CARBOHYDRATES: CHEMICAL STRUCTURE, CLASSIFICATION AND FUNCTIONS - BIOLOGY - 2025

The carbohydrates, carbohydrates or saccharides, are organic molecules that store energy in living beings. They are the most abundant biomolecules and include: sugars, starches and cellulose, among other compounds found in living organisms.

Organisms that carry out photosynthesis (plants, algae and some bacteria) are the main producers of carbohydrates in nature. The structure of these saccharides can be linear or branched, simple or compound, and they can also associate with biomolecules of another class.

For example, carbohydrates can bind to proteins to form glycoproteins. They can also associate with lipid molecules, thus forming glycolipids, the biomolecules that form the structure of biological membranes. Carbohydrates are also present in the structure of nucleic acids.

Initially, carbohydrates were recognized as cellular energy storage molecules. Subsequently, other important functions that carbohydrates fulfill in biological systems were determined.

All living things have their cells covered by a dense layer of complex carbohydrates. Carbohydrates are made up of monosaccharides, small molecules made up of three to nine carbon atoms attached to hydroxyl groups (-OH), which can vary in size and configuration.

An important property of carbohydrates is the tremendous structural diversity within this class of molecules, which allows them to perform a wide range of functions such as generating cell signaling molecules, forming tissues, and generating the identity of the different blood groups in humans.

Likewise, the extracellular matrix in higher eukaryotes is rich in secreted carbohydrates, essential for cell survival and communication. These cell recognition mechanisms are exploited by a variety of pathogens to infect their host cells.

Monosaccharides can be linked by glycosidic bonds to form a wide variety of carbohydrates: disaccharides, oligosaccharides, and polysaccharides. The study of the structure and function of carbohydrates in biological systems is called glycobiology.

Chemical structure

Carbohydrates are made up of carbon, hydrogen, and oxygen atoms. Most of these can be represented by the empirical formula (CH2O) n, where n is the number of carbons in the molecule. In other words, the ratio of carbon, hydrogen, and oxygen is 1: 2: 1 in carbohydrate molecules.

This formula explains the origin of the term "carbohydrate" since the components are carbon atoms ("carbo") and atoms of water (therefore, "hydrate"). Although carbohydrates are mainly made up of these three atoms, there are some carbohydrates with nitrogen, phosphorus or sulfur.

In their basic form, carbohydrates are simple sugars or monosaccharides. These simple sugars can combine with each other to form more complex carbohydrates.

The combination of two simple sugars is a disaccharide. Oligosaccharides contain between two to ten simple sugars, and polysaccharides are the largest carbohydrates, made up of more than ten monosaccharide units.

The structure of carbohydrates determines how energy is stored in their bonds during their formation by photosynthesis, and also how these bonds are broken during cellular respiration.

Classification

Monosaccharides

Monosaccharides are the elemental units of carbohydrates, which is why they are the simplest structure of a saccharide. Physically, monosaccharides are colorless crystalline solids. Most have a sweet taste.

From a chemical point of view, monosaccharides can be aldehydes or ketones, depending on where the carbonyl group (C = O) is located in linear carbohydrates. Structurally, monosaccharides can form straight chains or closed rings.

Because monosaccharides possess hydroxyl groups, most are soluble in water and insoluble in non-polar solvents.

Depending on the number of carbons in its structure, a monosaccharide will have different names, for example: triose (if it has 3 C atoms), pentose (if it has 5C) and so on.

Disaccharides

Disaccharides are double sugars that are formed by bringing two monosaccharides together in a chemical process called dehydration synthesis, because a molecule of water is lost during the reaction. It is also known as a condensation reaction.

Thus, a disaccharide is any substance that is composed of two molecules of simple sugars (monosaccharides) linked together through a glycosidic bond.

Acids have the ability to break these bonds, for this reason disaccharides can be digested in the stomach.

Disaccharides are generally water soluble and sweet when ingested. The three main disaccharides are sucrose, lactose and maltose: sucrose comes from the union of glucose and fructose; lactose comes from the union of glucose and galactose; and maltose comes from the union of two glucose molecules.

Oligosaccharides

Oligosaccharides are complex polymers made up of few simple sugar units, that is, between 3 to 9 monosaccharides.

The reaction is the same that forms disaccharides, but they also come from the breakdown of more complex sugar molecules (polysaccharides).

Most oligosaccharides are found in plants and act as soluble fiber, which can help prevent constipation. However, most humans do not have the enzymes to digest them, except for maltotriose.

For this reason, oligosaccharides that are not initially digested in the small intestine can be broken down by bacteria that normally inhabit the large intestine through a fermentation process. Prebiotics fulfill this function, serving as food for beneficial bacteria.

Polysaccharides

Polysaccharides are the largest saccharide polymers, they are made up of more than 10 (up to thousands) monosaccharide units arranged in a linear or branched manner. Variations in spatial arrangement is what gives these sugars their multiple properties.

Polysaccharides can be composed of the same monosaccharide or by a combination of different monosaccharides. If they are formed by repeating units of the same sugar they are called homopolysaccharides such as glycogen and starch, which are the storage carbohydrates of animals and plants, respectively.

If the polysaccharide is made up of units of different sugars, they are called heteropolysaccharides. Most contain only two different units and are usually associated with proteins (glycoproteins, such as gamma globulin in blood plasma) or lipids (glycolipids, such as gangliosides).

Features

The four main functions of carbohydrates are: providing energy, storing energy, building macromolecules, and preventing the breakdown of proteins and fats.

Carbohydrates are broken down through digestion into simple sugars. These are absorbed by the cells of the small intestine and are transported to all the cells of the body where they will be oxidized to obtain energy in the form of adenosine triphosphate (ATP).

The sugar molecules that are not used in the production of energy at a given moment are stored as part of reserve polymers such as glycogen and starch.

Nucleotides, the fundamental units of nucleic acids, have glucose molecules in their structure. Several important proteins are associated with carbohydrate molecules, for example: follicle stimulating hormone (FSH), which is involved in the ovulation process.

Because carbohydrates are the main source of energy, their rapid degradation prevents other biomolecules from being degraded for energy. Thus, when sugar levels are normal, proteins and lipids are protected from degradation.

Some carbohydrates are soluble in water, they function as a staple food in almost everyone, and the oxidation of these molecules is the main route of energy production in most non-photosynthetic cells.

Insoluble carbohydrates associate to form more complex structures that serve as protection. For example: cellulose forms the wall of plant cells together with hemicelluloses and pectin. Chitin forms the cell wall of fungi and the exoskeleton of arthropods.

Also, peptidoglycan forms the cell wall of bacteria and cyanobacteria. Animal connective tissue and skeletal joints are made up of polysaccharides.

Many carbohydrates are covalently bound to proteins or lipids forming more complex structures, collectively called glycoconjugates. These complexes act as tags that determine the intracellular location or metabolic fate of these molecules.

Foods that contain carbohydrates

Carbohydrates are an essential component of a healthy diet, as they are the main source of energy. However, some foods have healthier carbohydrates that offer a higher amount of nutrients, for example:

Starches

Starchy foods are the main source of carbohydrates. These starches are generally complex carbohydrates, that is, they are made up of many sugars joined together to form a long molecular chain. For this reason, starches take longer to digest.

There is a wide range of foods that contain starches. Grains include foods that are high in starch, for example: beans, lentils, and rice. Cereals also contain these carbohydrates, for example: oats, barley, wheat and their derivatives (flours and pasta).

Legumes and nuts also contain carbohydrates in the form of starches. Additionally, vegetables like: potatoes, sweet potatoes, corn, and squash are also rich in starch content.

Importantly, many carbohydrates are a source of fiber. In other words, fiber is basically a type of carbohydrate that the body can only partially digest.

Similar to complex carbohydrates, carbohydrate fibers tend to be digested slowly.

Fruits and vegetables

Fruits and vegetables are high in carbohydrates. In contrast to starches, fruits and vegetables contain simple carbohydrates, that is, carbohydrates with one or two saccharides attached to each other.

These carbohydrates, being simple in their molecular structure, are digested more easily and quickly than complex ones. This gives an idea of ​​the different levels and types of carbohydrates in foods.

Thus, some fruits have more carbohydrate content per serving, for example: bananas, apples, oranges, melons and grapes have more carbohydrates than some vegetables such as spinach, broccoli and kale, carrots, mushrooms and aubergines.

Milk

Similar to vegetables and fruits, dairy is foods that contain simple carbohydrates. Milk has its own sugar called lactose, a sweet-tasting disaccharide. One cup of this is equivalent to about 12 grams of carbohydrates.

There are many versions of milk and yogurt on the market. Regardless of whether you are consuming a whole or reduced-fat version of a particular dairy, the amount of carbohydrates will be the same.

The sweets

Sweets are another well-known source of carbohydrates. These include sugar, honey, candy, artificial drinks, cookies, ice cream, among many other desserts. All of these products contain high concentrations of sugars.

On the other hand, some processed and refined foods contain complex carbohydrates, for example: bread, rice and white pasta. It is important to note that refined carbohydrates are not nutritious like the carbohydrates found in fruits and vegetables.

Carbohydrate metabolism

Carbohydrate metabolism is the set of metabolic reactions that involve the formation, degradation and conversion of carbohydrates in cells.

Carbohydrate metabolism is highly conserved and can be observed even from bacteria, the main example being the Lac Operon of E. coli.

Carbohydrates are important in many metabolic pathways including photosynthesis, nature's most important carbohydrate formation reaction.

From carbon dioxide and water, plants use energy from the sun to synthesize carbohydrate molecules.

For their part, animal and fungal cells break down carbohydrates, consumed in plant tissues, to obtain energy in the form of ATP through a process called cellular respiration.

In vertebrates, glucose is transported throughout the body through the blood. If cellular energy stores are low, glucose is broken down through a metabolic reaction called glycolysis to produce some energy and some metabolic intermediates.

Glucose molecules not needed for immediate energy production are stored as glycogen in the liver and muscle, through a process called glycogenesis.

Some simple carbohydrates have their own degradation pathways, like some of the more complex carbohydrates. Lactose, for example, requires the action of the enzyme lactase, which breaks its bonds and releases its fundamental monosaccharides, glucose and galactose.

Glucose is the main carbohydrate consumed by cells, it constitutes approximately 80% of energy sources.

Glucose is distributed to cells, where it can enter through specific transporters to be degraded or stored as glycogen.

Depending on the metabolic requirements of a cell, glucose can also be used to synthesize other monosaccharides, fatty acids, nucleic acids, and certain amino acids.

The main function of carbohydrate metabolism is to maintain control of blood sugar levels, this is what is known as internal homeostasis.

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