Fundamentals of Biochemistry - A. A. Anisimov 1986

Carbohydrates
Structure, classification, and role in living nature

CARBOHYDRATES are defined as aldehydes and ketones of polyhydric alcohols and polymers of these compounds. The Water/144.html">Origin of the term "carbohydrates" stems from the fact that the first studied representatives of this Class appeared to be compounds of carbon and water, fitting the general formula C_n(H₂O)_n. As new carbohydrates were discovered, it became clear that not all of them fit this formula: in some, C, H, and O are present in different ratios, and N, S, or P are sometimes included. Conversely, some members of other classes share the same empirical formula, such as acetic acid, C₂H₄O₂.

In 1927, the International Commission for the Reform of Chemical Nomenclature proposed replacing the term "carbohydrates"—arguing that it reflects neither the chemical nature nor the composition of this class of compounds—with the term "glycides"; however, it did not gain widespread adoption. In the rules of biochemical nomenclature set by the International Union of Pure and Applied Chemistry (IUPAC) and published in 1978, this class of compounds continues to be referred to as "carbohydrates."

¹Carbohydrates are more abundant in the biosphere than all other Organic compounds combined. In the plant kingdom, they account for 80–90% of dry matter. In animal organisms, carbohydrates make up about 2% of body mass, yet their significance is universally vital for all living things, as evidenced by the crucial Functions they perform. Carbohydrates are the primary products synthesized in green plants from CO₂ and H₂O using solar energy, serving as the Starting Material for other organic substances in living organisms. The functions of carbohydrates are diverse and essential. Energy function: Upon oxidation during Respiration, carbohydrates release their stored energy, covering a significant portion of the Organism's energy demands. The oxidation of 1 g of carbohydrates yields ≈ 16.9 kJ of energy.

Plastic (structural) function: Carbohydrates are utilized in the synthesis of many vital substances for the organism, including Nucleic Acids, organic acids, and consequently Amino Acids, Proteins, Lipids, and more.

Protective function: Carbohydrates serve as the Main Components of Plant Cell Walls, participate in building the exoskeleton of insects and crustaceans, and contribute to The formation of bacterial cell walls and cell membranes across All living organisms (in complexes with proteins).

Supportive function: Cellulose and other Polysaccharides in plant cell walls not only protect Cells from external influences but also form the rigid framework of the plant, constituting its mechanical and supportive tissues. In complex with proteins, carbohydrates are part of Cartilage Tissues (chondroitin sulfates) and other Connective Tissue structures that perform supportive functions in humans and animals.

Regulatory function: Dietary fiber, by mechanically irritating the intestine, promotes peristalsis (motility) and thereby improves Digestion. The interconversion of starch and sugars in the guard cells of plant Stomata regulates their movement, namely the opening and closing of the stomata. Monosaccharides play a significant role in regulating osmotic processes.

The specific functions of carbohydrates are particularly noteworthy. As mentioned in the section on Glycoproteins, carbohydrate-containing compounds act as markers in cell-cell and molecule-molecule recognition processes, determine antigenic Specificity, and account for Blood group differences. Some carbohydrate-containing Biopolymers function as receptors for binding various toxins, bacterial cells, Viruses, Hormones, and more. In recent years, the Receptor Functions of carbohydrate-containing compounds in Nervous Tissue have been increasingly elucidated. A relatively high concentration of glycoproteins has been found in nerve terminal membranes, where they participate in Nerve Impulse propagation and act as receptors for certain pharmacologically active compounds.

Evidence has also been obtained confirming the involvement of carbohydrate-containing biopolymers in intercellular adhesion, aggregation, and morphogenesis. Furthermore, the carbohydrate component can determine Other properties of living cell molecules: enhancing the stability of many Enzymes—glycoproteins, acting as an antifreeze in the blood and Muscles of Antarctic fish, functioning as an anticoagulant, and exerting antitumor effects (such as certain β-1,3-glucans and the glycoprotein "TNF factor").

Carbohydrates also function as reserve nutrients. They can be stored in the body in the form of Glycogen (in humans and animals), as well as starch and fructosans (in plants), which are mobilized as needed. For instance, the Liver of a regularly fed animal can accumulate glycogen up to 10% of its tissue mass; during starvation, this content drops to 0.2%.

Carbohydrates are typically divided into monosaccharides, Oligosaccharides, and polysaccharides (glycans).

Monosaccharides include carbohydrates and their derivatives that cannot be hydrolyzed without losing their fundamental carbohydrate properties.

Oligosaccharides (first-order polysaccharides) undergo Hydrolysis to yield a small number of monosaccharide units (from 2 to 10).

Second-order polysaccharides (glycans) are high-molecular-weight polymers composed of MONOSACCHARIDES AND THEIR derivatives, varying in Composition and Structure. The number of monosaccharide units in these polymers ranges from 10 to several thousand.

Major contributions to The Study of carbohydrate structure, particularly monosaccharides, were made by the prominent German chemist E. Fischer (1891). In the Soviet Union, fascinating and successful research on carbohydrates was led for many years by B. N. Stepanenko.