active transport

Introduction to Transport Processes

  • Two main types of transport across plasma membranes:

    • Passive Transport: Movement of substances down their concentration gradient without energy use (previous video).

    • Active Transport: Requires energy to move substances against their concentration gradient.

Learning Objectives

  • Describe transport processes across plasma membranes.

  • Focus on:

    • Active Transport

    • Vesicular Transport

  • Differentiate transport processes based on:

    • Energy use

    • Source of energy

    • Direction of movement

    • Mechanism of transport

  • Reference: Saladin, pages 88-97, including Table 3.3 on page 97.

Primary Active Transport

  • Definition: Uses energy from ATP hydrolysis to transport substances against their concentration gradients.

  • Key Example: Sodium-Potassium ATPase (Pump).

Function of Sodium-Potassium ATPase

  • Steps of Action:

    1. Binding of Sodium

      • Protein binds to 3 sodium ions from the cytosol.

    2. ATP Hydrolysis

      • Hydrolyzes ATP to ADP and inorganic phosphate (Pi), providing energy.

      • Causes protein to change shape and face extracellular fluid.

    3. Release of Sodium

      • Sodium ions are released into the extracellular fluid.

      • Pi binds the ATPase, leading to a state called phosphorylation.

    4. Binding of Potassium

      • Now oriented to bind 2 potassium ions from the extracellular fluid.

      • Upon dephosphorylation (losing Pi), it returns to original conformation, releasing potassium into the cytosol.

  • Result: Pumps 3 sodium out and brings 2 potassium in, against their concentration gradients.

Electrochemical Gradient

  • Unequal Ion Distribution: Sodium is high outside the cell; potassium is high inside the cell.

  • Charge Separation:

    • Extracellular side slightly positive; cytosolic side slightly negative.

    • Result of unequal ion distribution and negatively charged proteins.

  • Electrochemical Gradient: Combines concentration gradient and charge separation, driving ion transport across the membrane.

Active Transport Discussion Question

  • What role does the sodium-potassium ATPase play in generating the difference in charge and ion concentrations across the membrane?

Secondary Active Transport

  • Definition: Uses stored energy from previously generated concentration gradients (indirect energy use).

  • Mechanism: Not directly using ATP but using ion gradients established by primary active transport.

Co-Transport Mechanisms

  • Examples:

    • Glucose Transport (Symport): Sodium moving in and glucose moving against its gradient into the cell.

    • Hydrogen Ion Transport (Antiport): Sodium moves into the cell while hydrogen ions move out.

  • Types of Coupled Transport:

    • Symport: Both substances move in the same direction; e.g., sodium and glucose.

    • Antiport: Substances move in opposite directions; e.g., sodium in, hydrogen out.

  • Importance: Allows import of nutrients, even against their concentration gradients.

Vesicular Transport

  • Definition: Transport method for larger molecules or cellular debris using vesicles or membrane-bound sacs.

  • Types of Vesicular Transport:

    • Exocytosis: Release of substances (e.g., neurotransmitters) from the cell.

      • Example: In neuron axon terminals, action potential triggers calcium influx, causing vesicles to fuse with the plasma membrane and release neurotransmitters.

    • Endocytosis: Substances moving into the cell, including:

      • Receptor-Mediated Endocytosis: Specific binding of ligands to receptors, leading to vesicle formation.

      • Phagocytosis: Engulfing of large particles or microbes.

      • Pinocytosis: Cell "drinking" of extracellular fluid.

Conclusion

  • Understanding these transport mechanisms is crucial for grasping how substances move across cellular membranes, influencing cell function and communication.