Diffusion and Osmosis with Tubing Workseet Lab directions below Question: 1) Was
ID: 166050 • Letter: D
Question
Diffusion and Osmosis with Tubing Workseet Lab directions below Question:
1) Was starch able to diffuse out of the sac? _____ How can you tell, what tests confirmed your findings?
A) Was iodine able to diffuse into the sac? _____ How can you tell, what tests confirmed your findings??
B) If iodine was able to diffuse into the sac, why did it diffuse into the sac?
C) Was glucose able to diffuse out of the sac? _____ How can you tell, what tests confirmed your findings??
D) If glucose was able to diffuse out of the sac provide 2 reasons why did it was able diffuse out?
E) What would allow the starch to diffuse out of the sac, what would you have to alter?
F) What are the two types of gradients that effect the diffusion of a substance across a cell membrane?
G) Besides the types of gradients what are 4 other factors, based from lecture notes, which will also effect the diffusionof something across a cell membrane? Do not include temperature and size in your answer.
H) Generally, what types of substances, if small enough, can easily diffuse through a cell’s membrane?
I) Generally, what types of substances, no matter the size, cannot easily diffuse through a cell’s membrane?
Molecules of liquids and gases are in constant motion, despite the fact that our illustrations have to show them asstationary. This motion is random—individual molecules move in straight-line paths until they bump into othermolecules and then bounce off, going in another direction. However, the net movement of many molecules of thesame type is directional. They go from areas where they are in higher concentration to areas where they are in lowerconcentration. This is called diffusion. To visualize this, think of a billiards or pool table, with a collection of ballsconcentrated in the center. Now imagine that the table begins to shake gently. An individual ball could move in anydirection. However, when the balls hit each other, they will bounce off and roll away from the other balls. When aball rolls away from the center of the table, toward an area of lower ball concentration, it will travel further in thatdirection, because its chance of bumping into others is reduced. Even though some balls may move toward the centerof the table, on average, more balls will end up traveling toward the outside where the concentration is lower.Eventually, the balls will end up spread evenly over the surface of the table.Membranes can act as barriers that block the diffusion of certain molecules. In the example above, the table edge actsas a barrier to prevent the billiard balls from spreading out further into the room. Selectively permeable membranesallow some substances to diffuse across them, but block the movement of others. A cell membrane is selectivelypermeable. Some substances can pass through the phospholipid bilayer itself. Others cannot, but proteins embeddedin the membrane facilitate their passage. If a substance cannot cross the phospholipid bilayer, travel through atransport protein or channel then the substance will not be able to diffuse into the cell. Artificial membranes can alsobe selectively permeable. For example, cellulose tubing is filled with numerous tiny pores. Thus it allows thediffusion of small molecules, but not those that are larger than the diameter of the pores. This concept applies to thetreatment of patients whose kidneys are not functioning properly. When their blood passes through the tubing in adialysis machine, urea, sulfate, and other small molecules that need to be excreted from the body cross the artificialmembrane and can be removed, while the blood cells remain and can be returned to the body.
Molecular size and diffusion through a selectively permeable membrane
In this exercise, we will test whether or not several different molecules are small enough to pass through the poresfound in cellulose tubing. If the molecules are too large, they will not be able to diffuse through it. Two of thesubstances involved in this experiment are starch and glucose, so we will use both the iodine test for starch and theBenedict’s test for reducing sugars to determine the presence of starch and glucose in a solution.
Supplies:
hot plate and 250 mL beaker, for water bath 20% glucose cellulose tubing, pre-soaked 1% soluble starch test tube “tongs” scale green plastic clamps iodine solution large beaker Benedict’s solution several standard test tubes distilled water
1. Obtain a pre-soaked piece of cellulose tubing, 15-20 cm long. Clamp one end of the tubing to form a sac.2. Fill the sac one-fourth full with starch solution. Then add an equal volume of glucose solution so that the tube isnow about ½ full.3. Close off the open end with a green clamp as demonstrated in lab.4. Once both ends are sealed, rinse the sac gently under running water. Then gently blot the outside of the sac withpaper towels so that it is no longer wet. Weigh the sac on your scale 3 times and take the average as your startingweight.
5. Insert the sac into the large beaker and add enough iodine solution to just cover the sac.
6. Examine the set-up and record any color changes that occurred in the solution or in the sac.7. Allow the sac to remain submerged for a minimum of 20 – 30 minutes.8. Remove the sac from the beaker, blot it dry, and weigh it again 3 times and take the average for your endingweight.9. Record your observations from the cellulose sac – How, if at all, did the sac change?10. Test the solution from the beaker (outside of the sac) for glucose by performing the Benedict’s test.
Explanation / Answer
1.No . starch is not able to diffuse out of the sac as the molecules of starch are large to pass through the membrane(semipermeable).This is confirmed by the iodine test when there is no change in the color of iodine (negative with iodine solution) in the beaker.
A.Yes . Iodine is able to diffuse into the sac as the molecules of iodine are much smaller (mol. weight. less than 100 for iodine) and hence can easily diffuse across the membrane.confirmed by iodine test It gives blue color.
B. Iodine molecules are smaller and can diffuse into the sac through the membrane .Concentration of iodine m,ore in the beaker(hyper tonic).
C. Yes glucose is able to diffuse out of the sac as the molecules are smaller in size and concentration of glucose more in sac than in beaker. Tests positive with benedicts reagent.
D.Glucose is able to diffuse out of the sac mainly because of concentration gradient and size of the glucose molecules(smaller) and hence can diffuse.
E.To facilitate the movement of starch it should be degraded into smaller molecules like glucose which can be done by adding an enzyme amylase.