vesicle size: ≈50–80 nm\approx50\text{–}80\,\text{nm}.
  • Nuclear pore diameter: ≈120 nm\approx120\,\text{nm}.
  • Microfilament diameter: 5–7 nm5\text{–}7\,\text{nm}; Intermediate filaments: 10–12 nm10\text{–}12\,\text{nm}; Microtubules: 25 nm25\,\text{nm}.
  • rER cisternal width: ∼20–30 nm\sim20\text{–}30\,\text{nm}; sER tubule diameter: 40–50 nm40\text{–}50\,\text{nm}.