Dialysis and Osmosis
Beitz 1
Introduction: The cell membrane is not a solid structure. It is made of millions of smaller molecules that create a flexible container. Proteins and phospholipids make up most of the membrane structure. “The proteins are found around the holes and help move molecules in and out of the cell. There are also proteins attached to the inner and outer surfaces of the membrane” (BiologyKids, 15). There are receptor proteins which receive specific molecules, and there are transport proteins. There are two types of transport proteins which only allow specific molecules to enter and exits, therefore selectively permeable. Releasing chemicals is called Exocytosis, and taking in chemicals is endocytosis.
“Scientists use the fluid mosaic model to describe the organization of phospholipids and proteins. The model shows you that phospholipid molecules are shaped with a head and a tail region. The head section of the molecule likes water (hydrophilic) while the tail does not (hydrophobic). Because the tails want to avoid water, they tend to stick to each other and let the heads face the watery (aqueous) areas inside and outside of the cell” (BiologyKids, 15). The two surfaces of molecules create the lipid bilayer. The interior of the lipid bilayer is “oily” and flexible like a soap bubble.
Osmosis is the diffusion of water across a selectively permeable membrane, which is a barrier that has holes large enough to allow water molecules to pass but small enough to block certain molecules like salt or glucose. “Water with solutes (substance dissolved in water) diffuses from an area of lower concentration of solutes to an area of higher concentration until the concentrations on both sides of the membrane are equal” (DiffsionBio, 15). Osmosis is of great importance in biological processes where the
Beitz 2 solvent is water. The transport of water and other molecules across biological membranes is essential to many processes in living organisms.
Dialysis is the movement of solutes. “It is also the separation of smaller molecules from larger molecules by using a differential permeable membrane (like the semi-permeable membrane for osmosis). This membrane has holes that allow smaller molecules to pass while blocking larger molecules in a solution” (WebMD, 15).
A simple solution is two substances that are evenly mixed together. One of them is called the solute and the other is the solvent. A solute is the substance to be dissolved (sugar). The solvent is the one doing the dissolving (water). There are 3 types of solutions; 1. Hypotonic Solution, 2. Hypertonic Solution, 3. Isotonic Solution. With a Hypotonic (low) solution the cell will gain water, therefore it will expand and could burst (lyse). However with a Hypertonic (high) Solution the cell will lose water, therefore it will shrivel. Luckily there is also Isotonic (same/equal) where the cell will gain and lose water at the same rate, therefore it will not gain or lose water overall!
Purpose: to observe the process of dialysis and osmosis.
Apparatus & Materials: 1. 3 Beakers (250ml) 6. 2 Cover Glasses 11. 1 Egg 16. Benedict’s Solution 2. 1 Graduated Cylinder 7. 1 Large Beaker 12. Scale (g) 17. Iodine 3. 2 Test Tubes 8. Water 13. Test Tube Holder 18. Hot Plate 4. Scissors 9. Salt 14. Starch 19. Knife
Beitz 3 5. Tweezers 10. 1 Potato 15. Glucose 20. Scoopula
Procedure: Part a – Dialysis 1. Remove top portion (smaller side) of the egg and discard the contents into the garbage. 2. Remove bottom portion (larger side) and expose the membrane. 3. Fill half the egg with 5% Starch and 5% Glucose. 4. Place the egg in a beaker containing 50ml of water. 5. Let it sit overnight. 6. Test solution to see if any molecules of glucose and starch passed through the membrane. 7. Take two 10ml samples from the beaker and perform the tests (Benedict’s solution & Iodine test). 8. Fill the dropper with Benedict’s solution and put it in the test tube. Stir, and the put the test tube in a beaker with boiling water that is sitting on a hot plate. 9. Record your results.
Part b – Osmosis 1. Cut your potato in half. Then cut two 2cm slices with identical sides with no peel left. 2. Take the mass of each potato and record it. 3. Place potato slice A in one beaker and slice B in a separate beaker. Add enough water to completely cover the potatoes. In beaker B add 1g of salt. Stir well. 4. Let both beakers sit overnight. 5. Take the mass of both potatoes and record the results.
Beitz 4
Results:
Table 1: Dialysis | Observations | Glucose Test: (+) | There was some glucose present. There was a little bit of change in colour (pink). As well you could see the solute and the bottom of the test tube. | Starch Test: (-) | There was no starch present. Starch is insoluble. Did not turn purple. |

Table 2: Osmosis | Beaker A: | Beaker B (Salt): | Before (g) | After (g) | Before (g) | After (g) | 19.7g | 22.1g | 19.6g | 17.2g |

Discussion: In part A the process of Dialysis was visible. During this lab the egg containing the 5% glucose and 5% starch somewhat allowed the process of dialysis. The egg membrane, with
Beitz 5 its selective permeable structure, allowed the movement of the solutes to travel through the membrane and into the beaker of 50ml of water. With the samples of 10ml of the substance either containing glucose or starch, the first test was performed; Benedict’s Solution. The Benedict’s solution is a blue substance which turns any substance containing glucose yellow/orange. The procedure in which this is possible is that you must heat up the sample containing a full dropper of the Benedict’s solution in a hot water bath and then wait. Over time the substance will either change colours, meaning there is glucose present, or, it will stay the same colour meaning there is no glucose present. The second test performed was to see if there was starch present. This test is called the Iodine test. The procedure in which this is possible is that the sample of 10ml of the substance either containing starch or not, will be mixed with 3 drops of Iodine. Mix well and then sit and wait for any reaction or change to occur, if any. This test really highlighted the process of dialysis because there was a clear movement of solutes through the membrane, resulting in a great and successful experiment.
Secondly, in part B the process of Osmosis was visible. During this lab the two potatoes were great examples of the movement of water. With the first potato, sitting in a beaker submerged in water (with no salt), the potato overnight absorbed most of the water and expanded, which is an example of a Hypotonic Solution. Furthermore with the second potato, it sat in a beaker submerged in water which contained 1g of salt. Overnight, the potato absorbed some of the water which contained salt (but also lost some water), then caused the potato to shrink, which is an example of a Hypertonic Solution.

http://www.ehow.com/how-does_5558199_osmosis-differ-dialysis.html http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/diffus.html http://www.webmd.com/a-to-z-guides/kidney-dialysis
http://www.biology4kids.com/files/cell_membrane.html