Learning Objective:
To show experimentally that light is necessary for photosynthesis.
Materials Required:
A potted plant
1 Beaker
1 Wired Gauze(Metallic)
1 Tripod
1 Bunsen Burner
1 Heatproof mat
1 Petri Dish
Iodine Solution
Water
A pair of Forceps
Ethanol
A boiling tube
A Dropper
Safety Precautions:
DO NOT EXPOSE THE ETHANOL TO DIRECT FLAMES AS IT IS A FLAMMABLE LIQUID. USE A WATER BATH* OR ELECTRIC HEATER INSTEAD.
*A water-bath can be constructed by heating a beaker containing water using a Bunsen Burner. The ethanol should be lowered into the water-bath using a separate test tube.
Chemical Reactions:
1. Iodine Test : In the presence of starch, Iodine solution changes from brown to blue-black Brown 🡪 Blue Black/Purple
2. Photosynthesis: 6H2O + 6CO2 🡪 C6H12O6 + 6O2
The chemical reaction above allows plants to synthesize their own food(glucose) and hence be photosynthetic autotrophs. The reaction above is dependent on various limiting factors including: light intensity, temperature and humidity, which affect the concentration gradients of key chemicals near the stomata of the leaf.
Methodology:
1. De-starch the plant: This is done in order to remove pre-existing starch in the plant’s leaves. The starch is removed by keeping the plant in the dark for 2-3 days(and works on the basis of the principal alluded to in the learning objective)
2. Pluck a green leaf(ensuring it has chlorophyll) and place a black strip on the leaf. Expose the leaf to sunlight for a couple of hours.
3. Boil the leaf in water to break its cell walls and membranes that may impede Iodine from reaching starchy regions(in the next step). Once this is done, boil the leaf gently in Ethanol(referring to any safety precautions listed above) to remove chlorophyll that may hinder the Iodine test from being successfully performed in the next step.
4. Remove the black strip from the leaf and place it in a Petri Dish. Perform the Iodine Test by dispensing Iodine Solution onto the leaf.
Observation
Regions of the leaf that were covered under the black strip will turn yellow whereas regions that were exposed to sunlight will turn blue-black/purple. This proves that light is necessary for photosynthesis as regions of the leaf that did not receive sunlight tested negative for starch(and hence did not photosynthesize). We can thus conclude that light is necessary for photosynthesis.
Experiment in Context:
There are various types of sugars that animals and plants need. While some are used for immediate acquirement of energy(most prominently glucose for respiration), others are used as storage molecules(such as starch in plants or glycogen in animals). Photosynthesis in plants helps produce sugars which can then be used as storage molecules.
Sugars belong to the Carbohydrate food class. Carbohydrates are:
Sugar molecules which are organic compounds that contain carbon, hydrogen and oxygen There are multiple types of sugars, namely:
(Mono/Di/Poly/Oligo) Saccharide – a carbohydrate consisting of one or more sugar molecules/carbon rings
Starch specifically, is a polysaccharide that is insoluble in water, and not sweet in taste. It is constituted by various repeating monomers/monosaccharides of glucose, which in contrast, are sweet in taste and soluble in water.
Begin by leaving the potted plant in the dark for 2-3 days. This will help to get rid of any starch formed in the leave before the experiment. Cut a piece of black paper and secure the cut on the leaf. Cover both surfaces of one of the leaf with this piece of black paper, and secure it to the leaf with paper clips.
As we can see, the parts of the leaf covered by the black paper will continue to be in the dark. Then, detach the leaf from the plant and remove the black paper from the leaf. Insert the leaf into the test tube containing the alcohol solution.
Insert the leaf into the test tube with the alcohol solution. To decolorize the chlorophyll pigments, immerse the tube in a boiling water bath. Once the leaf has been bleached, remove it from the alcohol with a forceps. Note: The burner flame was turned off before the test tube was added to avoid fire.
To soften the leaf, briefly immerse it in a hot water bath for a few seconds. Place the leaf in an iodine-containing petridish solution. After a few minutes, the exposed parts of the leaf that are not covered by the black paper will turn blue black in the iodine solution.