What are the possible ways of measuring rate of photosynthesis?
There are several methods available to you.
1) Dry mass of the plant is the usual method employed. 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.
2) 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.
3) There is a gadget called an Infra Red Gas Analyser (IRGA) which can compare the CO2 concentration of air entering a leaf chamber and leaving the chamber. The amount of CO2 taken up by the leaf can be measured. This system is likely to only be available in a research laboratory.
4) We wonder whether you could put a plant in a plastic bag and monitor the CO2 concentration in the bag. Naturally, the soil and roots must NOT be in the bag (as they respire). There are sensitive CO2 monitors now available and one was described in OSMOSIS 27. You could measure the concentration of CO2 concentration using the J-tube method (ask us if you are not familiar with this and you are interested in this method). 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.
How can I measure the rate of photosynthesis?
You could put 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 will find this method in most GCSE text books - it is called Audus apparatus. You can investigate the amount of gas produced at different distances from a lamp.
Here are some practical tips to help you make sure the experiment produces reliable results.
1) It is useful to set it up the apparatus at first with pure water and then slowly increase the bicarbonate solution to 0.5 molar. This willl enable you to find what is the best concentration to use. When you have found that, you can start to change the light levels.
2) It is vital to have a stabilising period of a minimum 5 minutes before any measurements are taken.
3) Don't forget that the Cabomba must be upside down so that the bubbles escape easily through the cut stem. A really clean cut with a sharp scalpel helps.
4) Sometimes the Cabomba fails to work in very bright light (for example if you put it very close to a mecury vapour lamp or a floodlight such as is used for security lighting), so a sunny windowsill would be best. However, without artificial light, you have the problem of making sure that you make every measurement at the same light intensity. In a fair experiment, you need to make sure that only one variable is changing and that could be light in your investigation.
What are the different methods of measuring rate of photosynthesis?
1) Measure the uptake of CO2
2) Measure the production of O2
3) Measure the production of carbohydrates
4) Measure increase in dry mass
The only one which won't work is to measure the uptake of water!
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
CO2 uptake can also be measured by following the uptake of carbon dioxide labelled with 14C
Oxygen can be measured by a) counting bubbles evolved from pond weed b) with the Audus apparatus
The rate of decolourisation of DCPIP in the Hill Reaction is a measure of the rate of the light-requiring stages of photosynthesis
The production of carbohydrates can be measured crudely by monitoring the dry mass of a disc cut out from one side of leaf (with a cork borer) and compared with the dry mass of a similar disc cut from the other side some weeks later.
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.
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.