Teacher Notes

Sodium Alginate Photosynthesis

Student Laboratory Kit

Materials Included In Kit

Bristol’s culture media concentrate, 100 mL*
Bromthymol blue indicator solution, 0.04%, 50 mL
Calcium chloride, CaCl2, 0.3 M, 500 mL
Sodium alginate, 10 g*
Cheesecloth, 1 square yard
Construction paper, black, 9" x 12", 2 sheets*
Dropping pipet, 23-mL
Pipets, extra-large, disposable, 15
Syringes, 20-mL, 15
Vials with screw tops, 30
Weighing dishes, 30
*for Prelab Preparation

Additional Materials Required

Water, distilled†
Beakers, 50-mL, 2*
Beaker, 250-mL†
Beaker, 400-mL*
Beaker, 600-mL†
Chlorella culture†
Funnel or strainer*
Glass jar or other clear container, 1.5 L†
Grow light (optional)*
Stir plate with stir bar†
Tape, transparent†
*for each lab group
for Prelab Preparation

Prelab Preparation

Part A. Culturing Chlorella

  1. To ensure an adequate population, begin culturing at least two weeks before the lab. The culture should be dark green before beginning.
  2. Add one liter of distilled water to a clear container, such as a large, glass jar or two-liter bottle with the top cut off. A volumetric or round bottom flask is also good for this purpose.
  3. Add ten mL of Bristol’s culture media concentrate and stir well.
  4. Add chlorella culture.
  5. Place the jar near a diffused light source or use grow lights or wide spectrum bulbs placed 12–18" from the container.
  6. Maintain the temperature near 21° C by adding small quantities of water or by adjusting the light source if it becomes too warm. Temperatures higher than 27° C can be fatal to the culture.
Part B. Preparing Sodium Alginate with Algae
  1. Add three grams of sodium alginate to a 250-mL beaker containing 100 mL of distilled water and stir until all the sodium alginate is incorporated and the solution is thick and uniform. This may take more than one hour. This step may be done ahead of time. Cover the beaker and store in a laboratory refrigerator for up to two weeks.
  2. Collect 400 mL of algae from near the bottom of the algae culture in a 600-mL beaker.
  3. Allow the algae to settle into the bottom of the beaker for about an hour, then pipet off the top 200 mL that is not as concentrated, leaving 200 mL of the most concentrated algae.
  4. Just before lab, combine the concentrated algae solution and the sodium alginate solution in the 600 mL beaker, stirring slowing to ensure the mixture is homogenous. You may choose to divide this stock sample into 20-mL quantities for each lab group or each lab group may use their syringe to remove a 15-mL portion from the stock.
Part C. Preparing Light-Blocking Tubes and Cheese Cloth
  1. Cut each sheet of black construction paper into four even strips, each 9" long.
  2. Cut each strip in half widthwise. You will have a total of 16 pieces that are each 3" x 4½".
  3. Make each into a loop and secure with a small piece of tape to complete the light-blocking tubes.
  4. Cut cheesecloth into 16 20 cm x 20 cm squares.
Part D. Preparing the Bromthymol Blue Indicator Solution.
  1. Add 25 mL of 0.04% bromthymol blue to 175 mL distilled water to make 0.005% bromthymol blue indicator solution. Adjust the pH using a dilute acid or base to achieve a green starting color. This corresponds to a pH of about 7.

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.


Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The leftover calcium chloride solution may be reused or rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The sodium alginate spheres and excess sodium alginate may be placed in the trash according to Flinn Suggested Disposal Method #26a. The bromthymol blue solution may be neutralized with sodium hydroxide according to Flinn Suggested Disposal Method #24b. Excess algae culture can be sterilized and handled according to Flinn Suggested Biological Waste Disposal Type 1.

Lab Hints

  • Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period, with the final color change being observed the following day or later the same day. The prelaboratory assignment may be completed before coming to lab, and the questions may be completed the day after the lab.
  • The rate of photosynthesis may be accelerated by placing the algae in the dark for 12 hours before the experiment.
  • A number of factors will affect the pH rate of change. A noticeable change in pH may take place in as little as thirty minutes or as great as several hours. Light quality, algae density and metabolic condition of the algae all may vary.
  • Rather than using bromthymol blue, a pH meter can be used to measure the pH at the beginning and end of the experiment.
  • For further information regarding proper algae care, see Culturing Algae, Publication No. 10583, available on the Flinn Scientific website.

Teacher Tips

  • This laboratory activity fits well when studying the metabolic processes of photosynthesis and respiration, during an ecology unit or when studying plants.
  • Sodium alginate is an edible polymer used to spherify food and drink in the practice of gastronomy.
  • To convert this activity to guided-inquiry, allow students to manipulate a different variable. Students could investigate different species of algae, different starting pH values of the solution, different types of light or filters over the vials or different sizes of spheres. There is enough sodium alginate to make three batches, and the calcium chloride can be reused.
  • The following student laboratory kits can be used to further explore productivity and metabolism: Energy Dynamics—Advanced Inquiry Kit (Flinn Catalog No. FB2049) or Plants in the Spotlight: A Photosynthesis Investigation—Student Laboratory Kit (Flinn Catalog No. FB1780).

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Obtaining, evaluation, and communicating information
Analyzing and interpreting data
Constructing explanations and designing solutions
Developing and using models

Disciplinary Core Ideas

MS-LS1.C: Organization for Matter and Energy Flow in Organisms
MS-PS3.D: Energy in Chemical Processes and Everyday Life
HS-LS1.C: Organization for Matter and Energy Flow in Organisms
HS-LS2.B: Cycle of Matter and Energy Transfer in Ecosystems
HS-PS3.D: Energy in Chemical Processes

Crosscutting Concepts

Cause and effect
Structure and function
Energy and matter
Systems and system models

Performance Expectations

MS-LS1-6: Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy into and out of organisms.
MS-LS1-7: Develop a model to describe how food is rearranged through chemical reactions forming new molecules that support growth and/or release energy as this matter moves through an organism
HS-LS1-6: Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.
HS-LS1-7: Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed, resulting in a net transfer of energy.
HS-LS2-3: Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions.

Answers to Prelab Questions

  1. How do the sodium alginate and algae spheres model a plant cell?

    The sodium alginate is permeable to gases as are the cell membranes of plants. The algae contain cellular machinery for photosynthesis and can carry out photosynthesis.

  2. How does dissolved carbon dioxide change the pH of the solution?

    A portion of dissolved carbon dioxide reacts with water to form carbonic acid. The carbonic acid decreases the pH of the solution.

  3. What is the difference between primary productivity and net primary productivity?

    Primary productivity is the amount of energy from the sun that is converted to chemical potential energy by plants. Net primary productivity is primary productivity minus the amount of energy used by the plant during respiration.

Sample Data

Change in pH


*pH was verified using a pH sensor.

Answers to Questions

  1. What was the change in pH of each of the samples?

    The sample that was not exposed to light had a change in pH from 7.7 to 7.0, for a total decrease of 0.7 in 45 minutes.
    The sample that was exposed to light had a change in pH from 7.7 to 7.4, for a total decrease of 0.3 in 45 minutes.

  2. Which sample experienced a higher rate of photosynthesis? Provide evidence to support your answer.

    The data collected shows that the vial exposed to light had a higher rate of photosynthesis. Both vials decreased in pH indicating that respiration was occurring at a faster rate than photosynthesis. However, the vial exposed to light experienced a smaller decrease in pH, indicating that more photosynthesis was taking place. The relationship between photosynthesis and respiration is expressed in the net primary productivity equation:

    NPP = rate of photosynthesis – rate of respiration

  3. Which sample had a respiration rate that was higher than the rate of photosynthesis? Provide evidence to support your answer.

    In the sample data, both had a higher rate of respiration than photosynthesis because the pH decreased in both samplesover time. This indicates that there was more carbon dioxide in solution at the end of the experiment than in the beginning. Carbon dioxide is released during respiration and absorbed during photosynthesis. This may vary if the conditions of the experiment change. In some cases, the vial in the light should have a higher rate of photosynthesis than respiration.

  4. Explain the role of light in photosynthesis.

    Light provides the energy needed to make glucose during photosynthesis. The energy from the light is transformed into chemical potential energy that is stored in the bonds of the glucose molecules. The pigments in chlorophyll absorb the energy from specific wavelengths of light in the blue and red portions of the visible spectrum. Light sources that have a higher amount of these wavelengths will encourage growth by increasing the rate of photosynthesis.

  5. Predict the outcome of an experiment in which red, green and blue filters are placed around different vials containing algae spheres rather than black paper.

    The color of the filter indicates the wavelengths of light that are transmitted by the filter. Therefore, these wavelengths of light will reach the algae. A green filter transmits green light, which is not absorbed by chlorophyll. Therefore, a green filter should reduce the amount of photosynthesis, and the pH should increase at a faster rate. The red filter transmits wavelengths in the red range and filters out others. Since red is absorbed by chlorophyll, the rate of photosynthesis will be higher than the green filter, but lower than no filter. A blue filter transmits wavelengths in the blue range and filters out others. Since blue is absorbed by chlorophyll, the rate of photosynthesis will be higher than the green filter. Students will probably not be able to make a prediction about whether the blue or red filter would provide a higher rate of photosynthesis.


Special thanks to Pam Bryer, Bowdoin College, Brunswick, ME, for sharing this activity with us.

Student Pages

Sodium Alginate Photosynthesis


Discover the effects of light on the rate of photosynthesis using algae encapsulated in sodium alginate. When carbon dioxide is taken up, the pH of the aqueous solution increases because carbon dioxide reacts with water to make carbonic acid. Measuring the changes in pH is an indirect way to measure the rate of photosynthesis.


  • Photosynthesis
  • Rate of reaction
  • Light reaction
  • Productivity


A novel method of controlling the amount of photosynthetic organisms during laboratory testing is by encapsulating algae in sodium alginate. Sodium alginate is a polysaccharide extracted from brown seaweed. When it contacts a solution containing calcium ions, it forms a gel. By dropping a mixture of sodium alginate and algae into the calcium solution using a syringe, small spheres are made in a process called spherification. These spheres are permeable to small molecules, including the reactants and products of photosynthesis. The algae remain alive throughout this process and can continue with photosynthesis and respiration.

Photosynthesis is the process of using light energy to convert water and carbon dioxide to glucose molecules with high chemical potential energy.

During photosynthesis, autotrophs, such as algae, remove carbon dioxide from the atmosphere and from aquatic environments. The removal of carbon dioxide from aquatic environments results in an overall increase in pH because aqueous carbon dioxide reacts with water to form carbonic acid (H2CO3) as shown in the following reaction. When carbon dioxide is removed, a proportional amount of carbonic acid reacts to reform carbon dioxide and water to maintain equilibrium.

CO2 (aq) + H2O → H2CO3 (aq)
Because of this, pH is a good indicator for measuring the relative rate of photosynthesis or respiration. When both are happening simultaneously, an increase in pH indicates that photosynthesis is occurring faster than respiration and a decrease in pH indicates that respiration is occurring faster than photosynthesis.

The relative rates of photosynthesis and respiration by the producers in an ecosystem are expressed by net primary productivity. Gross primary productivity is a measurement of the amount of energy converted from sunlight to chemical potential energy and is generally measured as the amount of carbon fixed into glucose during photosynthesis. Net primary productivity is the amount of photosynthesis minus the amount of respiration by the producers in an ecosystem. To simplify this concept, this experiment uses a controlled ecosystem containing one type of producer.

Experiment Overview

The purpose of this experiment is to measure the pH of a solution containing algae encapsulated in sodium alginate. The algae that is constantly exposed to light will be compared with algae that receives no light to see if the pH of the two solutions is different. The results will be connected to relative rates of photosynthesis and respiration, or net primary productivity.


Bromthymol blue indicator solution, 0.005% 10 mL
Calcium chloride, 0.3 M, 30 mL
Sodium alginate and algae mixture, 15 mL
Beakers, 50 mL, 2
Beaker, 400 mL
Cheesecloth, 20 cm x 20 cm square
Funnel or strainer
Graduated cylinder, 50 mL
Lamp with 150–200 W halogen bulb or 42 W CFL
Light-blocking tube
Meter stick
Support stand with clamp
Syringe, 20 mL
Vials with screw caps, 7 mL, 2
Weighing dishes, 2

Prelab Questions

  1. How do the sodium alginate and algae spheres model a plant cell?
  2. How does dissolved carbon dioxide change the pH of the solution?
  3. What is the difference between primary productivity and net primary productivity?

Safety Precautions

All materials are nonhazardous. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.


Part A. Preparing the Algae Spheres

  1. Add 30 mL of calcium chloride solution to a 50-mL beaker.
  2. Draw up 15 mL of sodium alginate and algae mixture into the syringe.
  3. Secure the syringe with a clamp attached to a support stand and place the calcium chloride solution beneath the syringe as shown in the Figure 1.
  4. Allow the sodium alginate and algae mixture to drip into the beaker until approximately 50 spheres of equal size have been made. You may need to apply a slight amount of pressure to the plunger.
  5. Fit the cheesecloth in the funnel or use a strainer with a fine mesh.
  6. Pour the algae spheres into the funnel and strain the calcium chloride solution into a clean, empty beaker.
  7. Pour 20 algae spheres into a weighing dish and record the mass values of the spheres. It should be close to one gram.
  8. Repeat step 7 with a second weighing dish and ensure the mass values of the spheres in the two dishes are the same within 0.1 g, and the spheres are of uniform size. Record the mass on the Sodium Alginate Photosynthesis Worksheet.
Part B. Testing the impact of light on pH
  1. Add 20 algae spheres to each vial.
  2. Using a pipet, fill the vial with bromthymol blue solution so that there is minimal space at the top. Screw on the cap.
  3. Record the color and corresponding pH of the solution in each vial on the worksheet.
  4. Arrange the vials so both are approximately 15 cm from the light source.
  5. Place the light-blocking tube over one of the vials.
  6. Place a 400-mL beaker of water between the light source and the vials to act as a heat sink.
  7. Turn on the light.
  8. After 30 minutes, check for color change. If time allows, check for color change every 30 minutes. It may take several hours to see a change in color.
  9. Once a color change is detected, record the total time elapsed, the color of the solution and the pH of each vial on the worksheet.
  10. Calculate the change in pH and record on the worksheet.
  11. Answer all analysis and calculation questions on the worksheet.
  12. Consult your instructor for appropriate disposal procedures. The calcium chloride solution can be reused many times.

Student Worksheet PDF


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