passive transport

Overview of Passive Transport Processes

  • Focus on energy-independent transport across plasma membranes.

  • Key processes: diffusion and osmosis.

  • Compare processes by energy use, transport direction, and mechanisms.

Structure of the Plasma Membrane

  • Composed of a phospholipid bilayer with:

    • Transmembrane proteins.

    • Cholesterol molecules for fluidity.

    • Carbohydrate molecules for signaling and protection.

  • Divides extracellular fluid (outside) from cytosol (inside).

Passive vs. Active Transport

  • Passive Transport: No energy required to move substances across membranes.

  • Active Transport: Requires energy to transport substances against concentration gradients.

Diffusion

  • Definition: Movement of substances from higher to lower concentration (down concentration gradient).

  • Examples:

    • Everyday experience: Smell of food spreading in a room.

    • Experiment with potassium permanganate diffusion in water.

  • Mechanism: Depends on the kinetic energy of molecules; substances may move directly through the membrane.

Types of Diffusion

  1. Simple Diffusion:

    • Movement through the plasma membrane without assistance.

    • Occurs for small nonpolar molecules and some polar/charged molecules, albeit infrequently.

  2. Facilitated Diffusion:

    • Requires the assistance of transmembrane proteins (channel or carrier proteins).

    • Still occurs down the concentration gradient; no energy required.

    • Incorporates both channel-mediated (for ions) and carrier-mediated transport.

    • Model:

      • As solute concentration increases, transport rate initially increases.

      • At saturation, transport reaches a maximum due to occupancy of transport proteins.

Graphical Representation of Diffusion

  • Facilitated Diffusion Graph:

    • X-axis: Solute concentration

    • Y-axis: Rate of transport

    • Initial increase in transport rate levels off when proteins saturate.

  • Simple Diffusion Graph:

    • Continues to increase linearly with solute concentration since no protein occupancy limits transport.

Osmosis

  • Definition: Net movement of water across a selectively permeable membrane from more water (or less solute) to less water (or more solute).

  • Importance: Key physiological concept influencing cell health and function.

  • Simulation: U-shaped tube experiment demonstrating water movement and equilibrium.

Tonicity

  • Concept: Describes a solution's effect on cell volume due to water movement.

  • Terms:

    • Hypotonic: Lower solute concentration outside; water moves into the cell, causing swelling.

    • Hypertonic: Higher solute concentration outside; water moves out of the cell, causing shrinkage (crenation).

    • Isotonic: Equal solute concentration inside and outside; no net movement of water.

  • Red blood cells serve as examples:

    • Normal shape in isotonic solution.

    • Swelling in hypotonic solution.

    • Shriveling in hypertonic solution.

Filtration

  • Definition: Movement of fluid and solutes through a membrane due to mechanical pressure.

  • Example: Coffee filter mechanism.

  • Biological relevance: Seen in capillary beds and kidney function for urine formation.