ATP-Driven Pumps and Active Transport

Sodium-Potassium Pump & ATP

• The sodium-potassium pump moves sodium and potassium against their concentration gradients.

  • Sodium is pumped from a low concentration inside the cell to a high concentration outside the cell.

  • Potassium is pumped from a low concentration outside the cell to a high concentration inside the cell.

Sodium-Potassium ATPase

• The sodium-potassium pump is also known as sodium-potassium ATPase due to its enzymatic function.

• It breaks down ATP through hydrolysis:

  • ATP \rightarrow ADP + P_i (inorganic phosphate).

  • The breaking of the bond releases energy.

  • The three phosphate groups in ATP are negatively charged and repel each other, storing potential energy like a "loaded spring."

Roles of Sodium-Potassium ATPase

• Maintaining chemical gradients:

  • High sodium concentration outside the cell and high potassium concentration inside the cell.

  • Essential for metabolic activity and cell depolarization/repolarization.

• Maintaining electrical differential (voltage) of the cell:

  • Positive charge outside and negative charge inside in resting cells.

  • The pump moves three sodium ions out for every two potassium ions in, making the outside more positive.

  • Ratio is 3:2 (Na:K).

  • There are thousands of sodium-potassium pumps in each cell.

• Maintaining cell volume:

  • Cells contain protein and phosphate anions.

  • These intracellular solutes can draw water into the cell.

  • If the pump moved 3 Na+ out and 3 K+ in, excess solutes would cause water influx, leading to cell swelling and lysis.

  • By pumping out more sodium than potassium, it balances solute concentration and prevents swelling. If swelling occurs anyway, pump activity will increase to move even more sodium out to draw water with it.

Reversal of Sodium-Potassium Pump

• Under specific conditions, the pump can work in reverse.

• When sodium moves into the cell and potassium moves out down their concentration gradients, kinetic energy is produced.

• This kinetic energy can be used by the sodium-potassium ATPase to synthesize ATP from ADP and inorganic phosphate:

  • ADP + P_i \rightarrow ATP (condensation).

  • Condensation is the opposite of hydrolysis.

Other examples of ATP-utilizing pumps:

• Calcium Pump (Calcium ATPase):

  • Found in the sarcoplasmic reticulum of muscle cells.

  • Pumps calcium against its gradient to store it in the sarcoplasmic reticulum.

  • Essential for muscle relaxation after contraction.

• Hydrogen-Potassium Pump (Hydrogen-Potassium ATPase):

  • Located in parietal cells of the stomach.

  • Pumps hydrogen ions into the stomach lumen, creating a low pH (pH ~2) for digestion and immunity.

  • Also known as a proton pump.

  • Individuals taking proton pump inhibitors block the activity of this pump to reduce stomach acidity.

Kinetic Energy and Secondary Active Transport

• Kinetic energy is the energy of motion created by ions or molecules moving down their gradients.

• Example: Sodium-Glucose Transporter (SGLT) in kidney cells (nephron).

  • Moves glucose into the cell against its gradient using the kinetic energy of sodium moving down its gradient.

  • No ATP is directly used.

  • SGLT is a symporter because it moves sodium and glucose in the same direction.

• The sodium-potassium pump maintains the sodium gradient, which is essential for the SGLT to function.

• Without the sodium-potassium pump, sodium would not move passively into the cell.

• When cells utilize kinetic energy to move things against their gradients, it is called secondary active transport.

• Primary active transport (using ATP) is required to establish the gradients for secondary active transport to occur.

Antiporters

• Sodium-Calcium Exchanger (NCX) in myocardial contractile cells:

  • Sodium moves into the cell down its gradient.

  • Calcium moves out of the cell against its gradient.

  • The NCX utilizes the kinetic energy created by sodium moving down its gradient to pump calcium out of the cell.

Summary

• Chemical energy (ATP hydrolysis) is used for primary active transport.

• Kinetic energy (ion gradients) is used for secondary active transport.

• Primary active transport is essential for establishing the gradients required for kinetic energy utilization in secondary active transport.