Lecture review of active/passive transport

Overview of Cellular Transport Mechanisms

Types of Transport

  • Active Transport

    • Requires energy

    • Primary method: ATP

  • Passive Transport

    • Does not require energy

Types of Passive Transport

  1. Simple Diffusion

    • Involves small, unpolar, and uncharged molecules directly crossing the membrane.

    • Example: Lipids

    • Phospholipid bilayer structure:

    • Middle layer: Nonpolar

  2. Facilitated Diffusion

    • Involves the use of protein channels:

      • Ion Channel

      • Shape: resembles a doughnut with a hole in the middle.

      • Example: Specific to potassium ions (only allows potassium to pass in/out; not calcium or chloride).

      • Carrier Protein

      • Structure: has a binding site for molecules.

      • Acts like a trapdoor allowing entry/exit of molecules after binding.

  3. Osmosis

    • Movement of the solvent (mostly water) from high to low concentration.

    • Non-penetrating solutes cannot move, but solvents can.

Types of Active Transport

  1. Primary Active Transport

    • Utilizes ATP directly.

    • Example: Sodium-Potassium Pump

      • Moves 3 sodium ions out and 2 potassium ions in against their gradients.

      • Movement against the gradient requires energy, akin to rolling a ball uphill.

  2. Secondary Active Transport

    • Utilizes the concentration gradient of ions created by primary active transport, thus not requiring ATP directly.

    • Types:

      • Cotransport (symport)

      • Two ions/molecules move in the same direction; e.g., sodium-glucose transporter.

      • Antiport

      • Two ions/molecules move in opposite directions.

      • Example: Ion moves down its gradient while another molecule moves against.

  3. Vesicular Transport

    • Involves transport mechanisms that utilize vesicles to move large molecules.

    • Types:

      • Endocytosis:

      • Phagocytosis: White blood cells engulfing pathogens via pseudopodia to form a vesicle.

      • Pinocytosis: Random uptake of fluid; analogy: "pinot" for drinking.

      • Receptor-mediated Endocytosis: Specific molecules trigger vesicle formation upon binding to receptors.

      • Exocytosis: Process of expelling materials from the cell.

      • Transcytosis: Combination of endo- and exocytosis whereby substances are moved through the cell to the opposite side.

Tonicity and Its Implications

  • Tonicity: Refers to the concentration of extracellular fluid relative to the intracellular fluid (ICF). Impacts cell behavior.

  • Hypotonic solution:

    • Definition: Lower solute concentration outside the cell compared to the inside.

    • Example: If intracellular concentration is 300 mOsm and the extracellular concentration is 275 mOsm, the solution is hypotonic, and cells will absorb water, leading to hemolysis (for red blood cells).

  • Isotonic solution:

    • Equal concentration of solutes outside and inside (e.g., 300 mOsm inside and outside).

  • Hypertonic solution:

    • Higher solute concentration outside than inside. This leads to water moving out of the cell, causing crenation (cell shrinks).

Summary of Key Terms

  • Active Transport: Needs energy (ATP).

  • Passive Transport: No energy needed; driven by concentration gradients.

  • Facilitated Diffusion: Transport with the help of protein channels or carriers.

  • Osmosis: Movement of water based on concentration differences.

  • Hemolysis: The bursting of red blood cells in hypotonic solutions.

  • Crenation: Shrinkage of cells in hypertonic solutions.

Important Concepts

  • Concentration Gradient: The difference in solute concentration across a membrane; critical for passive transport mechanisms.

  • Solvent vs. Solute:

    • Solvent: Typically water (moves in osmosis).

    • Solute: Particles dissolved in the solvent (cannot move in osmosis if non-penetrating).

  • Example Calculations in Tonicity: Comparing mOsm to determine the tonicity of solutions to predict cell behavior.