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By Will Stark, Head of Science

Blog published 9th January 2017 | Category: Supporting VI Students

A common practical when studying plant biology is investigating factors affecting the rate of photosynthesis. Plants produce oxygen which is invisible and odourless, so the practical is usually carried out with water plants; the bubbles of oxygen produced are observed. The practical is usually carried out with a common pond weed, and sighted students are instructed to count tiny bubbles - the faster the rate of bubble production, the faster the rate of photosynthesis.

This presents a problem for students with VI since the bubbles are often too small to see and therefore counting them and gathering valid results is not possible. A typical arrangement is shown below:

Photosynthesis practical set up

An alternative method is to invert a boiling tube which is full of water, collecting oxygen in the boiling tube. This requires leaving the plant for several hours, since the cross-sectional area of the boiling tube is large and requires a lot of gas to displace the water. This is not something that can be done during a single lesson. This arrangement is shown below:

Photosynthesis Practical alternative

This experiment can be adapted so that results can be gathered in 20 minutes or less. Firstly, add sodium hydrogencarbonate to the water which introduces carbon dioxide to the water to speed up the rate. Use an aquatic plant such as elodea, but plants such as hottonia palustris also work well. Cabomba may be used in some countries, but it is restricted in the UK, due to it being an invasive species and a risk to local plant populations if it is released into the environment. The gas collection can be modified by using a standard plastic pipette, cutting the bulb in half, then sealing the end with a flame.

Photosynthesis Modification

The two layers of glass provides protection from the heat from the lamp, and the water level moves down rapidly as the gas is collected, making it possible to gather results in well under an hour.

Image showing water level

For students who cannot see the water level, an audible light probe can be used to find the water – by pointing it at the lamp through the pipette, the pitch changes when the light probe points at the meniscus at the top of the water. This can then be marked with a tactile marker, and the amount of gas collected can be measured with a ruler and compared to other experiments. Light intensity can be varied by adjusting the distance between the lamp and the plant.

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