Osmosis and Diffusion
Molecules are in constant motion. Molecules randomly collide and move to fill
the available space. The movement of molecules from higher to lower
concentration is called diffusion. Diffusion does not require an energy input.
Certain factors influence the rate of diffusion such as temperature, molecule size,
and concentration gradient.
Exercise 1: Simple Diffusion
We will begin by exploring the effect of molecular weight on diffusion rate through
1. Obtain a petri dish filled with agar (semi-solid).
2. Using a straw, punch out 3 circular wells in the agar. Space them evenly
in the plate (refer to diagram below).
3. Place one drop of the following in each of the wells taking care not to
overflow the well:
b. Methylene blue
4. Place the lid on the dish and label the three wells with a wax pencil.
5. Measure the distance traveled from the edge of the well and record in the
6. Make a line graph (molecular weight vs. distance traveled) using your
LAST distance measurement for the two known dyes.
7. Using this graph, locate the distance traveled by the unknown dye on the
x-axis. Follow this distance up until it hits the line on your graph. Once
you reach the line graph, draw a line horizontally to determine the
predicted molecular weight.
Dist. (mm) 15 min Dist. (mm) 30 min Dist. (mm) 45 min
Graph: Relationship between molecular weight and diffusion rate
Distance Traveled (mm)
1. Based on the graph, what is the molecular weight of the unknown dye?
2. What is the general relationship between molecular weight and
3. What other factors may have influenced diffusion rate in this
4. What causes diffusion to occur?
5. Does diffusion occur fastest in semi-solid, liquid or gas?
It occurs fastest in gas.
6. Diffusion rate increases with an increase in temperature. WHY?
Exercise 2: Animal Cell: Osmosis across an egg membrane
Water also moves across a membrane from high to low concentration. The
diffusion of water is called osmosis. Although some of the water crosses the
plasma membrane directly, most water molecules cross through small channels
in the membrane called aquaporins. This is a passive process as it does not
In this exercise we will be placing a chicken egg in a hypertonic (10% NaCl)
environment and in a hypotonic (DI water) environment. We will be measuring
water gain and loss through osmosis. The shell of these eggs has been removed
and the outermost membrane left intact. This membrane is permeable to water
but not to salt or other solutes inside the egg.
1. Obtain two eggs and carefully weigh them. Record the initial weight in
the table below.
2. Submerge one egg in 10% NaCl solution and the other in DI water.
3. Every 15 minutes, carefully remove the egg and remove any excess
moisture prior to weighing. Weigh the egg and record the weight in the
4. Return the eggs to their respective treatments for another 15 minutes.
Distilled Water 10% Salt Solution
Time Weight (grams) Time Weight (grams)
0 min 95.4 grams 0 min 82.4 grams
15 min 97.1 grams 15 min 80.2 grams
30 min 97.3 grams 30 min 79.4 grams
45 min 45 min
60 min 60 min
1. How does water usually cross the plasma membrane?
2. What types of molecules contribute to the solute concentration inside
3. Can the molecules in question #2 cross the membrane freely?
4. Refer to the data table. Describe in detail what is happening to the
eggs in each treatment.
Exercise 3: Plant Cell: Osmosis and elodea
In this exercise, we will observe a plant cell placed in hypotonic and hypertonic
conditions. Because of the cell wall, plant cells can gain water without lysing.
Plant cells placed in salt water (hypertonic conditions) will lose water causing the
plasma membrane to pull away from the cell wall. This process is called
plasmolysis. Plant cells need hypotonic conditions in order to take water in from
the soil. The pressure inside the cells, which keeps the stems and leaves from
wilting is called turgor pressure.
1. Obtain two glass slides and place one elodea leaf on each.
2. Place a drop of DI water on one leaf and cover with coverslip.
3. Place a drop of 10% NaCl solution on the other leaf and cover with a
4. Wait 5 minutes before observing each under low and high power.
5. Draw and label your observations below.
Elodea in DI Water Elodea in 10% NaCl
1. Which treatment is hypertonic when compared to the cell?
2. Which treatment is hypotonic when compared to the cell?
3. In the 10% NaCl solution, what is the solute? Solvent?
4. If the membrane were permeable to NaCl, how would your
observations be different? Why?
5. Which conditions do plant cells prefer?
6. What would happen to red blood cells placed in distilled water?
7. Why is an isotonic saline solution used in an IV instead of pure water?
8. What structure is present in plant cells, that is not found in animal cells,
that allows it to withstand hypotonic conditions?
9. You have some wilted celery in your refrigerator. Which of the
following should you soak it in to make it crispy again?
a. salt water
b. DI water
c. isotonic saline solution
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Que3. What other factors may have influenced diffusion rate in this experiment?
Ans: The other factors which affects the diffusion rate are temperature (by increasing temperature diffusion rate increases), medium of diffusion (dense is the medium lower is diffusion), concentration gradient (higher is concentration is lead to greater diffusion).
Que4. What causes diffusion to occur?
Diffusion occurs to achieve equilibrium. A difference in concentration gradients causes the diffusion. Concentration gradient is basically a difference in concentrations. Particles move from higher concentrated areas to lower concentrated area to achieve equilibrium. As long as there is a difference in the concentration gradient (or basically difference in concentration of two things/areas), diffusion will occur to "balance out" the sides.
Que5. Does diffusion occur fastest in semi-solid, liquid or gas?
Ans: Diffusion occurs fastest in gas then liquid and then semisolid....