When leaves are tested for starch with iodine, what colour indicates the presence of starch?

Plants store glucose as the polysaccharide starch; the cereal grains (wheat, rice, corn, oats, barley) as well as tubers such as potatoes are also rich in starch. Starch can be separated into two fractions--amylose and amylopectin. Natural starches are mixtures of amylose (10-20%) and amylopectin (80-90%).

Amylose forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble. The structure of amylose consists of long polymer chains of glucose units connected by an alpha acetal linkage. Starch - Amylose shows a very small portion of an amylose chain. All of the monomer units are alpha -D-glucose, and all the alpha acetal links connect C #1 of one glucose and to C #4 of the next glucose. As a result of the bond angles in the α acetal linkage, amylose actually forms a spiral much like a coiled spring. See the graphic below, which show four views in turning from a the side to an end view.

Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. The iodine molecule slips inside of the amylose coil. Iodine - KI Reagent: Iodine is not very soluble in water, therefore the iodine reagent is made by dissolving iodine in water in the presence of potassium iodide. This makes a linear triiodide ion complex with is soluble that slips into the coil of the starch causing an intense blue-black color.

• Starch Test: Add Iodine-KI reagent to a solution or directly on a potato or other materials such as bread, crackers, or flour. A blue-black color results if starch is present. If starch amylose is not present, then the color will stay orange or yellow. Starch amylopectin does not give the color, nor does cellulose, nor do disaccharides such as sucrose in sugar.
• Iodine Test: When following the changes in some inorganic oxidation reduction reactions, iodine may be used as an indicator to follow the changes of iodide ion and iodine element. Soluble starch solution is added. Only iodine element in the presence of iodide ion will give the characteristic blue black color. Neither iodine element alone nor iodide ions alone will give the color result. This phenomenon is used in the iodine clock demonstration.

• Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook

The video above shows the steps in Testing a Leaf for the Presence of Starch. The Procedure outlined below is slightly different.

1. Remove a green leaf from a plant that has been exposed to sunlight for a few hours
2. Half-fill a 250cm3 beaker with water. Heat the water until it boils. Keep the water at boiling point.
3. Use the forceps to place the leaf in the boiling water. Boil for 2 minutes.
4. Turn off the Bunsen Burner. (If you are using a heat source without a naked flame - electric water bath or hot plate - this step is unnecessary.)
5. Place the boiled leaf in a boiling tube containing 90% ethanol.
6. Place the boiling tube in hot water and boil for 10 minutes or until the leaf decolourizes.    ( It may be necessary to replace the ethanol)
7. Gently remove the leaf and wash with a fine trickle of cold tap water.
8. Spread the leaf evenly on a white tile.
9. Add a few drops of iodine/potassium iodide solution to the leaf and note any observations.

• The leaf was flaccid (soft) after being boiled in water
• The ethanol changed from colorless to green
• The leaf was brittle after being boiled in ethanol
• The leaf become flaccid once more after being rinsed in cold water

• After iodine solution was added...
  

Describe Photosynthesis. Use a symbol equation.

Photosynthesis is the process by which light energy is harvested by chlorophyll and used to convert inorganic raw materials - carbon dioxide and water -  to products -  glucose and oxygen. Most photosynthesis takes places in the green parts of plants and more specifically, in  the leaves.

In the light - independent stage enzymes catalyse the reduction of carbon dioxide by hydrogen. The carbohydrate, glucose is first formed. Glucose may be immediately used, translocated in the form of fructose or sucrose or stored as starch granules.

The overall equation for photosynthesis is:

Why is starch tested for and not glucose? 

Glucose is the product of photosynthesis, and is rapidly converted to granules of starch  - a polymer of glucose  - for storage.  Starch granules have been visualized in the stroma of the chloroplast as  well as the cytoplasm.

It is advantageous to the plant to convert glucose to starch. Starch is insoluble in water and therefore not a solute in the solution component of the stroma. Glucose on the other hand is soluble in water and if it accumulated in the stroma, the stroma would become more concentrated  - hypertonic - relative to the  cytoplasm. Water  would move from the cytoplasm ( higher water potential) via osmosis into the stroma of the chloroplasts (lower water potential), causing them to swell and burst.

• Removes the waxy cuticle which prevents entry of iodine/potassium iodide solution.
• Ruptures cell membranes to make starch granules in cytoplasm and chloroplasts accessible to iodine/potassium iodide solution. Cell membranes are selectively permeable and do not readily allow the penetration of iodine.
• Denatures enzymes, particularly those which convert starch to glucose e.g. diastase. Boiling arrests all chemical reactions, since enzymes which catalyse the reactions are denatured. Denatured enzymes have altered or destroyed active sites due to heat, pH, ionic concentration

Account for the texture of the leaf after being boiled in water. The leaf was flaccid (soft) as cell membranes were ruptured and turgor pressure was lost.
Explain the significance of boiling the leaf in ethanolChlorophyll is a green pigment and so masks the colour change of the iodine test for starch. Chlorophyll needs to be removed from the leaf i.e. the leaf needs to be ' decolourized' for changes to be observed. A decolourized leaf is pale yellow or green. Ethanol is an organic solvent and so extracts chlorophyll from the leaf.

Account for the texture of the leaf after being boiled in ethanol. The leaf was brittle as ethanol dehydrated (extracted water) from the leaf.
Explain the significance of rinsing the leaf in water. The leaf was rinsed in water to rehydrate it. Iodine solution is an aqueous solution of iodine/potassium iodie - potassium tri-iodide; water is needed inside the leaf to enable penetration by diffusion.
State whether starch is present
Starch is present OR Starch is not present.

Which products of photosynthesis may be present but not revealed by the iodine test?Glucose, fructose and sucrose are all carbohydrate products of photosynthesis not revealed by the iodine test for starch.
Do your results indicate that starch is the first product of photosynthesis? The results do not indicate whether starch is the initial, intermediate or final product of photosynthesis.
How can this experiment be extended to demonstrate conclusively that photosynthesis has taken place?

We do not know whether starch has accumulated in the leaf as a result of photosynthesis or whether the starch is permanently present in the leaf. To improve this experiment, the plant needs to be de-starched, and the leaves tested for the presence of starch.

Iodine Test for StarchTesting a Leaf  for StarchWhy Destarch a PlantEffect of Light Intensity