2.9 - Mechanisms of Transport

Mechanisms of Cellular Transport

• Passive Transport:

  • Definition: The movement of substances across a cell membrane without the use of cellular energy.

  • Characteristics:

    • Substances move down/with the concentration gradient (from high to low concentration).

    • Typically involves small, nonpolar molecules and some water.

    • Does not require energy.

• Facilitated Diffusion:

  • Definition: A type of passive transport that requires membrane proteins to help move substances across the cell membrane.

  • Characteristics:

    • Aids the transport of small, charged molecules or larger amounts of water.

    • Substances still move down the concentration gradient.

    • Requires channel proteins for transport.

    • Examples include ion channels, which allow specific ions to pass through the membrane quickly and selectively.

• Active Transport:

  • Definition: The movement of molecules against their concentration gradient, from low to high concentration, which requires energy.

  • Characteristics:

    • Involves large, polar, and charged molecules.

    • Requires ATP or another source of energy.

    • Often utilizes specific membrane proteins.

    • Types of Active Transport:

      • Electrogenic Pumps: These are transport proteins that generate voltage across the membrane, such as proton pumps and the sodium-potassium pump (Na+/K+ pump).

      • Cotransport: Uses the energy stored in the concentration gradient of one molecule to transport another molecule against its gradient.

      • Exocytosis and Endocytosis: Mechanisms for moving large quantities of materials into or out of the cell via vesicles. Exocytosis involves the fusion of vesicles with the plasma membrane to secrete materials, while endocytosis is the process of taking materials into the cell by engulfing them in membrane vesicles.