BSCI 3234 Lecture 4: Lecture 4: Eukaryotic Cells
29 Apr 2018
School
Department
Course
Professor
1
Introduction to Microbiology--Lecture 4
Prokaryotic Cells Part 2 and Eukaryotic Cells
Bacterial Cell Walls
• Major function is protection from osmotic cell rupture
• Gram negative bacteria have cell wall components composed of lipopolysaccharide (LPS)
also called endotoxin. LPS can cause destructive responses by the immune system
• Some antibiotics act by inhibiting cell wall formation.
• Composition—[NAG + NAM] disaccharide repeating subunit attached by polypeptides to form a
lattice. The peptides have a four amino acid side chain
Who discovered penicillin?
- Alexander Fleming
Gram Positive Cells
• Thick peptidoglycan layer
• Sensitive to penicillin
• Cell wall contains teichoic acids
• Regulate movement of ions into and out of the cell
• Maintain cell integrity during cell division
• Major antigens for serotyping
• Mycobacteria are considered to be among the Gram positive bacteria, but they stain poorly by
the Gram stain method. Instead they are stained by the acid fast test.
• Cell walls of Mycobacteria are composed of 60% mycolic acid, a waxy lipid.
Review: What is the acid fast test?
⚫ The cell walls of Mycobacteria retain carbolfuchsin stain when washed with acid alcohol
because the stain is more soluble in the lipid than in the acid alcohol.
⚫ Mycolic acid is a lipid (long chain fatty acid)
⚫ This is the basis of the acid fast differential stain.
Gram Negative Cells
• Cell wall Includes an additional outer membrane
• The peptidoglycan layer in the cell wall is thinner than Gram positive cells
• Contains lipopolysaccharide (LPS) = endotoxin in outer membrane
• Periplasm is space between outer membrane and plasma membrane
• Outer membrane inhibits entry of penicillin
• Outer membrane inhibits actions of detergents and lysozyme (enzyme in mucous, tears—
breaks disaccharide bonds in peptidoglycans)
• Gram stain is negative because alcohol dissolves the outer membrane, leaves holes in the
peptidoglycan layer, lets stain leak out
Plasma Membrane
• Encloses cytoplasm
• Made of phospholipid bilayer
• Peripheral proteins are outside
• Integral proteins—may span membrane, some form pores
• Phospholipids and proteins not fixed, but move in a fluid medium
• Serves as a selective barrier
• Small molecules (oxygen, carbon dioxide, water) pass freely.
2
• Large molecules (proteins) do not pass.
• Some molecules, like glucose, moved by transporter molecules
• Some antimicrobial agents act by damaging membranes ; examples: alcohols, quaternary
ammonium compounds, polymyxins, antimicrobial peptides
Movement of Materials Across Membranes
1. Passive (requires no energy)
2. Active (requires energy)
Passive
• Molecules move from area of high concentration to one of low concentration
• Simple diffusion
• Facilitated diffusion
• Osmosis
Simple diffusion:
• Net overall movement of molecules or ions from area of high to low concentration.
• Movement continues until the ions and molecules are at equal concentrations on both sides of
membrane. This is equilibrium
Facilitated Diffusion:
• Movement of molecules by transporters (integral membrane proteins that serve as channels or
carriers) from area of high concentration to area of low concentration.
Does facilitated diffusion require energy?
- No
Osmosis:
• Net movement of solvent molecules across a selectively permeable barrier from an area with high
concentration of solvent molecules to an area of low concentration
• Solvent is a dissolving medium
• Solute is substance dissolved in another substance (examples sugar, or salt)
• In living systems, solvent is water.
• Osmosis describes the movement of water
Osmotic Pressure
• The force with which a solvent moves from a solution of lower solute concentration to a solution of
higher solute concentration.
Isotonic = isosmotic. Concentrations of solutes [dissolved molecules and ions] outside of cells is
equal to that of solutes in the cell
Hypotonic = solutes outside at a lower concentration than in the cell, water tends to move into the cell
Hypertonic = solutes outside the cell at a higher concentration. Water tends to move out of cell
Active Transport
• Bacterial cell needs rare nutrients
• Use energy (ATP) to get them
• Uses “transporters”
• The Molecule transported is not changed
Group Translocation (a special form of active transport)
• Molecules transported are changed, once inside, can’t get back out
