‘Algal balls’ – Investigating photosynthesis and respiration: post-16 version


Photosynthesis and respiration can be hard topics to demonstrate reliably in the lab, especially in winter. Using ‘algal balls’ (immobilised photosynthetic algae) makes this a reliable practical that can stretch students’ understanding of the topics as appropriate. The basic protocol can be used with lower secondary pupils, but here has been extended for post-16 students.

Algae (Scenedesmus quadricauda) can be purchased from our recommended suppliers, Darwin Biological. and Blades 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.

This page includes the post-16 versions of the resources only: for the full technical notes and basic student guide, see our standard Photosynthesis with Algal Balls pages.

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.

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