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GCSE Science Biology (9-1) Required Practical 6: Photosynthesis

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. The quantities are mind boggling. A hectare e. There is a total of x 10 9 tonnes of carbon dioxide in the atmosphere and photosynthesis fixes x 10 9 tonnes per year. 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. 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 a CO 2 monitor - More simply, you could put a plant in a plastic bag and monitor the CO 2 concentration in the bag using a CO 2 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.

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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 ppm CO 2 , so there is not much CO 2 to monitor and the plant will soon run out of CO 2 to fix. 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 NaHCO 3 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. 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. Possible practical work you may have encountered - methods of measuring the rate of photosynthesis.


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You can investigate the need for chlorophyll for photosynthesis with variegated leaves. Taking thin slices of potato and apple and adding iodine to observe under the microscope - test for starch. Investigating the effects of light, temperature and carbon dioxide levels using Canadian pondweed, Cabomba, algal balls or leaf discs from brassicas on the rate of photosynthesis. You can use computer simulations to model the rate of photosynthesis in different conditions. You can use sensors to investigate the effect of carbon dioxide and light levels on the rate of photosynthesis and the release of oxygen.


  • What are the different methods of measuring the rate of photosynthesis?;
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All experimental methods depend on measuring the rate of oxygen production as a measure of the rate of photosynthesis. The faster the oxygen production the faster the photosynthesis. It is assumed that the rate of oxygen production is proportional to the rate of photosynthesis. So, how can we measure the rate of photosynthesis? Next, methods of measuring the rate of photosynthesis.

Measuring the r ate of photosynthesis - experimental method 1 measuring the volume of oxygen produced. Method 1.

Photosynthesis Lab: Light Intensity by Anna Dang on Prezi

There are several aquatic plants you can use, the most popular seems to Canadian pondweed elodea canadensis , but this is regarded as an invasive species, so perhaps some other oxygenated aquatic plant should be used! In this 'set-up' you measure the rate of photosynthesis by measuring the rate of oxygen production as the gas is collected in the gas syringe.

From the graph of volume of oxygen versus time you measure the initial gradient to calculate the rate of production of oxygen as a measure of the rate of photosynthesis. The graph should be reasonably linear at first e. You can use sodium hydrogencarbonate NaHCO 3 as source of carbon dioxide and vary its concentration to vary the carbon dioxide concentration.

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You can use from 0. The light from the laboratory itself will contribute, but the total light should be constant. To vary temperature you need to immerse the conical flask in a water bath not shown of different, but carefully controlled constant temperatures. The concentration of NaHCO 3 and the light intensity should be both kept constant.

Lab 5: Rates of Photosynthesis in Response to Increasing Light Intensity

Varying the light intensity is quite difficult, you need to position a lamp at different measured distances away, but for accurate results you must take a light meter reading by the flask in the direction of the lamp. A lamp in position is shown in method 2. This simple experiment can readily show in principle the effect of changing the three controlling factors of the rate of photosynthesis. Ideally the experiments should be done in the dark, with the lamp the only source of light, not very convenient in a classroom situation but it is particularly important when varying the light intensity - I don't see how you can get accurate results for light intensity though using a light meter might just ok?

Do you swirl the flask so the NaHCO 3 concentration remains reasonably constant? When varying the temperature it is not easy to maintain a constant temperature - if it falls a little, you could use the average temperature, not as accurate, but better than nothing! A thermostated water bath would be ideal. The above apparatus is typical of that used in rate of reaction experiments in chemistry. How can we measure the speed or rate of a chemical reaction? You can use other experiment designs to look more conveniently, and hopefully more accurately at the three factors that influence the rate of photosynthesis eg.

Measuring the r ate of photosynthesis - experimental method 2 - timing the movement of a bubble. I've seen this sort of set-up in textbooks and on the internet and it seems ok in principle, but I have doubts about its use in practice?

Lab: Exploring the Rate of Photosynthesis

In this the Canadian pondweed elodea is enclosed in a boiling tube and placed in a large beaker of water that acts as a simple thermostated bath to keep the temperature constant. Again a thermostated water bath would be ideal. A lamp is positioned at suitable distances with a ruler. The oxygen bubbles are channelled into a capillary tube. From the rate of movement of the bubbles you get an estimate of the rate of production of oxygen as a measure of the rate of photosynthesis. It might ok just to measure the speed of a bubble down the capillary tube, BUT what happens if it fills with oxygen gas - you won't see any movement.

The general points about investigating the three variables were described in method 1.

You can measure the speed of an air bubble by the scale,. If you used a gas syringe here you would get a mixture of gas and liquid in the syringe - not very satisfactory, liquid in the syringe might make it quite stiff in movement and difficult to measure an accurate volume of oxygen gas formed. Further thoughts on the experimental methods described in methods 1. The 'set-up' probably the best system I can devise sitting at home in front of the computer screen!