Movement Across Membranes Study Guide

Movement Across Membranes Study Guide

Diffusion

  • Definition: Spontaneous movement of molecules from areas of high concentration to areas of low concentration.

Osmosis

  • Nature of Biological Membranes: Biological membranes are semi-permeable, meaning they allow certain molecules to pass while blocking others.
  • Molecule Diffusion:
    • Identify and understand the categories of molecules based on their ability to diffuse across membranes:
    • Easily Diffuse: Small, non-polar molecules, e.g., oxygen and carbon dioxide.
    • Difficulty Diffusing: Larger or polar molecules may require assistance.
    • Cannot Diffuse: Molecules that cannot pass through the membrane are referred to as osmotically active solutes.
  • Definition of Osmosis: The movement of water across a membrane in response to different concentrations of osmotically active solutes.
  • Mechanism:
    • Water moves from the solution with the lowest concentration of osmotically active solutes to the solution with the highest concentration.
    • This process will:
    • Concentrate the solution with the lowest concentration.
    • Dilute the solution with the highest concentration.
    • Ultimately, it brings the two solutions closer to equilibrium.

Uniports, Symports, and Antiports

  • Transport Mechanisms:
    • Uniports: Transport one type of molecule/ion across a membrane.
    • Symports: Transport two molecules/ions across a membrane in the same direction.
    • Antiports: Transport two molecules/ions across a membrane in different directions (e.g., one molecule enters a cell while the other leaves the cell).

Facilitated Diffusion, Ion Channels, and Active Transport

  • Transport Proteins: All three mechanisms require transport proteins to facilitate the movement of molecules across membranes.
  • Facilitated Diffusion:
    • Molecules move down their concentration gradient.
    • Does not require energy.
  • Ion Channels:
    • Allow specific ions to pass through the membrane following their concentration gradient.
    • Does not require energy.
  • Active Transport:
    • Carries molecules/ions across a membrane, either down or against their concentration gradient.
    • Requires an expenditure of energy (ATP).
    • Key Function: Understand and describe the function of the Na+/K+ pump, which actively transports sodium ions out of the cell and potassium ions into the cell against their gradients.

Co-Transport

  • Definition: Co-transport is a linked system that typically involves an active transport pump in conjunction with a facilitated diffusion symport/antiport.

  • Active Transport System: Despite facilitated transport appearing to move one molecule against its gradient, both substances are actually moving based on their net gradient across the membrane, maintaining the classification of co-transport as an active transport system.

  • Key Point: Understanding the co-transport mechanism requires analysis of the gradients of both substances involved.