Lecture 5: Cell Envelope Flashcards
Describe the modern model of a biological membrane.
50% protein and 50% lipid (semi-liquid/semi-solid). It is semipermeable.
Contrast how small and large, hydrophobic and hydrophilic compounds cross the membrane bilayer.
Semipermeable membrane have a hydrophobic interior but water enters freely via aquaporins (passive transport), small, non-polar solutes pass thru freely via direct diffusion, polar molecules (except H2O) cannot pass freely (Require gates), anything greater than 600 g/mol cannot pass freely (require transporters). Water flow across membranes lead to osmosis.
Explain why some cells placed in a hypoosmotic environment lyse, and why bacteria do not lyse.
Hypotonic environments have a high solute concentration inside the cell and low solute concentration outside the cell. Turgor pressure must be monitored because water flows inside the cell and it may lyse.
Hypertonic environment has low solute concentration inside the cell and high solute concentration outside the cell. Must use an osmoprotectant to equalize the solute because water will flow out of the cell and plasmolysis may occur.
Explain why most cells placed in hyperosmotic environment plasmolyze, and how some bacteria avoid plasmolysis.
If placed in a hypertonic environment, water will leave the cell and the cell may shrivel up. The cell can avoid this by equalizing the solute via an osmoprotectant.
Describe the two components of the proton motive force (PMF).
- Difference in charge: THe outside is positive due to Na+ and inside is negative due to HCO3- membrane potential. Protons will pump inward toward the negative charge.
- Follow the pH gradient; lower pH outside of the cell. Protons will pump toward the higher (more basic) pH.
Explain two ways in which cells can produce a proton gradient.
- Use reducing power: Chemical energy release is used to move protons; flow of electrons from electron donor releases chemical energy which can be turned into potential energy by pumping protons.
- Created by proton pumps: Respiratory chain uses energy from redox reactions. Bacteriorhodopsin uses light energy.
List three direct uses of the PMF.
- Flagella uses proton force to turn the rotor of the flagella.
- For transport.
- To make ATP => ATP Syntase; potential energy turned to kinetic energy to chemical energy.
Explain the basic differences between Gram negative, Gram postive, Mycobacterial, Mycoplasmal, and Archaeal cell walls.
Gram negative have only one layer of thin peptidoglycan. The outer membrane is a permeability barrier and things only enter through porin. A lipopolysaccharide attaches to the outer layer of peptidoglycan. Periplasm contains protein gradient. Lipopolysaccharide contains diglutamine with branched fatty acids, its an endotoxin, and core polysaccharides and o-antigens that are highly specific.
Gram positive bacterial has a thick peptidoglycan, teichoic acids, lipoteichoic acids, no nitrogenous bases,.
Mycobacteria contains a waxy mycelia acids. It has a peptidoglycan and inner membrane similar to gram negative cell. The outer membrane contains long chain mycolic acids (waxy and resistant to chemical damage).
Mycoplasma/Spiroplasma has no cell wall.
Archaea has a protein cell wall with pseudopeptidoglycan and chondroitin.
Recognize the structural components of peptidoglycan, and how they are assembled.
Sugars G and M linked with Beta-(1,4)-glycosidic bond. Peptides with amino acids: L-alanine, D-glutamine, diaminopimelic acid, and D-alanine (3rd amino acid is always a di-).
Peptide bonds can form from the amino group in one peptidoglycan to the carboxyl group in another peptidoglycan (transpeptidation).
Know how the antibiotics vancomycin, penicillin, and lysozyme alter the bacterial cell wall.
Penicillin prevents crosslink in peptidoglycan. Only kills cells that are trying to grow.
Lysozymes hydrolyzes the bond and kills already grown bacterial cell walls.
Vancomycin prevents transpeptidation and transglycosylation; only kills growing chains.
Be able to label the diagrams of Gram+ , Gram-, and mycobacerial cell walls.
Look at lecture 5.
Recognize why Lipid A is not a typical membrane lipid (o.e. compare with lipids from lecture 3).
Lipid A provides the anchor that secures the molecule within the membrane, while the polysaccharide component interacts with the external environment, including the defenses of the animal or plant host species. Lipid A is a unique and distinctive phophoglycolipid, the structure of which is highly conserved among species.
Explain the difference between Mycoplasma and Mycobacteria.
Mycoplasma refers to a genus of bacteria that lacks a cell wall. Gram negative bacteria and one interesting characteristic of these bacteria is that they don’t have a cell wall; which gives them an elastic shape that can vary easily.
Mycobacteria are acid-fast Gram positive bacteria that have a very thick, protective, waxy cell wall. Aerobic and slender, curved rod shaped species. Composed of waxes and mycolic acids, which makes them resistant to Gram staining.
Explain why mycoplasma do not have a defined shape.
Mycoplasma do not have a cell wall. It is invulnerable to drugs. Due to the lack of a rigid wall, Mycoplasmataceae can contort into a broad range of shapes, from round to oblong. They therefore cannot be classified as rods or cocci.
