Biology
Sensors:
Loggers:
Light, pH
Any EASYSENSE
Logging time: 24 hours (EasyLog)
09 How does photosynthetic activity vary with
light intensity?
Read
Green plants are called producers because they are able to make sugars like glucose by a process
called photosynthesis. The sugars are then used to make foods such as complex carbohydrates e.g.
starch. To carry out photosynthesis green plants need carbon dioxide gas (CO2), water and a source of
energy in the form of light. Oxygen (O2) is produced as a waste product, which is released into the air
(or water for an aquatic plant).
When carbon dioxide is dissolved in water the solution becomes more acidic (lowers the pH). If the
carbon dioxide in water is used up and replaced with oxygen from the plant then the water becomes
more alkaline (higher pH). A pH electrode can be used to measure the acidity of the water and
therefore monitor any changes in the level of dissolved carbon dioxide (as an indirect measurement of
photosynthesis). The electrode will record the changes in pH created by changes in CO2 levels in the
water due to respiration or photosynthesis.
In this investigation you will monitor the level of light to see how this affects the photosynthetic
activity of an aquatic plant as measured by the changes in pH.
L2 Biology
09 - 1 (V2)
What you need
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9.
An EASYSENSE logger that has EasyLog mode.
A Smart Q pH sensor (electrode and adapter).
A Smart Q Light level sensor – 100,000 Lux
Conical flask.
Retort stand and clamp.
Magnetic stirrer (optional).
Plasticine or Blu Tack.
Pond weed (Elodea or similar).
Water.
What you need to do
1.
Connect the pH and Light level sensor to the logger. Set up the apparatus as shown in the
diagram in a place it where there is plenty of sunshine or light e.g. on a windowsill.
2. Select EasyLog from the EASYSENSE logger’s menu and start logging.
3. After at least 24 hours stop the recording and connect the logger to the computer.
4. Start the EasySense software, select Retrieve Remote from the Home screen and download
the data.
5. Label the pH graph as carbon dioxide and mark its lowest and highest points.
Results and analysis
Add a Title to the graph and then Save the results. The results can be printed or copied into your
report document as required.
The graph will show two traces against time, one for the light levels and the other showing pH values.
Use Values to examine your graph. You may find it useful to rescale the axis using Sensor Settings
(Options).
It should be possible to correlate the light levels to the pH values. A rise in pH will indicate a
reduction in carbon dioxide levels and rise in oxygen levels. This is an indication of the change in rate
of photosynthesis.
Questions
1.
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Describe the pattern shown in the graph of pH change over the time period studied.
2. Explain how the change in pH tells you how the amount of dissolved carbon dioxide changed.
3. How does photosynthetic activity change with light levels?
4. What will happen to pH in bright light and darkness?
5. What is the relationship between light levels and photosynthetic activity? Use the graph to
explain if the plant carries out photosynthesis all the time.
6. What is the word equation that describes photosynthesis?
7. From where does the plant get the energy to carry out photosynthesis?
8. What process is taking place all the time in the plant but is more obvious in darkness?
L2 Biology
09 - 2 (V2)