BACKGROUND:

Starch grains are found in the chloroplasts of the higher plants and if leaves containing starch are kept in darkness for a moderate amount of time the starch grains will disappear. If these sun deprived leaves are exposed to light, starch reappears in the chloroplasts. Starch is the first visible product of photosynthesis.

The principle way that food is stored in plants is as starch. Starch can be found in all stems even the main trunk. Starch can be found in those layers that are many years old as well as those that are still living. Studies of food reserves in twigs of fruit trees show that starch accumulates toward the end of the growing season. A very large part of the food used by any individual bud in its early spring growth comes from the storage tissues in the immediate neighborhood of that bud. Tubers, rhizomes, and corn (underground stems) are especially adapted for food storage in the form of starch. Bulbs like garlic and onion store food in the form of sugar rather than starch.

The key principal in this lab is for students to realize that starch is an important by-product of photosynthesis, and is used through the life cycle of plants. When foods are stored, they are usually in the form of nonsoluble materials. Before such foods can be utilized, they must first be converted to a soluble form. For example, when a seed is planted and watered, the first thing that happens (after water is absorbed and swells the seed) is the digestion of starch, fats, and proteins. The resulting sugars, fatty acids, glycerol and amino acids are then available and are utilized in the subsequent production of new cells, which develop into roots, stems and leaves. Once the leaves have formed the plant no longer is dependent upon the foods stored in the seed. Photosynthesis can now take place and foods are produced.

PROCEDURE:

The rapid conversion of sugar into starch, a storage product, enables the use of iodine to test for starch as an indication of the occurrence of photosynthesis. In many leaves excess sugars (excess in that more sugars are produced during photosynthesis than are used by the leaf) are rapidly converted to starch, which yields a blue black color when treated with iodine-potassium iodide solution.

1. Instruct students to test each of the items listed on the worksheet with a drop of the tester solution. You may substitute items depending on the availability of materials.
     
2. Crush all the solid substances and place all liquid substances into containers before applying iodine. Make sure you have tincture of iodine. The clear "iodine" will not work on this experiment. Place the samples on a piece of wax paper or a recycled food tray.
     
3. Place one or two drops of iodine on the items. Instruct students to record what happens. In the space below, enter + or - to indicate whether it gave a positive or negative starch test result.
   
   bread - starch (from wheat)
   potato - starch
   candy - no starch
   banana - starch
   apple - starch
   cracker - starch (from wheat)
   meat - no starch
   potato chip - starch
   lettuce - starch
   water - no starch
   bean - starch
   orange juice - no starch
   butter - no starch
   oil - no starch
   leaves - starch
   paper - starch
   plastic - no starch
   onion - no starch

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