Topic 2-L6 - Nutrient uptake into bacterial cells Flashcards
The cytoplasmic membrane:
The gatekeeper of the cell
Which molecules can move freely across the cytoplasmic membrane?
Small, non polar, uncharged molecules
Some molecules can cross the CPM at a
meaningful rate (but significantly hindered by the membrane)
The CPM is impermeable to
Large, polar, charged molecules
How mcolueslenter bacterial cells
Passive transport (no E)
Active transport (uses E)
Passive transport
- simple diffusion
- facilitated diffusion
Active transport
- simple transport
- ABC transporters
- group transport
Diffusion:
The net movement of a chemical down it’s concentration gradient (from area of high concentration to area of low concentration).
- Entropically favorable!
Osmosis is the
diffusion of water along its concentration gradient. (A low concentration of solutes = a high concentration of water)
Facilitation diffusion:
Diffusion of molecules across the membrane via a membrane protein (permease) that acts as a channel. Porins of OM.
- can be specific or non specific
Any time a molecule is transported against its concentration gradient, this requires energy.
This can come from
stored chemical energy (e.g. ATP hydrolysis) or from dissipation of another concentration gradient (transporting another molecule along its concentration gradient)
Symporters & antiporters use the energy stored in
chemical gradients to power the transport of a different molecule against its gradient
Symport:
Both molecules travel same direction
Antiport:
One molecule in, the other out
Symporters and antiporters often use
proton motive force (H+ gradient)
Sodium proton antiporter
Exchanges protons (dissipation of proton gradient) for Na+ ions.
- Expel Na+ from cell under high salt conditions
- Lower pH of cell under alkaline conditions
Lac permease antiporter
Proton motive force is exploited to drive the uptake of lactose and some related disaccharides (high energy food source) into the cell
Group translocation
ACTIVE TRANPORT
Transported substance is bound by a transporter and is chemically modified during transport
Example of group translocation
glucose uptake using the phosphotransferase system.
glucose uptake using the phosphotransferase system.
Energy provided by hydrolysis of high energy phosphate in phosphoenolpyruvate (PEP)
- An important feature of this translocation mechanism is that the membrane is impermeable to the phosphorylated sugar molecule – remains in cell
ABC transporters
ATP binding cassette (ABC transporters) use ATP to power the transport of substances across the cytoplasmic membrane
ABC transporters proteins involved
- 2 ATPase domains (proteins) provide energy
- Transmembrane domain(s) (proteins) provides selective channel
- Substrate binding protein binds molecule with high affinity and delivers it to the channel
Prokaryotic ABC transporters are best studied in
Gram-negative bacteria – which use “periplasmic binding proteins” to capture their ligand within the periplasm
In archaea and Gram positive bacteria (lack OM) – the substrate binding protein is tethered to the
cytoplasmic membrane
Vitamin B12 is very
Large, precious, and too big to diffuse through porins.
How does vitamin B12 enter bacterial cells
ABC transporters
How does B12 use ABC transporters to enter cell
OM barrel protein BtuB binds B12 with high affinity, transports across OM using energy from To n B complex (via proton motive force)
Iron-binding siderophores
(molecules secreted to capture precious iron) are taken up in a synonymous manner as B12
