A Differentially Permeable Plasma Membrane - Lab Report Example

One of the more distinguishable features of cells is their semi-permeable plasma membrane, which increases efficiency of chemical processesand protecting it against other non-essential molecules. Because of this plasma membrane, water also moves dynamically into and out of the cell, depending on the concentration of the extracellular fluids. In this experiment, this condition was replicated using unshelled eggs as the cell and sucrose solutions as the extracellular fluid. It was observed that there are concentrations of sucrose which causes uptake or release of water, while there is a solution that did not change the amount of water inside the semi-permeable membrane. Because of these changes that extracellular conditions may incur, animals such as humans have in them mechanisms like kidneys by which the interstitial fluid which bathe the cells remain isotonic to the cytoplasm. Introduction A differentially permeable plasma membrane is an important feature of cells as it encloses all the organelles to increase the concentration of reactants (by decreasing the volume) available for chemical processes specific to the organelle. In addition, it protects the cell from the constantly changing external milieu. The unregulated entry of big and ionic molecules is prevented by the hydrophobic interior of the membrane. Impermeable molecules that are nonetheless essential are transported through proteins embedded in the membrane. The most common example is water, which passes through the transmembrane aquaporins. The water molecule is an 18 g/mol molecule, which is small compared to a 32 g/mol O2 that can pass through the membrane much more freely than water. So how come water molecules still need aquaporins to be able to pass through the amphiphilic plasma membrane? Despite their neutral charge, water molecules are polar molecules which have a transient negative on the O side and a transient positive in between the two H molecules. This polarity makes them attractive to other polar solutes, producing a transient solute-water binding that decreases the thermodynamic activity (or movement). The more impermeable solutes are present the less is its activity. Because energy spontaneously flow from high to low thermodynamic activity, water movement, or osmosis, should go from a low (hypotonic solution) to high concentration (hypertonic solution) of solutes. If no osmosis was observed, the cytoplasm is said to be isotonic to the surrounding solution. Osmosis happening in cells was replicated in this experiment, with unshelled eggs representing the cells with differentially permeable plasma membrane. The objective of this experiment was to observe the effects of changing the extracellular concentration s to the amount of water within the cell. Methods Six unshelled eggs of predetermined weight were subsequently submerged in 0%, 10%, 20%, 30%, 40%, and x % sucrose for 15 min. The sucrose was the choice for the non-permeating solution because of its bulk and charge (glucose dimer). The eggs were removed from the solution, wiped dry, and weighed again. The eggs were placed back into their respective solutions, and the process was repeated until three more weight recordings were made. Data was recorded in p. 2107 E-4 of the workbook. The cumulative weight changes during the six 15 min. intervals were plotted per sucrose solution. This was done to determine which solutions were hypertonic, hypotonic or isotonic with respect to the contents of the egg. In addition, the graph can give insights as to the concentration of impermeable solutes inside the egg. Lastly, the differences in total weight change caused by the increasing sucrose concentrations were plotted. The relation that can be derived from the plot was used to determine the concentration of sucrose solution into which the 6th egg was submerged. Results A cumulative weight change over time was plotted for each egg and sucrose concentration (Figure 1). A linear correlation (R > 0.95) between length of immersion and weight change was found in all set-ups, with eggs in 0% and 10% sucrose gaining weight steadily, while those in 30% and 40% sucrose lost weight. On the other hand, the egg in 20% sucrose did not change in weight throughout the experiment. The change in weight of the egg in the unknown concentration of sucrose was also increasing, with its slope (m = 0.0332) closer to that of 10% sucrose (m = 0.0506) than to the plot of 20% sucrose (m = 0). These findings were reinforced when the resulting change in weight of unshelled eggs after 60 min. of immersion versus different concentrations of sucrose was plotted (Figure 2). To determine the concentration of the unknown, the equation of this plot was used. The egg gained 2.04 g after immersion in the unknown concentration of sucrose. Taking that value as y, the x (sucrose concentration) is computed to be 15%, which is as predicted by the plots in Figure 1. Discussion In this experiment it was obvious how dynamic a solution bounded by a semi-permeable membrane can be. Because the unshelled eggs in 0%, 10%, and 15% sucrose steadily gained weight throughout the experiment, their contents are hypertonic or had a higher impermeable solute concentration with respect to their surrounding solution. Because the extracellular solution had more thermodynamically active water molecules than inside the egg, they were able to rush into the egg, causing an increase in weight. In contrast, the eggs in 30% and 40% sucrose decreased in weight because the contents of the egg had less impermeable solutes as compared to the sucrose solution, causing water to rush out. If a 50% sucrose was added as a set-up, it can be predicted that the amount of weight loss would be greater than that observed in the egg submerged in 40% sucrose. When the solution bounded by a semi-permeable membrane is isotonic to that on the other side of the membrane, no water transport can be observed because the thermodynamics of water molecules is equal between the membrane. Similar things happen in the cells. When the surroundings become hypertonic, water rush out of the cells and causes cell shrinkage. When the surroundings become hypotonic, water rushes in, causing cells to become flaccid. This is the reason why animals such as humans have regulatory mechanisms such as the kidneys to regulate the solution of interstitial fluid that bathe their cells. Conclusion As observed from the experiment, changes in the condition of external surroundings of a solution bounded by a semi-permeable membrane (ex. cell) affect the interior solution. Hypertonic extracellular solutions cause cell shrinkage, hypotonic ones result to cell flaccidity. Isotonic solutions do not cause water movement, and thus maintain the concentration of both intracellular and extracellular concentration References Campbell, NA and Reece, JB 2002, Biology, Fourth edition, Benjamin Cummings, San Francisco. Read More