Skip to 0 minutes and 6 secondsBEVERLEY GOODGER: This experiment uses hydrogen carbonate indicator to investigate the uptake or release of carbon dioxide by living organisms. Before starting any experiments, the indicator will need to be equilibrated with the carbon dioxide in the atmosphere. This can be done by bubbling the indicator with a clean air stone, or by pouring it between clean beakers. It should be cherry red when equilibrated with the atmosphere. We will demonstrate the use of hydrogen carbonate indicator with three organisms, pond weed, algae, and basil leaves. To investigate gas exchange in pond weed, take four transparent containers that can be easily sealed. We're using 7 ml bijou bottles. Rinse the containers with a small amount of indicator to ensure that they are free from contaminants.

Skip to 0 minutes and 59 secondsFill each container with bicarbonate indicator that has been equilibrated with atmospheric carbon dioxide.

Skip to 1 minute and 14 secondsIn two of the containers, place equal length sprigs of pond weed. The bottles must be filled to the top to avoid the effect of any atmospheric carbon dioxide in the bottle. Seal all of the containers. Place one of the containers of pond weed into a dark cupboard together with one of the tubes containing indicator only. You can also use black paper to cover these tubes. Place the other tube of pond weed next to a white light source together with the fourth tube that only contains indicator. You need a bright light that is over 2,000 lumens. Make sure that the lamp will not cause your pond weed to overheat.

Skip to 1 minute and 57 secondsModern LED bulbs have a negligible heating effect, but you will need to make sure that the students understand that a heat sink, such as a transparent screen between the light and the plant, or a flat-sided container of cold water to absorb the heat, could be needed with some bulbs, as this is still mentioned in exam questions. Leave the tubes in position until there is a noticeable change in colour in the tube of bicarbonate indicator that contains pond weed in the light. Compare the colours in all four tubes. The observed colours change can now be compared against the bicarbonate indicator colour change chart or a set of buffer solutions with bicarbonate indicator.

Skip to 2 minutes and 40 secondsThe exchange of gases in a photosynthetic organism can also be investigated using immobilised green algae. This intriguing and reliable practical investigation can be used with students from 11 to post 16, offering quantifiable and replicable results. You can find out how to make algal balls using the video on the SAPS website and YouTube channel.

Skip to 3 minutes and 6 secondsTo investigate rates of respiration and photosynthesis using algal balls in hydrogen carbonate indicator, set up four sealed tubes of algal balls in equilibrated bicarbonate indicator solution, as in the previous investigation. We recommend 7 ml bijou bottles, or even Eppendorf tubes. Anything bigger than 7 ml and you will not see a clear colour change in all the indicator over the period of the lesson. You can also observe similar colour changes using whole leaves detached from a plant in a vial with a small amount of indicator, but you would need larger tubes. We've used basil here. Using this method, the colour changes are not as obvious because the leaf is detached, and this setup will only last for a few hours.

Gas exchange in plants

This video demonstrates how to set up investigations into the uptake or release of carbon dioxide by pondweed, algae immobilised in alginate beads and whole leaves detached from a plant, using bicarbonate indicator.

This indicator helps us understand the relative levels of carbon dioxide in the experimental set up and then use the colour change to look at the uptake or release of carbon dioxide by the organisms in the set up. The colour changes of the indicator can therefore be used to determine whether the amount of carbon dioxide taken up by a photosynthesising organism is greater than, less than or equal to the amount of carbon dioxide released by its respiration. This in turn allows the common misconception that plants only respire at night to be tested first-hand.

Hydrogen carbonate indicator is used to measure carbon dioxide levels in aquatic systems. It is red in equilibrium with atmospheric air.

It becomes more orange/yellow with increased carbon dioxide levels so with higher rates of respiration than photosynthesis, it will turn yellow.

It changes from red through magenta to deep purple as carbon dioxide is removed so that with higher rates of photosynthesis than respiration, it will turn purple.

Before starting any experiments the indicator will need to be equilibrated with the carbon dioxide in the atmosphere. This can be done by bubbling the indicator with a clean air stone or by pouring it between clean beakers. It should be “cherry red” when equilibrated with the atmosphere. If you purchase the indicator it will likely be a concentrated solution and need diluting before use.

We have provided full instructions detailing how to set up the investigation using immobilised algae, including a bicarbonate indicator colour chart:

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What conceptual leaps must the students make if they are to use bicarbonate indicator to investigate rates of respiration and photosynthesis in plants?

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Teaching Biology: Inspiring Students with Plant Science

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