Photosynthesis - Starch
This practical lesson covers:
- The relationship between the rate of photosynthesis and A. light intensity, B. carbon dioxide, and C. chlorophyll.
Investigating factors that affect the rate of photosynthesis.
Aim:
To investigate the rate of photosynthesis, the production of starch and the requirement for light, carbon dioxide and chlorophyll.
Background information:
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, transforming carbon dioxide and water into glucose and oxygen. This practical investigates the effects of light intensity, carbon dioxide concentration, and chlorophyll presence on the rate of photosynthesis. You will measure oxygen production from pondweed as an indicator of photosynthesis and test for the presence of starch in variegated leaves to assess the role of chlorophyll.
Variables:
- Independent variable - Light intensity, concentration of carbon dioxide, or presence of chlorophyll.
- Dependent variable - Rate of photosynthesis.
- Control variables - Temperature of water, type of pondweed, duration of experiment.
Equipment:
- Boiling tube
- 10 cm piece of pondweed
- Light source
- Ruler
- Test tube rack
- Timer
- Sodium hydrogen carbonate (varying concentrations needed including 0.2%)
- Glass rod
- White tile
- Iodine solution
Method 1 - light intensity
- Place the light source on the table and measure 10 cm away from the lamp. Place the test tube rack containing a boiling tube at this distance.
- Add 0.2% sodium hydrogen carbonate solution to the boiling tube.
- Place the pondweed into the boiling tube with the cut end at the top. Use the glass rod to push the pondweed into the boiling tube.
- Leave the boiling tube to rest for 5 minutes.
- Start the timer and count the number of bubbles produced in one minute.
- Repeat twice and calculate a mean.
- Repeat steps 1-6 for distances of (15, 20, 25, 30, 35, 40 cm) of the boiling tube from the light source.
- Calculate the light intensity at each distance using the inverse square law.
- Plot a graph of light intensity against the rate of photosynthesis.
Method 2 - requirement for carbon dioxide
- Measure 10 cm from the light source using a ruler. Place the test tube rack containing a boiling tube at this distance.
- Add sodium hydrogen carbonate solution (0.2%) to the boiling tube.
- Place the cut pondweed into the boiling tube with the cut end at the top. Gently push the pondweed down with the glass rod.
- Leave the boiling tube to rest for 5 minutes.
- Start the timer and count the number of bubbles produced in two minutes.
- For each sodium hydrogen carbonate concentration, repeat the count twice more and take a mean.
- Repeat steps 1-7 for 3 more concentrations of sodium hydrogen carbonate solution.
- Plot a graph of the rate of photosynthesis (given by the no. of bubbles) against sodium hydrogen carbonate concentration.
Method 3 - requirement for chlorophyll
- Fill a beaker with boiling water.
- Submerge a variegated leaf in the boiling water for 30 seconds.
- Place the leaf on a white tile so the colour change can be seen clearly.
- Using a dropping pipette, add iodine solution to the leaf.
- The green parts of the leaf should turn blue-black due to the presence of starch. The white areas should not.
What is the purpose of the sodium hydrogen carbonate solution in Method 1?
to measure the light intensity
to provide carbon dioxide for photosynthesis
to test the requirement for chlorophyll
to calculate the rate of photosynthesis
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What is the dependent variable in Method 1?
rate of photosynthesis
concentration of sodium hydrogen carbonate
light intensity
distance of the boiling tube from the light source
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What is the purpose of the iodine solution in Method 3?
To measure the light intensity
To calculate the rate of photosynthesis
To provide carbon dioxide for photosynthesis
To test the presence of starch
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What is the independent variable in Method 2?
Concentration of sodium hydrogen carbonate
Distance of the boiling tube from the light source
Rate of photosynthesis
Light intensity
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