Structure and Function of 3 Most Important Carbohydrates

Carbohydrates are the most available biological macromolecules found in living species on Earth that are one of the three major sources of energy for the living organism.

Carbohydrates, which include sugars, fibers, and starches, are necessary nutrients. They can be found in cereals, vegetables, fruits, as well as milk and other dairy products. They are the primary dietary types that are essential for living a healthy life.

Carbohydrate-containing foods are transformed into glucose or blood sugar by the digestive system throughout the digesting process.

Carbohydrates are one of the most important members of 5 Major Biomolecules in Life | Chemical Constituents of Life.

Definition of Carbohydrates:

Carbohydrates are polyhydroxy aldehydes or ketones, or chemicals that hydrolyze to produce such compounds. Carbohydrates are primarily made up of carbon, hydrogen, and oxygen.

The empirical formula for most carbohydrates is (CH2O)n, where n is the number of atoms.

Some carbohydrates, on the other hand, may contain nitrogen, phosphorus, or sulfur in their chemical composition.

These are vital components of food that give the necessary energy in the form of glucose. Carbohydrates are distinct from other biological macromolecules in the sense that they may bind to protein and lipids to produce glycoprotein and glycolipid, respectively.

Examples of Carbohydrates:

Hexoses, glucose, and fructose are the major monosaccharides found in fruits and vegetables.

Mannose, galactose, xylose, and arabinose are examples of minor monosaccharides.

The most prevalent pentoses found are arabinoses and xyloses.

So the common examples of carbohydrates are:

Glucose:

Glucose is the most important and abundant monosaccharide in the human body; it is the principal cell fuel and may be found unbound in body tissues and fluids. Glucose is included in a variety of polysaccharides.

Galactose:

Galactose is a monosaccharide produced by the hydrolysis of lactose, a sugar present in milk. It is also produced by the human body from glucose. Galactose is a glycolipid component that is common in nerve cells and can be used as cell fuel.

Maltose:

Maltose is a reducing sugar, therefore it can transfer electrons in chemical reactions. Maltose is utilized for fuel by seed growth in nature. As Maltose is easily absorbed, animals transform starch from plant sources into Maltose. Maltose further breaks down into glucose, which is then used to produce energy.

Fructose:

Fructose (levulose) is a sugar present in honey and fruits that can be used to power cells. Fructose is the most delectable monosaccharide.

Sucrose:

Our body needs a consistent supply of energy. Sucrose, as a carbohydrate, provides your body with the energy it needs to perform physical and mental functions. Your body transforms foods like sucrose and starch into fructose and glucose during digestion.

Lactose:

Lactose, which is synthesized in the mammary gland, is nature’s means of obtaining a significant amount of carbs in milk, and it significantly affects the volume of milk produced as well as providing a significant share of slow-release energy.

Starch:

Humans depend on starch as their primary source of energy. The body digests starch by converting it into glucose, which enters the bloodstream and circulates throughout the body. Glucose fuels practically every cell, tissue, and organ in the body. The liver stores excess glucose as glycogen.

Cellulose:

Cellulose is the principal structural component that accounts for the majority of the cell wall’s mechanical strength. Cell development is influenced by the distribution and orientation of cellulose microfibrils within the cell wall.

Chitin:

Chitin is the most prevalent renewable polymer in the waters and a significant carbon and nitrogen source for marine species. Chitin breakdown is an important phase in the cycling of nutrients in the oceans, and chitinolytic bacteria play an important part in this process.

Sources of Carbohydrates:

Monosaccharides are found in the cytosol (cell sap) most of the time. Some fruits and vegetables, such as corn, peas, and sweet potatoes, have a high concentration of them.

Fructose:

Most fruits contain simple sugars in a kind of fructose.

Galactose:

It may be found in nearly all dairy products.

Lactose:

It is frequently found in milk and other dairy products.

Maltose:

It may be found in breakfast cereal, beer, potatoes, processed cheese, pasta, and other foods.

Sucrose:

It originates organically from sugar and honey, which contain small quantities of vitamins and minerals.

These simple carbohydrates with minerals and vitamins are typically found in milk, fruits, and vegetables. Many refined and processed foods, such as white flour, white rice, and sugar, lack essential nutrients. It is perfectly safe to consume vitamins, carbs, as well as other organic substances in their original state.

Significance of Glucose:

Glucose is the most nutritionally significant and abundant monosaccharide that is easily absorbed in the colon. it is the primary cell fuel in the human body and can be found unbound in body tissues and fluids. Glucose is a component of several polysaccharides. Cell fuels galactose and fructose are also used.

Functions of Carbohydrates:

• Carbohydrates’ primary job is to provide energy and food to the body and nervous system.
• Carbohydrates, which include sugars, starch, and fiber and are abundant in grains, fruits, and milk products, are recognized as one of the basic components of the diet.
• It also helps with fat metabolism and prevents ketosis.
• Amylase, an enzyme, aids in the breakdown of starch into glucose, which is then converted into energy for metabolism.

Carbohydrates play a vital role in the texture, flavor, color, and nutritional value of horticultural goods. Plant cells require celluloses, hemicelluloses, and pectins to function properly. Starch is a polymer found in unripe fruits and vegetables that is converted into simple carbs during ripening. Sugars help to make fruits and vegetables sweeter. The flavor of horticulture commodities is determined by the balance of sugars, organic acids, and other substances like phenolic compounds. The majority of the flavonoids responsible for the red and blue hues of horticultural goods are glycosides. Through their breakdown during the respiration process, hexose carbohydrates provide energy to the cell.

Ascorbic acid (vitamin C) is classified as a sugar derivative.

Requirements for Carbohydrates:

• Carbohydrates (Carbs) are employed as an energy source, as a precursor for the production of glycoproteins and glycolipids, and as a general precursor for the majority of complex chemical molecules in the body.
• Carbohydrates in plant meals are starches and sugars. Glycogen, as well as minor amounts of glucose (Glc) and other sugars, are found in animal meals.
• While there are no specific criteria, a balanced diet should give more than half of the energy as carbohydrates. A large portion of this should come from whole grains and other types of complex, slowly digested carbohydrates.

Deficiency:

• The health hazards of low carbohydrate intake are still debated, and they are heavily influenced by alternate energy fuel sources (protein, saturated fat, monounsaturated fat, etc.).
• The risk is greatest when complex carbs are consumed in little amounts. The most frequently mentioned side effects are related to an increased risk of atherosclerosis.
• Starvation is caused by a low intake combined with an insufficient overall energy intake.

Excessive intake:

• The danger is greatest when high carbohydrate consumption is primarily in the form of simple sugars and occurs in the setting of excessive total energy fuel consumption.
• High amounts encourage the non-enzymatic production of protein-sugar adducts, which can impair protein function.
• Long-term risks include an increased risk of obesity, diabetes, and atherosclerosis.
• The consumption of simple sugars on a regular basis, combined with poor oral hygiene, increases the risk of dental caries and tooth loss.

Types of Carbohydrates:

Carbohydrates are categorized into three types based on their molecular structure: monosaccharides (the simplest units of carbohydrates), oligosaccharides (made by 2-10 units of monosaccharides joined by glycosidic linkages), and polysaccharides (theoretically created by more than 10 units).

So, carbohydrates are classified into four types based on their molecule size: monosaccharide, disaccharide, oligosaccharide, and polysaccharide.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates. As basic carbohydrates, they can’t be split into smaller forms. They are aldehydes or ketones with two or more hydroxyl groups that are essential as building blocks for the synthesis of nucleic acids as well as fuel molecules, such as in glycolysis.

Monosaccharides can unite to generate bigger carbohydrates via glycosidic linkages. Monosaccharides’ primary role is to produce and store energy. The most abundant monosaccharides in nature are glucose and fructose.

Classification of Monosaccharides:

The positioning of the carbonyl group, the number of carbon atoms present, and the chirality of monosaccharides are used to classify them. When the carbonyl group is present then an aldehyde is called an aldose and when the carbonyl group is a ketone, then it is called a ketose respectively.

The two simplest monosaccharides are glyceraldehyde, an aldotriose, and dihydroxyacetone, a ketotriose.

Number of carbon atoms	Kind of carbonyl group
	Aldehyde	Ketone
3	triose	triulose
4	tetrose	tetrulose
5	pentose	pentulose
6	hexose	hexulose
7	heptose	heptulose
8	octose	octulose
9	nonose	nonulose

The monosaccharide is made up of a single unit with a three to six-carbon chain.

Trioses are the smallest monosaccharides, with three carbon atoms, and include dihydroxyacetone and d- and l-glyceraldehyde.

Those with four carbon atoms are referred to as tetroses, those with five carbon atoms are referred to as pentoses, those with six carbon atoms are referred to as hexoses, and so on.

Formula and examples of some monosaccharides:

Trioses (C₃H₆O₃):

It has three carbon atoms per molecule. Example: Glyceraldehyde

Tetroses (C₄H₆O₄): 

Contains four carbon atoms per molecule. Example: Erythrose.

Pentoses(C₅H₁₀O₅):

Composed of five carbons. Examples: ribose in RNA and deoxyribose in DNA. There are 2 types of pentoses- ketopentoses and aldopentoses.     

Hexoses(C₆H₁₂O₆):

With six carbon atoms. Examples: D-Glucose. D-Fructose.

Heptoses(C₇H₁₄O₇):

They have seven carbon atoms. Examples: L-glycero-D-manno-heptose.

Disaccharides

Disaccharide is made up of two monosaccharide units connected by a glycosidic bond created by the condensation reaction of one monosaccharide’s hydroxyl group with the hydrogen of another monosaccharide.Disaccharides are made up of two monosaccharide units connected by glycosidic linkages in the orientation.

Examples:

Sucrose, lactose, and maltose are especially significant. These most common disaccharides are sucrose, lactose, and maltose, sometimes known as table sugar, milk sugar, and malt sugar.

Sucrose:

It is the most abundant and is made up of a molecule of glucose and a molecule of fructose joined together.

Lactose:

This is present in milk and dairy products and is made up of galactose and glucose that are bonded together by a -1,4-glycosidic bond.

Maltose:

This is mostly formed from the partial hydrolysis of starch and is composed of two glucose units connected by a -1,4-glycosidic bond.

.

Class	Species	Significance
Disaccharide	Sucrose	Constituent of fruits, vegetables, and sweeteners
	Lactose	Milk and dairy products
	Maltose, isomaltose	Constituent of starch
	Trehalose	Food additive and constituent of mushrooms
	Lactulose	Lactose derivative, laxative
Disaccharide alcohols	Maltitol	Constituent of starch, sweetener
	Lactitol	Constituent of lactose, sweetener

Oligosaccharides

Oligosaccharide is a short chain of monosaccharide units made up of 3-10 molecules linked together by glycosidic linkage. Oligosaccharides are carbohydrate chains that comprise three to ten sugar units. Some authors, however, include carbohydrates with up to 20 residues and even disaccharides.

Oligofructose and Oligogalactose:

Oligosaccharides can be synthesized from any sugar monomer, however, the majority of study has focused on fructooligosaccharides (for example, oligofructose) and galactooligosaccharides (for example, raffinose, human milk oligogalactose).

Examples of Oligosaccharides:

Maltotriose:

Few oligosaccharides (e.g., maltotriose) are digested and absorbed in the small intestine, but virtually all enter the colon intact (nondigestible oligosaccharides)

Raffinose:

It is the most frequent type of oligosaccharide. It is a trisaccharide consisting of galactose, glucose, and fructose that is present in many plants.

                                                                                                        

Common name	Simplified structures	Source
Lactose	Galβ1→4Glc	Milk, milk products
Maltose	Glcα1→4Glc	Glucose syrups, hydrolysis of starch
Sucrose	Fruβ2→1Glc	Table sugar
Cellobiose	Glcβ1→4Glc	Hydrolysis of cellulose
Trehalose	Glcα1→1Glc	Mushrooms, yeast

Polysaccharides

Polysaccharides are biopolymers composed of long chains of monosaccharide molecules. It is made up of hundreds to thousands of monosaccharide molecules, which can be straight or branched. Based on the kind of monosaccharide, polysaccharides are divided into two types: homopolysaccharides and heteropolysaccharides.

Homopolysaccharides:

These are composed of one type of monosaccharide, such as cellulose, starch, and glycogen, which include glucose units.

The fructan, xylan, galactan, and chitin, which contain fructose, xylose, galactose, and N-acetyl glucosamine respectively.

Heparin is composed of glucosamine, glucuronic acid, and iduronic acid.

Heteropolysaccharides:

These types of carbohydrate contains two or more different types of monosaccharides. 

The hyaluronic acid is composed of hundreds of units of N-acetyl glucosamine and glucuronic acid.

Characteristics of Polysaccharides:

Polysaccharides are the most common organic biopolymer with unique chemical, physical, and biological characteristics. The backbone of these polymers is formed by monosaccharide building units and glycosidic connections, which define the diversity and complexity of the polysaccharides.

Polysaccharides are monosaccharide polymers2. (Polymers are long sequences of structural units [poly meaning many in Greek].) Polysaccharides are thus high-molecular-weight carbohydrate compounds with many monosaccharide units. Most polysaccharides are substantially larger than the oligosaccharide limit of 20 units.

Polysaccharide monosaccharide units are connected in a head-to-tail pattern by glycosidic connections, as in most oligosaccharides. Polysaccharide molecules, like oligosaccharide molecules, can be either linear or branched. As a result, all polysaccharides have one and only one decreasing end. Nonreducing endings are found on branched polysaccharides.

Functions of Polysaccharides:

Polysaccharides’ primary role is energy storage or structural support of the organism. Starch and glycogen are employed for energy storage, whereas cellulose and chitin are used for plant and animal structural support, respectively.

Homoglycans:

If all of the glycosyl units are of the same sugar type, the polysaccharide is homogenous in terms of monomer units and is referred to as a homoglycan. Homoglycans can be straight or branched. Homoglycans include cellulose and the linear amylose component of starch, as well as the branching amylopectin component of starch; each of these polysaccharides is made up entirely of d-glucopyranosyl units.

Heteroglycans:

A heteroglycan is a polysaccharide that is made up of two or more distinct monosaccharide units. A diheteroglycan is a polysaccharide that comprises two different monosaccharide units; a triheteroglycan contains three different monosaccharide units, and so on.

Carbohydrates play a pivotal role in sustaining life and supporting various physiological functions within the human body. Serving as the primary source of energy, carbohydrates undergo intricate processes to provide the fuel necessary for cellular activities. The diversity of carbohydrates, ranging from simple sugars to complex polysaccharides, underscores their significance in nutrition.