Active Transport Flashcards
Essential Feature of Active Transport
Moves solute against electrochemical gradient
- energy is required in the form of ATP hydrolysis
Coupled Transporters
Couple the transport of one solute w/ the gradient to another against the gradient
- Secondary or indirect active transport
ATP-driven Pumps
Energy derived from hydrolysis of ATP is used to drive the transport of a solute against its gradient
- Primary or direct active transport
Light-driven pumps
Couple the transport of a solute against gradient from the energy from light
Why do we need active transport?
To maintain electrochemical gradients (differences in conc of ions across a membrane)
- ~70% of energy is used to maintain the gradients
Why are electrochemical gradients maintained? - Osmosis
Electrical forces inside and outside of the cell must be balanced
- Without active transport to maintain electrochemical gradients ions would flow
down their gradients through channels, disturbing osmotic balance - cell could shrivel up
Why are electrochemical gradients maintained? - Na+ movement
- Movement of Na+ down gradient drives active movement of other substances against their gradient
- Not maintained by many transport systems would fail
The Sodium Potassium ATPase (pump)
- Uses energy from ATP hydrolysis to transport 3 Na out +2 K in against electrochemical gradients
- Operates constantly to get rid of Na+ that enters through carrier proteins and channels
-
Steps of the Na/K Pump
(1) 3 Na+ taken in
(2) ATP phosphorylates alpha subunits
(3) A conformational change after phosphorylation takes the 3 Na+ out and pumps are open to outside
(4) 2 K+ accepted from outside
(5) Dephosphorylation triggers conformational change
(6) 2 K+ expelled to outside
Coupled transport - Symport
- A solute moving against conc gradient is coupled with another solute doing the same
e.g. Na+/Glucose Symporter
2Na+ down electrochemical gradient coupled to transport of glucose against conc. gradient
Coupled transport - Antiport e.g. Na+/Ca2+ Antiporter
- Cardiac muscle cell contraction is triggered by a rise in intracellular [Ca2+]
- Operation of the Na+/Ca2+ antiporter reduces intracellular [Ca2+] and thereby reduces strength of cardiac muscle contraction
The Proton Pump
An ATP-driven pump
Whereas higher eukaryotes use Na+ electrochemical gradients to create ionic gradients across membranes and drive transport processes, prokaryotes use proton gradients
- Proton pumps are used in lysosomes for catalysing breakdown of substrates
Problem with transport of glucose across gut epithelial cells - intracellular conc of glucose is higher than in the gut
- So, down the gradient would get rid of sugar from epithelial
- Therefore, active transport mechanism to transport glucose
into the epithelial cells lining gut - This involves the Na+/glucose symporter
Problem with transport of glucose across gut epithelial cells - Intracellular conc. of Na+ rises disturbing electrochemical gradient
- Therefore, Na+ must be pumped out of the cell
- This involves the Na+/K+ antiporter which couples movement of Na+ out of the cell to movement of K+ into the cell.
- ATP, is required as both ions are being moved against
their electrochemical gradient (primary active transport)
Problem with transport of glucose across gut epithelial cells - How glucose can be transported out of epithelial cell + into blood
- Concentration of glucose in the cell is higher than in the blood
- Therefore glucose can move down its concentration gradient by
facilitated diffusion, down conc gradient - This involves the Glut1 uniporter
