Synthesis and Content in Living Organisms
The process of carbohydrate formation differs fundamentally among representatives of different kingdoms of living nature.
- Plant organisms: They are primary producers. Carbohydrate synthesis occurs directly in chloroplasts during photosynthesis. The initial substrates for building these molecules are carbon dioxide ($CO_2$) and water ($H_2O$). This is why plants are so rich in these compounds—in specialized storage organs (tubers, seeds), their concentration can reach up to 90% of the total mass.
- Animal organisms: Incapable of primary synthesis from inorganic substances on their own. Carbohydrates enter their bodies exclusively with consumed food. In animal cells, their content is relatively low, making up only 1–5%. In the bloodstream, this figure is even lower—0.1–0.12%, reflecting strict homeostasis.
Simple Carbohydrates (Monosaccharides)
Monosaccharides are basic structural units (monomers) from which more complex chains are built. Their classification is based on the number of carbon atoms in the carbon skeleton of the molecule.
- Trioses (contain three carbon atoms). A typical representative is glyceraldehyde, an important intermediate in cellular metabolism.
- Tetroses (four carbon atoms). An example is erythrose.
- Pentoses (five carbon atoms). This group includes ribose and deoxyribose. They play a critically important role as components of nucleic acids (RNA and DNA), as well as the universal energy source—the ATP molecule.
- Hexoses (six carbon atoms). The most well-known and physiologically significant representatives are glucose and fructose.
Complex Carbohydrates (Polymers)
Complex carbohydrates are formed through the polymerization of monosaccharides. Individual units in such macromolecules are securely linked together by a glycosidic bond. Depending on the chain length, two major groups are distinguished.
Oligosaccharides
These are compounds whose polymer chain includes from 2 to 10 monosaccharide residues. Physicochemical properties of oligosaccharides: they are highly soluble in water and have a pronounced sweet taste. Disaccharides (consisting of exactly two monomers) are of the greatest importance in nutritional physiology:
- Sucrose (table sugar) — is formed by combining a glucose molecule and a fructose molecule.
- Maltose — consists of two glucose molecules.
- Lactose — includes glucose and galactose residues.
Polysaccharides
These are high-molecular-weight compounds whose molecules combine hundreds or even thousands of monomers. Unlike oligosaccharides, they are insoluble in water and completely devoid of a sweet taste. The spatial structure of polysaccharides can be either linear or highly branched.
Based on the composition of their monomer units, polysaccharides are divided into:
- Homopolysaccharides — built from identical monosaccharide residues. These include starch, glycogen, and cellulose.
- Heteropolysaccharides — consist of different types of monomers. A classic example is heparin.
Biological Functions
In living systems, carbohydrates perform a wide range of vital tasks, providing both the energy and plastic (structural) needs of cells.
- Energy function: This is the primary "fuel" resource. The oxidation of just 1 gram of carbohydrates releases 17.6 kJ of energy required for biochemical reactions to occur.
- Storage function: Excess monosaccharides are stored in the form of polymers. In plants, the main storage carbohydrate is starch, while in animals it is glycogen.
- Structural (building) function: Carbohydrates form a strong framework. They form the cell wall in plants, bacteria, and fungi.
- Component function: Carbohydrate residues (specifically pentoses) are an integral part of essential molecules: DNA, RNA, ATP, as well as the coenzymes FAD, $NAD^+$, and $NADP^+$.
- Protective function: Carbohydrates participate in the formation of tissue compatibility receptors on the surface of membranes. Additionally, they form the chitinous cover, which serves as a reliable exoskeleton and protection for arthropods.
Briefly: answers to topical questions
Which enzymes break down carbohydrates in the digestive tract?
Carbohydrates are broken down in the digestive tract by enzymes involved in the hydrolysis of glycosidic bonds:
- Amylase: salivary $\alpha$-amylase and pancreatic amylase.
- Enzymatic complexes on the brush border / apical membrane of enterocytes:
- Sucrase-isomaltase complex — includes sucrase, maltase, and isomaltase activities.
- Glucoamylase complex — includes enzymes that break down oligosaccharides and maltose.
- $\beta$-glycosidase complex — exhibits lactase activity.
Carbohydrate digestion stops in the stomach: gastric juice does not contain carbohydrate-digesting enzymes, and salivary amylase is inactivated.
By means of which transporters is glucose absorbed in the intestine?
Glucose absorption in the intestine occurs with the participation of carrier proteins via facilitated diffusion and secondary active transport. The specific names of glucose carrier proteins are not provided in the materials, but the mechanisms of their action are described:
- Facilitated diffusion — transport from the intestinal lumen into the enterocyte and subsequent exit into the blood along a concentration gradient.
- Secondary active transport — Na-dependent symport with $Na^+$ ions, transporting glucose against a concentration gradient.
The energy for the work of the secondary active transporter is provided by the sodium gradient created by $Na^+, K^+$-ATPase on the basolateral membrane.
To remember the composition of disaccharides: Sucrose = Glucose + Fructose (SGF), Maltose = Glucose + Glucose (MGG), Lactose = Glucose + Galactose (LGG).
Class and exam questions
What chemical bond unites monosaccharides into polymers?
Monosaccharides in complex carbohydrates (oligosaccharides and polysaccharides) are connected to each other by a glycosidic bond.
Which carbohydrates are part of nucleic acids?
Nucleic acids contain pentoses (five-carbon sugars): ribose is present in RNA, and deoxyribose is in DNA.
What else to study on this topic
- Mechanism of glycosidic bond formation
- Spatial structure of linear and branched polysaccharides
- The role of heparin as a heteropolysaccharide in the body
- Carbohydrate components of tissue compatibility receptors
- Coenzymes FAD, $NAD^+$, and $NADP^+$: structure and carbohydrate residue
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