1OSMOSISINTRODUCTIONOsmosis is the process of water movement across a concentration gradient. Cells needwater uptake, and a way to do so is through osmosis. Cell membranes are selectivelypermeable, only allowing certain molecules to pass through. If there is a concentration gradientbetween the cell and the solution, water will be able to move either into or out of the celldepending on the concentration gradient. Similar to diffusion, osmosis moves from a higherconcentration to a lower concentration. When the surrounding environment has greater solutesolution, water will move out of the cell causing it to shrink. This is called a hypertonic solution.Meanwhile, a greater solute concentration inside the cell would cause the water molecules tomove into the cell, creating a hypotonic solution. However, if both the surrounding environmentand the cell has equal solute concentration, then the amount of water molecules that flows out isequal to the amount of water molecules that goes into the cell. We call this an isotonic solution.Objective1.To demonstrate the process of osmosis2.To describe the effects of osmosis in plants cellsMaterials●Potato●Dialysis Tubing(cellulose membrane)●Thistle tube●Iron stand●500mL beaker●10% sucrose solution●Rubber band●Cork borer●Scalpel●2 petri dishes●Tap water●Distilled waterMethodsPhysical process of osmosis
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The cell membrane maintains the cell a separate entity; it holds the cell contents within, and acts as a barrier to the external environment. It is selectively permeable and has various mechanisms to allow for the exchange of gases and nutrients. These mechanisms allow for the intake of anything that is required and allows for the expulsion of waste and toxins. This membrane does not resemble a sheet or bag; rather, it is many molecules of Phospholipid Bilayers held together by the combined forces of attraction and repulsion. They are comprised of a Phosphate head; which is hydrophilic (water-loving), and a Lipid (fatty acid) tail which is hydrophobic (repelled by water). As the internal and external environments of a cell are aqueous, these molecules arrange themselves into two layers; one with the Phosphate heads oriented out into the external fluid, and the other with the heads oriented inwards into the internal fluid (the Cytoplasm). The Lipid tails are between the two layers of Phosphate heads; thereby, protected from the water, and the strength of this attraction/repulsion mechanism keeps the molecules together as though the membrane were a single entity. In this practical, dialysis tubing is used as a surrogate cell membrane for a visual demonstration of osmosis and diffusion. A solution containing large molecules (Starch) and small molecules (Glucose) is placed inside the tubing; which is then placed in a solution containing iodine. Students are able to observe as the solution inside the tubing turns dark blue, while the surrounding solution it is submerged in does not. From this, students can use their prior knowledge of the Starch-Iodine complex to surmise that Iodine is able to pass through the membrane while starch is not. The Glucose-testing strips indicate that glucose has been able to pass out of the tubing and into the external fluid. Thus proving the tubing allows movement in both directions. This inexpensive and simple experiment provides students with a clear visual result that effectively demonstrates how the size of a molecule can affect its ability to be transported into or out of a cell. It also illustrates the mechanics of diffusion and osmosis by which a cell will attempt to create homeostasis, or equilibrium between its inner and outer environments. PREPARATION - BY LAB TECHNICIAN
Glucose/ Starch Solution
Iodine Solution
Preparing the "cell" tubing
Observing changes in the “cell”
OBSERVATION AND RESULTSAfter fifteen minutes, the solution inside the tubing should begin to turn blue while the surrounding liquid remains yellow/brown. Starch molecules are too large to pass through the membrane; however, the Iodine molecules are small enough. This results in a Starch-Iodine complex that is confined to where the Starch is trapped - ie, inside the “cell”. Conversely, the Glucose molecules are free to pass through the membrane, and thus will begin to diffuse out in an attempt to equilibrate the Glucose concentration of the two solutions. Below is an example of possible results for the experiment.
The concentration of Glucose in this practical is quite high to enable shorter waiting times for students. This allows them to more readily measure the glucose which has diffused out of the “cell” using their test strips. However, this also means that the initial concentration is too high to show that the concentration inside the cell has decreased in line with the increase outside the cell. To manage this, students are asked to take a sample of the original combined Glucose/ Starch solution prior to being placed in the “cell” and also a sample of the now-blue solution inside the “cell” at the end of the prac. Both solutions are diluted by a factor of ten to bring the Glucose concentration into the range of the Uriscan strips.
To observe the process of cell diffusion and osmosis over an extended period of time, make an extra “cell” and keep it in solution until the next class. By the beginning of next class, the Glucose inside and outside the cell should have somewhat equalised. This could be conducted as a class demonstration, or each student may make an extra cell. Once again, dilute both solutions by a factor of ten prior to measuring. TEACHER TIPS:
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