Membrane transport: Solute Transport

Benefits of permeability barrier

• lipid bilayer is impermeable to most hydrophilic molecules

• higher intracellular concentration: faster enzyme reactions and retention of metabolic intermediates

• maintenance of the electrochemical potential and sodium ion gradient that drive ATP synthesis, solute transport, and other membrane activities

• transport rakes place through proteins integrates in the membrane

• by controlling which transporters are expressed the cell selects which solutes it transports→ can exclude toxins and transport linked to metabolic events

cells import

• macronutrients

• micronutrients

• growth factors

macronutrients

nutrients needed in large amounts (not in size)→ carbon, nitrogen, phosphorus, sulfur, potassium and magnesium

micronutrients

nutrients needed in small amounts→ iron, magnesium, cobalt and other metals

growth factors

organic molecules that are needed, but the species can’t make→ amino acids, nucleotides and vitamins (depends on species)

cells export

all have to cross the cytoplasmic membrane

• proteins

• cell wall/outer membrane components

• signal/ internaction molecules

proteins

OM proteins, periplasmic proteins, extracellular enzymes, carrier proteins, toxins, S-layer proteins

Cell wall/outer membrane components

peptidoglycan monomers, LPS components, teichoic acids, capsule polysaccharides, extracellular polysaccharides

signal/interaction molecules

quorum sensing auto inducers, antibiotics

Nonspecific/ general porins

Linear relation between translocation rate and solute concentration gradient (passive diffusion). The inside of the barrel (beta-barrel) is lined with charged residues.

Specific porins

Michaelis-Menten kinetics for the transport of specific solutes. Lining of barrel is different. LamB→maltose

Ligand-gated porins

Highly specific, channel is blocked by an extra protein domain (plug), very specific low concetration nutrients (Fe3+ and B12)

passive/simple diffusion

directly through the lipid bilayers (water, gases,ethanol)→ slow

facilitated diffusion

Needs facilitated protein. For uptake: aquaporin and export: formate

passive transport limitations

• most nutrients don’t move freely across the membrane

• little control over which solutes enter the cell or leave

• in nature most nutrients are present outside the cells in very low concentrations→ must selectively accumulate nutrients against a concentration gradient

primary active transport

uses chemical energy (ATP) to drive the transport

Secondary Active transport

Uses energy from the proton motive force

• uniporter

• synporters

• antiporters

uniporter

Synporter

antiporters

Group translocation

Uses energy from an energy rich organic compound other than ATP. The molecule transported is chemically modified during transport (phosphorylated)

undirectional transporters

• medium affinity uptake

• high affinity uptake

• micronutrient uptake

• exporters

Medium affinity uptake

Ions, sugars, amino acids, short peptides and oligosaccharides, not very low concentrations

High affinity uptake

Large, low concentration, hydrophobic substrates (iron chelates and B12). Very low concentration molecules

Micronutrient uptake

water soluble vitamins

Exporters

peptides, toxins, other proteins. Flip lipids, LPS components, other polysaccharides

ABC transporters

function in evasion of or resistance to host defenses, as well as helping with host colonization (adhesion and nutrient scavenging)

ABC exporters

LPS (O-antigen), techoic acids, capsule polysaccharides, proteins(toxins, proteases, lipases, S-layer), hemophores and siderophores, xenobiotic (antimicrobials and bile salts)

ABC importers

metal chelates (Fe3+, Mn, Zn2+), hemophores, siderophores, amino acids, osmoproectants