6.1.2. Investigations for photosynthesis - chlorophyll, light, carbon dioxide

 

Variables tested: chlorophyll, light, carbon dioxide

We use each variable as a limiting factor, which limits the amount of glucose made (product). Some of the glucose is converted into starch. 

We determine the necessity of the variable by conducting a starch test to see how much starch was made.

The Starch Test

For a thorough explanation, click here.

Otherwise, here are the main steps to the test:

1. Chlorophyll is necessary for photosynthesis

Process

• Take a potted plant with variegated (green and white) leaves.
• Destarch the plant by keeping it in complete darkness for about 48 hours.
• Expose the plant to the sunlight for a few days.
• Test one of the leaves for starch with iodine solution.

Observations

     - Areas with previously green patches test positive (turn blue black).

     - Areas with previously pale yellow patches test negative (remain brown). 

Conclusion

• Photosynthesis takes place only in green patches because of the presence of chlorophyll. 
• The pale yellow patches do not perform photosynthesis because of the absence of chlorophyll.

2. Light is essential for photosynthesis

Process 

• Take a potted plant.
• Destarch the plant by keeping it in complete darkness for about 48 hours.
• Test one leaf for starch, to check that is does not contain any.
• Fix a leaf of this plant in between two strips of a thick paper on leaf.
• Place the plant in light for a few days.
• Remove the cover from the leaf and test it for starch.

Observations

Positive starch test will be obtained only in the portion of the leaf exposed to light and negative test in parts with paper strip.

Source: freeexamacademy

Conclusion

Light is necessary for photosynthesis.

3. Carbon dioxide is essential for photosynthesis 

Process

• Take two destarched potted plants.
• Cover both the plants with bell jars and label them as A and B.
• Inside Set-up A, keep water.
• Inside Set-up B, keep NaOH (Sodium hydroxide). It absorbs CO₂.
• Keep both the set-ups in the sunlight for at least 1 day.
• Perform the starch test on both of the plants.

Observations

Leaf from the plant in which water has been placed gives positive test.

Leaf from the plant in which NaOH has been kept give negative test.

Conclusion

Plant in Set up A gets CO₂ whereas plant in Set-up B does not get CO₂.

It means CO₂ is must be used for photosynthesis.

Investigating effects of variables on photosynthesis aquatic plants:

- light intensity

- carbon dioxide

- temperature

Hydrogencarbonate indicator solution - investigate effect of light and darkness on gas exchange in an aquatic plant

Hydrogencarbonate indicator shows carbon dioxide concentration in solution. 

• Prepare 4 boiling tubes with hydrogencarbonate solution (1/3 full). 
• Record the colour of the indicator in each tube.
• Place an aquatic leaf in 2 tubes.
• Use black paper to wrap around one of the tubes with a plant in it, and one without. 
• Leave tubes in the light for at least 1 hour.
• When the indicator has changed colour in at least one of the tubes without a covering, remove the black paper from the two covered tubes. Record the colour of the indicator in each tube.

Possible indicator colors:

• yellow =  a lot of carbon dioxide
• red = little carbon dioxide = ordinary air
• purple = no carbon dioxide

Other source: mastermindtutor.com, BBC bitesize

For the original post, click here

For past papers, click here