You design an experiment to test the effect(s) various compounds have on the osmotic potential of a model cell. You know that substances dissolved in aqueous or gaseous solutions tend to diffuse from regions of higher concentration to regions of lower concentration.
You fill each of three (20mL) dialysis bags half full with one of these substances:
i. 5% by weight of glucose in distilled water
ii. 5% by weight of egg albumin (protein) in distilled water
iii. 5% by weight of glass bead (one glass bead) in distilled water
The dialysis bag is permeable to water but impermeable to glucose, albumin, and glass bead.
a) If the final weight of each prepared bag is 10g, how many grams of glucose, albumin, and glass bead were added to each bag?
b) The molecular weight of the protein is about 45 kilodaltons, and the molecular weight of glucose is about 180 daltons. How can you estimate the number of molecules of glucose in its 5% solution compared to the number of albumin molecules in its 5% solution?
c) You put the dialysis bags into three separate flasks filled with distilled water. After 2 hours, you remove the bags and record these weights:
Dialysis Bag Weight
Glucose 13.2g
Albumin 10.1g
Glass bead 10.0g
How do you explain these results? (hint: consider the surface-area-to-volume ratio, or concentration, of each substance based on its molecular weight)
d) What results would you predict if you set up a similar experiment but used 5% glucose and 5% sucrose (MW 342 daltons)?
II. How is the structure of a cell membrane related to its function?
1. Substances can move across the membrane via simple diffusion, facilitated diffusion, or active transport. Fill in the table below with information about each process.
a) Where does it occur in the membrane? b) Does it require a transport protein? c) Does it require an input of energy?
Simple Diffusion
Facilitated Diffusion
Active Transport
d) What functions might each of the three types of diffusion serve in an independent cell, such as Paramecium or an amoeba?
e) What functions might each of the three types of diffusion serve in a multicellular organism, such as a human or a tree?
2. What would you need to observe or measure to determine whether a substance was moved across a membrane via each type of diffusion? Fill in the table below.
Simple Diffusion Facilitated Diffusion Active Transport
3. The ratios of saturated to unsaturated phospholipids in an organism’s membranes can change in response to changes in environmental conditions.
a) How do the properties of a membrane that contains a low percentage of unsaturated phospholipids compare with those of a membrane that contains a high percentage of unsaturated phospholipids?
b) Considering what you know about the properties of saturated and unsaturated fatty acids, would you expect an amoeba that lives in a pond in a cold northern climate to have a higher or lower percentage of saturated fatty acids in its membranes during the summer as compared to the winter? Explain your answer.
4. A fish is removed from a contaminated lake. You determine that a particular toxin (X) is present in its cells at concentration X = 1,500 μg/L. You place the fish in a tank full of clean water (X = 0 μg/L), and measure the toxin concentration in the fish cells each day for the next 10 days.
a) How would you expect graphs of toxin concentration (in μg/L) in the fish and in the water to change over the 10 day period? (ex: concentration would start around 1,500 μg/L, then gradually decrease to 0, or toxin concentration would start and remain at 0, etc.) Describe (in as much detail as you can) what you would expect each graph of toxin concentration to look like:
i. In the fish if the toxin is water soluble?
ii. In the water if the toxin is water soluble?
iii. In the fish if the toxin is fat soluble?
iv. In the water if the toxin is fat soluble?
b) After making your hypothesis, you test it by measuring the toxin levels in the fish at various times during its 10 days in the tank. You observe that the level of toxin in the fish drops from 1,500 μg/L to 750 μg/L and then stabilizes at 750 μg/L. You test the water in the tank and find that after it stabilizes, toxin is present in the water at concentration 750 μg/L also.
i. Which of your predictions fits these data?
ii. Which of the following processes is most likely eliminating the toxin from the fish?
(Delete the incorrect answers)
-Passive transport
-First active, then passive transport
– First passive, then active transport
– Active transport
c) Given the situation in part b), what should you do, in the short term, to continue to reduce the toxin level in the fish below 750 μg/L?
