Carrier-mediated transport Flashcards
What do all types of carrier-mediated transport have in common?
All display:
- specificity: Each carrier protein is specialised to transport one or at most a few closely related substances
- saturation: Limited number of carrier binding sites in the membrane for a particular substance, therefore can become full - transport maximum Tm
- competition: If closely related substances can use the same carrier they will compete for the use of that carrier.
What is carrier-mediated transport and what are the four types?
The cell membrane is studded with proteins.
These proteins enable trans-membrane solute movement of four kinds:
- SImple diffusion (of ions through channels)
- Facilitated diffusion (larger molecules)
- Active transport - primary and secondary (required energy to move solutes against their concentration gradient)
Explain simple diffusion via ion channels
Channels are an easy way to go for ions.
They come in many types depending on ion selectivity and gating (ungated vs gated).
These gates are downhill only!
What are the concentration gradients for typical ions?
ICF ECF K+ 150 5 Na+ 15 150 Ca++ <10-4 2 Cl- 5 110 Thus when appropriate ion channels are open K+ will move out and the rest will move in.
What is facilitated diffusion?
Step 1: Transported solute binds weakly to a carrier protein. (concentration gradient does exist for solute high outside low inside)
Step 2: Binding of solute molecules induces change in conformation of the carrier protein. It flips inside out. X conformation –>Y conformation
Step 3: Transported solute detaches from carrier protein in area of low concentration.
Step 4: Carrier protein reverts to original shape.
Define primary and secondary active transport.
Cells needs to move a solute in or out but does not have a favourable concentration gradient.
The movement is uphill so it needs energy - ATP.
Primary active transport happens when the energy comes directly from ATP hydrolysis.
Secondary active transport is when the energy is derived from existing concentration gradient of another solute.
Both involve are protein carrier that binds one or more solutes thus substrate specific and saturable.
What are the steps in primary active transport?
Step 1: Carrier protein splits ATP into ADP plus phosphate. The phosphate binds to the carrier which increase the affinity for the ion.
Step 2: Ion binds to carrier on low concentration side:
Step 3: In response to binding the carrier changes shape which expose its opening to other side and also causes the ion to be less attracted.
Step 4: Carrier released ion to side of higher concentration. Phosphate is also released.
Step 5: carrier reverts to original shape.
What is the most important example of primary active transport in the body?
The Na+/K+ pump. To maintain the high K+ and low Na+ concentration in cells.
This pump has three high affinity sites for Na+ when exposed to ICF. When these bind it cause the ATP split which gives it the energy to change shape (phosphorylation = phosphate binding to protein).
Releases Na+ into ECF and repeats the process with two K+ ions.
When the K+ bind it causes phosphate to release (dephosphorylation) which causes protein to revert to original shape. K+ are released and process starts again.
What are the steps in secondary active transport?
Similar to facilitated diffusion but extra solute transported against concentration gradient.
Eg. Na+/glucose symport.
In this process the Na is being moved by facilitated diffusion along its concentration gradient. But when the Na+ binds on the ECF side its causes the protein to have an affinity for glucose as well even though it is against glucoses’s concentration gradient. So glucose catches a ride as well!
What are some important active transporters?
Na+/K+ pump
K+/H+ gastric acid pump
Ca2+ calcium extrusion pump
H+ Acid extrusion pump
Na+/H+ antiport - removes acid from body, makes urine acidic
Na+ coupled amino acid transporters which allow cells to take in amino acids to make proteins
