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

ATP DRIVEN PUMPS

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)

Coupled transporters

Couple movement of one solute against its gradient with movement of another solute down its gradient

Light-driven pumps

Occur in bacteria and couple active transport with light energy

COUPLED TRANSPORT

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

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

CLINICAL CORRELATION
Digoxin: disrupts Ca2+ gradient to increase force of cardiac cell contraction

• Inhibits sodium-potassium pump, which increases intracellular Na+
• Sodium-calcium antiporter cannot function: intracellular Ca2+ increases and produces stronger contractions