Scientific Paper on Membranes

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The Semipermeability of Cell Membrane to Different
Osmotic Environments Using Dialyzing Bag Model1

Sittie Johaynnah M. Sambarani
Group 3 Sec. I-1L

December 17, 2012

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1 A scientific paper in partial fulfillment of the requirements in Biology 10 Laboratory under Professor Junaldo Mantiquilla, 2nd sem., 2012-2 ABSTRACT

The semipermeability of the cell membrane was determined using dialyzing bag models which were exposed into different osmotic conditions. In order to observe the response of animal and plant cells into these conditions, red blood cells (RBCs) and Hydrilla leaf were used and were exposed into different concentrations. Both cells exhibited hypertonic and hypotonic solutions when exposed to high and low concentrations respectively, while exhibiting isotonic when concentrations are equal. Three experiments using dialyzing bags were conducted. The first experiments used three set-ups. The first set-up was a dialyzing bag filled with saturated NaCl solution immersed in distilled water. The second set-up was a dialyzing bag filled with water immersed in saturated NaCl solution, and the last set-up was a dialyzing set-up was dialyzing bag filled with saturated NaCl immersed in saturated NaCl solution. The set-ups were weighed every five minutes for fifty minutes. The next experiment used gelatin granules and distilled water to test if macromolecules could pass through a semipermeable membrane. The change of weight was observed. The last experiment used dialyzing bag filled with NaCl-methylene blue solution. As the Silver nitrate (AgNO3) was dropped, clouds were formed first, proving that salt reacted first than Methylene blue. As observed on the changes of the weight of the dialyzing bags and also on the reactions of plant and animal cells, it was then concluded that cell membranes are semipermeable, allowing water molecules and even macromolecules to pass from regions of lower concentration to regions of higher concentration (osmosis).

INTRODUCTION
A semipermeable membrane is a membrane which allows the passage of solvent molecules but blocks the passage of soluble molecules. The cell membrane is semipermeable. Osmosis is the selective passage of solvent molecules through a porous membrane form regions of lower concentration to a more concentrated one until equilibrium is achieved. The osmotic pressure of a solution is the pressure required to stop osmosis. Relatively little is known about how the semipermeable membrane stops some molecules yet allows others to pass. In some cases, it is simply a matter of size. A semipermeable membrane ay have pores small enough to let only the solvent molecules pass through. (Chang, 2012).

The selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane. The diffusion of substance across a biological membrane is called passive transport because the cell does not have to expand energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion. However, membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. Some adaptations such as osmoregulation help prevent unicellular organisms living in hypotonic environments to burst in order to survive. Osmoregulation is the control of water balance. One example is the paramecium, its contractile vacuole is an evolutionary adaptation for osmoregulation. This kind of vacuole is an organelle that functions as bile pump to force water out of the cell as fast as it enters by osmosis. Paramecium has a plasma membrane that is much less permeable to water than the membranes of most other cells, this slows the uptake of...
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