Cell: The Unit of Life – Comprehensive Study Notes

INTRODUCTION

  • 1665: Robert Hooke observes cork under his self-made compound microscope; publishes “Micrographia” and coins the term “cell” (from Latin cella = small room/compartment).

  • Anton van Leeuwenhoek (1670s-80s): first to see live single-celled organisms; calls them “animalcules” (protists, bacteria, RBCs, sperm).

  • 1831: Robert Brown discovers the nucleus in orchid root cells.

  • Definition – Cell: fundamental structural & functional unit of all living organisms.

  • Unicellular examples: Amoeba, Paramecium, bacteria, yeast – capable of independent existence and performing all life functions.

CELL THEORY

  • 1838 Schleiden (botanist): all plants composed of cells forming tissues.

  • 1839 Schwann (zoologist): animal cells have thin outer “plasma membrane”; plant cells uniquely possess a cell wall. Proposes bodies of plants & animals are made of cells and cell products.

  • Joint formulation (Schleiden & Schwann):
    • Cell = fundamental unit of structure & function.
    • All organisms = one or more cells.

  • Limitation: mechanism of new-cell formation unexplained.

  • 1855 Rudolf Virchow: \textit{Omnis\;cellula\, e\,cellula} – cells arise only from pre-existing cells. Final modern statements:
    • All living organisms composed of cells & products.
    • All cells come from pre-existing cells.
    • Cells share basic chemical composition & metabolism.
    • Functions of an organism = collective activities/interactions of its cells.

  • Branches: Cytology – study of cells; Cell biology – structural, biochemical, genetic, developmental, pathological & evolutionary aspects.

EXCEPTIONS TO CELL THEORY

  • Viruses – acellular yet show life properties in host.

  • Prokaryotes – genetic material without nuclear membrane.

  • Enucleate but functional cells: mammalian RBC, sieve tubes.

  • Multinucleate, non-septate cytoplasm: Rhizopus, Vaucheria.

CELL SHAPE, SIZE & LIFE-SPAN

  • Shapes: isodiametric parenchyma, dumb-bell guard cells, pipe-like xylem vessels, elongated sieve tubes, biconcave RBC, amoeboid WBC, long-axon neurons.

  • Size range:
    • Mycoplasma (PPLO): 0.1\,–\,0.3\,\mu m.
    • Typical bacteria: 1\,–\,5\,\mu m.
    • Eukaryotic cell: 10\,–\,20\,\mu m.
    • Single-cell alga Acetabularia: \sim 10\,cm.
    • Ostrich egg: largest single cell (≈150 mm).
    • Human neuron axon: up to 1\;m.

TYPES OF CELLS

Prokaryotic Cells

  • Representatives: Bacteria, Cyanobacteria (BGA), Mycoplasma/PPLO, Spirochaetes, Rickettsiae.

  • General traits: Smaller, rapid division (e.g. E. coli 30-min generation), haploid genome.

  • Definition: Cell with undeveloped nucleus (nucleoid = single circular naked DNA) & no double membrane organelles.

Bacterial Morphology

  • Shapes: Cocci (mono-, diplo-, strepto-, staphylo-, tetra-), Bacilli (rod), Vibrio (comma), Spirillum (spiral).

  • Flagellar number: Atrichous (none), Monotrichous, Lophotrichous, Amphitrichous, Peritrichous, Amphilophotrichous.

Prokaryotic Cell Envelope

  1. Glycocalyx – outermost; slime layer (loose glycoprotein) or capsule (firm polysaccharide); protective & antiphagocytic.

  2. Cell Wall – peptidoglycan (murein) framework of \text{N-acetyl-muramic acid} + \text{N-acetyl-glucosamine} cross-linked by peptides; absent in Mycoplasma.

  3. Plasma Membrane – phospholipid bilayer + proteins; cholesterol absent, some bacteria contain pentacyclic sterols (hapnoids); forms mesosomes.

Membranous Specialisations

  • Mesosomes – PM infoldings (vesicles/tubules/lamellae); septal & lateral types. Functions: cell-wall formation, DNA replication partition, respiration (oxidative enzymes), secretion, increased surface area.

  • Chromatophores/Thylakoids – in cyanobacteria, purple & nitrifying bacteria; carry photosynthetic or chemoautotrophic pigments.

Cytoplasm & Inclusions

  • No cyclosis; granular due to 70 S ribosomes & inclusion bodies.

  • Inorganic inclusions – Volutin (metachromatic PO_4^{3-} granules: energy reserve), sulphur, iron, magnetite.

  • Gas vacuoles – protein-bounded, gas-filled; buoyancy in aquatic photoautotrophs.

Ribosomes

  • 70 S (50 S + 30 S); attached to membrane or free; site of protein synthesis; polysomes translate mRNA concurrently.

Nucleoid & Plasmids

  • Circular dsDNA coiled with non-histone proteins; plasmids = extra-chromosomal, self-replicating, antibiotic resistance vectors.

Flagellum Ultra-structure

  • Parts: Filament (flagellin), Hook, Basal body.
    • Gram-negative basal body: L-P-S-M rings (outer → inner).
    • Gram-positive: only inner & outer pairs.

  • Movement via rotary motor.

Surface Appendages

  • Pili (pilin protein, conjugation), Fimbriae (adhesion), Spinae (rigid hairs in Gram+).

Gram Staining (Christian Gram 1884)

  • Gram+ retain crystal violet–iodine (thick peptidoglycan, teichoic acid, low lipid, exotoxin susceptibility).

  • Gram– decolorised, counter-stain pink (thin peptidoglycan, outer LPS, porins, high lipid, endotoxin).

Eukaryotic Cells

  • Members: Protists, fungi, plants, animals.

  • Features: true nucleus with envelope, multiple linear chromosomes, extensive compartmentalisation (endomembrane system), 80 S cytoplasmic ribosomes, complex cytoskeleton, mitotic/meiotic division.

CELL WALLS

  • First seen by Hooke (1665); thickness 0.1\,–\,10\,\mu m.

  • Composition:
    • Bacteria – peptidoglycan.
    • Fungi – chitin (N-acetyl glucosamine polymer).
    • Plants – cellulose, hemicellulose, pectin, proteins; lignin/suberin added in woody tissues.
    • Algae – cellulose + galactans, mannans, CaCO_3, silica.

  • Layers: Middle lamella (Ca/Mg-pectate), Primary wall (thin, extensible, living), Secondary wall (thick, lignified; may have tertiary xylan wall). Pits (simple/bordered) enable transport; plasmodesmata provide cytoplasmic continuity.

PLASMA MEMBRANE

  • Models:
    • Davson–Danielli “sandwich” (1935).
    • Robertson unit membrane (1953).
    • Singer–Nicolson Fluid Mosaic (1972) – lipid bilayer + embedded/integral & peripheral proteins; amphipathic components, quasi-fluid nature permits lateral mobility, growth, endocytosis.

  • Transport mechanisms:
    • Simple diffusion.
    • Facilitated diffusion (carrier/channel proteins).
    • Osmosis.
    • Active transport (ATP-driven e.g. \text{Na}^+/\text{K}^+ pump: 3 Na⁺ out / 2 K⁺ in per ATP).
    • Bulk transport – endocytosis (phagocytosis, pinocytosis) & exocytosis.

CYTOPLASM & CYTOSOL

  • Colloidal matrix (water, ions, enzymes, metabolites, RNA, proteins); site for glycolysis, fatty-acid & nucleotide biosyntheses; enables cyclosis; contains organelles + cytoskeleton.

ENDOMEMBRANE SYSTEM

Endoplasmic Reticulum (ER)

  • Discovered Porter & Thomson 1945.

  • Continuous with nuclear envelope; cisternal (RER), tubular/vesicular (SER).

  • RER: ribosome-studded, protein synthesis, glycosylation, forms membranes.

  • SER: lipid & steroid synthesis, glycogen metabolism, detoxification (liver), sarcoplasmic reticulum stores Ca^{2+} in muscles.

Golgi Apparatus (Camillo Golgi 1898)

  • Parallel cisternae (cis forming face, trans maturing face), associated vesicles & vacuoles (dictyosomes in plants).

  • Functions: modification (glycosylation, sulfonation), sorting & packaging of proteins/lipids; secretory vesicle formation; lysosome genesis; cell-wall polysaccharide synthesis; acrosome, cortical & nematocyst formation.

Lysosomes (Christian de Duve 1955)

  • Single-membrane vesicles from Golgi; contain \sim50 acid hydrolases (protease, lipase, nuclease, phosphatase, sulphatase).

  • Types: Primary (storage), Secondary/heterophagosome (digestive), Autophagic vacuole, Residual body.

  • Autolysis = self-destruction; hence “suicidal bags”.

Vacuoles

  • Membrane (tonoplast) bound; large central vacuole in mature plant cells (up to 90 % volume).

  • Types: Sap (osmotic turgor, storage), Food (protozoans), Contractile (osmoregulation in freshwater protists), Gas (buoyancy in prokaryotes).

MITOCHONDRIA – “POWER HOUSE”

  • Observed by Kolliker 1880; Benda coined term 1898.

  • Double membrane; outer smooth, inner folded into cristae bearing F0F1 particles (ATP synthase).

  • Matrix: circular DNA, 70 S ribosomes, enzymes of \beta-oxidation & Krebs cycle.

  • Function: aerobic respiration, \text{ADP}+Pi\rightarrow ATP (oxidative phosphorylation), heat generation (thermogenic tissue).

  • Semi-autonomous: self-replicating by binary fission; maternal inheritance.

PLASTIDS

  • Types:
    • Chloroplasts – green, photosynthesis; double membrane, stroma, grana (thylakoid stacks), stroma lamellae; pigments chlorophyll a, b & carotenoids; own DNA, 70 S ribosomes; light reaction on thylakoid, Calvin cycle in stroma.
    • Chromoplasts – coloured (carotene, xanthophyll), in petals, fruits; formed from chloroplasts.
    • Leucoplasts – colourless storage: Amyloplast (starch), Aleuroplast/Proteinoplast (proteins), Elaioplast (oils).

  • Special giant plastids: Gerontoplast (senescent), Etioplast (dark-grown), Proplastid (precursor).

RIBOSOMES – “PROTEIN FACTORIES”

  • Discovered by G. Palade 1953.

  • Non-membranous rRNA-protein particles; two subunits held by Mg^{2+}.
    • 80 S (60 S + 40 S) – cytosolic of eukaryotes.
    • 70 S (50 S + 30 S) – prokaryotes, mitochondria, chloroplasts.

  • Multiple ribosomes on one mRNA = polysome; peptidyl transferase (ribozyme) resides in large subunit rRNA.

CYTOSKELETON

  • Microfilaments (actin, 6-10 nm): muscle contraction, cytoplasmic streaming, microvilli support, cleavage furrow.

  • Microtubules (tubulin, 25 nm): spindle fibres, axoneme, intracellular transport, shape.

  • Intermediate filaments (keratin, vimentin etc., 8-12 nm): tensile strength.

Cilia & Flagella (9 + 2 axoneme)

  • Basal body (centriole derived) anchors axoneme; dynein arms produce sliding -> bending; cilia short/many, flagella long/few.

Centrioles & Centrosome

  • 9 × triplet microtubules (9 + 0); perpendicular pair + PCM = centrosome (MTOC) organising mitotic spindle, basal bodies.

MICRO-BODIES & INCLUSIONS

  • Peroxisomes – H2O2 metabolism (oxidases, catalase), \beta-oxidation, photorespiration.

  • Glyoxysomes – in germinating fatty seeds; glyoxylate cycle (acetate → succinate → carbohydrates).

  • Spherosomes – lipid droplets in plants (phospholipid synthesis & storage).

  • Storage inclusions: starch grains, glycogen, fat droplets; crystals (Ca-oxalate etc.).

NUCLEUS – “COMMAND CENTRE”

  • Discovered Robert Brown 1831.

  • Components:
    • Nuclear envelope (double, perinuclear space 10-50 nm, outer continuous with RER; nuclear pores with pore complex allow bidirectional traffic of mRNA, tRNA, ribosomal subunits, proteins).
    • Nucleoplasm (matrix with enzymes, nucleotides).
    • Chromatin: DNA + histone & non-histone proteins + RNA.
    – Euchromatin (light, loosely packed, transcriptionally active).
    – Heterochromatin (dark, condensed, inactive; constitutive vs facultative).
    • Nucleolus (no membrane; rRNA synthesis & ribosome assembly; disappears in prophase).

  • Chromosome morphology: two sister chromatids joined at centromere (kinetochore). Types based on centromere position: Metacentric, Sub-metacentric, Acrocentric, Telocentric. Satellite/secondary constriction forms NOR.

Special Chromosomes

  • Polytene (salivary glands of dipterans) – endoreduplication → giant, banded, puffs (active transcription sites).

  • Lampbrush (amphibian oocytes) – extended loops with intense transcription.

Karyotype / Idiogram

  • Ordered display of metaphase chromosomes; human: 46 (44 autosomes + XY/XX) – tool for cytogenetics, aneuploidy detection.

COMPARATIVE TABLES (high-yield)

Plant vs Animal Cell

  • Cell wall present / absent.

  • Plastids present / absent.

  • Vacuole large single / small multiple.

  • Lysosomes rare / common.

  • Centrioles absent (except lower plants) / present.

  • Reserve food: starch & oils / glycogen & fats.

Prokaryote vs Eukaryote

  • Size (0.1–5 µm vs 5–100 µm), nucleoid vs nucleus, 70 S vs 80 S ribosomes, absent vs present membrane organelles, peptidoglycan vs cellulose/chitin walls, haploid vs diploid cycles, mesosomes vs mitochondria, no cyclosis vs cyclosis.

KEY EQUATIONS & NUMBERS

  • Oxidative phosphorylation: ADP+Pi+\frac12O2+NADH\rightarrow ATP+H_2O+NAD^+

  • Na⁺/K⁺ pump stoichiometry: 3\,Na^{+}{\text{in}}+2\,K^{+}{\text{out}}+ATP\rightarrow3\,Na^{+}{\text{out}}+2\,K^{+}{\text{in}}+ADP+P_i

  • Sedimentation coefficients: 70\,S=50\,S+30\,S(non-additive) ; 80\,S=60\,S+40\,S.

ETHICAL / PHILOSOPHICAL NOTES

  • Cell theory unified biology: life’s continuity via replication of cells, not spontaneous generation.

  • Modern medicine leverages cell principles—e.g., targeting bacterial ribosomes (70 S) with antibiotics sparing human (80 S), exploiting lysosomal enzymes in drug delivery, mitochondrial diseases via maternal inheritance counselling.

CONNECTIONS & APPLICATIONS

  • Mesosomes vs mitochondrial cristae – analogous respiratory roles.

  • Plasmids foundational for recombinant DNA technology & antibiotic resistance crisis.

  • Photorespiration & peroxisomes linked to plant efficiency, crop engineering (C₄ pathway introduction).

  • Cytoskeletal dysfunction → neurodegenerative diseases; microtubule inhibitors used as anti-cancer drugs.

  • Karyotyping essential for prenatal diagnostics (Down, Turner syndromes).

MEMORY AIDS

  • “OIL RIG” – Oxidation Is Loss (of electrons/H), Reduction Is Gain – apply to respiratory chain.

  • “RER – Rough = Ribosomes; SER – Smooth = Steroids.”

  • “COPS” for Gram+: Cell wall thick, Outer LPS absent, Porins absent, Susceptible to penicillin.

  • 9+2 axoneme rhyme: “Nine pairs outside, two singlets inside; dynein slides, flagellum glides.”

PREVIOUS-YEAR QUESTION HIGHLIGHTS (NEET / Boards)

  • Cell theory proponents: Schleiden & Schwann; modified by Virchow.

  • Detoxification site in hepatocytes: SER.

  • Organelle forming acrosome: Golgi apparatus.

  • Photorespiration site: Peroxisomes.

  • Polymorphic organelle with hydrolases: Lysosome.

  • Mitochondrial cristae function: \text{Oxidation–reduction} & ATP synthesis.

  • Ribosome synthesis locale: Nucleolus.

END OF NOTES