Integrated Science 3
Name:
Per:
Algal Bead Lab
Background
The human species and all other heterotrophs are dependent on photosynthesis carried out by
autotrophs for virtually all food/energy needs. Whether we eat plants directly or indirectly (by eating
animals), it is the photosynthesis conducted in the leaves of those plants that limits the food resources and
ultimately, the population of our species.
The overall equation for photosynthesis is written as:
6 CO2 + 6 H2O  light energy 
 C6H12O6 + 6 O2
In words, this says that carbon dioxide combines with water to form glucose and oxygen. This chemical
change will take place as long as light energy is present. As the plant is producing glucose through
photosynthesis, at least half of this glucose is used to meet the plant’s own energy needs (for growth and
reproduction) through cellular respiration. If people (or other animals) eat a plant, they perform cellular
respiration and use glucose that the plant has not used as an energy source for their own growth and
reproduction.
The overall equation for cellular respiration is written as:
C6H12O6 + 6O2 
 6 CO2 + 6H2O + ATP
Oxygen is produced in photosynthesis as a waste product and is given off as a gas. All organisms
performing cellular respiration use this Oxygen and produce Carbon Dioxide as a waste product. Carbon
Dioxide is used in turn by organisms performing photosynthesis.
Photosynthesis is the only biological process that can
capture light energy and convert it into chemical
compounds (glucose) that all organisms – from bacteria to
humans – can use to power metabolism, growth and
reproduction. Cellular respiration is the process that all
organisms require to convert glucose into ATP, a type of
energy usable by cells. The two processes are
interdependent since the products of photosynthesis are
the reactants of cellular respiration, and vice versa.
In this lab, you will use algae beads. These are algae
cells (from the microalgae Scenedesmus obliquus) that are
encapsulated in alginate. All algae are autotrophs so each
algae cell contains both a large central chloroplast where
photosynthesis takes place, and smaller mitochondria
where cellular respiration occurs.
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Pre-Lab Questions
1. What is autotrophy? Provide an example of an autotrophic organism.
2. What is heterotrophy? Provide an example of a heterotrophic organism.
3. In your own words (or using chemical reactions), describe how photosynthesis and cellular respiration
are interdependent.
4. What type of organism would you need to use to be able to monitor both photosynthesis and cellular
respiration? Why are algal cells a good choice?
5. You will indirectly measure the rates of photosynthesis and cellular respiration by monitoring product
generation. Considering this, what products might you monitor to determine the rate of
photosynthesis? Of cellular respiration?
6. Which process (photosynthesis, cellular respiration, or both) do you expect the algae cells to perform
when incubated in the light? In the dark? Explain.
7. Draw a pH scale. Label appropriate areas as acidic, neutral, and alkaline/basic.
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Procedure
The algae beads used in the following investigation allow you to observe both photosynthesis and cellular
respiration simultaneously. You will incubate the algae beads in a CO2 indicator solution that is sensitive to
changes in pH caused by gaseous CO2 dissolving in water to form carbonic acid: CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+
When the CO2 indicator is at equilibrium with the atmosphere, it is dark orange. When the CO2 levels
increase, it changes to yellow, and when CO2 levels decrease, it changes to purple. The CO2 indicator spans
the range of pH change that will be seen in the algae beads (pH 6.9–9.1), making it a convenient way to
measure photosynthesis and cellular respiration.
In this investigation, you will design an experiment to demonstrate photosynthesis and cellular respiration. In
the process, you will compare the rates of color change of the CO2 indicator caused by algae beads incubated
under bright light and in complete darkness, over time. The color/pH change of the CO2 indicator can be
determined using the Indicator Color Guide.
Experimental Organizer
Complete the Experimental Organizer below for the experiment you will conduct.
Title:
Hypothesis:
Independent Variable (include units): _________________________________________
☐ continuous
☐ discontinuous
Levels (treatments) of I.V.:
Number of Trials:
Dependent Variable (include units):
_________________________________________
☐ Quantitative
☐ Qualitative
Constants:
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Data Table 1.
Use Google Sheets to design a data table for the data. Remember: data is being recorded over time (every 5
minutes)
Protocol
1. Use the wash transfer pipet to remove the distilled water
from the cuvette. Discard the water into the waste
container.
2. Label a new transfer pipet indicator and use it to
transfer 1 ml of CO2 indicator to each cuvette. Cap
cuvettes tightly.
3. Wrap the cuvette labeled dark in aluminum foil. Place both the
cuvettes labeled light and dark on their sides 15-25 cm from the
lamp. Have a white sheet of paper under them. Ensure that the all
beads are distributed evenly throughout the cuvette (in one layer)
and the clear side of the cuvette faces the light.
4. Collect data starting at time = 0 min. Every 5 min, thoroughly mix the CO2
indicator in the cuvettes and determine the color. This can be done by
comparing the color of the CO2 indicator in your cuvette to the provided
Indicator Color Guide. Be quick about taking this reading and immediately
return the cuvettes to the experimental conditions. Record your data in
Data Table 1.
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After data collection and before cleaning up:
5. Use the indicator pipet and use it to remove and discard the
CO2 indicator. Discard the CO2 indicator into the waste
container.
6. Use the wash pipet to add 1 ml of distilled water
to each of the cuvettes.
Graph
Use Google Sheets to plot the data. Refer to the Graph Choice Chart attached to determine the best type of
graph.
Analysis Questions
1. Are your slopes positive or negative for light and dark conditions? What does this mean about the
change in CO2?
2. Under which condition did the CO2 indicator turn more alkaline? Why?
3. How does cellular respiration impact the observed rate of photosynthesis? Is your calculated rate of
photosynthesis accurate? Why or why not?
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4. Look up current ocean pH values. How do the current values compare to those from previous years?
Consider what you’ve just learned about algae and how the chemistry of the indicator used in the
experiments you just performed works. Hypothesize why oceans are at their current pH. How is the
pH of the ocean changing and why? How might this affect the organisms that live in the ocean?
5. The algae beads provide a convenient experimental system because they are uniform in size and
contain roughly the same number of algal cells per bead. Why are these advantages for the experiments
you performed?
6. Why is it important to keep the cuvettes at a consistent distance from the lamp as you perform this
activity?
7. Scientists can measure the extent of reactions by monitoring either reactant depletion or product
generation. What other substrates or products might you be able to monitor to determine the rate of
reaction in this lab?
8. Photosynthesis uses CO2 and cellular respiration produces CO2. We call the point when the two
processes are in balance — when there is no net production of CO2 — the compensation point. How
might you limit one of the processes in order to achieve a compensation point?
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