LAB 4 OSMOSIS AND DIFFUSION

OBJECTIVES SELECTIVE PERMEABILITY AND OSMOSIS CHARACTERISTICS THAT AFFECT MEMBRANE PERMEABILITY OF A SUBSTANCE

SELECTIVE PERMEABILITY AND OSMOSIS Molarity = # of moles of solute / 1 liter of solution (mass of one mole of a substance = the molecular weight of the substance in grams) Osmolarity = # of osmoles of solute / 1 liter of solution (the # of osmoles = the # of moles times the number of particles that the substance breaks into when put into solution) Osmolality = # of osmoles of solute / 1 liter of water

DIRECTION OF TRANSPORT Factors affecting movement across membranes: - Chemical driving force (concentration difference) - Electrical driving force (membrane potential Vm = -70mV)

TONICITY Tonicity describes the ability of solutions to pull water across a membrane, and is determined by the concentration of impermeant solutes (solutes that do not cross the plasma membrane). The impermeant solutes can balance the impermeant solutes inside the cell. Hypotonic: lower concentration of impermeant solutes than the ICF Hypertonic: higher concentration of impermeant solutes than the ICF Isotonic: same concentration of impermeant solutes as the ICF

TONICITY A solution can also contain permeant solutes. Permeant solutes can cross the plasma membrane. They may initially cause water flow into or out of the cell, but the concentration of permeant solutes can equilibrate. Eventually, the concentration of permeant solutes will be the same inside and outside the cell. ICF total osmolarity is 320 mOsm

TONICITY Tonicity is relative measure, comparing the concentration of impermeant solutes in the intracellular fluid (ICF) to the concentration of impermeant solutes in the fluid surrounding the cell, that is, the extracellular fluid (ECF) ICF = intracellular fluid (compartment) Fluid within cells ECF = extracellular fluid Fluid outside cells but within body Is the “internal environment” Subdivisions Plasma: fluid around blood cells Tissue fluid: fluid surrounding other cells called interstitial fluid (ISF)

ECF AND ICF ECF: ISF or Plasma ICF Na+ Mg2+ K+ PO42- HCO3- Cl- Glucose Ca2+ K+ PO42- ATP Amino Acids Proteins ECF: ISF or Plasma ICF

DIALYSIS AND OSMOSIS Dialysis: the diffusion of “solutes” across a semi-permeable membrane. Osmosis: the diffusion of “water” across a semi-permeable membrane. Factors that determine rate of osmosis: 1. the osmolality of the two solutions separated by the membrane 2. the permeability of the membrane to the solutes involved

RED BLOOD CELLS ACTING AS OSMOMETERS The hemolysis or crenation of the red blood cells are used to determined the tonicity of the solution: Red blood cells will hemolyze (swell/burst) ----- Hypotonic solution Red blood cells will crenate (shrink) ------- Hypertonic solution Red blood cells will remain the same ------ Isotonic solution

DIALYSIS TUBING The direction and rate of osmosis will allow you determine if the solution in the thistle tube is hypotonic, isotonic, or hypertonic to the solution in the beaker. Using Excel you will construct a graph showing time vs distance moved and analyze data

CHARACTERISTICS THAT AFFECT MEMBRANE PERMEABILITY OF A SUBSTANCE Some factors that can affect rate of diffusion: - Molecular size --- the lower molecular weight, the faster it will diffuse across the cell membrane - Lipid solubility --- the more soluble in lipids, the easier and faster it will diffuse across the cell membrane. Alcohols are lipid soluble. * The degree of lipid solubility is more important than the size of the alcohol molecule in determining whether the molecule can diffuse across the plasma membrane. Larger alcohols are more lipid soluble than smaller alcohols, they actually cross more easily than smaller alcohols.