Movement across cell membranes
A selectively permeable barrier is one of the defining features of a living cell. The cell membrane and the associated transport proteins found in the membrane are responsible for regulating the movement of thousands of different types of molecules into and out of the cell. All molecular motion is influenced by diffusion, the tendency for particles to spread from higher to lower concentrations until they are evenly distributed. Smaller molecules diffuse more rapidly than larger ones. If the molecules are separated into compartments, diffusion will be restricted by the ability of the molecules to move across the barrier. A selectively permeable barrier (e.g., a cell membrane) will allow some molecules to move through but not others. In this series of experiments, we will investigate the properties of various types of molecules as they relate to membrane permeability.
The effect of molecular size on membrane permeability
You will place red blood cells into various solutions of alcohols of increasing molecular size. The solutions initially are isoosmotic to the cells, i.e., the solute concentration in the solution is the same as that of the cells. If the cell membrane is permeable to alcohol, the alcohol molecules will diffuse through the membrane because of the concentration gradient. The cells will expand and eventually lyse.
The solutions are prepared from alcohols of increasing molecular size: methanol (CHO), glycerol 4(CHO) and mannitol (CHO). Smaller molecules will diffuse faster than larger ones, so lysis will occur 3836146more quickly. If the membrane is not permeable to the alcohol, the cells will not lyse because the solutions are isoosmotic.
The effect of molecular polarity on membrane permeability
Polarity is a chemical property that affects the solubility of molecules. Water soluble molecules are polar in that they have charged reactive groups that interact with water molecules, which are themselves polar. In contrast, lipids are nonpolar; the lipids of the cell membranes serve as a barrier to the passage of polar molecules through the cell membrane. Nonpolar molecules, as well as small polar molecules, may pass through pores in the lipid bilayer, but larger polar molecules, such as proteins and polysaccharides, are restricted by the pore size.
Molecules with –OH groups, like alcohols, tend to be polar because of the electronegative
properties of oxygen. The number of –OH groups on a molecule affects the degree of polarity a molecule
will exhibit. Note the structures of isopropanol and glycerol. Each has a backbone of three carbon atoms and is about the same overall size. Isopropanol has one –OH group while glycerol has three. Which is the
more polar molecule? To which molecule will the membrane be LESS permeable?
Methods and materials
; Six clean test tubes and rack
; 0.3 M solutions of methanol; glycerol; isopropanol; and D-mannitol
; Isoosmotic solutions of 0.3 M D-glucose and 0.15 M NaCl
; Sheep blood in saline. Cells must be mixed gently before use.
Which alcohol solution will cause the most rapid lysis of cells?
Mannitol and glucose are of similar size and polarity. Do you expect similar hemolysis times? Why or why not?
Solute # carbon # -OH Molecular Molarity of Osmolality g/dL
molecule atoms (polar) weight solution
NaCl 58.4 150 mM
Methanol 1 1 32.0 300 mM
Glycerol 3 3 92.1 300 mM
2-Propanol 3 1 60.1 300 mM
Mannitol 6 6 182.2 300 mM
glucose 6 6 180.2 300 mM
Procedure- have data table ready
1. Pipet 5 ml of each solution into a separate tube.
2. Mix the sheep blood gently before taking samples. Then add two drops of blood to each tube,
immediately cap and invert to mix, and record the time the blood was added in the data table.
(minutes and seconds).
3. The tubes will be turbid, and as lysis occurs, the solution in the tube will become transparent.
Record the time at which the tube becomes transparent in the appropriate column.
4. Calculate the hemolysis time by subtracting the time the blood was added from the time the tube
became transparent and record the number of minutes and seconds.
Tube number Solute molecule Time blood was Time solution Hemolysis time
added became transparent (minutes: seconds)
1. How do propyl alcohol and glycerol differ from each other
2. Why were the solutions made isoosmotic?
3. Why does the blood solution become clear?
4. How do ethylene glycol, glycerol and ribose differ from each other?
5. Based on the data, how does a molecule’s size affect its ability to cross a cell membrane?
6. Based on the data, how does a molecule’s polarity affect its ability to cross a cell membrane?
7. Is every isoosmotic solution isotonic? Is every isotonic solution isoosmotic? Explain.