3. Transport Flashcards
What is facilitated diffusion? what determines the direction of the flow for uncharged substrates vs. charged substrates?
- Facilitated diffusion can only transport solutes down a gradient.
- For uncharged substrates, the concentration gradient alone will determine the direction of the flow.
- For charged substrates, the charge and the concentration gradient – the electrochemical gradient – will determine the direction of the flow.
multicellular vs.unicellular organisms acquire nutrients by___?
- multi: diffusions or facilitated
- uni: active transport
When external concentration of solute increases, rate of solute entry by Simple diffusion increases/decreases?
vs. transport using transporters?
- rate of solute entry by simple diffusion increases
- the rate of uptake by transporters reaches a maximum and addition of more substrate does not increase the rate. because transporters are saturated with substrate
Facilitated diffusion: Channel-mediated vs. Carrier-mediated
• Channel-mediated:
– Specificity is relatively low
– Can be closed by the cell (gated channel)
• Carrier-mediated:
– The binding of the substrate on one side of the membrane induces a conformational change in the carrier.
– The substrate is released on the other side.
– Tends to be more specific than channel-mediated diffusion.
– No energy required.
Active transport
- against the concentration gradient
- requires energy
- Group transloca0on and ABC transporters are found in bacteria, archaea.
uniporter
antiporter
symporter
uniporter: 1 substrate
antiporter: 2 substrates; 1 in 1 out
symporter; 2 substrates; both in or both out
pmf-dependent transport – bacteria, archaea
what types of transporters ?
- When respiration is possible, a terminal
electron acceptor is present, so proton motive force is generated. - When respiration is not possible. ATPases can be used to generate the pmf. ATPases are reversible.
- symporters or antiporters
- Pmf can be used to generate a gradient of Na+ that is used by some transporters as a source of energy. (common in marine organisms)
H+ transporter
Cation (+), ex: potassium
Cation uniporter
(+) net charge
–>driving force: membrane potential
Anion (-), ex: sulfate
Proton-Anion symporter
0 net charge
–>proton concentration
X (Neutral), ex: lactose
Proton-X symporter ex:lac permease
+ net charge
both membrane potential & proton gradient
Na+ transporters
Anion (-), ex: glutamate
Na+-Anion symporter ex: sodium-proton symporter
0
–>proton gradient
X (Neutral), ex: melobiose
Na+-X symporter ex:
+
both membrane potential & proton gradient
membrane potential
out: more H+ acid positive
in: alkaline, negative
* * net charge is never (-) if using pmf
ABC transporters
- ATP-binding cassette
• ATP-binding cassette, 3 components:
– A membrane-spanning protein, the ac0ve
transport carrier.
– An ATP-hydrolyzing protein that provides the energy for ac0ve transport.
– A substrate-specific binding protein that has very high affinity for a specific substrate (or a class of substrates).
- The binding protein binds to its substrate and transfers it to the transporter. at very low substrate concentrations
the binding protein are located at___ in Gran(-) vs.(+) ?
- In Gram-negetive: the binding protein is free in the periplasm.
- In Gram-positive: the binding protein is anchored in the cytoplasmic membrane.
Group translocation
- the substrate is modified as it passes through the transporter, across the membrane
e. g.phosphoenolpyruvate-dependent sugar phosphotransferase system. -used by bacteria to transport common monosaccharides such as glucose, fructose and mannose. - modify things to make them they stay in plasma membrane, make sure glucose does NOT diffuse away
- responsible for the first step of glycoly0c pathway (in prokaryotes).
Transport in unicellular Eukaryotes - pmf
- H+-substrate symporters are used
- pmf is created inside mitochondria at the level of the cytoplasmic membrane.
- In Eukaryotes, a proton-transloca0ng ATPase – P-type ATPase – in the cytoplasmic membrane uses ATP to pump out protons.
- The pmf generated at the level of the cytoplasmic membrane can now be used to power-up symporters in the cytoplasmic membrane.
-Eukaryotes: P-type ATPase = 1H+ / ATP hydrolyzed
Prokaryotes / Mitochondria: F-type ATPase = 3H+ / ATP hydrolyzed
Transport in eukaryotes - endocytosis
- only eukaryotes are able to do endocytosis
NO bacteria or prokaryotes - Actin filaments are essential for endocytosis
- Phagocytosis
- Pinocytosis
- Receptor-mediated endocytosis
