Life Cycle - Plants (5A)
Post Lab 

  • Demonstrating photosynthesis.
  • Analyzing the mechanism of photosynthesis.
  • chlorophyll
  • chloroplast
  • photosynthesis
  • respiration
  • elodea plant
  • light source
  • Bromothymol Blue
  • straws
  • five - 25 ml graduated cylinders
  • demonstration prepared the night before - see step #5 in Procedure

Students demonstrate the oxygen is produced by plants.



The production of food from carbon dioxide and water in the presence of chlorophyll utilizing light energy and releasing oxygen is called photosynthesis. Plants produce food in the form of simple sugars (carbohydrate). In order for photosynthesis to occur sunlight and chlorophyll must be present. There are different types of chlorophyll, but the one involved in photosynthesis is called chlorophyll-A. Chlorophyll is a pigment which is stored in chloroplasts. During autumn and winter chlorophyll is not produced allowing the other pigments of brown and red to predominate giving plants their autumn colors.

  1. Add 1-2 drops of bromothymol blue to 15mL of water in a 25 ml graduated cylinder. Gently blow into the tube using a straw until it changes color to yellow. Have the students discuss the change. [The change occurs because CO2 , has been added to the solution, making it more acidic therefore causing a color change. Remember, humans produce carbon dioxide and utilize oxygen whereas plants utilize carbon dioxide and produce oxygen]
  2. Place a piece of elodea in the 25mL graduated cylinder with the yellow solution and insert a cork stopper in the beak of the cylinder. Expose the cylinder to sunlight for 20 minutes. [The yellow solution will change back to blue because the plant will utilize the carbon dioxide, making the solution less acidic, converting the color back to blue. ]
  3. In another cylinder, add 1-2 drops of bromothymol blue to water and gently blow with a straw until the solution changes color. Plug the top of the cylinder with a stopper and place it in the sunlight for 20 minutes without elodea. [No change will occur because the carbon dioxide remains in solution and the acidity of the solution does not change.]
  4. Prepare another cylinder in the same manner as the previous one, but do not expose it to sunlight, wait 20 minutes. [Again, no change will occur because no plant is present and photosynthesis is not occurring therefore the acidity of the solution remains the same.]
  5. A final cylinder containing an elodea plant bromothymol blue and water should have been prepared before this activity and left in a dark place for 24 hours. [No change will occur because sunlight is required for photosynthesis to occur.] Ask the students to describe what they observe.
  6. Write the following reaction on the board:

    [carbon dioxide + water - yields - carbohydrate + oxygen + water]

Explain the exchange of gases and how humans and plants live together in a mutual relationship. Without plants humans would not have oxygen and without humans plants would have no carbon dioxide. Through photosynthesis, plants produce food for both themselves and other living organisms. Remind students that when animals eat plants they too are eating the food that the plants produced.

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