Photosynthesis Experiments

• Photosynthesis experiments demonstrate the necessity of chlorophyll, light, and carbon dioxide.

Photosynthesis Equation

• The word equation for photosynthesis involves carbon dioxide plus water, which, under light and chlorophyll produce glucose and oxygen.
• The symbol equation for photosynthesis is 6CO2 + 6H2O, which, under light and chlorophyll, produces C6H12O6 + 6O2.
• The raw materials include CO2, H2O, and light energy.
• The products include glucose (starch) and O2.

Experiments to Study Photosynthesis

• Experiments examine whether chlorophyll, light, and CO2 are necessary for photosynthesis, and whether O2 is produced during photosynthesis.
• Leaves used in these experiments must not contain starch at the start, so it is possible to tell whether starch has formed during the experiment.
• To ensure leaves do not contain starch plants can be destarched by leaving them in the dark for 2-3 days, and the starch is used for respiration.

Testing for Starch in a Leaf

• Testing for starch is a way to tell whether photosynthesis has occurred.
• Testing a leaf for starch cannot be done by pouring iodine solution because it runs off due to the waxy cuticle.
• Testing a leaf for starch involves 4 steps: -Dip leaf in boiling water for 30 seconds. -Boil leaf in alcohol. -Dip leaf in boiling water. -Spread leaf on a white tile and place iodine solution on the leaf.
• Boiling in alcohol removes the chlorophyll from the leaf.
• A white tile makes it easier to view subtle color changes on the leaves.

Is Chlorophyll Needed for Photosynthesis?

• To demonstrate the requirement for chlorophyll, a plant with variegated leaves is destarched.
• Then the plant is left in sunlight for 2-3 hours, and the outline of the leaf drawn.
• Test the leaf for starch using the steps above, the presence of starch will be revealed by iodine turning blue black.
• The result will be that the parts of the leaf with chlorophyll present will show starch production, where as these leaves deficient in chlorophyll (white) will be starch free.
• This demonstrates the necessity of chlorophyll for photosynthesis.

Is Light Needed for Photosynthesis?

• To demonstrate the requirement for light, a plant is destarched
• A black paper is placed on one of the leaves, then the plant is left in sunlight for 2-3 hours.
• The leaf is tested for starch using the steps above.
• The portion exposed to the sun is blue-black (has starch) and the covered portion is yellow (no starch).
• Thus, light is needed for photosynthesis.

Effect of Light Intensity on Photosynthesis

• Changing light intensity affects the rate of photosynthesis.
• A lamp at different distances from a beaker of pondweed can be used to measure oxygen production.
• When the lamp is further away the light intensity is reduced.
• Measuring the volume of oxygen produced at different light intensities shows the effect of light intensity on the rate of photosynthesis.
• Further the concentration of sodium hydrogen carbonate, the temperature and aquatic plant are control variables.

Is CO2 Needed for Photosynthesis?

• To demonstrate the requirement for CO2, two destarched plants are watered and set up as shown.
• Leave plants in sunlight for 2-3 hours, test a leaf from each plant for starch.
• One plant uses soda lime [sodium hydroxide],which absorbs CO2
• The second plant uses sodium hydrogen carbonate, which releases CO2
• Result shows the is leaf without CO2 present will be starch free, where as the the leaf with CO2 will contain starch.
• CO2 is thus needed for photosynthesis.

Concepts

• During photosynthesis, light energy is converted into chemical energy stored in glucose.
• Factors affecting the rate of photosynthesis include chemical, light intensity, and amount of the the chemical iodine.

Group Tasks Answers

• Boiling a leaf in water softens it and kills the cells.
• Boiling it in alcohol removes the chlorophyll, making the leaf pale or white, so iodine solution can show any color change.
• Needed materials to demonstrate necessity of CO2 for photosynthesis include: -Two potted plants, soda lime (to absorb CO2) -Water -Vaseline or sealing material (to make airtight setups) -Iodine solution
• The uncovered part of the leaf turns blue-black because it received light
• The covered part does not turn blue-black because it did not receive light.
• The rate of photosynthesis increases as the light intensity increases
• After a certain point, the rate of photosynthesis plateaus even if the light intensity continues to rise.

Explore photosynthesis through experiments demonstrating the necessity of chlorophyll, light, and carbon dioxide. Understand the word and symbol equations for photosynthesis, identifying raw materials and products. Learn how to test for starch in a leaf.