Active Transport | Transport Across the Cell Membrane | Sodium Potassium Pump | Cell Physiology

Transport Mechanisms

• Active Transport: Requires energy (ATP) to move substances against an electrochemical gradient (from low to high concentration).

• Passive Transport: Does not require energy; solutes move along their gradient (from high to low concentration).

Carrier Proteins

• Role: Essential for both active and passive transport.

• Types: In active transport, carrier proteins are often referred to as pumps.

Primary Active Transport

• Definition: Direct use of ATP to transport molecules.

• Examples:

  • Sodium-Potassium ATPase: Pumps 3 sodium ions out and 2 potassium ions in (3:2 coupling ratio).

    • Maintains resting membrane potential and cell volume.

    • Alpha subunit binds ions; undergoes phosphorylation and conformational changes.

  • Calcium ATPase: Keeps cytoplasmic calcium levels low by pumping calcium ions out of cells or into the sarcoplasmic reticulum in muscles.

  • Proton-Potassium ATPase: Pumps hydrogen ions into the stomach (important for gastric acid production) and regulates acid-base balance in the kidneys.

Secondary Active Transport

• Definition: Indirect use of energy; utilizes the gradient established by primary active transport.

• Mechanisms:

  • Co-transport (Symport): Both solutes move in the same direction. Example: In kidneys, glucose and sodium are reabsorbed together from the proximal convoluted tubule (PCT).

    • Sodium-potassium pump creates sodium gradient.

    • Sodium moves back into the cell along its gradient, bringing glucose with it against glucose's gradient.

  • Counter-transport (Antiport): Solutes move in opposite directions. Example: Sodium-hydrogen exchanger in the PCT.

Summary

Both primary and secondary active transport are critical for maintaining cellular functions, including concentration gradients, membrane potential, and transporting essential molecules across cell membranes.