Starch is called a polysaccharide. This means that it consists of monosaccharides bound in long chains. This is actually a mixture of two different polymeric substances: starch consists of amylose and amylopectin. The monomer in both chains is a glucose molecule, but they differ significantly in structure and properties.
General composition
As already mentioned, both amylose and amylopectin are alpha-glucose polymers. The difference is that the amylose molecule has a linear structure, and amylopectin has a branched structure. The first is a soluble fraction of starch, amylopectin is not, and in general starch in water is a colloidal solution (sol), in it the dissolved part of the substance is in equilibrium with undissolved.
Here, for comparison, the general structural formulas of amylose and amylopectin are given.
Amylose is soluble due to the formation of micelles - these are several molecules assembled together in such a way that their hydrophobic ends are hidden inward, and hydrophilic - outward for contact with water. They are in equilibrium with molecules not assembled into such aggregates.
Amylopectin is also able to form micellar solutions, but to a much lesser extent, and therefore is practically insoluble in cold water.
Amylose and amylopectin in starch are in a ratio of approximately 20% of the first to 80% of the second. This indicator depends on how it was obtained (in different starch-containing plants, the percentages are also different).
As already mentioned, only amylose can dissolve in cold water, and even then partially, but in hot starch a paste forms, a more or less uniform sticky mass of swollen individual starch grains.
Amylose
Amylose consists of glucose molecules bonded to each other via 1,4-hydroxyl bonds. This is a long unbranched polymer, the number of individual glucose molecules is on average 200.
In starch, the amylose chain is coiled: the diameter of the “windows” in it is approximately 0.5 nanometers. Thanks to them, amylose is able to form complexes, guest-host inclusion compounds. They include the well-known reaction of starch with iodine: the amylose molecule is the "host", the iodine molecule is the "guest" placed inside the spiral. The complex is intensely blue in color and is used to detect both iodine and starch.
In different plants, the percentage of amylose in starch can vary. In wheat and corn, it is standardly 19-24% by weight. Its rice starch contains 17%, and only amylose is present in apple starch - 100% mass fraction.
Amylose forms a soluble part in the paste, and this is used in analytical chemistry to separate starch into fractions. Another way of fractionating starch is to precipitate amylose as complexes with butanol or thymol in boiling solutions with water or dimethyl sulfoxide. In chromatography, the property of amylose to adsorption on cellulose (in the presence of urea and ethanol) can be used.
Amylopectin
Starch has a branched structure. This is achieved due to the fact that, in addition to 1 and 4-hydroxyl bonds, the glucose molecules in it also form bonds in the 6th alcohol group. Each such “third” bond in the molecule is a new branch in the chain. The general structure of amylopectin in appearance resembles a bunch, the macromolecule as a whole exists in the form of a spherical structure. The number of monomers in it is approximately equal to 6000, and the molecular weight of one amylopectin molecule is much larger than that of amylose.
Amylopectin also forms an inclusion compound (clathrate) with iodine. Only in this case the complex is colored red-violet (closer to red).
Chemical properties
The chemical properties of amylose and amylopectin, excluding the already discussed interaction with iodine, are exactly the same. They can be conditionally divided into two parts: reactions characteristic of glucose, that is, occurring with each monomer separately, and reactions affecting the bonds between the monomers, for example, hydrolysis. Therefore, further we will talk about the chemical properties of starch as a mixture of amylose and amylopectin.
Starch refers to non-reducing sugars: all glycosidic hydroxyls (hydroxyl group at the 1st carbon atom) participate in intermolecular bonds and therefore cannot be present in oxidation reactions (for example, the Tollens test - a qualitative reaction to the aldehyde group, or the interaction with the Felling reagent - freshly precipitated copper hydroxide). Preserved glycosidic hydroxyls, of course, are available (at one end of the polymer chain), but in small quantities they do not affect the properties of the substance.
However, as well as individual glucose molecules, starch is able to form esters with the help of hydroxyl groups not participating in the bonds between the monomers: they can be hung on the methyl group, the residue of acetic acid and so on.
Starch is also able to oxidize with iodic (HIO 4 ) acid to dialdehyde.
Hydrolysis of starch can be of two types: enzymatic and acid. Hydrolysis using enzymes belongs to the section of biochemistry. The amylase enzyme breaks down starch into shorter polymer chains of glucose - dextrins. Acid hydrolysis of starch is complete in the presence of, for example, sulfuric acid: starch is broken down immediately to a monomer - glucose.
In wildlife
In biology, starch is primarily a complex carbohydrate, and therefore is used by plants as a way to store nutrients. It is formed during photosynthesis (first in the form of individual glucose molecules) and is deposited in the cells of the plant in the form of grains - in seeds, tubers, rhizomes, etc. (so that later it can be used as a “food warehouse” by new embryos). Sometimes starch is found in the stems (for example, the saga palm has a powdery starchy core) or leaves.
In the human body
Starch in the composition of food first enters the oral cavity. There, the enzyme contained in saliva (amylase) breaks down the polymer chains of amylose and amylopectin, turning the molecules into shorter ones - oligosaccharides, then it also breaks them down, and finally maltose, a disaccharide consisting of two glucose molecules, remains.
Maltose breaks down maltase to glucose, a monosaccharide. And already glucose is used by the body as a source of energy.