AP Biology 12
ACIDS BASES AND CELLS
Name _____________
Organisms are often very sensitive to the effect of acids and bases in their environment. They
need to maintain a stable internal pH in order to survive—even in the event of environmental
changes. Many naturally occurring biological, geological, and man-made chemicals are capable
of stabilizing the environment’s pH. This may allow organisms to better survive in diverse
environments found throughout the earth.
It is very important for cells to maintain the correct pH in order to survive and function properly;
however, many of the metabolic reactions that occur in cells tend to shift the pH away from the
ideal position.
Objective of this lab:
Use different methods of pH measurement.
Measure the pH of some biological chemicals.
To investigate the extent to which certain substances resist changes in pH.
To investigate how cells maintain the correct pH in spite of metabolic activities that shift
the pH towards either the acidic or basic end of the scale? In other words, how do cells
maintain homeostasis with respect to pH?
Prelab To be completed before starting the lab.
Living cells need to maintain a fairly constant internal environment. This is called
_________________________________. The cell will not function, for example, if it becomes
too acidic (too many __________ ions, H+) or to basic (too many ___________ ions, OH-).
Unless the pH is correct, the chemical reactions that must take place inside the cell are disrupted,
and the cell could die.
When the concentrations of hydrogen ions and hydroxide ions in a solution are equal, the
solution is neither acidic nor basic; it is said to be ______________________.
We measure the hydrogen ion concentration of a solution by means of a scale called the pH scale.
Neatly draw a diagram of the pH scale to illustrate your understanding of it.
The pH scale is geometric. This means that the numbers on the scale differ by factors of 10.
How much more acidic will a solution of pH 5 be than a solution of pH 7? _________
How many times more basic is a solution with a pH of 12 than one with a pH of 8? ________X
What are substances that resist change in pH called? _________________
AP Biology and Biology12: pH lab
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Write a formal hypothesis for the following questions:
(Use your textbook to refresh your memory about writing hypotheses, page 9 and 10.
Remember a formalized hypothesis focuses on two variables that may be related. It makes a
prediction of how manipulating one variable, independent, will affect the other variable,
dependent. For example: ‘If’ and ‘then’ may be used. )
1. Part A: What is the pH of most biological materials?
________________________________________________________________________
________________________________________________________________________
2. Part B: When acid or base are added to water or to a biological sample, how would the
rate of change of pH compare in the two samples?
________________________________________________________________________
________________________________________________________________________
______________________________________________________________________
AP Biology and Biology12: pH lab
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Part A. Measuring pH and Determining the pH of Some Living Materials
1. Dip pieces of blue and pink litmus paper into the solutions of 0.1 M HCl (an acid), 0.1 M
NaOH (a base), tap water and distilled water. Record the colour of the indicator papers in
Table 1 below. Also, use the pH indicator paper and the Vernier pH meter to measure the
actual pH of the solutions
2. Use the pH indicator paper to determine the pH of each of the biological materials listed
in Table 2. Record the pH of each sample, and indicate whether it is acidic or basic.
Table 1. Measure pH using various indicator papers and Vernier pH meter
0.1 M
HCl
0.1 M
NaOH
Tap water
Distilled
water
Colour of blue litmus paper
Colour of pink litmus paper
pH determined from pH
indicator paper
pH determined from Vernier pH
meter
Table 2. pH of Various Living Materials
Biological Sample Tested
pH
(use pH paper)
Acidic or Basic?
Saliva
Tears
Urine
Lemon juice
Milk
Liver, ground
Potato, ground
Egg
AP Biology and Biology12: pH lab
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Part B. Investigating how cells maintain homeostasis in acidic and basic environments
1. Pour 25 ml of tap water into a 50 ml beaker. Record its initial pH using the Vernier pH
meter. Add 0.1 M HCl one drop at a time and determine the pH after each drop has been
added, until 10 drops of acid have been used. Then record the pH after every 5 drops,
until 30 drops have been added in all.
2. Rinse the beaker and repeat step 1 using 25ml 0.1 M NaOH.
3. Rinse the beaker and repeat step 1 using 25 ml of liver homogenate instead of tap water.
Liver is one of the most metabolically active tissues of the body and has been blended
into a homogenate for your use (10 g of liver per 100 ml of water and put in a blender).
4. Repeat step 3, but use NaOH.
5. Repeat the test with potato homogenate (10 g potato blended with 100 g water).
6. Repeat the test with an inorganic solution of sodium phosphate.
7. Make a simple graph, plotting two lines for the change of pH in tap water against the
number of drops of acid and base solutions added. Use a solid line for acid changes and a
dashed line for changes with NaOH. On the same graph, plot the data for liver
homogenate, potato homogenate, and inorganic sodium phosphate solution, using a
different colour for each different solution, and solid lines for acid and dashed lines for
base. Put an appropriate title on your graph.
AP Biology and Biology12: pH lab
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Table 3. Maintaining homeostasis in acidic and basic environments
pH Measurements
Number of Drops
of Acid or Base
Added
Tap Water
Liver
Homogenate
Potato
Homogenate
HCl
HCl
HCl
NaOH
NaOH
NaOH
Sodium
Phosphate
Solution
HCl
NaOH
0
1
2
3
4
5
6
7
8
9
10
15
20
25
30
Questions
1.
List two different methods of measuring the pH of a solution.
________________________________________________________________________
________________________________________________________________________
2.
In general, is the pH of the biological samples you tested acidic, basic or neutral?
________________________________________________________________________
3.
Use a reference to find out the pH of the following biological materials:
Stomach juice __________________________________________
Intestinal juice __________________________________________
AP Biology and Biology12: pH lab
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Blood
4.
__________________________________________
In part B, what was the ‘control setup’? _______________________________________
What were the ‘experimental setups’? _________________________________________
_______________________________________________________________________
Why was the control setup required in this experiment? __________________________
_______________________________________________________________________
In part B, what was the ‘independent variable’?
________________________________________________________________________
What was the ‘dependent variable’?
___________________________________________
Which variable, independent or dependent, is always placed on the X-axis?
_______________________________________________________________
Identify some factors that were kept constant in all the setups. ____________________________
______________________________________________________________________________
5.
Did the biological samples (such as the liver and potato homogenates) respond in the
same way as the tap water when acid or base was added. Explain.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
AP Biology and Biology12: pH lab
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6.
Does the inorganic sodium phosphate solution respond to additions of acid or base more
like the tap water, or more like the organic (biological) samples? Explain how you know.
Did your results in this activity support your original hypothesis?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
7.
The sodium phosphate solution is called a buffer. There are many different kinds of
buffers, but they all show the same property as the sodium phosphate solution when acids
or bases are added. Based on your observations, do you think that there are buffers in
living systems? How do you know? (Your answer must be based on your observations in
this lab.)
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
8.
How do buffers affect homeostasis in a living cell when the environmental pH changes?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
AP Biology and Biology12: pH lab
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9.
Examine the results of your experiment carefully. Can you identify any errors and if so
explain the source of the errors and make suggestions as to how these can be avoided.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
___________________
AP Biology and Biology12: pH lab
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AP Biology and Biology12: pH lab
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Biology 12: Rubric for pH and Buffer lab
4
Rubric Component
Name: _______________
3
2
1
Accurate and written
with insight, goes
beyond the problem.
Accurate and written
with insight.
Accurate
Partially accurate
Identifies and
summarizes the
problem/questions
at issue.
Accurately identifies
the
problem/questions and
provides welldeveloped
explanations.
Accurately identifies
the
problem/questions
and provides a brief
explanation.
Identifies the
problem/
questions and
provides a poor
summary or identifies
an inappropriate
problem/question.
Does not identify or
summarize the
problem/questions
accurately if at all.
Hypothesis
Clear explanation of
purpose; educates by
providing context.
Gives a correct
purpose with some
framework
Identifies and
assesses the
quality of
supporting
data/evidence
Provides a welldeveloped
examination of the
evidence/data and
questions its accuracy,
relevance, and
completeness. Clearly
distinguishes between
fact and opinion.
Examines evidence
and
questions the quality.
Distinguishes
between
fact and opinion.
Identifies and
evaluates
conclusions,
implications, and
consequences
Accurately identifies
conclusions,
implications, and
consequences with a
well-developed
explanation. Provides
an objective reflection
of own assertions.
Accurately identifies
conclusions,
implications, and
consequences with a
brief evaluative
summary.
Clarity of data
tables and graphs
Graph correctly titled
and labeled
Graph correctly titiled Graph labeled but
but not labeled.
contains errors in
units, axes or
headings. No pencil.
Completion of
prelab.
Declares a purpose
that is correct.
Merely repeats
information
provided. Does not
justify position or
distinguish between
fact and opinion.
Does not explain,
provides inaccurate
information, or
merely provides a list
of ideas; or only
discusses one area.
Purpose is incorrect.
Does not identify or
assess the quality of
supporting
evidence.
Does not identify or
evaluate any
conclusions,
implications or
consequences.
Graph present but
not titled nor
labeled.
Focused for entire lab/ Partly focused and
followed all instructions partly cleaned up
and cleaned up.
Watched others and
did gave instructions
did not clean up.
Very little
participation/ mind
on other things
100% Accurate
Adequate
Complete but
inaccurate
Incomplete
Writing skills
Full sentences, good
grammar, answer the
questions accurately.
Full sentences, poor
grammar, answers
questions accurately
Point form and
Illegible and
answers the
incomplete
questions accurately.
Handed in on
time
4
Participation
during lab
Research
0
Total mark:
AP Biology and Biology12: pH lab
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