Lecture 5: Cell Envelope

Aquaporin

A membrane protein channel that facilitates the transport of water across cell membranes.

Bacteriorhodopsin

A light-driven proton pump found in archaea that uses sunlight to generate a proton gradient.

Core polysaccharide

A conserved portion of lipopolysaccharide (LPS) in Gram-negative bacteria that connects lipid A to the O-antigen.

Gated channel

A membrane channel protein that opens or closes in response to stimuli, controlling ion or molecule passage.

Gram negative

Bacteria with a thin peptidoglycan layer and an outer membrane containing LPS; stain pink with Gram stain.

Gram positive

Bacteria with a thick peptidoglycan layer and teichoic acids but no outer membrane; stain purple with Gram stain.

Hyper osmotic (hypertonic)

A solution with a higher solute concentration than the cell, causing water to leave the cell.

Hypo osmotic (hypotonic)

A solution with a lower solute concentration than the cell, causing water to enter the cell.

Iso osmotic (isotonic)

A solution with the same solute concentration as the cell, resulting in no net water flow.

Lipid A

The endotoxic component of lipopolysaccharide that anchors LPS to the outer membrane of Gram-negative bacteria.

Lipopolysaccharide

An important component of the Gram-negative outer membrane composed of lipid A, core polysaccharide, and O-antigen.

Lipoprotein

A protein covalently bound to lipids that helps anchor the outer membrane to peptidoglycan in Gram-negative bacteria.

Lipoteichoic acid

Polymers of glycerol or ribitol phosphate anchored into the cytoplasmic membrane of Gram-positive bacteria.

Lysis

The breaking apart or rupture of a cell due to damage or osmotic imbalance.

Membrane potential (ΔΨ)

The electrical potential difference across a membrane resulting from ion gradients.

Mycobacteria

A genus of bacteria with waxy cell walls containing mycolic acids, including M. tuberculosis.

Mycolic acid

Long-chain fatty acids in the cell walls of mycobacteria that make them waxy, resistant, and acid-fast.

Mycoplasma

Bacteria that naturally lack a cell wall and have flexible plasma membranes.

N-acetyl glucosamine (NAG)

A sugar monomer that alternates with NAM in the glycan chains of peptidoglycan.

N-acetyl muramic acid (NAM)

A sugar monomer cross-linked with peptides in peptidoglycan, alternating with NAG.

O-antigen

A variable polysaccharide chain extending from lipopolysaccharide that provides antigenic diversity in Gram-negative bacteria.

Osmoprotectant

A compound that helps organisms survive hypertonic stress by balancing osmotic pressure.

Osmoprotection

Mechanisms by which cells prevent dehydration and plasmolysis in high-solute environments.

Osmosis

The passive diffusion of water across a semipermeable membrane from low to high solute concentration.

Outer membrane

The outer phospholipid and LPS-containing layer of Gram-negative bacteria that provides protection.

Pentapeptide cross bridge

A five-amino-acid bridge that links glycan chains in peptidoglycan of Gram-positive bacteria.

Peptide crosslinking

The peptide bonds that connect glycan chains, giving peptidoglycan its structural rigidity.

Peptide side chain

Short chains of amino acids attached to NAM residues in peptidoglycan that enable crosslinking.

Peptidoglycan

A mesh-like polymer of sugars and amino acids forming the bacterial cell wall.

Plasmolysis

The shrinking of the cytoplasm away from the cell wall due to water loss in a hypertonic environment.

Porin

A protein channel in the outer membrane of Gram-negative bacteria that allows small molecules to diffuse through.

Proton gradient (ΔpH)

A difference in hydrogen ion concentration across a membrane that contributes to proton motive force.

Proton motive force (PMF)

The combined effect of proton gradient (ΔpH) and membrane potential (ΔΨ) that drives ATP synthesis and transport.

Protein pump

A membrane protein that actively transports molecules using energy, often from ATP or PMF.

Respiratory chain

A series of electron carriers in the membrane that transfer electrons and pump protons to generate PMF.

Semipermeable membrane

A membrane that allows selective passage of water and some molecules while blocking others.

Teichoic acid

Polymers found in Gram-positive bacterial cell walls that provide rigidity and contribute to cell shape.

Transglycosylation

The enzymatic process that links sugar monomers (NAG and NAM) into glycan chains in peptidoglycan synthesis.

Transpeptidation

The enzymatic reaction that forms peptide cross-links between glycan chains in peptidoglycan.

Turgor pressure

The pressure exerted by the cell contents against the cell wall, helping maintain cell shape and rigidity.