The role and physicochemical properties of starch as a reserve polysaccharide in plants

The main representatives of non-sugar-like polysaccharides are starch, glycogen, cellulose, inulin, lichenin, hemicellulose, pectic substances, chitin, gums, and mucilages.

Starch is one of the most common reserve polysaccharides of plants. It is formed during photosynthesis in plant leaves and, as it accumulates, it is broken down and converted into soluble compounds, and then re-deposited as a reserve substance in roots, seeds, and fruits. Subsequently, starch is used by the plant cell as an energy and building material.

It is contained in large quantities in the endosperm of cereals – 65-85% of its mass, and in potatoes – up to 20% (Table 18; Sheudzhen A.Kh., Onishchenko L.M., Korsunova M.I. et al., 2004).

Starch is not a chemically individual substance. In addition to polysaccharides, its composition includes:

• mineral substances, mainly represented by phosphoric acid;
• lipids;
• high-molecular-weight fatty acids – palmitic, stearic, and some other compounds adsorbed by the carbohydrate polysaccharide structure of starch.

In endosperm cells, starch is found in the form of grains (granules). Starch grains have an oval, spherical, or irregular shape. The diameter of starch grains ranges from 0.002 to 0.15 nm. The largest starch grains are found in potatoes, and the smallest in rice and buckwheat.

Starch grains are divided into simple and complex ones: simple grains represent homogeneous formations; complex grains are a combination of smaller particles. Simple starch grains are characteristic of potatoes, wheat, and rye, while complex ones are characteristic of rice and oats.

A characteristic property of starch is its ability to turn blue when an aqueous solution of potassium iodide is added. The appearance of the blue color is explained by the formation of complex and adsorption compounds between starch and iodine.

In cold water, starch grains swell due to the addition of water, but they do not dissolve. However, upon gradual heating, starch forms a viscous colloidal solution – starch paste. The temperature at which this change in starch occurs is called the gelatinization temperature.

Potato starch gelatinizes at a temperature of 55–65°C, corn starch – at 64–71°C, wheat starch – at 60–80°C, and rice starch – at 70–80°C.

Table 18 – Starch content in fruits and seeds

The carbohydrate part of starch consists of two polysaccharides: amylose and amylopectin, which differ in their physical and chemical properties.

In terms of chemical structure, amylose is a long, unbranched chain of glucose residues connected by (14) glycosidic bonds (Fig. 23; Shcherbakov V.G., Lobanov V.G., Prudnikova T.N. et al., 2003).

Each amylose molecule contains from hundreds to several thousand glucose residues connected mainly by α1→4 glycosidic bonds. Its molecular weight is 3105–1106. Amylose has a reducing end (A) and a non-reducing end (B). The chain molecule of amylose has a helical shape and a diameter of 1 nm, with each turn of the helix consisting of six glucose residues. Molecules with a diameter of less than 1 nm can penetrate the inner channel of the helix, forming complexes called inclusion compounds, or clathrates. The molecular complex of amylose with iodine has a blue color (max=620–680 nm). This is used for analytical purposes to determine starch and iodine.

Amylopectin is a branched component of starch. In its molecule, glucose residues are connected by glycosidic bonds not only between the 1st and 4th carbon atoms but also between the 1st and 6th. Amylopectin is stained blue-violet by an iodine solution. Each amylopectin molecule has one reducing end (A) and a large number of non-reducing ends (B). The structure of amylopectin is three-dimensional; its branches are located in all directions and give the molecule a spherical shape. The molecular weight of amylopectin ranges from 106 to 109.

Fig. 23. Starch, its structure. Amylose and amylopectin. a – diagram of glucose molecule connection in amylose; b – spatial structure of amylose; c – diagram of glucose connection in amylopectin; d – spatial molecule of amylopectin

Starch, as a typical representative of polysaccharides, forms simple and complex ethers through its hydroxyl groups. Acidic and enzymatic hydrolysis, γ-irradiation, and thermal treatment of dry starch yield products of its partial cleavage – dextrins. During the hydrolysis of starch under the action of acids, there is first a weakening and breaking of associative bonds between amylose and amylopectin macromolecules. This is accompanied by the disruption of the starch grain structure and the formation of a homogeneous mass. Next, the –D–(1,4)– and –D–(1,6)– bonds are broken, with a water molecule added at the site of the break. During the process of hydrolysis, the number of free aldehyde groups increases, and the degree of polymerization decreases. As hydrolysis progresses and the content of reducing substances increases, the dextrin content decreases, the glucose content increases, and the concentration of maltose, tri- and tetrasaccharides first increases and then decreases. The final product of hydrolysis is glucose. Dextrins, tri- and tetrasaccharides, and maltose are formed at intermediate stages.

The biological role of starch lies in the fact that it represents a form of storage for nutrients in plants. If a need for energy and a carbon source arises, starch is easily released from granules and hydrolyzed by enzymes.

Most plants contain two different types of hydrolytic enzymes, traditionally called -amylase and -amylase. Both of these enzymes break down both the amylose and amylopectin fractions of starch at the (14)-linkage, but into different fragments:

• -Amylase acts on the entire starch granule, attacking it, loosening the surface, and forming channels and grooves, i.e., it essentially splits the granule into parts. Gelatinized starch is hydrolyzed by it to form low-molecular-weight dextrins.
• As dextrins accumulate, -amylase comes into play, breaking them down into sugars, which are the direct building material for the developing seedling. Unlike -amylase, -amylase practically does not hydrolyze native starch; gelatinized starch is hydrolyzed into maltose in the -configuration.

Cellulose is the most common organic compound. It is found everywhere in the plant world as a structural component of the cell wall. In some parts of certain plants, it is present in almost pure form.

Cellulose consists of polymer chains of D-glucose molecules (up to 1000 units), linked together by (14)-glycosidic bonds. These chains combine to form fibers. When glucose molecules in the C1-configuration form (14)-chains, a -structure is formed. This is due to the fact that sugar residues linked by glycosidic bonds lose complete freedom of rotation around the 1C–O– and O–C4– bonds due to the presence of massive 6CH2OH groups, and the polymer acquires a configuration favorable for the formation of interchain hydrogen bonds when the chains are arranged antiparallel to each other.

The structure of cellulose is well adapted to its biological role. The ability to form hydrogen bonds between individual cellulose chains is very high, as each residue is capable of participating in their formation with its three OH groups. This gives intact fibers high strength and is the reason for the insolubility of cellulose in water. In plant cell walls, these cellulose fibers are tightly packed into layers, which are further stabilized by the presence of other polysaccharide compounds, such as hemicellulose, pectin, and lignin, which play the role of a binding material.

Cellulose is a very stable chemical compound, does not dissolve in most common solvents, but can be dissolved by the action of Schweizer's reagent — an aqueous solution of the copper-ammonia complex [Cu(NH3)4](OH)2. When boiled with concentrated hydrochloric or sulfuric acid, hydrolysis of cellulose occurs. Complete hydrolysis produces -D-glucose. The hydrolysis of cellulose to form cellobiose molecules occurs under the action of the enzyme cellulase.

Glycogen is a polysaccharide found in human and animal body tissues, in fungi and yeasts, and in sweet corn grain. Plant glycogen, found in corn grain, is sometimes called phytoglycogen. Structurally, glycogen is a branched polyglucose molecule, analogous to the amylopectin fraction of starch, however, glycogen is more highly branched.

Inside cells, glycogen molecules are hydrolyzed by glycogen phosphorylase – an enzyme that sequentially cleaves one glucose residue at a time from any of the non-reducing ends, leading to the formation of glucose-1-phosphate. Special enzymes are used to break down the polymer at branch points at (14)-linkages and (16)-linkages.

Inulin is a high-molecular-weight carbohydrate (5000–6000), soluble in water, precipitating from aqueous solutions when alcohol is added. Fructose molecules in inulin are linked by 1,2-bonds. Usually, an inulin molecule contains 35–42 fructose residues. When inulin is hydrolyzed with acids, fructofuranose and a small amount of glucopyranose are formed.

This polysaccharide is found in large quantities in the following crops, where it replaces starch:

• tubers of Jerusalem artichoke and dahlia;
• roots of dandelion, kok-saghyz, and chicory;
• artichokes;
• roots, leaves, and stems of the guayule rubber plant.

Protective polysaccharides: how callose, chitin, and hemicelluloses save plants from damage

Plants protect themselves from mechanical injuries and pathogen invasion using structural polysaccharides. The main mechanism for emergency sealing of vessels is callose. This glucan consists of approximately 100 glucose residues linked by β-1,3-bonds. Under normal conditions, callose lines the sieve plates of the phloem in a thin layer, but when damage occurs, its role changes dramatically.

In biotechnologies, preparations based on chitin and its derivative, chitosan, are used to stimulate plant immunity. Chitin is a linear polysaccharide composed of N-acetyl-β-D-glucosamine residues with β(1→4) linkages. Its structure is similar to cellulose, characterized by high rigidity and chemical resistance. Chitin and chitosan preparations activate defensive cells, enhance antibody synthesis, stimulate macrophage activity, and accelerate the healing of plant wound surfaces.

Hemicelluloses are responsible for stem strength and crop resistance to lodging. They form the cell wall matrix together with pectins and lignin, filling the space between cellulose microfibrils. Depending on their structure, these polysaccharides are divided into three groups.

Pectins, lichenin, and gums: role in ripening, storage, and harvest processing

The state of polysaccharides in fruits directly determines their shelf life and transportability. Pectin substances play a crucial role here — polymers based on α-D-galacturonic acid residues with 1,4-linkages. During fruit development, insoluble protopectin, bound to galactan and araban, accumulates in the cell walls. During ripening and storage, it turns into soluble pectin, free from cellulose, which leads to tissue softening.

For fodder production in northern regions, lichenin — a lichen polysaccharide consisting of glucose residues with 1,4- and 1,3-linkages — is of key importance. The human body does not digest lichenin, but for reindeer, it serves as a primary energy source.

Under stress conditions, plants secrete gums and mucilages. These are colloidal water-soluble polysaccharides that form extremely viscous and sticky solutions. They perform a protective function, sealing wounds on the bark and retaining moisture in plant tissues.

• Number of glucose residues in a callose molecule — about 100
• Molecular weight of arabans — up to 6000
• Digestibility of lichenin in the reindeer GI tract — 78 %
• Yield of D-glucose during acid hydrolysis of lichenin — 98–99 %

The appearance of glue exudates (gum) on the trunks of cherry, plum, or almond trees is not just a sign of mechanical damage. This is how the tree's protective reaction manifests, where cell walls at the site of injury are dissolved by the action of enzymes. At the same time, in herbaceous crops such as flax and rice, reserve polysaccharides are stored in the form of mucilages. They are contained in significant quantities in flax seeds and rice grain.

The main practical property of these substances is their ability to swell intensely in water and then dissolve, forming highly viscous solutions. From a chemical perspective, the composition of these polysaccharide groups differs:

• Gums consist of galactose, mannose, arabinose, n-glucuronic acid, and a small amount of xylose residues.
• Mucilages are generally represented by pentosans: xylose, arabinose, and galactose.

Agar-agar: properties and application of seaweed polysaccharide

This high-molecular-weight polysaccharide accumulates in certain marine algae belonging to the genera Gelidium, Cracilaria, Pterocladia, and Ahufeltia. The key practical feature of agar-agar lies in its relationship with temperature. It is insoluble in cold water but dissolves easily when heated. Upon subsequent cooling, the resulting solution solidifies into a firm gel.

The substance consists of two polysaccharides — agarose and agaropectin. Agarose is built from D-galactose and 3,6-anhydro-L-galactose residues, which are connected by α-1,3- and β-1,4-glycosidic linkages. Agaropectin consists of D-galactopyranose chains, some of which are linked by an ester bond to residues of sulfuric acid.

Due to its unique physicochemical properties, agar-agar is indispensable in laboratory practice and the processing industry. In bacteriology, this polysaccharide is used to prepare solid culture media. In confectionery production, it is used as an effective gelling agent for making marmalade, pastila, jams, and jellies.