Starch
Starch, common name
applied to a white, granular or powdery, odorless, tasteless, complex
carbohydrate, (C6H10O5)x, abundant
in the seeds of cereal plants and in bulbs and tubers. Molecules of starch are
made of hundreds or thousands of atoms, corresponding to values of x, as
given in the formula above, that range from about 50 to many thousands.
Starch molecules are of two kinds. In the first kind,
amylose, which constitutes about 20 percent of ordinary starch, the
C6H10O5 groups are arranged in a continuous but
curled chain somewhat like a coil of rope; in the second kind, amylopectin,
considerable side-branching of the molecule occurs.
Starch is manufactured by green plants during the process
of photosynthesis. It forms part of the cell walls in plants,
constitutes part of rigid plant fibers, and serves as a kind of energy storage
for plants, because its oxidation to carbon dioxide and water releases energy.
The granules of starch present in any plant have size, shape, and markings
characteristic of the species of plant in which the starch is made.
Starch is almost insoluble in cold water and in alcohol,
but with boiling water it gives a colloidal suspension that may form a jelly on
cooling. Hot water changes starch slowly into smaller molecules called dextrins
(see Dextrin). This reaction, an example of
hydrolysis, is catalyzed by acids and by some enzymes. Dextrins,
like starch, react with water, giving still simpler molecules, the ultimate
products being maltose, C12H22O11,
a disaccharide, and glucose, C6H
12O6, a monosaccharide (see Sugar).
The digestion of starch in the human body follows this
course: The hydrolysis begins in the mouth under the action of salivary ptyalin,
but is completed in the small intestine. The body does not use immediately all
the glucose absorbed from the digestion of starch, but converts much of it to
glycogen, which is stored in the liver. (Glycogen, called animal starch, has a
structure nearly identical with that of amylopectin.) As the body requires
glucose, hydrolysis of glycogen releases it into the bloodstream. Glycogen
provides, therefore, an energy reserve for animals in the same way that ordinary
starch does for plants.
See Carbohydrate; Human Nutrition.
See Carbohydrate; Human Nutrition.
