Cell: The Fundamental Unit of Life

Cell Basics

• Cell = basic structural & functional unit of life
• Performs all vital processes to sustain an organism

Cell Theory

• $1$ All organisms are composed of $ext{cell(s)}$
• $2$ Cells arise only from pre-existing cells (cell division)
• $3$ Cell is the fundamental unit of structure & function

Historical Milestones

• Compound microscope (Janssen) $1590$–$1595$
• Robert Hooke coined "cell" $1665$
• A. van Leeuwenhoek saw live cells (magnification $270\times$) $1674$
• Robert Brown discovered nucleus $1833$
• Schleiden (plants) & Schwann (animals) proposed cellular makeup $1838$–$1839$
• Virchow: "Omnis cellula e cellula" $1855$

Microscopy

• Compound microscope ≈ $2000\times$
• Electron microscope ≈ $500\,000\times$ (reveals ultrastructure)

Classification – Domains & Kingdoms

• $3$ Domains: Bacteria, Archaea (both prokaryotic), Eukarya (eukaryotic)
• Criteria: cell type, nutrition mode, cell number
• Eukarya subdivided into Protists, Fungi, Plants, Animals

Cellular Organization

• Unicellular = single cell (e.g., Euglena, yeast)
• Multicellular = many cells (plants, animals, fungi)

Size, Shape & Examples

• Unit: $\mu m$ (micrometer) $1\,\mu m = \dfrac{1}{1000}$ mm
• Smallest cell: Mycoplasma $0.1\,\mu m$
• Largest cell: Ostrich egg $18$ cm
• Human extremes: sperm $5\,\mu m$, ovum $120\,\mu m$, nerve cell length $1$ m
• Shape relates to function: RBC biconcave, nerve branched, WBC amoeboid

Cell Types Compared

• Prokaryote: undeveloped nucleus, $1$ chromosome, no membrane organelles, size $0.5$–$5\,\mu m$
• Eukaryote: true nucleus, multiple chromosomes, membrane organelles, size $5$–$100\,\mu m$
• Animal vs Plant: cell wall & plastids absent/present; vacuoles small/large; centrioles present/absent

Fundamental Cell Structures

• Plasma membrane: lipid bilayer + proteins; semi-permeable; maintains homeostasis
• Cell wall (plants, bacteria): cellulose; rigidity & protection
• Nucleus: double membrane with pores; contains chromatin (DNA + histone); nucleolus makes rRNA; controls activities & heredity
• Cytoplasm: cytosol (water $80\%$) + organelles

Major Organelles & Functions

• Endoplasmic Reticulum
  • Rough ER: ribosomes, protein synthesis
  • Smooth ER: lipid synthesis, detoxification, $Ca^{2+}$ storage
• Golgi Body: modifies, packages, directs molecules; forms lysosomes & vesicles
• Lysosomes: hydrolytic enzymes; intracellular digestion & autolysis
• Peroxisomes: oxidative enzymes (catalase); β-oxidation & peroxide detox
• Vacuoles: storage & turgor; large & permanent in plants, small & temporary in animals
• Mitochondria: double membrane, cristae, own DNA; ATP production – “powerhouse”
• Plastids (plants)
  • Chloroplasts: grana (thylakoids with chlorophyll), stroma; photosynthesis
  • Chromoplasts: pigment storage (carotene, xanthophyll, lycopene)
  • Leucoplasts: food storage (starch, oil, protein)
• Centrosome (animals): $2$ perpendicular centrioles; spindle fibre, cilia/flagella formation
• Cytoskeleton: microtubules (tubulin) + microfilaments (actin); shape, support, movement

Key Comparisons

• Prokaryotic (bacterial) cell lacks nucleus & organelles; has nucleoid, plasmids, ribosomes, plasma membrane, cell wall, capsule, pili, flagella
• Plant cell unique features: cell wall, chloroplasts, large central vacuole
• Animal cell unique features: centrioles, smaller temporary vacuoles, no cell wall/plastids