Diffusion Across a Sheep Red Blood Cell Membrane

Introduction

As important and powerful as the plasma membrane is, it makes up only a fraction of the total amount of membrane in a cell. Most animal cells, especially those that have specialized functions, are so packed with membranes that there may be 50 to 100 times as much membrane inside them as on their surface. (In plant cells the proportion of inner membrane is even greater, going up to 150 times the amount of membrane on their surface). Yet the plasma membrane is best known to scientists. (Tse, Lux, Samuel, 2-13) The main problem in studying membranes has been the difficulty of obtaining pure specimens. Until recently, this was possible only with the plasma membrane of the mammalian red blood cell, which has no nucleus. (Abramoff, Thomson, 109-121) When biochemists grind up cells and centrifuge their components, only fragments of the cells' membranes can be recovered usually in the form of small, closed vesicles or ragged pieces of unidentified origin. These can be sorted out and identified, so that by now the membranes of practically every organelle can be isolated, but they still vary in purity. Such brutal treatment is not necessary with the mammalian red blood cell, however. Since it contains mostly hemoglobin and has neither nucleus nor mitochondria, nor any internal membranes, it can be emptied with relative ease: It is simply induced to swell up until temporary, large pores open in its plasma membrane, letting the fluid and hemoglobin seep out. The plasma membrane is then allowed to shrink back to its normal size and the pores close, leaving researchers with an intact outer membrane or "ghost." Although other kinds of membranes can now be isolated by various means, the mammalian red blood cell "ghost" remains the favorite subject of many scientists who work on membranes.

Studying these membranes under an electron microscope proved frustrating, however. Since electrons can pass only through ultra-thin sections, the cell was usually sliced finely with a straight knife to prepare samples for the microscope. Yet cell membranes are far from straight, and it was practically impossible to get a fullface view of a membrane in this fashion. Nor could one learn much about the membrane's inner layer.

Material and Methods

Laboratories in Animal Physiology are divided into five investigative laboratory blocks consisting of 2-3 weeks per block. These blocks include: (1) cell membrane physiology; (2) neurophysiology; (3) muscle physiology; (4) exercise physiology and human electrophysiology; and (5) hormonal physiology. The introduction to techniques and preliminary collection of data occurs during the introductory laboratory within each laboratory block. During the subsequent investigative laboratory session(s), student research teams (composed of three to four students) use the acquired techniques to design, perform, and analyze an investigative project and then report results to other research teams. The sequence of presentation of physiological techniques is designed to be cumulative, so that the techniques acquired in the first laboratory block are assumed in subsequent laboratory blocks. This cell membrane physiology laboratory was originally designed ...