Measuring the rate of photosynthesis

Without photosynthesis life as we know it would not exist. It’s worth a moment’s reflection…

There would be no biology without photosynthesis. Plant biomass is the food and fuel for all animals. Plants are the primary producers. These amazing organisms are capable of capturing the energy of sunlight and fixing it in the form of potential chemical energy in organic compounds. The organic compounds are constructed from two principle raw materials; carbon dioxide and water (which is a source of hydrogen). These compounds are stable and can be stored until required for life processes. Hence animals, fungi and non-photosynthetic bacteria depend on these for the maintenance of life.

But how can we measure the rates at which photosynthesis takes place?

The quantities are mind boggling. A hectare (e.g. a field 100 m by 100 m) of wheat can convert as much as 10,000 kg of carbon from carbon dioxide into the carbon of sugar in a year, giving a total yield of 25,000 kg of sugar per year.

There is a total of 7000 x 10⁹ tonnes of carbon dioxide in the atmosphere and photosynthesis fixes 100 x 10⁹ tonnes per year. So 15% of the total carbon dioxide in the atmosphere moves into photosynthetic organisms each year.

What are the different methods of measuring the rate of photosynthesis?

There are a few key methods to calculate the rate of photosynthesis. These include:

1) Measuring the uptake of CO₂

2) Measuring the production of O₂

3) Measuring the production of carbohydrates

4) Measuring the increase in dry mass

As the equation for respiration is almost the reverse of the one for photosynthesis, you will need to think whether these methods measure photosynthesis alone or whether they are measuring the balance between photosynthesis and respiration.

Measuring photosynthesis via the uptake of carbon dioxide

Using 'immobilised algae' - It's easy and accurate to measure the rate of photosynthesis and respiration using immobilised algae in hydrogen carbonate indicator solution - known as the 'algal balls' technique. Read the full protocol on using immobilsed algae to measure photosynthesis.

Using an IRGA - Uptake of CO₂ can be measured with the means of an IRGA (Infra-Red Gas Analyser) which can compare the CO₂ concentration in gas passing into a chamber surrounding a leaf/plant and the CO₂ leaving the chamber.

Using a CO₂ monitor - More simply, you could put a plant in a plastic bag and monitor the CO₂ concentration in the bag using a CO₂ monitor. Naturally, the soil and roots must NOT be in the bag (as they respire). Alternatively, you could place some Bicarbonate Indicator Solution in the bag with the plant and watch the colour change. This would best be done with a reference colour chart to try to make the end-point less subjective. This could give a comparison between several plants. There are difficulties with this method, as I'm sure you can appreciate. The leaf area of the plants should be measured so you can compensate for plant size. Atmospheric air is only 400ppm CO₂, so there is not much CO₂ to monitor and the plant will soon run out of CO₂ to fix.

Measuring photosynthesis via the production of oxygen

Oxygen can be measured by counting bubbles evolved from pondweed, or by using the Audus apparatus to measure the amount of gas evolved over a period of time. To do this, place Cabomba pondweed in an upside down syringe in a water bath connected to a capillary tube (you can also use Elodea, but we find Cabomba more reliable). Put the weed in a solution of NaHCO₃ solution. You can then investigate the amount of gas produced at different distances from a lamp. Read a full protocol on how to investigate photosynthesis using pondweed.

Measuring photosynthesis via the production of carbohydrates

There is a crude method where a disc is cut out of one side of a leaf (using a cork borer against a rubber bung) and weighed after drying. Some days (or even weeks later), a disk is cut out of the other half of the leaf, dried and weighed. Increase in mass of the disc is an indication of the extra mass that has been stored in the leaf. This is very simple to do and enables you to investigate plants growing in the wild. However, you can probably think of several inaccuracies in this method.

Measuring photosynthesis via the increase in dry mass

Dry mass is often monitored by the technique of 'serial harvests' where several plants are harvested, dried to constant weight and weighed - this is repeated over the duration of the experiment. If you harvest several plants and record how much mass they have accumulated you will have an accurate measure of the surplus photosynthesis over and above the respiration that has taken place. As with most methods, you need several plants so you have replicate measurements and you can find an average and a standard deviation if necessary.

Investigating the light-dependent reaction in photosynthesis

The rate of decolourisation of DCPIP in the Hill Reaction is a measure of the rate of the light-requiring stages of photosynthesis