The Cell Envelope
• External covering outside the cytoplasm
• Composed of two basic layers:
– Cell wall and cell membrane
• Maintains cell integrity
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Insert figure 4.12
Comparative cell envelopes
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Structure of Cell Walls
• Determines cell shape, prevents lysis
(bursting) or collapsing due to changing
osmotic pressures
• Peptidoglycan is primary component:
– Unique macromolecule composed of a
repeating framework of long glycan chains
cross-linked by short peptide fragments
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Peptidoglycan
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Gram-Positive Cell Wall
• Thick, homogeneous sheath of peptidoglycan
– 20-80 nm thick
– Includes teichoic acid and lipoteichoic acid:
– Some cells have a periplasmic space, between the
cell membrane and cell wall
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Gram-Negative Cell Wall
• Composed of an outer membrane and a thin
peptidoglycan layer
• Outer membrane is similar to cell membrane bilayer
structure
– Outermost layer contains lipopolysaccharides and
lipoproteins (LPS)
• Lipid portion (endotoxin) may become toxic when released
during infections
• May function as receptors and blocking immune response
• Contain porin proteins in upper layer – regulate molecules
entering and leaving cell
– Bottom layer is a thin sheet of peptidoglycan
• Periplasmic space above and below peptidoglycan
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Comparison of Gram-Positive and Gram-Negative
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The Gram Stain
• Differential stain that distinguishes cells with a grampositive cell wall from those with a gram-negative cell
wall
– Gram-positive - retain crystal violet and stain purple
– Gram-negative - lose crystal violet and stain red from
safranin counterstain
• Important basis of bacterial classification and
identification
• Practical aid in diagnosing infection and guiding drug
treatment
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