Biology
Standard: SB1d
Movement Across Cell Membranes
A selectively permeable barrier is one of the defining features of a living cell. The cell membrane and the
associated transport proteins found in the membrane are responsible for regulating the movement of
hundreds, if not thousands, of different types of molecules into and out of the cell. All molecular motion
is influenced by diffusion, which is the tendency for particles to spread from higher concentrations to
lower concentrations until they are evenly distributed, or reach equilibrium. This movement towards
equilibrium is the driving force behind a majority of physiological processes, from neuronal impulses to
renal function. Today we will investigate the movement of several different types of molecules across a
cell membrane, including water, and we will examine the physical properties of these different molecules
to see how they influence this movement.
Today’s Objectives
1. Observe the movement of water across a membrane in model cells (decalcified eggs) and examine the
environmental conditions that determine the direction of osmosis.
2. Compare the rate of osmosis when the concentration gradient varies.
3. Observe the effect of molecular size on the movement of solutes across a membrane.
4. Observe the effect of polarity on the movement of solutes across a membrane.
Osmosis
When a selectively permeable membrane can inhibit the movement of some types of solutes and a
concentration gradient exists, water will diffuse towards the higher solute concentration to equalize the
concentration on both sides of the membrane. If you have a hard time remembering which way water
moves in the presence of an osmotic imbalance (concentration gradient), just remember that SOLUTES
SUCK!
Water will always be drawn towards more concentrated solutes. A solution can be described by its
tonicity. Tonicity describes how a solution affects cell volume. A hypotonic solution will cause a cell to
stretch and swell as water enters because it has a lower solute concentration (hypo = below) than a cell. A
hypertonic solution will draw water out of a cell and make it shrink because it has a higher relative
solute concentration (hyper = above). An isotonic solution produces no change cell volume because there
is no difference in concentration (iso = same); an isotonic solution is said to be in osmotic equilibrium
with the cell.
You have observed this phenomenon when your fingers get wrinkled after soaking in bath water, a
hypotonic solution. Your skin wrinkles because the skin cells swell with water and your skin becomes too
large to fit smoothly on your fingertips. Conversely, your skin may feel dry and tight after a day
swimming in the ocean, a hypertonic solution, as the salt from the sea water draws water out of your skin
cells. In the following experiment, you will be using decalcified eggs as model cells. The eggs have been
treated with vinegar to remove the calcium from the shell, leaving behind a membrane that is permeable
to water (solvent), but not to other molecules (solutes).
Materials:
Biology
Standard: SB1d
• 5 decalcified eggs
• 5 weigh boats, one for each egg
• 3 Beakers or plastic containers with solutions A, B, & C
• 2 Beakers or plastic containers with solutions 1 and 2
• Paper towel
• Gram scale
Procedure IA: Determining the Tonicity of Extracellular Fluid:
1. Fill three beakers with enough of solution A, B, or C to cover an egg, about 300 ml.
2. Obtain three decalcified eggs. Gently dry and weigh each egg before immersing it in Solution A, B, or
C. Dry the egg by gently rolling it on a paper towel. Do not dry the egg for too long because the paper
towel will begin to draw out water from inside the egg and will change the weight of the egg. Record the
weight of each egg in Table 1.
3. Let the three eggs soak in solutions A, B, and C for 20 minutes. Go on to Procedure IB while you are
waiting. The soak time should be at least 20 minutes, but can be longer if it is more convenient.
4. After at least 20 minutes, dry and weigh each egg and record your results. Use a "+" sign to indicate an
increase in weight and a "–" sign to indicate a decrease in weight.
5. The change in weight reflects the movement of water into or out of the egg. Based on the movement of
water, determine if the Solutions A, B, and C are hypotonic, isotonic, or hypertonic.
Questions:
1. Compare the presoak weight for eggs A, B, and C with their weights after the 20 minute soak. What is
the tonicity of each solution?
Solution 1:
Solution 2:
2. Explain the physiological cause of the change in the weight of each egg.
3. What physical conditions are required to cause the water to move in a particular direction, into or out of
the egg?
Biology
Standard: SB1d
4. Explain why osmotic homeostasis must be closely regulated for the all the body fluid compartments.
Procedure IB:
The rate of osmosis is dependent on the difference in solute concentration across the membrane. Water
will diffuse more quickly if the concentration gradient is steeper (the difference in concentration is
greater). In this experiment, you will be soaking your eggs in two different hypotonic solutions and
measuring the rates of osmosis.
1. Gently dry and weigh the last two eggs. Record the results in Table 2 as your zero time points. 2.
Immerse one egg in solution 1 and the other in solution
2. Dry and reweigh each egg after two minutes. Return the eggs to their solutions after weighing.
Reweigh the eggs every 2 minutes for about a half hour.
3. Plot your results on graph paper (not binder paper!). Plot the data for both solutions on the same graph
using different symbols or colors. You do not need to begin your Y-axis at zero.
4. Compare the rate of water movement into each egg by calculating the slope of each line using the
formula below, where ∆Y is the change in the egg weight and ∆X is the change in the time.
slope = ∆Y/∆X
Table 1. Weight of Eggs and Tonicity of Solutions A, B, and C
Solutions
Egg in Solution
A
Egg in Solution
B
Egg in Solution
C
Weight Before
Soaking (g)
Weight After
Soaking (g)
Difference in
Weight (g)
Tonicity of
Solution (hyper-,
hypo-, or iso-)
Biology
Standard: SB1d
Time
Table 2. Weight of Egg every 2 minutes in Solutions 1 and 2
Solution 1
Solution 2
Egg Weight
Change in
Egg Weight
Change in
(grams)
Weight (Relative
(grams)
Weight (Relative
to 0 Time Point)
to 0 Time Point)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Questions:
5. How does the rate of osmosis differ for the two solutions?
6. Which solution is more hypotonic, solution 1 or 2?
Biology
Standard: SB1d
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