Cells and Heredity Lab
22 OCTOBER 2012
Membrane Permeability Decreases as Molecular Size Increases
Red blood cells are vital to organisms functioning properly. They are microscopic cells that carry oxygen from the lungs to all the tissues throughout the body. Upon transporting oxygen, red blood cells also exports waste, such as carbon dioxide, to the lungs where it can be expelled. Red blood cells are made up of hemoglobin which is surrounded by a cell membrane (Barrilleaux 2012).
Organisms also have white blood cells, also referred to as leukocytes, which combat foreign antibodies in the immune system. White blood cells are complex in structure, and in contrast to red blood cells, have a nucleus. They include such cells as lymphocytes, monocytes, eosinophils, neutrophils and basophils. While some cells such as lymphocytes make antibodies, others attack foreign objects, such as leukocytes, and others have several support jobs that help the immune system perform more efficiently. The immune system also consists of platelets. They are produced in the bone marrow of animals by megakaryocytes (bone marrow cells) which continuously go into the blood system and help clot blood (Barrilleaux 2012).
Cell membranes are composed of a phospholipid bilayer, making them hydrophobic. Membranes have many functions, most importantly holding the cytoplasm and organelles. Cell membranes often contain protein channels that allow substances to enter the cell (Bowe et al., 1997). Cell membranes are selectively permeable, meaning that some substances and chemicals can enter the cell, but not others. Most often, hydrophobicity and size determines permeability rates (Barrilleaux 2012).
If too much of a substance rushes into the cell, then they create an osmotic imbalance, meaning that the pressure inside the cell compared to outside the cell differs so much that the cell membrane bursts. This process is called hemolysis (Ivanov 1999).
Hemolysis is the process in which red blood cells are disrupted. The cells then release their cytoplasm and organelles. Since the cells are microscopic, we cannot view one cell undergoing hemolysis by the naked eye, however we can view a solution of them undergoing hemolysis without any specific equipment. However you can also view a specific number of cells using a phase contrast microscope, which will not only magnify the cells, but also shows depth and contrast (Barrilleaux 2012).
We can also measure hemolysis by a spectrophotometer. A spectrophotometer measures how much light is absorbed by the solution. If a solution is more turbid (cloudy) then it will have a higher absorbance.
Throughout this experiment, we wanted to test the membrane permeability of mammalian red blood cells by using hemolysis. We would view it under phase contrast microscopes, spectrophotometers and our eyes. We don’t know what the exact partition coefficients are yet of all the chemicals we will be testing. We will test the membrane permeability of 12 different chemicals, and our hypothesis is that they will differ by their molecular composition, structure, size and whether or not they are ionic.
Barrilleaux, A. (2012). Cells and Heredity Laboratory Manual. (pp. 90). New Orleans, LA: Loyola University.
Bowe, C.L., Mokhtarzadeh, L., Venkatesan, P., Babu, S., Axelrod, H.R., Sofia, M.J., Karkarla, R., Chan T.Y., Kim, J. W., Lee, H.J., Amidon, G.L. Choe, S.Y., Walker, S., Kahne, D. (1997). Design of Compounds that Increase the Absorption of Polar Molecules. Proceedings of the National Academy of Sciences of the United States of America, 94, 2218-12223.
Ivanov, I.T. (1999). Low pH-Induced hemolysis of erythrocytes is related to the entry of the acid into cytosole and oxidative stress on cellular membranes. Biochimica et Biophysica Acta-Biomembranes, 1415, 349-360.
Reece, J.B., Urry, L.A, Cain, M.L., Wasserman, S.A., Minorsky, P.V., Jackson, R.B. (2011). Membrane...