‘Algal balls’ – Photosynthesis using algae wrapped in jelly balls


Photosynthesis can be a hard topic to demonstrate reliably in the lab, especially in winter. This fun and reliable practical makes investigating photosynthesis easy, with a technique that can be used with students from KS3 to post-16, and offering quantifiable and replicable results.

This page includes the core information on the protocol and the students’ notes for 11-16 students. For post-16 students, please see our updated post-16 notes.

Algae (Scenedesmus quadricauda) can be purchased from our recommended supplier, Darwin Biological. We suggest that a 30ml sample will need to be grown for 2-3 weeks using the growth medium recipe to ensure there is a sufficient amount for a class of 30. Please note that the algae can be purchased from other school suppliers.

Green algae photosynthesise in a way similar to that seen in C3 higher plants. In this practical, students use algae to look at the rate of photosynthesis. Since algae are tiny and are difficult to work with directly in the water,  the first part of the practical involves ‘immobilising’ the algae as algal balls. This effectively traps large numbers of algal cells in ‘jelly like’ balls made of sodium alginate. Sodium alginate is not harmful to the algae, and they will continue to photosynthesise once immobilised.

When these algae are ‘wrapped up’ in the jelly balls they are excellent to use in experiments on photosynthesis. These algal balls are:

  • cheap to grow and easy to make – you will be able to make hundreds in a very short time
  • easy to get a standard quantity of plant material because each of the balls is approximately the same volume
  • easy to keep alive for several weeks so you can keep them for future experiments

A kit, produced in collaboration with the NCBE, is available for this practical.

Further details to support those doing further investigations with this protocol with post-16 students are also available.

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