Starch is called a polysaccharide. This means that it consists of monosaccharides linked in long chains. In fact, it is a mixture of two different polymeric substances: starch consists of amylose and amylopectin. The monomer in both chains is a glucose molecule, however, they differ significantly in structure and properties.
Total squad
As already mentioned, both amylose and amylopectin are polymers of alpha-glucose. The difference lies in the fact that the amylose molecule has a linear structure, and amylopectin is branched. The first is a soluble fraction of starch, amylopectin is not, and in general, starch in water is a colloidal solution (sol), in which the dissolved part of the substance is in equilibrium with the undissolved one.
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 inside, and their hydrophilic ends are hidden outside 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 practically insoluble in cold water.
Amylose and amylopectin in starch are in a ratio of approximately 20% of the former to 80% of the latter. 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 only partially, but in hot water a paste is formed from starch - a more or less homogeneous sticky mass of swollen individual starch grains.
Amylose
Amylose consists of glucose molecules linked to each other by 1, 4-hydroxyl bonds. It is a long, unbranched polymer with an average of 200 individual glucose molecules.
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, compounds-inclusions of the "guest-host" type. The well-known reaction of starch with iodine belongs to them: the amylose molecule is the "host", the iodine molecule is the "guest", placed inside the helix. The complex has an intense blue 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 standard 19-24% by weight. Rice starch contains 17% of it, and only amylose is present in apple starch - 100% mass fraction.
In the paste, amylose forms the soluble part, and this is used inanalytical chemistry for the separation of starch into fractions. Another way, starch fractionation is the precipitation of amylose in the form of complexes with butanol or thymol in boiling solutions with water or dimethyl sulfoxide. Chromatography can use the property of amylose to adsorb to cellulose (in the presence of urea and ethanol).
Amylopectin
Starch has a branched structure. This is achieved due to the fact that, in addition to 1 and 4-hydroxyl bonds, glucose molecules in it also form bonds at the 6th alcohol group. Each such "third" bond in the molecule is a new branch in the chain. The general structure of amylopectin resembles a bunch in appearance, 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 molecule of amylopectin is much larger than that of amylose.
Amylopectin also forms an inclusion compound (clathrate) with iodine. Only in this case the complex is colored in red-violet (closer to red) color.
Chemical properties
The chemical properties of amylose and amylopectin, except for the interactions with iodine already discussed, 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 monomers, such as hydrolysis. Therefore, further we will talk about the chemical properties of starch as a mixture of amylose and amylopectin.
Starchrefers 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, Tollens test - a qualitative reaction for an aldehyde group, or interaction with Felling's reagent - freshly precipitated hydroxide copper). Preserved glycosidic hydroxyls, of course, are available (at one of the ends of the polymer chain), but in small quantities and do not affect the properties of the substance.
However, just like individual glucose molecules, starch is able to form esters with the help of hydroxyl groups that are not involved in the bonds between monomers: they can be "hung" with a methyl group, an acetic acid residue, and so on.
Also, starch can be oxidized with iodine (HIO4) acid to dialdehyde.
Hydrolysis of starch is of two types: enzymatic and acidic. Hydrolysis with the help of enzymes belongs to the section of biochemistry. The enzyme amylase breaks down starch into shorter polymeric chains of glucose - dextrins. Acid hydrolysis of starch is complete in the presence of, for example, sulfuric acid: starch is broken down immediately to the monomer - glucose.
In wildlife
In biology, starch is primarily a complex carbohydrate and is therefore used by plants as a way to store nutrients. It is formed during photosynthesis (at first in the form of individual glucose molecules) and deposited in plant cells in the form of grains - in seeds, tubers, rhizomes, etc. (to be used later as"food warehouse" with new embryos). Sometimes starch is found in the stems (for example, the sago palm has a mealy starchy core) or leaves.
In the human body
Starch in the composition of food enters the oral cavity first. There, an enzyme contained in saliva (amylase) breaks down the polymer chains of amylose and amylopectin, turning the molecules into shorter ones - oligosaccharides, then breaks them down, and finally m altose remains - a disaccharide consisting of two glucose molecules.
M altose is broken down by m altase to glucose, a monosaccharide. And already glucose is used by the body as an energy source.
