When a plant receives adequate sunlight and water and takes in carbon dioxide, chloroplasts in the plant's cells convert the reactants (water and carbon dioxide) into oxygen and glucose. This is the process of photosynthesis. The chloroplasts also synthesize starch, which is made of glucose molecules linked in long chains.
Beer and whiskey producers use their knowledge of starch degradation and fermentation in cereal grains to make their products. Plants have to produce starch to store energy for cell metabolism. Human bodies, on the other hand, do not synthesize starch. When a human eats starchy plant material, some of the starch breaks down into glucose for energy; any unused remnant is stored as fat deposits.
When the plant cell requires energy for a cell process, enzymes are released to degrade part of the starch chain. When starch in plant cells degrades, carbon is released to be utilized in producing sucrose. At the same time, the carbon produced allows cells to continue to grow and maintain themselves.
Starch Storage Location
In some plants, starch is stored in cell organelles called amyloplasts. Some plant roots and embryos, in the form of seeds and fruit, also serve as storage units for starch. Cells in plant leaves produce starch in the presence of sunlight.
The test for presence of starch is to apply tincture of iodine to a cut surface of a fruit or vegetable. If starch is present in the plant's "juices", the iodine will change color from dark brown to dark bluish-purple or black. Solid parts of plants like leaves and stems must be pulverized with a mortar and pestle. Then drops of tincture of iodine may be added to a test tube containing the crushed plant parts and sap.
When an ear of corn is picked, glucose in the kernels converts into starch over time, making the corn lose its flavor. Each year, new hybrids of sweet corn are produced that allow the kernels in an ear of corn to retain their sweetness for a longer period post-picking.
Genetic researchers are studying ways to increase the quality and quantity of starch in plant cells. The food processing industry continues to see great demand for plant starch used in things like high fructose corn syrup and other foods. Scientists are studying the way plant cell walls are built. They hope to genetically alter plants so that cellulose from formerly unusable plant parts like corn husks and stems can be fermented for ethanol production. This will reduce the need to use plant starch in ethanol and may reduce its cost.