passive transport lab
How is the relationship between solutes and solvents described?
The simplest method is the percent solution -- this refers to the number of grams of solute per 100 mL of solution. For example, a 5% glucose solution contains 5 g glucose per 100 mL solution. A 0.9% NaCl solution contains 0.9 g NaCl per 100 mL solution.
We also use molar solutions in lab. A one molar solution of any substance contains 1 mole of solute in 1 liter of water. One mole of a solute equals the molecular weight of the solute in grams. A one molar (1M) solution of glucose would contain 180 g glucose/liter of water. A one molar (1M) solution of NaCl would contain 58.5 g NaCl/liter of water. In terms of molarity, 1M glucose is equivalent to 1M NaCl.
Solute concentrations in the body fluids are relatively low, so their concentrations are measured in milligrams per 100 mL (mg/dL or mg%) or in millimoles (mM). One milligram is what fraction of one gram? ______________. One millimole is what fraction of one mole? _______________ If a 1 M solution contains 1 mole solute in 1 liter of water, a 1 mM solution would contain what fraction of a mole of solute in 1 liter of water? _______
What is the relationship between osmosis and osmolarity?
Osmosis is the diffusion of water through a selectively permeable membrane, and is caused by an unequal distribution of solutes on the two sides of the membrane. The solute is not able to cross the membrane (=nonpenetrating solute). Water is “drawn” towards the side of the membrane with the higher solute concentration. When the solute concentrations on the two sides of the membrane are equal, no osmosis occurs.
Osmolarity is a measure of concentration based on the number of particles per liter of solution. Osmolarity applies to solutes that can cross a membrane (=penetrating solutes) and solutes that cannot cross a membrane (=nonpenetrating solutes).
For a particular solute, the number of particles per liter of solution depends on whether the solute dissociates in water or not. Molecules like glucose do not dissociate in water, and only contribute one particle per molecule. NaCl does dissociate in water (into one Na+ and one Cl-), so there are two particles per molecule in solution. CaCl2 does dissociate in water (into one Ca+ and two Cl-), so there are three particles per molecule in solution.
If osmolarity (OsM) = # particles/mole X # moles/liter, then
OsM of 1 M glucose = 1 particle/mole X 1 mole/liter = 1 OsM, and
OsM of 1 M NaCl = 2 particles/mole X 1 mole/liter = 2 OsM
OsM of 1 M CaCl2 = 3 particles/mole X 1 mole/liter = 3 OsM
So, solutions can have the same molarity but different osmolarities.
For nonpenetrating solutes, the higher the osmolarity, the greater the osmotic pressure of the solution. The greater the osmotic pressure, the more water is pulled across the membrane.
What is tonicity?
Tonicity describes the effect of an extracellular solution on the volume of a cell and is determined by the relative concentration of nonpenetrating solute molecules. If the total concentration of nonpenetrating solutes inside a cell (ICF) is equal to the concentration of nonpenetrating solutes of a surrounding solution, the solution is called an isotonic solution (iso- means same). Some percent solutions that are isotonic to ICF are 0.9% NaCl and 5.0% glucose. A hypotonic solution has a lower concentration of nonpenetrating solutes than the ICF (hypo- means below or under), and a hypertonic solution has a higher concentration of nonpenetrating solutes than the ICF (hyper- means above or over).
What information can we learn by adding RBCs to a solution?
We can tell just by looking at suspensions of RBCs whether the cells are intact (normal or crenated) or hemolyzed. (You can’t tell if RBCs are normal or crenated without looking at them under a microscope.) A suspension of intact RBCs is cloudy, and you cannot see through it easily. A suspension of hemolyzed RBCs is red but transparent, and you can see through it. How do other factors affect the rate of diffusion of penetrating solutes?
Lipid soluble (nonpolar) molecules enter cells by diffusing through the lipid bilayer, but molecules that are not lipid soluble (polar) have to pass through channels. The rate of diffusion of nonpolar molecules depends on how soluble they are in lipids. The rate of passage of polar molecules depends on the size of the molecule (which is proportional to its molecular weight). In general, a nonpolar molecule would pass through a membrane faster than a polar molecule of the same size. Small nonpolar molecules would pass through a membrane faster than larger nonpolar molecules. Small polar molecules would pass through a membrane faster than larger polar molecules (if they can pass through). You would expect a small nonpolar molecule to pass through a membrane faster than a larger, polar molecule.