Lab 4 Homeostasis and Osmosis

Lab 4 Homeostasis and Osmosis
Objectives:
At the end of this lab you should be able to:
1. use litmus paper to measure pH and interpret that measurement.
2. define the terms diffusion, osmosis, hypertonic, hypotonic, and isotonic.
3. describe what happens to cells in a hypertonic, hypotonic or isotonic
solutions.
Introduction:
Homeostasis is the ability to maintain a steady state. The ability to maintain
homeostasis is an important characteristic of life. The type of traits maintained in
homeostasis varies from species to species. For example, mammals and
birds maintain a relatively constant internal temperature; fish, amphibians, and
reptiles do not regulate body temperature. Today we explore two characteristics
for which every organism must maintain homeostasis: water content and
hydrogen ion concentration.
Plasma membranes, diffusion, and osmosis.
The constant random movement of molecules allows the process called diffusion
to take place. That is,the net movement of the same kind of molecules from an
area of higher concentration to an area of lower concentration. Molecules move
along a concentration gradient (from high to low concentration) until the
molecules are equally distributed. In the case of a cell, this means, that the net
movement results in the same concentration of dissolved materials on both sides
of the plasma membrane.
Cells are surrounded by a water solution that contains food molecules, waste
products, gases, salts, and other substances. This is the external environment
for every cell. The outer surface of the plasma membrane is in contact with this
external environment, while the inner surface of the plasma membrane is in
contact with the cytoplasm. The plasma membrane is responsible for controlling
what enters and leaves the cell. Thus, the plasma membrane is responsible for
maintaining homeostasis in the cell. Cell membranes are semipermeable. That
means that the plasma membrane permits passage of some materials in or out of
the cell and does not permit passage of other materials. For example, water can
easily cross the plasma membrane by simple diffusion; sugar, salt and proteins
can not cross the plasma membrane by simple diffusion.
This diffusion of water across a membrane is called osmosis. A hypertonic
solution has a higher concentration of solutes. Cells in a hypertonic solution will
lose water. If a cell is placed in a hypotonic solution where the concentration of
solutes is lower outside the cell, then water will diffuse into the cell. When the
concentration of solutes is the same on both sides of the plasma membrane
there is no net movement of water and the solution is called isotonic.

Osmosis in synthetic cells.
In this experiment a synthetic plasma membrane (dialysis tubing) is used to study
osmosis. The solutions used inside the synthetic cells are simple sugar water
(sucrose solution). Each team uses a different temperature and different
concentration of sugar. At the end of class you will use the data collected by
every team to explore the effect of solute concentration and temperature on
osmosis.
Experimental Design
o Obtain a piece of dialysis tubing and secure one end with a plastic clip.
o Fill about half the tubing with the proper solution.
o Secure the other end of the tubing.
o Completely dry your tubing and check for leaks.
o Weigh your tubing on the appropriate scale.
o Record your results on the chalk board and on the next page.
o Place your tubing in the assigned water bath.
o Dry and weigh your tubing at 10 minute intervals, each time recording your
results on the chalk board and on the next page.
Use the same scale for all measurements.
Why is it important to use the same scale
Sketch the experimental set up and make predictions:
Cell soaking in a 40%
sugar solution
Cell soaking in a 20%
sugar solution
Cell soaking in a 5%
sugar solution
Will this cell gain water or
loose water ________
Will this cell gain water or
loose water ________
Will this cell gain water or
loose water ________

Record the results for the entire class in the following charts.
Warm
Room
Temp
Cold
# 1
# 2
# 3
# 1
# 2
# 3
# 1
# 2
# 3
0 min 10 min 20 min 30 min 40 min 50 min 60 min
total
weight
change
1. What are the two independent variables for this experiment
____________________________
____________________________
2. What is the dependent variable for this experiment
____________________________
3. What are some variables for this experiment
____________________________
____________________________
____________________________
4. Which solution had the lowest concentration of sugar (5%)
__________________________________________________
5. How could you tell
________________________________________________________________
_______________________________________________________________
8. Which solution had the highest concentration of sugar (40%)
_______________________________________________
9. How could you tell
________________________________________________________________
_______________________________________________________________

10. Explain how concentration effects diffusion and osmosis.
________________________________________________________________
________________________________________________________________
_______________________________________________________________
11. Does your data support this explanation
________________________________________________________________
________________________________________________
_______________________________________________________________
12. Explain how temperature effects diffusion and osmosis.
________________________________________________________________
________________________________________________________________
_______________________________________________________________
13. Does your data support this explanation
________________________________________________________________
________________________________________________
_______________________________________________________________
Based on what you saw with the dialysis tubing: describe how it can be used for
kidney patients:

Osmosis in onion skin cells.
Changes in the amount of water inside a cell look very different in cells that have
a cell wall. This experiment provides an opportunity to observe the effect of
osmosis in onion cells. The solute is sugar.
o Obtain a piece of red onion skin no larger than 1/4" square and one cell
layer thick. (For best results, snap a single onion layer and then peel off
the red cells.)
o Place the onion skin on the slide, add a cover slip. This is your control.
o Prepare a second slide as above except this time use a drop of 40% sugar
solution. Make sure to use a cover slip!
o Prepare a third slide as above except this time use a drop of 20% sugar
solution. Make sure to use a cover slip!
o Prepare a third slide as above except this time use a drop of 0% sugar
solution. Make sure to use a cover slip!
o Make sure you can tell your slides apart!!!! Label them somehow.
o Using proper microscope techniques; view slides under high power (some
slices may be too thick; you may have to use medium power).
Plain onion cell with no solution
Onion cell in
a 40%
solution
Onion cell in
a 20%
solution
Onion cell in
a 0%
solution
14. Compared to the onion cell, is the 0% solution hypertonic, hypotonic, or
isotonic _____________________
15. Compared to the onion cell, is 20% sugar water hypertonic,
hypotonic, or isotonic _____________________
16. Compared to the onion cell, is 40% sugar water hypertonic,
hypotonic, or isotonic _____________________
17. Why did the plain slide act as a control in this experiment
_________________________________
18. Describe the independent variable in this experiment.
_________________________________
19. Describe the dependent variable in this experiment.
_________________________________

Osmosis in red blood cells.
Changes in the amount of water inside a cell look very different in cells that do
not have a cell wall. This experiment provides an opportunity to observe the
effect of osmosis in red blood cells. The solute is salt.
o Place a drop of sheep blood on the slide, and add a cover slip. This is
your control.
o Prepare a second slide as above except this time use a drop of 10% salt
solution. Make sure to use a cover slip!
o Prepare a third slide as above except this time use a drop of 0.9% salt
solution. Make sure to use a cover slip!
o Prepare a third slide as above except this time use a drop of 0% salt
solution. Make sure to use a cover slip!
o Make sure you can tell your slides apart!!!! Label them somehow.
o Using proper microscope techniques; view slides under high power. Call
me over to check if your focus is correct; blood can be difficult to see.
Plain blood cells with no
solution
Blood cell in a
10% solution
Blood cell in a
0.9% solution
Blood cell in a
0% solution
20. Compared to the blood cell, is the 0% solution hypertonic, hypotonic, or
isotonic _____________________
21. Compared to the blood cell, is 0.9% salt water hypertonic,
hypotonic, or isotonic _____________________
22. Compared to the blood cell, is 10% salt water hypertonic,
hypotonic, or isotonic _____________________
23. . Why did the plain slide act as a control in this experiment
_________________________________
24. Describe the independent variable in this experiment.
_________________________________
25. Describe the dependent variable in this experiment.
_________________________________

pH, acidity and alkalinity.
Perhaps no single change has a greater effect on our well being than a change in
pH. If the pH of our blood is too low (acidic conditions) we feel depressed,
sluggish and can even go into a coma. If the pH of our blood is too high (basic or
alkaline conditions) we find that our nerve impulses increase and we become
irritable or if the impulses get too fast we become rigid. Variations in pH of just a
few tenths can be fatal.
Interpreting pH
26. Indicate what regions on the ph scale would be a strong acid, a strong base,
a weak acid and a weak base.
0 7 14
27. Identify the following pH values as either acid or base and rank them in order
of increasing strength.
a. 9 b. 3 c. 12 d. 5
e. 10 f. 7.3 g. 1.5 h. 6.5
STRONG ACID _____ _____ _____ _____ WEAK ACID
STRONG BASE _____ _____ _____ _____ WEAK BASE
Measuring pH
Chemicals that change color depending upon the presence of an acid or a base
are called pH indicators. Litmus paper contains such a chemical and is
commonly used to measure pH. By placing a drop of liquid on the paper and then
comparing the paper's color to the accompanying color chart, you can estimate
the pH. To see how this works, measure the pH of water. Now, being careful not
to get the HCl or NaOH on your body or your clothing, measure the pH of the
HCl and NaOH solutions.
H 2 O: _____ HCl: _____ NaOH: _____

Measuring pH
o In the lab today you will find a set of test tubes containing various liquids.
Measure the pH for each liquid.
o Do not attempt to taste these liquids and use caution if you try to smell
these liquids.
1. description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
2 description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
3. description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
4. description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
5. description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
6. description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
7. description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
8. description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
9. description:
_______________________________________________________________
pH = _____ , identity of liquid: ______________________________________
10. description:
_______________________________________________________________

pH = _____ , identity of liquid: ______________________________________