Measuring pH in Zygocactus
A Laboratory Exercise in Photosynthesis
Prelab
pH is the measure of acidity/alkalinity of a substance. The pH system is based on the hydrogen
ion (H+) concentration in the solution. The pH ranges from 0 (strongly acidic) to 14 (strongly
alkaline), and 7 being neutral.
pH Determination
The pH number is actually based on a power of 10, a logarithm, which indicates the hydrogen
ion concentration [H+]. A pH 1 = [H+] of 10–1, pH 3 = [H+] of 10–3, etc. Therefore, a pH of 1 has
more hydrogen ions and is more acidic than a pH of 3.
Since pH is based on powers of 10, a pH 1 is 10 times more acidic than pH 2, a pH 2 is 10 times
stronger acid than a pH 3, etc. A pH 1 is 100 times stronger of an acid than pH 3.
Crassulacean Acid Metabolism (CAM) in Zygocactus
Plants in the family Crassulaceae have adapted to dry climates by closing their stomata during
the day, which is opposite of most C3 plants. This is beneficial to survival since less water is lost
through transpiration. When the stomata are open at night, CO2 is fixed and then passed through
a series of organic acids, eventually arriving at the Calvin Cycle. This process is called
Crassulacean Acid Metabolism (CAM).
The Zygocactus, also known as the Christmas Cactus, can be used to demonstrate CAM. The pH
can vary two points from morning to evening, depending on the level of CO2 contained within
the leaves.
Prelab Questions-Complete these questions before beginning the experiment.
1. What is the [H+] of pH 5?
2. What is the pH if [H+] = 10-11?
3. A pH 3 acid is how much stronger than pH 6 acid?
4. How much stronger is a base of pH 10.6 than a base of 8.2?
5. When do CAM plants open their stomata? Why?
6. What is being taken in by CAM plants and fixed into organic acids?
7. What is the ultimate destination of the CO2?
8. Predict the pH level in the morning compared to it in the evening. In other words, when
will the pH be more acidic?
Laboratory-Measuring pH in Zygocactus
Materials
Zygocactus plant
Deionized water
Cheesecloth
pH meter (digital is best)
50 mL beakers
pH buffers
mortar & pestle
Procedure
1. Calibrate the pH meter using buffers of known pH (ask your instructor for assistance).
2. Remove one leaf from the Zygocactus and grind it up in the mortar and pestle.
3. Add 10 mL of deionized water, and strain the mixture through cheesecloth into a 50 mL
beaker.
4. Insert the pH meter and record the pH for the correct time of day. Record the class
average in Table 1. (The pH will be measured once an hour throughout the day by other
classes and once in the late afternoon/early evening.)
5. Next Day-Record the average pH measurements from the other classes in the table
provided.
Time
pH
Table 1.
Analysis Questions
1. On a separate sheet of paper, graph the average pH vs. time for the Zygocactus. Don’t
forget to label axes and title.
2. When did the Zygocactus have low pH? Why? (Discuss the part of CAM photosynthesis
occurring during this time of day?)
3. When did the Zygocactus have high pH? Why? (Discuss the part of CAM photosynthesis
occurring during this time of day?)
4. How much stronger was the acid at the lowest vs. the highest pH? Show your work!
5. Why is it necessary for plants, such as cacti, to do CAM photosynthesis?
6. How does CAM photosynthesis differ from regular C3 plant photosynthesis?
Revised from: Spectrum Publications