Structure and Function of the Bacterial Cell Envelope

Cell Envelope

• Refers to the outer structure of the bacterial cell.
• Includes:
  • Plasma membrane.
  • Cell wall.
  • Sometimes includes a second phospholipid membrane.

Types of Bacterial Cells

• Gram-positive
• Gram-negative
• Differentiation through the Gram staining technique.

Function of the Cell Wall

• Maintains internal hydrostatic pressure.
• Bacterial cells are in a hypertonic environment.
  • More solute inside than outside.
  • Water flows into the cell by osmosis, increasing hydrostatic pressure.
• The cell wall resists this pressure to prevent cell bursting.

Structure of the Cell Wall

• Composition: peptidoglycan.
  • Peptide: peptide bonds.
  • Glycan: sugar molecules.
• Polysaccharides and amino acids form a mesh-like structure.
• Disaccharides in peptidoglycan contain amino acids linked to other sugar chains.

Peptidoglycan Structure

• Disaccharide units contain several amino acids.
• Polysaccharide chains are linked by beta-1,4 glycosidic bonds.
• Polysaccharide chains are connected by cross-links (inter-bridges) between amino acids.

Periplasmic Space

• Contains important hydrolytic enzymes.
• Helps in molecule breakdown and digestion.

Differentiation between Gram-positive and Gram-negative Bacteria

• Gram-positive
  • Thick peptidoglycan cell wall.
  • Remain violet after staining and washing.
• Gram-negative
  • Thin peptidoglycan cell wall.
  • Decolorize when washed and appear pink.
  • Contain a second phospholipid membrane.

Outer Membrane in Gram-negative Bacteria

• More permeable than the inner plasma membrane.
• Contains lipopolysaccharides (LPS) which provide protection against drugs and antibodies.
• Helps create a protective barrier.

Importance of Peptidoglycan Bonds

• Drugs that break cross-links can kill bacteria.
• Destroying the cell wall leads to cell bursting.

Conclusion

• The cell envelope contains the plasma membrane, cell wall, and sometimes an outer membrane.
• Differentiation between gram-positive and gram-negative is done through the structure of the cell envelope, notably the thickness of the peptidoglycan wall.
• Peptidoglycan is crucial for resisting hydrostatic pressure and providing stability to the bacterial cell.