Active Transport
ACTIVE TRANSPORT
- Movement of solutes against their electrochemical gradients
- Extracellular space is positively charged
- Intracellular space is negatively charged
- Requires energy to overcome solute's gradient
- Facilitated by transporters NOT channel proteins
Primary Active Transport
ATP hydrolysis fuels transport
Sodium-potassium pump
- Na+ electrochemical gradient: large and directed into cell
- K+ electrochemical gradient: small (chemical and electrical gradients oppose each other) and directed out of the cell
- Pumps K+ and Na+ against their gradients
- Hydrolyzes ATP (account for 30% of animal cell's ATP consumption)
Secondary Active Transport
Does not directly require ATP
Glucose-sodium symport protein
- Apical surface of intestinal epithelial cell
- [Glucose] greater in cytosol
- [Na+] greater in extracellular space
- Transports glucose and sodium into cell
- Electrochemical gradient of sodium drives transport of glucose against its own gradient
- Cell can absorb glucose from the intestines even when intracellular glucose is high
Secondary Active Transport
Does not directly require ATP
Sodium-calcium antiporter
- Surface of a cardiac muscle cell
- [Ca2+] greater in extracellular space
- Couples movement of Na+ down an electrochemical gradient with the movement of Ca2+ against its gradient and out of the cell
- Influx of Ca2+ triggers a contraction and antiporter restores gradient for next contraction