Using Algae Beads as a Model for Photosynthesis

A Carolina Essentials™ Activity

Total Time: 45 mins

Prep: 30 mins | Activity: 45 mins

Life Science


Middle/High School


Students are introduced to photosynthesis in a hands-on activity with fresh water algae. By creating algae beads (made of algae and sodium alginate solution), they indirectly observe the change in concentration of oxygen in algae beads that are exposed to different amounts of light. Students make qualitative and quantitative observations of the algae culture tubes for a period of 4 days. Students develop a model of photosynthesis as oxygen is trapped in the algae beads, changing the density of the beads and causing them to rise to the surface.

Essential Question

How does photosynthesis transform light energy into stored chemical energy?

Activity Objectives

  1. Create green algae spheres for use in photosynthesis experiments.
  2. Observe algae beads for evidence of photosynthesis.

Next Generation Science Standards* (NGSS)

Science & Engineering Practices

Developing and Using Models

Disciplinary Core Ideas

LS1.C: Organization for Matter and Energy Flow in Organisms

Crosscutting Concepts

Energy and Matter

Cause and Effect


Safety and Disposal

Wash contents down a lab sink/drain using plenty of water. Dispose of any waste in accordance with local regulations.


Use a highly concentrated algae culture that appears dark green. If beginning with a non-concentrated culture, allow it to grow for 3 to 4 weeks under constant lighting to achieve desired population density. It should appear dark green prior to use. The algae beads can be stored in the distilled water and refrigerated for approximately 2 weeks.

Student Procedures

  1. Place 5 mL of green algae culture into a culture tube.
  2. Add 2.5 mL of 2% sodium alginate solution to the tube, place the cap on the tube, and mix for 1 to 2 minutes.
  3. Collect some of the mixture into your pipet.
  4. Hold the pipet over a beaker containing 3% calcium chloride (CaCl2) solution. The calcium chloride solution should be cold.
  5. Gently depress the bulb on your pipet to release the algae mixture drop by drop into the beaker. Do this slowly and uniformly. As the algae mixture drops in the calcium chloride, the algae will be immobilized inside of a bead. Your mixture should allow you to make approximately 100 algae beads.
  6. Collect your beads using the plastic spoon provided and rinse them using the distilled water in the wash bottle.
  7. Transfer the beads to your clean culture tubes and fill the tubes with distilled water.
  8. Tightly wrap one tube with aluminum foil so no light can penetrate.
  9. Place the tubes in a well-lit area designated by your teacher.
  10. Observe the algae beads daily for 3 to 4 days. Rinse the tubes when finished.
  11. Observe the algae beads daily for 3 to 4 days. Rinse the tubes when finished.

Teacher Preparation and Tips

  1. Prepare the algae culture 3 to 4 weeks prior to the activity to ensure a high concentration of algae. The culture should be dark green.
  1. Prepare the CaCl2 solution a day prior to use and refrigerate it. Keep it cold throughout the procedure.
  2. Make sure students are not squirting algae into the CaCl2 solution. Each drop should form a bead.
  1. Use a light bank or lamp with a flexible neck. The light should remain on for the duration of observations.

Data and Observations

Analysis & Discussion

  1. Explain the differences in results between the covered and uncover tubes.

    The uncovered tube has the light necessary to photosynthesize, but the covered one does not.

  2. What causes the algae beads to float to the surface?

    The build-up of oxygen trapped inside the beads.

  3. Use the evidence you gathered to develop a model explaining the relationship between the position of algae beads in the tube and photosynthesis.

The oxygen gas produced during photosynthesis in the unwrapped tube is trapped in the algae beads. As oxygen gas builds up, the density of a bead decreases, allowing the bead to float to the surface. Algae beads do not come to the surface in the wrapped tube because without light, the algae cannot photosynthesize.

*Next Generation Science Standards® is a registered trademark of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of, and do not endorse, these products.

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