Carbohydrate: Definition, Types, and Examples

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Carbohydrate Definition

Carbohydrate is any of a class of organic molecules made up of carbon, hydrogen, and oxygen in a 1:2:1 ratio, as in the general formula: Cn(H2O) n. “Saccharide” and “carbohydrate” are synonyms.

What is Carbohydrate?

Any molecule generated by living creatures is referred to as a biomolecule. As a result, the vast majority of them are organic compounds. Amino acids and proteins, carbohydrates (particularly polysaccharides), lipids, and nucleic acids are the four primary categories of biomolecules.

A carbohydrate is any of a class of organic molecules that contain carbon, hydrogen, and oxygen in a 1:2:1 ratio, as indicated by the general formula: Cn (H2O) n. Carbohydrates are the most plentiful of the major biomolecule classes.

Characteristics of Carbohydrate

Carbohydrates are Organic Compounds

An organic molecule is one in which carbon is covalently linked to other atoms, particularly Carbon-Carbon (C-C) and Carbon-Hydrogen (C-H) (C-H). Carbohydrates are one sort of chemical substance among many. Carbon, hydrogen, oxygen, and nitrogen are the four main elements that make up carbon.

The majority of them have the general formula Cn (H2O) n, which is whence they get their name, carbs (which means hydrates of carbon). This is due to the fact that the ratio of hydrogen to oxygen atoms is frequently 2:1.

This formula, however, does not apply to all carbs. In essence, they are aldehydes or ketones with a large number of hydroxyl groups attached to each carbon atom that is not part of the aldehyde or ketone functional group.

Carbohydrates are Energy-rich Biomolecules

They are one of the most important nutrients for many living creatures since they give chemical energy to the body. ATP is a chemical energy that is created during cellular respiration through a number of metabolic activities. In a nutshell, glucose (a monosaccharide) is “churned” to extract energy in the form of ATP.

To begin, glucose is converted to pyruvate by a sequence of processes. It then converts pyruvate to acetyl coenzyme A for oxidation via the Krebs cycle, an enzyme-driven cyclic process. Finally, the electron transport chain is involved in a series of events (redox reactions) that result in the synthesis of ATPs (via chemiosmosis).

Glycolysis requires glucose molecules, which come from a carbohydrate-rich diet. During digestion, saccharification breaks down complex carbohydrates into simpler monosaccharides like glucose.

Carbohydrates are an important source of nourishment for both animals and humans. Many other carbs, on the other hand, come in the form of fibres. It is also difficult for humans to digest as fibre. Mucilages, pectins, gums, and insoluble components present in lignin and cellulose are examples of fibrous carbohydrates.

Ruminants, such as cattle, sheep, deer, and goats, may digest plant matter that would otherwise be indigestible to humans. Ruminococcus, Fibrobacter, Streptococcus, and Escherichia coli are symbiotic bacteria that live in their rumen and may breakdown cellulose materials into simpler carbs for ruminants.

Classification of Carbohydrates

Polymers make up a large portion of carbs. Polymerization produces a polymer, which is a molecule made up of multiple repeating units (monomers) or protomers. The carbohydrates’ structural (monomeric) unit is the saccharide.

Carbohydrates are characterized as monosaccharides, disaccharides, oligosaccharides, or polysaccharides depending on the number of saccharide units. The most basic kind is monosaccharides, which are simple sugars. These simple sugars can be combined to produce more complex sugars.

Glucose, galactose, and fructose are examples. A disaccharide is a sugar molecule comprises of two simple sugars. Sugars include sucrose, maltose, and lactose, among others.

Oligosaccharides are carbohydrates made up of three to ten simple sugars. Raffinose, maltotriose, and maltotetraose are among the examples. Polysaccharides are carbohydrates made up of multiple saccharide units.

When a polysaccharide is made up of saccharide units of the same type, it is termed a homopolysaccharide (or homoglycan), but when a polysaccharide is made up of saccharide units of different types, it is called a heteropolysaccharide (or heteroglycan) (or heteroglycan). Polysaccharides encompass starch, cellulose, and glycogen.

Carbohydrates are divided into two categories in terms of nutrition: simple and complex. Simple carbohydrates, also known as “sugar,” are easily digestible carbohydrates that provide a quick source of energy.

Complex carbs take longer to digest and metabolise than simple carbohydrates. They are frequently abundant in fibre and, unlike simple carbs, are less prone to induce blood sugar increases.

Function of Carbohydrate

One of the most important roles of carbohydrates, as previously stated, is to supply energy to organisms. Monosaccharides, in particular, are the body’s primary source of energy. They are transformed into energy-storing polysaccharides, such as starch in plants and glycogen in animals, when they are no longer needed.

Starch is prevalent in amyloplasts within the cells of numerous plant organs, such as fruits, seeds, rhizomes, and tubers, in plants. Glycogen is housed in animals’ livers and muscle cells. Aside from that, carbohydrates play an essential role in the structure of the body.

Polysaccharides (such as cellulose) are components of the cell walls of plant cells and many algae at the cellular level. Structure and mechanical damage are more likely in cells without cell walls. The cell wall in plants protects the cell from bursting in a hypotonic solution. Water diffuses into the cell due to osmotic pressure.

The osmotic pressure is resisted by the cell wall, which prevents the cell from bursting. The structural carbohydrate in bacterial cell walls is murine, whereas the polysaccharide chitin is a cell wall component in fungus.

Some bacteria contain a polysaccharide “capsule” that helps them avoid immune cell identification. Chitin exoskeletons are found in some animals and offer strength and protection to soft-bodied creatures.

Nucleic acids, such as RNA and DNA, include ribose and deoxyribose, respectively, as a sugar component. Many other biological molecules, including as glycoproteins, glycolipids, and proteoglycans, include sugar components as well, and play important roles in immune response, detoxification, blood coagulation, fertilisation, biological identification, and so on.

Carbohydrate Bio-reaction

Some of the most frequent biological responses involving carbs are listed below.

Photosynthesis: Photosynthesis is used to make simple sugars (such as glucose) in plants and other photosynthetic autotrophs. To generate glucose, water, and oxygen molecules, the process requires carbon dioxide, water, inorganic salts, and light energy (from sunlight) absorbed by light-absorbing pigments like chlorophyll and other accessory pigments.

Dehydration Synthesis: Dehydration synthesis is the process through which monosaccharides come together in glycosidic linkages to create carbohydrates. The combining of two monosaccharides to produce a disaccharide, for example, leads in the release of water as a by-product.

Polysaccharides, on the other hand, are produced from a lengthy chain of monosaccharide units by a dehydration process. The compounds produced, such as starch and glycogen, are energy-dense. When the body demands extra energy, these complex carbohydrates are digested into simpler forms (e.g. glucose). Saccharification is the term for this procedure.

Saccharification: Saccharification is the degradation of complex carbohydrates into simpler carbohydrates including such glucose. It entails the process of hydrolysis. Enzymatic activity is involved in humans and other higher animals. Through the action of salivary amylase, glucose-containing complex carbohydrates are broken down into simpler forms in the mouth.

The digestion of complex carbohydrates continues in the small intestine. Maltase, lactase, and sucrase are enzymes that break down disaccharides into monosaccharide components. Another class of enzymes is glucosidases, that also catalyse the removal of the terminal glucose from either a polysaccharide made consisting of long glucose chains.

Assimilation: The epithelial cells of the small intestine absorb monosaccharides from digested carbs. The sodium ion-glucose symport system transports them from the intestinal lumen to the cells (via glucose transporters or GluT).

GluTs are proteins that aid in the transport of monosaccharides into cells, such as glucose. Then, by enhanced diffusion, they are released into the capillaries. Tissue cells need GluTs to re-absorb them from the circulation. When glucose enters the cell, it gets phosphorylated to keep it there. As a result, glucose-6-phosphate can be utilised in any of the metabolic processes listed below:

(1) glycolysis, where glucose is transported to the liver via the vena portae and stored as cellular glycogen,

(2) glycogenesis, where glucose is transported to the liver via the vena portae and stored as cellular glycogen, or

(3) NADPH for lipid synthesis and pentoses for nucleic acid synthesis are produced through the use of the pentose phosphate pathway.

Cellular Respiration: Cellular respiration is the mechanism through which glucose is digested by the cell. The three primary stages or processes in cellular respiration are (1) glycolysis, (2) the Krebs cycle, and (3) oxidative phosphorylation.

A sequence of events in the cytosol culminate in the conversion of a monosaccharide, generally glucose, into pyruvate and the creation of a tiny quantity of high-energy biomolecules, such as ATP, in the first stage (glycolysis). Also generated is NADH, an electron-carrying molecule. The pyruvate from glycolysis is transformed into an organic molecule that can be fully oxidised inside the mitochondrion if there is enough oxygen.

The electrons are shuttled along the electron transport chain by electron carriers (such as NADH and FADH2). Along the chain, a succession of redox reactions occur, culminating in the ultimate electron acceptor, i.e. molecular oxygen. More ATP is generated by chemiosmosis in the inner mitochondrial membrane, which is a coupling process. The net ATP produced by glycolysis is two (from substrate-level phosphorylation).

The net ATP produced by oxidative phosphorylation is about 34. As a result, the total net ATP per glucose is around 36. 2 Anaerobic catabolism happens when there is no or insufficient oxygen (e.g. by fermentation).

Fermentation is an anaerobic process that uses glycolysis to create ATP. Instead of shuttling electrons along the electron transport chain, NADH delivers electrons to pyruvate, replenishing NAD+ and allowing glycolysis to continue. 2 Fermentation produces just approximately two ATPs per gram of glucose.

Gluconeogenesis: Gluconeogenesis appears to be the inverse of glycolysis in that glucose is turned into pyruvate, whereas pyruvate is converted into glucose in gluconeogenesis. Gluconeogenesis is a metabolic process during which non-carbohydrate precursors such as pyruvate, lactate, glycerol, and glucogenic amino acids get converted to glucose.

Gluconeogenesis occurs mostly in the liver cells of humans and many other animals. Fasting, low-carbohydrate diets, and strenuous exercise are all common causes. The process starts in the mitochondria and ends in the endoplasmic reticulum lumen, according to cytology. Glucose is shuttled from the endoplasmic reticulum into the cytoplasm when the enzyme glucose-6-phosphatase hydrolyzes glucose-6-phosphate.

Glycogenesis: Glycogenesis is the metabolic process of converting glucose into glycogen for storage, mostly in the liver and muscle cells, in response to high blood glucose levels. Short glucose polymers, particularly exogenous glucose, are synthesized into long polymers, which are subsequently stored inside cells, mostly in the liver and muscle. The process of glycogenolysis breaks down glycogen into glucose subunits when the body demands metabolic energy. As a result, glycogenesis is the inverse of glycogenolysis.

Glycogenolysis: Glycogenolysis is the disintegration of stored glycogen in the liver to produce glucose that can be used in energy metabolism. Glycogen is broken down into glucose precursors in the liver cells. A single glucose molecule is separated from glycogen and turned to glucose-1-phosphate, which is then converted to glucose-6-phosphate, which may be used in glycolysis.

Pentose Phosphate Pathway: The pentose phosphate route is a glucose metabolic process in which the cytosol produces five-carbon sugars (pentoses) and NADPH. In the breakdown of glucose, the pentose phosphate pathway is an alternative metabolic route. It may be found in the liver, adrenal cortex, adipose tissues, testis, and other organs in animals.

In neutrophils, this is the primary metabolic route. As a result, infection sensitivity is caused by a congenital defect in the route. Part of the route is involved in the photosynthesis of hexoses from carbon dioxide in plants.

Leloir Pathway (Galactose Metabolism): Galactose enters glycolysis through this metabolic route by being phosphorylated by the enzyme galactokinase and subsequently transformed to glucose-6-phosphate. Lactose is the source of galactose (milk sugar comprised of a glucose molecule and a galactose molecule).

Fructose 1-Phosphate Pathway: Instead of glucose, fructose enters glycolysis in this metabolic route. However, before fructose can enter glycolysis, it must go through a few more stages. It may be found in the muscles, fat tissues, and the kidney in animals.

Glucoregulation: The correct absorption and breakdown of carbs inside the organism requires appropriate carbohydrate metabolism. Glucoregulation refers to the body’s ability to maintain constant glucose levels. Hormones from the pancreas, such as insulin and glucagon, control glucose metabolism.

The quantity of glucose circulating in the body is referred to as blood sugar. Whenever blood glucose levels are low, glucagon is emitted. A high blood glucose level, on the other hand, promotes the production of insulin. Insulin controls carbohydrate (and fat) metabolism by increasing glucose absorption from the circulation into skeletal muscles and adipose tissues, where it is stored as glycogen for later use in glycogenolysis.

Glucagon, in turn, works by increasing sugar production. It causes the liver’s stored glycogen to be transformed into glucose, which is then released into the circulation. Diabetes mellitus, lactose intolerance, galactosemia, glycogen storage disease, and fructose malabsorption are all examples of metabolic illnesses or disorders caused by improper carbohydrate metabolism.

Carbohydrate Citations
  • Emergence and significance of carbohydrate-specific antibodies. Genes Immun . 2020 Aug;21(4):224-239.
  • Carbohydrate-based adjuvants. Drug Discov Today Technol . 2020 Dec;35-36:57-68.
  • Carbohydrate terminology and classification. Eur J Clin Nutr . 2007 Dec;61 Suppl 1:S5-18.
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