5.2.1 Study Notes Passive Transport

A. Passive Transport

• Diffusion is the movement of molecules from higher to lower concentration (i.e., “down” the concentration gradient)

  • A solution contains a solute, usually a solid, and a solvent, usually a liquid

    • In the case of a dye diffusing in water, the dye is a solute and water is the solvent

    • Once a solute is evenly distributed, random movement continues but with no net change

  • Membrane chemical and physical properties allow only a few types of molecules to cross by diffusion

    • Gases readily diffuse through the lipid bilayer

    • e.g., the movement of oxygen from air sacs (alveoli) to the blood in lung capillaries depends on the concentration of oxygen in alveoli

    • Temperature, pressure, electrical currents, and molecular size influence the rate of diffusion. 

B. Osmosis

• the diffusion of water across a differentially (selectively) permeable membrane

• Osmosis is illustrated by the thistle tube example:

  • A differentially permeable membrane separates two solutions.

  • The beaker has more water (lower percentage of solute) and the thistle tube has less water (higher percentage of solute).

  • The membrane does not permit passage of the solute; water enters but the solute does not exit the membrane.

  • The membrane permits passage of water with a net movement of water from the beaker to the inside of the thistle tube.

• Osmotic pressure is the pressure that develops in such a system due to osmosis

  • results in water being absorbed by the kidneys and water being taken up from tissue fluid.

• Tonicity is strength of a solution with respect to osmotic pressure

  • isotonic solutions occur where the relative solute concentrations of two solutions are equal

    • a 0.9% salt solution is used in injections because it is isotonic to red blood cells (RBCs)

  • hypotonic solution has a solute concentration that is less than another solution

    • when a cell is placed in a hypotonic solution, water enters the cell and it may undergo cytolysis (“cell bursting”)

    • swelling of a plant cell in a hypotonic solution creates turgor pressure

      • this is how plants maintain an erect position

  • hypertonic solution has a solute concentration that is higher than another solution

    • when a cell is placed in a hypertonic solution, it shrivels (a condition called crenation)

    • plasmolysis is shrinking of the cytoplasm due to osmosis in a hypertonic solution

      • as the central vacuole loses water, the plasma membrane pulls away from the cell wall

C. Facilitated Transport

• the transport of a specific solute “down” or “with” its concentration gradient (from high to low)

• use of a carrier protein

  •  glucose and amino acids move across the membrane in this way