Structural Carbohydrates

When you think of Carbohydrates, do you think of foods like pasta, rice, and grains? If so, that makes sense since people often talk about carbohydrates in the context of the foods that make up our diets.

Celebrities in the media and online regularly mention low-carb and high-protein or keto diets, but Carbohydrates are found all around us and inside us. They can come in many forms. Besides processing them, our bodies also use them to structure and stabilize things.

In the following, we will explore structural carbohydrates, the different types, some examples, and how they compare to non-structural ones!

Types of Structural Carbohydrates

Let's start by looking at the definition of carbohydrates and the types of structural carbohydrates that exist.

The arrangement of carbon to hydrogen and oxygen gives carbohydrates their name. One carbon atom (\(C\)), two hydrogen atoms (\(H\)), and one oxygen atom (\(O\)) are what make up the foundation of carbohydrates.

Carbohydrates are used to store energy. Every ounce of this energy is used by the human body when ingested, as carbohydrates are essential to our nutrition.

Besides fueling your body, carbohydrates also help make lipids and build important macromolecules, and their byproducts aid the Immune System, reproduction, and energy production.

You may have noticed that this combination of particles makes it a pairing of carbon (carbo-) and water (-hydrate). How we differentiate carbohydrates is by the length of these atomic chains.

These chains allow us to categorize carbohydrates into three main groups:

• Monosaccharides

• Disaccharides

• Polysaccharides

All of these carbohydrates store energy for some time in some form. Monosaccharides and disaccharides are known as simple sugars or carbohydrates.

Examples of monosaccharides include glucose and fructose, as shown in Figure 1.

Figure 1: Examples of monosaccharide structures shown. Wikimedia. DsChanz.

Fructose and glucose are isomers, or molecules with the same formula \(C_6H_{12}O_6\) but different structural arrangements. Many foods we consume contain glucose or fructose, such as apples, grapes, pears, and raisins.

When two monosaccharides bond together, it’s called a disaccharide. The most common disaccharide that many of us use today is in the form of table sugar or sucrose. Sucrose is made up of the monosaccharides glucose and fructose linked together, as illustrated in Figure 2.

Figure 2: Disaccharide structure of sucrose shown. Wikimedia. NEUROtiker.

Polysaccharides and oligosaccharides are considered complex carbohydrates. The molecules are formed when many groups of simple sugars are chained together. Polysaccharides consist of multiple monosaccharides or more complicated structures, making them complex carbohydrates. Examples of polysaccharides would be starch, glycogen, cellulose, and chitin.

• Oligosaccharides are carbohydrates usually consisting of three to ten monosaccharides, while polysaccharides are, on average, longer. Examples of oligosaccharides include raffinose and oligofructose.

• Polysaccharides consist of multiple monosaccharides or more complicated structures, making them complex carbohydrates.

• Cellulose: The most abundant biopolymer in the world, comprising glucose bonds that can form into dense fibers and compose the cell walls of Plants and vegetables. It can be found in wood and cotton. Biopolymers are substances produced by living organisms' Cells.

• Chitin: A structural polysaccharide rich in nitrogen and present in the structure of Fungi and exoskeletons of arthropods. It is the most abundant natural Polymer in the world after cellulose.

• Peptidoglycan: Also known as murein, it is a resistant copolymer that makes up the Cell Structure in numerous Bacteria. Copolymers consist of more than one type of monomer in their chains.

Some structural carbohydrates that are oligosaccharides are:

• Fructooligosaccharides (FOS): Found naturally in Plants and are usually used by humans as alternative sugars because they have fewer calories than table sugar or sucrose.

• Raffinose: Raffinose are oligosaccharides found in many plant products, such as beans and cabbage, and can protect seeds from drying out.

Structural Carbohydrates in Plants

Some of the most critical structural carbohydrates in plants are:

• Cellulose

• Hemicellulose

• Pectins

• Fructans

• Gums

Cellulose

Cellulose are polysaccharides or complex carbohydrates made of glucose linkages.

Generally, cellulose consists of the formula \((C_6H_{10}O_5)n\) where n is the number of monomers present in the compound. Cellulose is a linear, branched polymer when compared to starch and has \(\beta\) glycosidic linkages. It also consists of glucose Monomers. Specific properties of cellulose depend on the number of Monomers or chain length of the molecule.

Cellulose makes up the cell walls of green plants, such as Algae and land plants. Land or terrestrial plants are most of the plants that make up the earth.

Hemicellulose

Hemicellulose is a polysaccharide that's a copolymer. It also makes up the cell walls of land plants like cellulose.

Compared to cellulose, hemicellulose has shorter chains and is branched. Hemicelluloses can be found as cross-linked fibers bound to pectin and cellulose to form a matrix or connection of networks to support plants, as shown in Figure 3.

Since it's a copolymer, it consists of multiple monosaccharides: glucose, galactose, xylose, arabinose, and mannose.

Pectin

Pectins are polysaccharides that make up the cell walls of land plants, like cellulose. It's mainly found in the non-woody parts of plants.

Pectin makes up fruits and vegetables and is usually broken down as fruits and vegetables ripen. Fruits and veggies become softer as they mature because the matrix shown in Figure 3 is broken down. This is the same process that occurs during the fall when leaves change colors and fall to the ground.

In desert plants, pectin has been shown to repair DNA. Foods that have been shown to contain large amounts of pectin include apples, oranges, pears, and carrots.

In industrial settings, we mainly use pectin as a thickening agent. For example, we use pectin to thicken jams into the consistency we're familiar with. Pectin is mainly made up of galacturonic acid derived from galactose.

Figure 3: Cellulose, Hemicellulose, and Pectin cross-linked to form the structure of plant cell walls. Wikimedia. LadyofHats.

Fructans

Fructans are polysaccharides made up of fructose monomers. Fructans that have a shorter chain length are known as fructooligosaccharides. They can be found in grass and other plants such as garlic, onions, and agave.

They are considered structural polysaccharides because they aren't easily digestible but have storage functions. They help plants with cold and drought tolerance and regulate osmotic pressure. Humans can also use them as alternative sweeteners, as they have lower calories than table sugar.

Gums

Natural gums are polysaccharides usually found in woody parts of plants. Natural gums can act as binding or thickening agents extracted from green Algae such as seaweed. One of the most famous examples of natural gums is agar.

Agar can be used in labs to culture Bacteria in Petri dishes and diagnose infections, as illustrated in Figure 4. It can also be used in desserts such as jellies and ice cream. Agar consists of monosaccharides galactose, agarose, etc.

Usually, gums are made from a mix of monosaccharides.

Figure 4: Blood agar plate used to assess for infections. Wikimedia. NIH.

Structural Carbohydrates in Animals

Some of the most critical structural carbohydrates in Animals are chitin and Glycosaminoglycans.

Chitin

• They are polysaccharides found in the exoskeleton of arthropods such as insects and the cell walls of Fungi.

• The structure of chitin is similar to that of cellulose, and it's behind cellulose as the second most abundant biopolymer in the world.

• It consists of glucose monosaccharides, except these glucose monosaccharides are amide derivatives called N-Acetylglucosamine.

• Chitin is a tough complex carbohydrate that helps it serve as shells for crustaceans. The shell or exoskeletons of most invertebrates combines with calcium carbonate to make a more robust component.

• In butterflies, chitin can be found in their wings in a gyroid pattern allowing light to move through it, making them iridescent.

Glycosaminoglycans (GAGs)

• Non-branched polysaccharides that are made of disaccharide units. Their structure differs because they are altered by Proteins known as Enzymes.

• Glycosaminoglycans (GAGs) attach themselves to Extracellular Matrix Proteins to synthesize proteoglycans. The extracellular matrix is a three-dimensional connection of networks or matrix mainly composed of Proteins that aid cells in support and structural integrity.

• Some examples of proteoglycans are Heparan sulfate, a linear polysaccharide that attaches itself to proteins like collagen, and Keratin sulfate, which are sulfated glycosaminoglycans that can be found in the bones and cartilage of Animals.

Difference between Structural and Non-structural Carbohydrates

Structural and non-structural carbohydrates are two general types of carbohydrates. Examples of structural carbohydrates are cellulose in plants, chitin in arthropods, and peptidoglycan in bacteria.

Examples of non-structural carbohydrates are starch and glycogen. Starch stores energy in plants, and glycogen stores energy in animals.

Non-structural carbohydrates are the rest of the carbohydrates that do not aid in the structural integrity of cells. Non-structural carbohydrates are usually storage carbohydrates, and some examples of those are:

• Starch: The main form of energy storage for plants such as potatoes that are made of numerous glucose units combined. Starch exists in two structural forms: amylose is straight, and amylopectin is branched.

• Glycogen: Glycogen is the primary energy reserve of animals, lodged in bodily tissues and, to a lesser extent, in the liver. It can be broken down to glucose by hydrolysis in situations of energetic necessity and serves as a way to store energy for animals, fungi, and bacteria.

Although structural carbohydrates can be built from monosaccharides such as glucose and fructose and disaccharides such as sucrose, most simple sugars aren't structural, as their primary function is to store sugars. Structural carbohydrates can essentially be compounds that living organisms use to maintain their shape or structural integrity. Similarly, in this way, proteins can also be structural.

Proteins are another type of organic compound, like carbohydrates, but their main functions include acting as Antibodies to protect our Immune System, Enzymes to speed up chemical reactions, etc.