ACTIVITY II-PH
ORGANISMS MAINTAIN A NORMAL pH RANGE DESPITE CHEMICAL
REACTIONS THAT TEND TO SHIFT pH TOWARD EITHER ACIDIC OR BASIC
ENDS OF SCALE
PH INVESTIGATION II
Organisms and pH
Individual organisms and cells must maintain an internal homeostasis, but many factors
can affect that stability-for example, the relative concentrations of hydrogen ions (H+)
and hydroxide ions (OH-). The biochemical activities of living tissues frequently affect
pH, yet life depends on maintaining a pH range that is normal for each tissue or system.
Using a pH probe or pH paper you can compare the responses of several materials to the
addition of an acid and a base.
Before you begin, study the investigation and develop a
hypothesis that answers the question “How do organisms survive
and function despite metabolic activities that tend to shift pH
toward either acidic or basic ends of the scale.
Materials (per team of 4)
4 pairs of safety goggles
50 ml beaker or small jar
100 ml graduated cylinder
pH probe or pH paper
tap water
0.1M HCL and dropper
0.1M NaOH and dropper
Sodium phosphate pH 7 buffer
Liver homogenate
Potato homogenate
Egg white
Warm gelatin suspension 2%
Procedure:
1. Pour 25 ml of tap water into beaker or jar
2. Record the initial pH by using a pH probe or pH paper
3.Add 0.1M HCL a drop at a time. Gently swirl the mixture after each drop. Determine
the pH after 5 drops have been added. Repeat this procedure until 30 drops have been
used. Record the pH measurements in the table below.
4.Rinse the beaker thoroughly and pour into it another 25 ml of tap water. Record the
initial pH of the water and 0.1M NaOH drop by drop, recording the pH changes
in exactly the same way as for the 0.1M HCL.
5. For each biological material (liver, potato, egg white, and gelatin) repeat steps 1-4
except add 25 ml of biological material instead of water. Record the data in your
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table.
6.Test the buffer solution (a nonliving chemical solution) using the same method outlined
in steps 1-4 except add 25 ml of buffer instead of water. Record the data in your
table.
7. Wash your hands
Measuring the pH of Biological Material After The Addition of HCL or NaOH
Tests with 0.1M HCL
Tests with 0.1M NaOH
pH after addition of # of drops of HCL
Solution
tested
0
5
10
15
20
25
pH after addition of # of drops of NaOH
30
0
5
10
15
20
25
Tap water
Liver
Potato
Egg white
Gelatin
Buffer
2
30
DISCUSSION QUESTIONS:
1. Summarize the effects of HCL and NaOH on tap water
2.What was the total pH change for the 30 drops of HCL added to the biological
material? For the 30 drops of NaOH added? How do these data compare with the
changes in tap water?
3. Make two graphs of your data
a. pH versus the number of drops of acid. Create a line for tap water, liver,
potato, and buffer
b. pH versus the number of drops of base. Create a line for tap water, liver,
potato, and buffer
4. How do biological materials respond to changes in pH?
5. How does the buffer system respond to HCL and NaOH?
6. Is the pH response of the buffer system more like that of water or the biological
material?
7. How does the reaction of the buffer solution serve as a model for the response of
biological materials to pH changes?
8. Would buffers aid or hinder the maintenance of homeostasis within a living cell in a
changing environment?
9. What does the model suggest about the mechanism for regulating pH in an organism?
10. What served as the control?
11. What served as the experimental variables?
12. What was the independent and dependent variable?
LAB WRITE UP:
1.
2.
3.
4.
Data table
Two graphs (from question 3 under discussion)
Word processed responses to Discussion questions 1-12
Web-based research:
An important buffer system in humans that helps maintain the pH of the
blood at 7.4 is known as the carbonic acid- bicarbonate buffer system. This is
extremely important because a slight deviation from this blood pH for too long is
deadly. Maintaining the pH of blood at 7.4 is part of maintaining homeostasis.
Look up the carbonic acid- bicarbonate buffer system on the Internet and explain
how it works.
1. How does exercise affect the pH of the blood?
2. How does increased breathing during exercise help off set the change in
blood pH from exercise?
3. What role do kidneys play in maintaining blood pH?
4. What equation describes the carbonic acid-bicarbonate system? Explain
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5. Which way would the equation shift if excess hydrogen ions were added to
the blood? Why?
6. Which way would the equation shift if hydroxide ions were added to the
blood? Why?
7. If blood pH can’t be maintained and the pH raises it is called Alkalosis. What
are the symptoms of alkalosis?
8. If the blood pH can’t be maintained and the pH lowers it is called acidosis?
What are the symptoms of acidosis?
Cite your sources
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