Cell Structure & Function – Comprehensive Study Notes

Cosmic and Biological Origins

• Universe formation sequence:
  • Sub-atomic particles (PSA) ➔ atoms ➔ molecules ➔ compounds ➔ Solar system (Sun, constellations, planets)
• Emergence of life:
  • Harold Urey’s hypothesis: complex organic compounds (proteinaceous) arose abiotically
  • Gradual hierarchy: protein compounds ➔ organelles ➔ cells ➔ tissues ➔ organs ➔ organ systems ➔ individuals (humans, animals, plants)

Hierarchical Levels of Life

• Aggregation of individuals forms successive ecological levels:
  • Population ➔ Community ➔ Ecosystem ➔ Biome ➔ Biosphere

Cell Theory – Historical Milestones

• 1665 Robert Hooke: observed cork (Quercus suber); coined “cell” for empty chambers
• 1835 Felix Dujardin: living cells contain liquid substances
• 1839 Johannes Purkinje: named that substance “protoplasm”
• Robert Brown, R. Strasburger & C. Bernard: discovered nucleus; Bernard emphasized its importance

Four “Aspects” of Cell Theory

• Structural unit (M. Schleiden – botany; T. Schwann – zoology)
• Functional unit (M. Schultze; T. Huxley)
• Reproductive unit (R. Virchow – “Omnis cellula e cellula”)
• Hereditary unit (Boveri)

Fundamental Definition of a Cell

• Smallest structural & functional living unit
• Contains organelles, performs metabolism, exhibits reproduction & heredity

Unicellular vs. Multicellular Organisms

• Unicellular: one cell only – e.g., Amoeba, Paramecium
• Multicellular: many cells – e.g., humans (≈ 6\times10^{13} cells; ≈100 distinct cell types)

General Architecture of a Cell

• Exterior: plasma membrane (plus cell wall in plants/prokaryotes)
• Interior: cytoplasm (sitosol) + organelles
• Genetic center: nucleus

Chemical Composition

• Water – reaction medium
• Carbohydrates – cell wall constituent
• Lipids – membranes
• Proteins – membranes & organelles
• Nucleic acids – nuclear genetic material

Prokaryotes vs. Eukaryotes

• Prokaryote: no nuclear envelope (nucleoid); incomplete organelles; example – bacteria
• Eukaryote: true nucleus, complete endomembrane system; examples – plant & animal cells

Comparative Features

Plant vs. Animal Cells

| Organelle | Plant | Animal |
| D-wall | Present | Absent |
| Plastids | Present (chloroplast etc.) | Absent |
| Lysosome | Generally absent | Present |
| Centriole | Absent | Present |
| Golgi | Dictyosomes | Golgi bodies |
| Vakuole | Large central (mature); numerous small (young) | Small |
| Flagella/Cilia | Rare | Common |

Catalogue of Organelles

Plasmalemma, ER, Ribosomes, Golgi, Lysosomes, Mitochondria, Plastids, Centrioles, Cytoskeleton, Peroxisomes, Nucleus, Vacuoles/Vesicles, Cilia/Flagella

Plasma Membrane (Plasmalemma)

• Composition: 52\% protein / 40\% lipid / 8\% carbohydrate
• Lipid bilayer: hydrophilic heads + hydrophobic tails
• Carbohydrates form glycocalyx via glycolipids & glycoproteins
• Functional integral/ peripheral proteins: mechanical support, transporters, receptors, enzymes, antigens
• Key functions:
  • Boundary & protection
  • Selective permeability; regulates influx/efflux
  • Reaction surface & signal reception

Cytoplasm (Sitosol)

• Colloidal, jelly-like medium between membrane & nucleus
• Site for majority of metabolic reactions

Endoplasmic Reticulum (ER)

• Network of cisternae, tubules, vesicles; membrane continuous with nuclear envelope
• Enzymes located within membranes & lumen – e.g., ATPase, peptidase, glucose-6-phosphatase
• Rough ER (RER): ribosome-studded
  • Synthesizes secretory proteins, glycogen, glycosylates proteins
• Smooth ER (SER): ribosome-free
  • Synthesizes non-secretory proteins, steroids, lipids
  • Lipid transport & glycogen metabolism
  • Detoxifies drugs; cooperates with RER to produce antibodies (plasma cells)

Ribosomes

• Non-membranous; 80S in eukaryotes (60S + 40S subunits)
• Types: bound (to RER) vs. free (cytosolic)
• Constituents: rRNA + proteins
• Functional sites: 40S – mRNA decoding; 60S – peptidyl transferase
• Aggregates (polyribosomes/polysomes) enable rapid protein synthesis

Golgi Apparatus (Dictyosome)

• Components:
  • Cisternae/sisternae (stacked saccules)
  • Vesicles (secretory), arise from RER or nuclear envelope
  • Vacuoles (microvesicles) containing dense secretory material
• Functions:
  • Activates enzymes, modifies & packages proteins
  • Forms lysosomes
  • Maintains plasma membrane; synthesizes carbohydrates & lipoproteins

Lysosomes

• Single-membrane vesicles rich in hydrolytic enzymes (≈80 types: proteases, lipases, nucleases, etc.)
• Abundant in phagocytes (leukocytes, macrophages)
• Digestive pathway: Golgi-produced primary lysosome ➔ fusion with endosome ➔ secondary lysosome ➔ residual body
• Roles: intracellular digestion, secretion, absorption/reabsorption, recycling, defense (phagocytosis), detoxification, fertilization (sperm acrosome)

Mitochondria

• Double-membrane, inner membrane folded into cristae bearing ATP synthase; matrix contains enzymes + circular mtDNA
• Semi-autonomous protein synthesis
• Functions:
  • Aerobic respiration & ATP generation
  • Lipid metabolism & steroid biosynthesis
  • Heat production (thermogenesis)
  • \text{Ca}^{2+} storage & buffering in active tissues (liver, muscle)
• Representative enzymes: monoamine oxidase, cytochrome chain, ATP synthase, citrate synthase, etc.

Plastids (Plant-specific)

• Double-membrane organelles with stroma (enzymes, DNA, ribosomes)
• Internal thylakoid system forms grana; chlorophyll embedded
• Types:
  • Chromatophores (colored): chloroplasts, chromoplasts
  • Leucoplasts (colorless): amyloplasts (starch), elaioplasts (lipid), proteinoplasts (protein)
• Functions: photosynthesis, storage of starch/oil/protein

Centrioles

• Cylindrical; nine triplet microtubules in a circle
• Organize spindle fibers (mitotic apparatus), orient cell division, nucleate cytoskeletal elements, regulate organelle movement & chromosome segregation

Cytoskeleton

• Protein network: microtubules (25 nm), microfilaments (6–10 nm), intermediate filaments (7–10 nm)
  • Microtubules: structure of cilia/flagella; mitotic spindle
  • Actin filaments: cell motility structures (pseudopodia, microvilli), smooth muscle
  • Intermediate filaments: keratin (epidermis), neurofilaments (neurons)
  • Myosin filaments (<10 nm): cooperate with actin for contraction
• Provides shape, structural support, intracellular transport, motility

Peroxisomes & Glyoxysomes

• Peroxisome: single membrane; enzymes catalase & oxidases from ER
  • Functions: 2\,H2O2 \rightarrow 2\,H2O + O2, amino acid & sugar phosphate synthesis, gluconeogenesis, C-3 acid metabolism to acetyl-CoA
• Glyoxysome (plant): synthesizes glycine/serine, fatty-acid β-oxidation during seed germination

Nucleus

• Usually singular, spherical/oval, double envelope with pores
• Components:
  • Nuclear envelope: outer ribosome-studded, inner chromatin-lined; perinuclear space; nuclear pores regulate traffic
  • Nucleolus: rRNA gene region, ribosome assembly center; parts – pars granulosa, pars fibrillosa, amorphous matrix
  • Chromatin: DNA-protein complex
    • Peripheral, chromatin islands, nucleolus-associated regions
    • Heterochromatin (condensed, stainable) vs. Euchromatin (extended)
  • Karyoplasm: viscous nucleoplasmic matrix continuous with cytosol via pores

Vacuoles & Vesicles

• Membrane-bound sacs (vacuole larger than vesicle)
  • Transport vesicles: intracellular cargo carriers
  • Food vacuoles: engulfed solids (phagocytosis)
  • Contractile vacuoles: freshwater protozoa – pump excess water out
  • Central vacuole (plants): storage, digestion, pigment deposition, water uptake

Cilia & Flagella

• Surface motile organelles; structure "9 + 2" microtubule axoneme (nine doublets + two central)
• Few & long = flagella; numerous & short = cilia
• Powered by dynein; provide locomotion, create fluid currents

Metabolic vs. Structural Organelles

• Metabolically Active: Ribosome, Mitochondrion, ER, Golgi, Lysosome, Vacuole/Vesicle
• Structural/Supportive: Centriole, Microtubule, Filaments, Microbodies

Model Cell Examples (Key Features)

• Bacterial cell: capsule, cell wall, plasma membrane, nucleoid DNA, ribosomes, plasmid, pili, flagella
• Plant cell: cell wall, central vacuole, chloroplast, plasmodesmata, tonoplast, ER (smooth/rough), Golgi, mitochondria, peroxisome, cytoskeleton
• Animal cell: plasma membrane, nucleus, centriole, lysosome, mitochondria, SER/RER, Golgi, peroxisome, cytoskeleton, flagella

Essential Diagrams Recap (visual references)

• Plasma membrane: lipid bilayer, proteins (integral/peripheral), glycolipids, cholesterol
• ER: rough & smooth regions contiguous with nuclear envelope
• Ribosome: 40S + 60S subunits, mRNA & tRNA binding sites
• Golgi flow: cis-face ➔ medial cisternae ➔ trans-face ➔ secretory vesicles
• Lysosome: single membrane, enzyme complexes
• Mitochondrion: outer/inner membranes, intermembrane space, cristae with F0/F1 ATP synthase
• Chloroplast: double envelope, grana (stacked thylakoids), stroma
• Centriole: microtubule triplets
• Cytoskeleton: integrated actin, microtubules, intermediate filaments
• Peroxisome: crystalline enzyme core within single membrane
• Nucleus: envelope, pores, chromatin, nucleolus
• Cilium cross-section: “9 + 2” arrangement, dynein arms, radial spokes, nexin links

Numerical & Molecular Highlights

• Human cell count: 6\times10^{13} (approx.)
• Plasma membrane composition: protein 52\%, lipid 40\%, carbohydrate 8\%
• Ribosome sedimentation: 80S (60S + 40S) in eukaryotes; 70S in prokaryotes
• Mitochondrial size & DNA: possesses circular mtDNA enabling semi-autonomy
• Vacuole osmotic role: central vacuole maintains turgor via water uptake (\psi_{p} pressure)

Conceptual & Practical Connections

• Cell theory provides foundation for histology, pathology, genetics, biotechnology
• Membrane selective transport principles underpin pharmacology & nutrient uptake
• Organelle dysfunction relates to diseases: lysosomal storage disorders, mitochondrial myopathies, peroxisomal syndromes
• Cytoskeletal elements are drug targets: taxol (stabilizes microtubules), cytochalasin (inhibits actin polymerization)
• Photosynthetic efficiency in plastids informs renewable energy research
• Ethical/philosophical: understanding cellular life prompts discussion on origin of life, synthetic biology boundaries, cloning and gene editing responsibilities