Cells – The Working Units of Life | Comprehensive Bullet-Point Notes

Concept 5.1 Cells Are the Fundamental Units of Life

  • Cell Theory (unifying framework of biology)

    • Cells are the fundamental units of life.
    • All organisms are composed of cells.
    • All cells arise only from pre-existing cells.
    • Modern cells descended from a common ancestral cell.
    • Significance: establishes continuity of life, underpins modern medicine, genetics, biotechnology.

  • Why Cells Are Small

    • Functionality requires a large surface-area-to-volume ratio (SA : V).
    • Surface area ∝r2\propto r^2, volume ∝r3\propto r^3; thus as radius increases, SAV\frac{SA}{V} decreases.
    • More volume → more metabolic demand for resources & waste removal; limited surface slows exchange.
    • Large multicellular organisms therefore consist of many small cells rather than a few giant ones.

  • Microscopy — Seeing Cells

    • Key parameters
    • Magnification = enlargement of apparent size.
    • Resolution = minimal distance between two points that can still be distinguished as separate.
    • Instrument classes
    • Light microscopes: glass lenses + visible light; resolution ≈ 0.2 μm0.2\,\mu m (≈1 000× human eye).
    • Electron microscopes: electromagnets focus electron beam; resolution ≈ 0.2 nm0.2\,nm (≈1 000× light scope).
    • Staining (chemical or fluorescent) enhances contrast and reveals structures (e.g., Figure 5.3).
    • Practical implication: technology choice dictates the level of detail (organelle vs macromolecule) accessible to researchers.

  • Basic Internal Terminology

    • Cell membrane: universal phospholipid bilayer with embedded proteins.
    • Selectively permeable barrier; maintains homeostasis; site of cell–cell communication & adhesion.
    • Cytoplasm: everything inside plasma membrane except nucleus.
    • Cytosol = aqueous matrix not enclosed by organelles.

Comparison of Cell Types

  • Prokaryotic Cells (Domains Bacteria & Archaea)

    • No membrane-bound organelles; DNA resides in nucleoid region.
    • Typical diameter: 0.5–5 µm; structurally simpler yet metabolically diverse.
    • Universal components
    • Plasma membrane
    • Cytoplasm with 70S ribosomes (protein synthesis)
    • Often a rigid cell wall (peptidoglycan in Bacteria; pseudo-peptidoglycan or S-layers in Archaea).
    • Optional/variable structures
    • Outer membrane (Gram-negative bacteria) for extra protection.
    • Capsule: polysaccharide layer that aids adhesion & defense (e.g., against immune system).
    • Internal photosynthetic membranes (thylakoid-like) in cyanobacteria.
    • Cytoskeleton: protein filaments maintaining shape & aiding division (FtsZ, MreB, etc.).
    • Flagella (protein flagellin): rotary motor enabling motility (Figure 5.7).
    • Pili/Fimbriae: hair-like appendages for attachment, DNA transfer (conjugation).
    • Real-world relevance: prokaryotes drive global biogeochemical cycles, serve as pathogens & biotech tools.

  • Eukaryotic Cells (Domain Eukarya)

    • Approx. 10× larger diameter (10–100 µm); contain numerous membrane-enclosed organelles enabling compartmentalization.
    • 80S ribosomes (larger; distinct antibiotic sensitivity profile).
    • Universal organelles/functions
    • Nucleus: double-membrane nuclear envelope encasing most DNA; controls gene expression.
    • Endomembrane system (ER, Golgi, vesicles, lysosomes).
    • Mitochondria: ATP production via aerobic respiration; double membrane; own DNA; replicate autonomously.
    • Cytoskeleton:
      • Microfilaments (actin)
      • Intermediate filaments
      • Microtubules (tubulin)
        Functions: structural support, organelle positioning, intracellular transport, cell motility, cytoplasmic streaming.
    • Plant/Algal specifics
    • Cell wall (cellulose matrix) providing rigidity, pathogen barrier, and shaping growth.
    • Chloroplasts: photosynthesis; double membrane + internal thylakoids; own DNA; independent division.
    • Plasmodesmata: membrane-lined channels connecting adjacent cells for molecular exchange (water, ions, RNA).
    • Animal specifics
    • Extracellular matrix (ECM): collagen fibers + proteoglycans; provides tissue integrity, transmits signals, anchors cells.
    • Applied angle: organelle dysfunction underlies many diseases (e.g., mitochondrial disorders, lysosomal storage diseases).

Ribosomes – Protein Factories

  • Composition: rRNA + dozens of proteins; non-membranous.
  • Size difference crucial for antibiotic design: many drugs (e.g., tetracycline) selectively bind 70S but not 80S.

Cytoskeleton – Dynamic Scaffolding

  • Roles: shape maintenance, organelle anchoring, vesicle & chromosome movement, ciliary/flagellar motion.
  • Enables cytoplasmic streaming (especially in plants) improving intracellular transport over long distances.

Extracellular Materials & Structures

  • Bacteria: peptidoglycan wall (target of penicillin).
  • Plants: cellulose wall (rigid yet flexible) – supports vertical growth, mitigates pathogen ingress, shapes organ expansion.
  • Animals: ECM (collagen + proteoglycans) mediates mechanical support & cell signaling.
    • Biomedical relevance: ECM remodeling linked to cancer metastasis, fibrosis, wound healing.

Origin of Eukaryotic Complexity – Endosymbiosis

  • Endosymbiosis Theory
    • Mitochondria & plastids originated as free-living prokaryotes engulfed by ancestral host cell.
    • Evidence: double membranes, circular DNA, prokaryotic-like ribosomes, independent fission.
    • Gene transfer to nucleus explains reduced organelle genomes while retaining key metabolic genes.
  • Endomembrane System/Nuclear Envelope Formation
    • Proposed to arise via infolding of plasma membrane, later pinching off and fusing (Figure 5.25 A).
  • Evolutionary impact: enabled aerobic respiration → energy surplus → genome expansion → multicellularity.

Practical, Ethical & Philosophical Notes

  • Medical diagnostics: Microscopy & staining (e.g., Gram stain, histological dyes) remain cornerstone techniques.
  • Antimicrobial strategy: Exploiting structural distinctions (cell wall, ribosome type) yields selective drugs.
  • Biotechnology: Harnessing bacterial flagella motors, CRISPR systems (nucleoids) for nanoscale engineering & gene editing.
  • Environmental stewardship: Understanding microbial roles in carbon & nitrogen cycles informs climate policy.
  • Astrobiology: Cell theory guides search for extraterrestrial life; expectations of cell-like entities.
  • Ethical implication: Synthetic biology challenges definition of "cell" & life’s common ancestry.

Figures referenced (study tip: review textbook images for visual reinforcement):

  • Figure 5.1 – Logarithmic scale of biological sizes.
  • Figure 5.2 – Graphical proof of diminishing SAV\frac{SA}{V}.
  • Figure 5.3 & 5.4 – Microscopy & staining examples.
  • Figure 5.5 – Prokaryotic vs. eukaryotic schematic.
  • Figure 5.6 & 5.7 – Anatomy & motility in prokaryotes.
  • Figure 5.9 – Electron micrographs of plant vs. animal cell structures.
  • Figure 5.25 – Diagrams of endomembrane evolution & endosymbiosis.

Study Checklist

  • Can I recite the four tenets of cell theory?
  • Can I mathematically explain why cells stay small?
  • Do I know the resolution limits of light vs electron microscopy?
  • Can I list universal vs optional prokaryotic features?
  • Can I match each eukaryotic organelle with its function?
  • Can I outline evidence supporting endosymbiosis?
  • Am I aware of real-world applications stemming from each concept?