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CELL ENVELOPE AND ITS MODIFICATIONS

Ribosomes and Inclusion Bodies

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CELL ENVELOPE

  • • Chemically complex, three-layered structure:
  • 1. Glycocalyx
  • 2. Cell Wall
  • 3. Plasma Membrane
  • • Acts as a single protective unit
  • • Basis of Gram staining

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GLYCOCALYX

  • • Outermost layer
  • • Slime layer: loose sheath
  • • Capsule: thick, tough layer
  • • Function: Protection & attachment

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CELL WALL & PLASMA MEMBRANE

  • Cell Wall:
  • • Determines cell shape
  • • Provides structural support
  • • Prevents bursting/collapsing

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  • Plasma Membrane:
  • • Selectively permeable
  • • Similar to eukaryotic membrane

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SPECIAL MEMBRANOUS STRUCTURES

  • Mesosomes:
  • • Extensions of plasma membrane
  • • Functions: cell wall formation, DNA replication, respiration, secretion

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  • Chromatophores:
  • • Found in cyanobacteria
  • • Pigment-containing, for photosynthesis

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SURFACE APPENDAGES

  • • Flagella: Motility (filament, hook, basal body)
  • • Pili: Tubular structures, conjugation
  • • Fimbriae: Bristle-like, attachment to surfaces

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RIBOSOMES

  • • Prokaryotic ribosomes: 70S (50S + 30S)
  • • Associated with plasma membrane
  • • Function: Protein synthesis
  • • Polysome = many ribosomes on one mRNA

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INCLUSION BODIES

  • • Reserve materials stored in cytoplasm
  • • Not membrane-bound
  • • Types:
  • - Phosphate granules
  • - Cyanophycean granules
  • - Glycogen granules
  • • Gas vacuoles: found in photosynthetic bacteria

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EUKARYOTIC CELLS

Cell Membrane and Cell Wall

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GENERAL FEATURES OF EUKARYOTIC CELLS

  • • Found in protists, plants, animals, fungi
  • • Cytoplasm compartmentalised by membrane-bound organelles
  • • Nucleus with nuclear envelope
  • • DNA organised into chromosomes
  • • Cytoskeleton & locomotory structures present
  • • Plant cells: cell wall, plastids, vacuole
  • • Animal cells: centrioles

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CELL MEMBRANE: STRUCTURE

  • • Studied in detail with electron microscopy (1950s)
  • • Composed of lipids, proteins, carbohydrates, cholesterol
  • • Phospholipid bilayer: polar heads outward, nonpolar tails inward
  • • RBC membrane: ~52% protein, 40% lipids
  • • Proteins: integral (inside bilayer), peripheral (on surface)

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FLUID MOSAIC MODEL (SINGER & NICOLSON, 1972)

  • • Lipid bilayer is quasi-fluid
  • • Proteins move laterally across bilayer
  • • Fluidity important for:
  • - Cell growth
  • - Intercellular junctions
  • - Secretion & endocytosis
  • - Cell division

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TRANSPORT ACROSS CELL MEMBRANE

  • • Selectively permeable
  • • Passive transport (no energy):
  • - Diffusion (solutes)
  • - Osmosis (water)
  • - Facilitated diffusion (via proteins)
  • • Active transport (ATP required):
  • - Against concentration gradient
  • - Example: Na+/K+ Pump

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CELL WALL

  • • Rigid outer covering in plants & fungi
  • • Functions: shape, support, protection, cell interaction
  • • Composition:
  • - Algae: cellulose, galactans, mannans, CaCO3
  • - Plants: cellulose, hemicellulose, pectin, proteins

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PRIMARY AND SECONDARY CELL WALLS

  • • Primary wall: thin, flexible, present in young cells, allows growth
  • • Secondary wall: thick, formed inside primary wall as cell matures

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MIDDLE LAMELLA & PLASMODESMATA

  • • Middle lamella: layer of calcium pectate
  • - Holds neighbouring cells together
  • • Plasmodesmata: cytoplasmic connections between adjacent cells