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Diffusion and Solutes (Net Movement of Solutes)

Diffusion: Net Movement of Solutes

  • Definition: Diffusion is the net movement of solutes (substances dissolved in a solvent) driven by their random thermal motion.
  • Solute vs. solvent:
    • Solvent: the medium in which solutes are dissolved; in biology, the solvent is usually water.
    • Solute: substances dissolved in the solvent.
    • Examples of solutes in water (in typical diffusion scenarios): salts (e.g., NaCl), sugars (e.g., glucose), gases (e.g., O₂, CO₂), amino acids, minerals.
  • Solvent context given in transcript:
    • The solvent is usually water.
    • Solutes are the substances that are in that solvent.
  • Mechanism and driving force:
    • Solutes move due to random motion, creating a net movement from regions of higher concentration to regions of lower concentration.
    • This movement continues until equilibrium (uniform concentration) is approached.
  • Key equations (Fick's laws):
    • Fick's first law: \mathbf{J} = -D \nabla C
    • (\mathbf{J}) is the diffusion flux (amount per unit area per unit time).
    • (D) is the diffusion coefficient (depends on temperature, solvent, and solute properties).
    • (\nabla C) is the concentration gradient.
    • Fick's second law: \frac{\partial C}{\partial t} = D \nabla^2 C
    • Describes how concentration changes with time.
  • Factors influencing diffusion:
    • Temperature: increasing temperature generally increases molecular motion, increasing (D).
    • Molecular size: smaller solutes diffuse faster than larger ones.
    • Medium viscosity and properties: higher viscosity or crowded environments slow diffusion.
    • Distance: diffusion time increases with the square of the distance; longer distances slow effective diffusion.
    • Barriers and membranes: semi-permeable barriers can restrict diffusion.
  • Related concepts:
    • Osmosis: diffusion of water across a semi-permeable membrane; water moves toward higher solute concentration.
    • Diffusion vs. active transport: diffusion is passive (no energy input required), whereas active transport requires energy.
  • Biological relevance:
    • Diffusion drives gas exchange (O₂ and CO₂) and transport of small molecules in tissues.
    • Diffusion alone limits cell size and distance over which nutrients and wastes can diffuse; this underpins the need for circulatory or transport systems in larger organisms.
  • Connections to foundational principles:
    • Brownian motion underlies diffusion.
    • Concentration gradients as a fundamental driving force in chemistry and biology.
    • Equilibrium concepts and mass balance.
  • Practical and real-world implications:
    • Diffusion is exploited in dialysis, drug delivery design, filtration, and various laboratory separation techniques.
  • Notes on the excerpt:
    • The transcript defines diffusion as the net movement of solutes and clarifies that solutes are the substances in the solvent, with water typically serving as the solvent.
  • Common clarifications:
    • Diffusion does not require energy input (passive process).
    • It is distinct from osmosis, though both involve movement driven by gradients.