How Do Living Cells Deal With Change in pH?
(Acids, Bases, and Cells)
Introduction: Enzymes are essential to the chemical reactions that occur in cells, but they are sensitive to
environmental factors, such as heat and pH. The pH (how acidic or basic something is) of tissues can affect
the activity of enzymes. You will recall that a pH of 7 is neutral; below 7 is acidic; and above 7 is basic. Most
cells maintain a pH close to neutral. If the enzymes inside a cell can't function properly, many biochemical
reactions would cease to occur.
The chemical reactions that occur within living cells often cause the pH to change within the cell. Yet, the life
of the cell depends upon its ability to maintain a pH that allows the enzymatic reactions of the cell to continue.
The question that you will be exploring in this lab is: Can living tissues (cells) control the pH of their
environment?
***Form a hypothesis: What do you think will happen to cells that are exposed to changes in the
pH of their environment? (Write your response in your own lab notebook)
Materials:
pH meter
.1N HCl
.1N NaOH
Liver and potato homogenates
Graduated cylinder
50 mL Beaker
250 mL beaker
400 mL Beaker
Graph Paper
Safety Precautions:
1. Safety goggles must be worn during the entire lab
2. Care should be taken when working with chemicals. If a spill occurs, flush with
water and alert the instructor.
Procedure:
Before you begin: In your lab notebook draw a data table like the one you see on the next
page and add a title – this is where you will record all your data.
1.
2.
3.
4.
5.
Obtain a 50 mL beaker. Using the numbers on the side of the beaker, fill the beaker with 30 mL of tap
water.
Turn on the pH meter and place it in the water in the beaker. Allow it to stabilize. Please use the pH
meters with care, as they are rather fragile.
Record the initial pH of the tap water in the data table below.
Add 5 drops of hydrochloric acid. VERY gently stir the solution. Allow the pH meter to stabilize.
Record the pH in the data table.
Repeat step 4. Record the pH after every 5 drops until a total of 30 drops has been reached. Remember to
stir the solution after each addition of drops of acid.
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Number of
drops
Water + acid
Water + base
Liver + acid
Liver + base
Potato + acid
Potato + base
0
5
10
15
20
25
30
6.
7.
8.
9.
Fill the large 400 mL beaker about half full with tap water. Place your pH meter in the water and gently
swish it around. You are trying to rinse it completely before going to the next part of the experiment.
Rinse the small 50 mL beaker and pour into it another 30 ml of tap water.
Record the initial pH and record it in the data table above.
Add 5 drops of sodium hydroxide base, recording the pH in exactly the same way as you did for the
hydrochloric acid. Continue until a total of 30 drops has been added. Remember to stir the solution after
each addition of base.
Observations:
In your lab notebook make a graph, plotting two lines (one for the acid data and one for the base data – use a
different color for each line) showing the relationship between the change of pH in tap water and the drops of
acid and base solutions added. Write the questions below into your lab notebook and then answer them in full
sentences
1.
What is the independent variable?
2.
On which axis is the independent variable placed?
3.
Which is the dependent variable?
4.
What makes this the dependent variable?
5.
On which axis of your graph will the dependent variable be placed?
6.
Summarize the effects of HCl (acid) and NaOH (base) on tap water. Does there seem to be a steady
increase or a sudden jump? Is there any correlation between the number of drops and the change in pH?
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Procedure:
10. Before using your pH meter again, rinse it again in tap water as you did before.
11. Now try tissue -- living -- from a living system. Rinse your 50 mL beaker and pour 30 ml of liver
homogenate into it.
12. Record the initial pH.
13. Add 5 drops of hydrochloric acid and record the pH as you did before. Continue until 30 drops have been
added. Remember to stir the solution after each addition of acid.
14. Thoroughly rinse the pH meter in the 400 mL beaker of water.
15. Rinse the 50 mL beaker. Add another 30 ml of liver homogenate. Repeat the procedure above except use
base instead of the acid.
Observations:
Graph the results for the liver on the same graph as for the tap water, using a different colored pencil. Label
the lines on the graph. Write the questions below into your lab notebook and then answer them in full sentences
7.
8.
9.
What was the total pH change for the 30 drops of acid added to the liver?
What was the total pH change for the 30 drops of base added?
How do these differ from the tap water?
10. Explain the difference in the liver and tap water. Why did the liver behave differently than the tap water?
Procedure:
16.
17.
18.
19.
Try plant tissues -- potato homogenate -- to see if plant tissues react the same way as animal tissues.
Rinse your pH meter thoroughly in the beaker of water.
Rinse your small 50 mL beaker, and pour 30 ml of potato homogenate into it.
Record the initial pH. Add drops of hydrochloric acid as you did before until 30 drops have been reached.
Repeat the whole procedure using the sodium hydroxide base.
Observations:
Graph this data on the same graph with the liver and the tap water. Use a different color pencil and label your
lines. Write the questions below into your lab notebook and then answer them in full sentences
11. Does the potato react more like the liver or the tap water?
12. Explain why the potato reacted the way it did.
13. What are buffers?
14. Do you think that living cells have built in buffers?
15. What evidence do you have from this lab that indicates that cells can produce buffers?
16. What do you think would be the advantage of having organic buffers in cells?
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