The effect of temperature on photosynthetic rate of Elodea (pondweed).

We investigated in which temperature plants will produce the biggest amount of oxygen. So then on nwe planet people will have the amount of oxygen needed to live.


In this experiment, the relationship between the photosynthesis rate and temperature on Elodea is investigated.  Photosynthesis is the chemical change by which plants use carbon dioxide and water, with the aid of sunlight, to make glucose and oxygen.

The equation for photosynthesis is as follows:

Carbon dioxide + water ->glucose + oxygen

Photosynthesis is a vital process in the production of energy, occurring in the leaves of most plants. It allows them to convert light energy into chemical energy. This information is quite useful knowledge for farmers as in order to grow their crops they need the optimum temperature for photosynthesis. There are three main limiting factors: light intensity, carbon dioxide concentration and temperature. As shown by the equation, it is necessary that a plant have carbon dioxide in order to photosynthesize, as well as light for growth. It is yet known, however, if an increase in temperature has a positive or negative impact on photosynthesis. Rate is the change occurring over time.

The hypothesis of this experiment is: if temperature increases, the rate of photosynthesis will also increase. To have a high photosynthesis rate, the plants need an optimum temperature.

A few variables need to be taken into consideration. The more controlled variables there are, the more accurate the experiment will be.

In order to complete this experiment, the following apparatus are required.



-Cutting tile

-Pondweed – Elodea (1 small branch)

-Capillary tube




-Lux meter

– 500ml Beaker


-Thermometer (0-100 degrees)

-Kettle Boiler



1)    Choose a branch of Elodea and cut the stem by 5mm to ensure the xylem and phloem vessels are not blocked.

2)    Fill a sink with water as the experiment must occur under water at all times. This is to keep all air bubbles out of the capillary tube as to not falsify our results due to the miscounting of an air bubble, which was not produced by photosynthesis.

3)    Use a centimetre syringe to empty the capillary tube of all air bubbles by putting it to one end  and pulling through until no gas is left. Do this under water ensuring both ends of the tube remain under water.

4)    Attach the empty syringe to one end of the capillary tube.

5)    Put the stem of the pondweed in the other side of the capillary tube as far in as the branches allow. Keep it under water at all times.

6)    Fill a beaker with water and keeping the whole arrangement under water, place the pondweed inside the beaker (keeping the stem in the capillary tube).

7)    Once everything is put in place, the syringe in one end of the capillary tube and the stem of the pondweed in the other end, with the pondweed in the beaker full of water; take the whole setup out of the water and place the capillary tube on the stand.

8)    To change the temperature take out some water from the beaker using pipettes putting it into an empty beaker and refill the beaker with warmed up water from kettle

9)    Place the thermometer in the beaker without disturbing the pondweed.

10) Install the lux meter near the plant and watch out for any variation throughout the experiment.

11) Start a timer as soon as the water bath is filled and the correct temperature is obtained

12) Record data every minute by pulling the syringe slightly until air bubbles released by the Elodea appear in the capillary tube for measurement.

13)  Repeat the reading 5 times for each of the 5 different temperatures.



Health and Safety:

-Be careful whilst using electricity and water next to one another. Always dry your hands and try to work as far as possible from a plug socket.

-Be careful whilst touching the lamp head, it can get very hot.

-Beware whilst handling hot water

-In order to be ethical, once the experiment is over, the Elodea will be recycled by releasing it back into its original habitat, a river or stream.

-The capillary tube can be very fragile and breaks easily so take care whilst handling the glass.


Data Collection and Processing


It was observed that as the temperature increases, the rate of photosynthesis increases as well. When the water bath was at 25 degrees Celsius, the rate of photosynthesis was very low and few bubbles were released. However, as the temperature increased, the number of bubbles released increased as well. There were fewer time intervals between each bubble. It was also observed that as the temperature reached over 50 degrees, the amount of bubbles produced started decreasing.


Data Presentation:

Temperature (degrees Celsius)
Photosynthetic rate (mm^3) 25 °C 35 °C 45 °C 55 °C 65 °C
Trial 1 7.0 45 80 44 36
Trial 2 6.0 40 70 38 34
Trial 3 4.0 40 71 31 24
Trial 4 2.3 26 69 24 20
Trial 5 2.1 25 66 20 19
Average Bubbles released (mm^3/min 4.28 35.2 71.2 31.4 26.6
Standard Deviation 2.19 9.09 5.26 9.48 7.92


The original hypothesis of this experiment was that as the temperature increases, the rate of photosynthesis will also increase. This is only partly true because as we can see from quantitative data, the increase of temperature does increase the photosynthetic rate but only up to 45 °C. Photosynthesis itself is a chemical reaction. There are five main ways to speed up a reaction. One can increase the surface area, increase the pressure, increase the concentration, use a catalyst and increase the temperature. According to the above statement, the greater the temperature, the greater rate of photosynthesis, however this only partially applies to this experiment because after a certain temperature, the rate begins to fall. This is due to the use of enzymes during the reaction of photosynthesis. Enzymes are used as catalysts. In order for them to function properly, they require a specific temperature, or optimum temperature. If it is too cold, as seen in table with a temperature of 25°C, the rate of reaction is low, the enzymes have low kinetic energy and fewer collisions occur. At the optimum temperature however, the enzymes have the most collisions occurring at any given time. If the optimum temperature is exceeded, the enzymes begin to deform and can no longer collide as they have changed shape, they have denatured.

Data shows very clear results for this experiment. Between 25°C to 35°C, the photosynthetic rate gradually increases. The minimum point is found at 25°C when no bubbles are released. From 35°C to 45°C there is a rapid yet steady increase before reaching its maximum photosynthetic rate at 45°C with an average of 71.2 mm^3. From that point onwards, the photosynthetic rate decreases rapidly going from around 70 to 26.6 mm^3 in a span of 20°C.


Whilst performing the experiment, trying to keep the water temperature in the beaker constant was difficult, there were slight fluctuations. In order to have data at exactly the same temperature for each reading, one could take water with higher speed. This would avoid having results for 24°C, 25°C, 26°C (when what you want is 25°C) and would just keep it constant for one minute instead of five consecutive ones. Also, there was an accuracy of half a cubic millimetre in the capillary tube. In order to avoid this, one could use a more precise capillary tube with smaller readings. When it was time to pull the syringe in order to make the air bubbles go to the capillary tube, not all bubbles would be together to form one big bubble. The solution of this would be to count the smaller bubbles next to one another in order to obtain the total air released. There were a few gas bubbles, which were left behind and therefore were not part of the reading. Next time, we could ensure to pull the syringe a little further, in order to have all the gas released during that period of time in the capillary tube, and adding the spare bubbles together. In order for a farmer to grow his plants, he would need an optimum temperature of 45°C, high levels of sunlight and abundant irrigation as well as optimum levels of CO2. Even though the pH of the experiment was not controlled, this did not affect the temperature or the rate of photosynthesis as it did not change since the same variables were kept (water, light, etc…)


The data collected during this experiment supported the hypothesis that if temperature increases, the rate of photosynthesis will also increase, only partially. The temperature does increase the rate of photosynthesis up to 45°C. After the optimum point, the rate begins to decrease due to the denaturing of the enzymes(as they are proteins they denature in 40 °C) , causing fewer collisions between the enzyme and its substrate. This knowledge is useful for us to know as if we wish to grow plants or crops, it is necessary that we do not exceed 45°C.




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