5.16: Structure and Function of Carbohydrates - Biology

5.16: Structure and Function of Carbohydrates - Biology

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Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. Carbohydrates also have other important functions in humans, animals, and plants.

Carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.


Monosaccharides (mono– = “one”; sacchar– = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix –ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R′), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). See Figure 1 for an illustration of the monosaccharides.

The chemical formula for glucose is C6H12O6. In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants.

Galactose and fructose are other common monosaccharides — galactose is found in milk sugars and fructose is found in fruit sugars. Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon (Figure 2).

Practice Question

What kind of sugars are these, aldose or ketose?

[practice-area rows=”2″][/practice-area]
[reveal-answer q=”972235″]Show Answer[/reveal-answer]
[hidden-answer a=”972235″]Glucose and galactose are aldoses. Fructose is a ketose.[/hidden-answer]

Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms (Figure 3). Glucose in a ring form can have two different arrangements of the hydroxyl group (−OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the process of ring formation). If the hydroxyl group is below carbon number 1 in the sugar, it is said to be in the alpha (α) position, and if it is above the plane, it is said to be in the beta (β) position.


Disaccharides (di– = “two”) form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond (Figure 4). Glycosidic bonds (also called glycosidic linkages) can be of the alpha or the beta type.

Common disaccharides include lactose, maltose, and sucrose (Figure 5). Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose.


A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly– = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be 100,000 daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.

Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose, and the excess glucose, beyond the plant’s immediate energy needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.

Starch is made up of glucose monomers that are joined by α 1-4 or α 1-6 glycosidic bonds. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond. As illustrated in Figure 6, amylose is starch formed by unbranched chains of glucose monomers (only α 1-4 linkages), whereas amylopectin is a branched polysaccharide (α 1-6 linkages at the branch points).

Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.

Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers that are linked by β 1-4 glycosidic bonds (Figure 7).

As shown in Figure 7, every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. While the β 1-4 linkage cannot be broken down by human digestive enzymes, herbivores such as cows, koalas, buffalos, and horses are able, with the help of the specialized flora in their stomach, to digest plant material that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria and protists reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase. The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in the digestive systems of ruminants. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal. Termites are also able to break down cellulose because of the presence of other organisms in their bodies that secrete cellulases.

Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have an outer skeleton, called the exoskeleton, which protects their internal body parts (as seen in the bee in Figure 8).

This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing nitrogen. It is made of repeating units of N-acetyl-β-d-glucosamine, a modified sugar. Chitin is also a major component of fungal cell walls; fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya.

Learning Objectives

Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that are formed as a result of dehydration reactions, forming disaccharides and polysaccharides with the elimination of a water molecule for each bond formed. Glucose, galactose, and fructose are common monosaccharides, whereas common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be branched or unbranched. Cellulose is an example of an unbranched polysaccharide, whereas amylopectin, a constituent of starch, is a highly branched molecule. Storage of glucose, in the form of polymers like starch of glycogen, makes it slightly less accessible for metabolism; however, this prevents it from leaking out of the cell or creating a high osmotic pressure that could cause excessive water uptake by the cell.

5.16: Structure and Function of Carbohydrates - Biology

Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.

3 different types of carbohydrates

General formula of carbohydrates can be represented by the stoichiometric formula (CH2O)n , where n is the number of carbons in the molecule.

In carbohydrate molecules the ratio of carbon to hydrogen to oxygen is 1:2:1.

The general formula of carbohydrates and the ratio of their atoms also explain the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”).

There are 3 different types of carbohydrates:

Building Macromolecules

Although most absorbed glucose is used to make energy, some glucose is converted to ribose and deoxyribose, which are essential building blocks of important macromolecules, such as RNA, DNA, and ATP (Figure 3.4.3). Glucose is additionally utilized to make the molecule NADPH, which is important for protection against oxidative stress and is used in many other chemical reactions in the body. If all of the energy, glycogen-storing capacity, and building needs of the body are met, excess glucose can be used to make fat. This is why a diet too high in carbohydrates and calories can add on the fat pounds&mdasha topic that will be discussed shortly.

Figure 3.4.3: The sugar molecule deoxyribose is used to build the backbone of DNA.© Shutterstock

General structure and function of phospholipids

Phospholipids are an one of the main types of lipids and major constituents of the plasma membrane.

Like triglycerides, phospholipid structure consist of fatty acid chains attached to a glycerol or sphingosine backbone.

Instead of three fatty acids attached as in triglycerides, however, there are two fatty acids forming diacylglycerol, and the third carbon of the glycerol backbone is occupied by a modified phosphate group.

A phosphate group alone attached to a diaglycerol does not qualify as a phospholipid it is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids.

The phosphate group is modified by an alcohol.

Phosphatidylcholine and phosphatidylserine are two important phospholipids that are found in plasma membranes.

3. Types of carbohydrates

3.1. Monosaccharides, disaccharides and polyols

Simple carbohydrates &ndash those with one or two sugar units &ndash are also simply known as sugars. Examples are:

  • Glucose and fructose: monosaccharides that can be found in fruits, vegetables, honey, but also in food products like glucose-fructose syrups
  • Table sugar or sucrose is a disaccharide of glucose and fructose, and occurs naturally in sugar beet, sugar cane and fruits
  • Lactose, a disaccharide consisting of glucose and galactose, is the main carbohydrate in milk and dairy products
  • Maltose is a glucose disaccharide found in malt and starch derived syrups

Monosaccharide and disaccharide sugars tend to be added to foods by manufacturers, cooks and consumers and are referred to as &lsquoadded sugars&rsquo. They may also occur as &lsquofree sugars&rsquo that are naturally in honey and fruit juices.

Polyols, or so-called sugar alcohols, are also sweet and can be used in foods in a similar way to sugars, but have a lower calorie content compared to normal table sugar (see below). They do occur naturally, but most polyols that we use are made by the transformation of sugars. Sorbitol is the most commonly used polyol in foods and drinks, while xylitol is frequently used in chewing gums and mints. Isomalt is a polyol produced from sucrose, often used in confectionery. Polyols can have a laxative effect when eaten in too large quantities.

If you want to find out more about sugars in general, read our &lsquoSugars: addressing common questions&rsquo article, the &lsquoAddressing common questions about sweeteners&rsquo article, or investigate the opportunities and difficulties in replacing sugar in baked goods and processed foods (&lsquoSugars from a food technology perspective&rsquo).

3.2. Oligosaccharides

The World Health Organization (WHO) defines oligosaccharides as carbohydrates with 3-9 sugar units, although other definitions allow for slightly longer chain lengths. The most well-known are oligofructans (or in proper scientific terms: fructo-oligosaccharides), which consist of up to 9 fructose units and naturally occur in low sweetness vegetables such as artichokes and onions. Raffinose and stachyose are two other examples of oligosaccharides found in some pulses, grains, vegetables, and honey. Most of the oligosaccharides are not broken down into monosaccharides by human digestive enzymes and are utilised by the gut microbiota instead (see our material on dietary fibres for more information).

3.3. Polysaccharides

Ten or more &ndash and sometimes even up to several thousand &ndash sugar units are needed to form polysaccharides, which are usually distinguished in two types:

  • Starch, which is the main energy reserve in root vegetables such as onions, carrots, potatoes, and whole grains. It has different length chains of glucose, more or less branched, and occurs in granules which size and shape vary between the plants that contain them. The corresponding polysaccharide in animals is called glycogen. Some starches can only be digested by the gut microbiota rather than our own body&rsquos mechanisms: these are known as resistant starches.
  • Non-starch polysaccharides, which are part of the dietary fibre group (although a few oligosaccharides such as inulin are also considered dietary fibre). Examples are cellulose, hemicelluloses, pectins and gums. The main sources of these polysaccharides are vegetables and fruits, as well as whole grains. A hallmark feature of non-starch polysaccharides and actually all dietary fibres is that humans cannot digest them hence, their lower average energy content compared to most other carbohydrates. Some types of fibre can, however, be metabolised by gut bacteria, giving rise to compounds beneficial for our body, such as short-chain fatty acids. Find out more about dietary fibres and their importance for our health in our article on &lsquowhole grains&rsquo and &lsquodietary fibre&rsquo.

From here onwards, we will be referring to &lsquosugars&rsquo when talking about mono- and disaccharides, and &lsquofibres&rsquo when talking about non-starch polysaccharides.

Monosaccharide Structure

All monosaccharides have the same general formula of (CH2O)n, which designates a central carbon molecule bonded to two hydrogens and one oxygen. The oxygen will also bond to a hydrogen, creating a hydroxyl group. Because carbon can form 4 bonds, several of these carbon molecules can bond together. One of the carbons in the chain will form a double bond with an oxygen, which is called a carbonyl group. If this carbonyl occurs at the end of the chain, the monosaccharide is in the aldose family. If the carboxyl group is in the middle of the chain, the monosaccharide is in the ketose family.

Above is a picture of glucose. Glucose is one of the most common monosaccharides in nature, used by nearly every form of life. This simple monosaccharide is composed of 6 carbons, each labeled in the image. The first carbon is the carbonyl group. Because it is at the end of the molecule, glucose is in the aldose family. Typically, monosaccharides with more than 5 carbons exist as rings in solutions of water. The hydroxyl group on the fifth carbon will react with the first carbon. The hydroxyl group gives up its hydrogen atom when it forms a bond with the first carbon. The double bonded oxygen on the first carbon bonds with a new hydrogen when the second bond with the carbon is broken. This forms a fully connected and stable ring of carbons.


Polysaccharides can be composed of hundreds to thousands of monosaccharides combined together. These monosaccharides are joined together through dehydration synthesis. Polysaccharides have several functions including structural support and storage. Some examples of polysaccharides include starch, glycogen, cellulose, and chitin.

Starch is a vital form of stored glucose in plants. Vegetables and grains are good sources of starch. In animals, glucose is stored as glycogen in the liver and muscles.

Cellulose is a fibrous carbohydrate polymer that forms the cell walls of plants. It composes about one-third of all vegetable matter and cannot be digested by humans.

Chitin is a tough polysaccharide that can be found in some species of fungi. Chitin also forms the exoskeleton of arthropods such as spiders, crustaceans, and insects. Chitin helps to protect the animal's soft internal body and helps to keep them from drying out.

16.5: Cyclic Structures of Monosaccharides

So far we have represented monosaccharides as linear molecules, but many of them also adopt cyclic structures. This conversion occurs because of the ability of aldehydes and ketones to react with alcohols:

You might wonder why the aldehyde reacts with the OH group on the fifth carbon atom rather than the OH group on the second carbon atom next to it. Recall that cyclic alkanes containing five or six carbon atoms in the ring are the most stable. The same is true for monosaccharides that form cyclic structures: rings consisting of five or six carbon atoms are the most stable.

Figure (PageIndex<1>): Cyclization of D-Glucose. D-Glucose can be represented with a Fischer projection (a) or three dimensionally (b). By reacting the OH group on the fifth carbon atom with the aldehyde group, the cyclic monosaccharide (c) is produced.

When a straight-chain monosaccharide, such as any of the structures shown in Figure (PageIndex<1>), forms a cyclic structure, the carbonyl oxygen atom may be pushed either up or down, giving rise to two stereoisomers, as shown in Figure (PageIndex<2>). The structure shown on the left side of Figure (PageIndex<2>), with the OH group on the first carbon atom projected downward, represent what is called the alpha (&alpha) form. The structures on the right side, with the OH group on the first carbon atom pointed upward, is the beta (&beta) form. These two stereoisomers of a cyclic monosaccharide are known as anomers they differ in structure around the anomeric carbon&mdashthat is, the carbon atom that was the carbonyl carbon atom in the straight-chain form.

It is possible to obtain a sample of crystalline glucose in which all the molecules have the &alpha structure or all have the &beta structure. The &alpha form melts at 146°C and has a specific rotation of +112°, while the &beta form melts at 150°C and has a specific rotation of +18.7°. When the sample is dissolved in water, however, a mixture is soon produced containing both anomers as well as the straight-chain form, in dynamic equilibrium (part (a) of Figure (PageIndex<2>)). You can start with a pure crystalline sample of glucose consisting entirely of either anomer, but as soon as the molecules dissolve in water, they open to form the carbonyl group and then reclose to form either the &alpha or the &beta anomer. The opening and closing repeats continuously in an ongoing interconversion between anomeric forms and is referred to as mutarotation (Latin mutare, meaning &ldquoto change&rdquo). At equilibrium, the mixture consists of about 36% &alpha-D-glucose, 64% &beta-D-glucose, and less than 0.02% of the open-chain aldehyde form. The observed rotation of this solution is +52.7°.

Figure (PageIndex<2>): Monosaccharides. In an aqueous solution, monosaccharides exist as an equilibrium mixture of three forms. The interconversion between the forms is known as mutarotation, which is shown for D-glucose (a) and D-fructose (b).

Even though only a small percentage of the molecules are in the open-chain aldehyde form at any time, the solution will nevertheless exhibit the characteristic reactions of an aldehyde. As the small amount of free aldehyde is used up in a reaction, there is a shift in the equilibrium to yield more aldehyde. Thus, all the molecules may eventually react, even though very little free aldehyde is present at a time.

Commonly, (e.g., in Figures (PageIndex<1>) and (PageIndex<2>)) the cyclic forms of sugars are depicted using a convention first suggested by Walter N. Haworth, an English chemist. The molecules are drawn as planar hexagons with a darkened edge representing the side facing toward the viewer. The structure is simplified to show only the functional groups attached to the carbon atoms. Any group written to the right in a Fischer projection appears below the plane of the ring in a Haworth projection, and any group written to the left in a Fischer projection appears above the plane in a Haworth projection.

The difference between the &alpha and the &beta forms of sugars may seem trivial, but such structural differences are often crucial in biochemical reactions. This explains why we can get energy from the starch in potatoes and other plants but not from cellulose, even though both starch and cellulose are polysaccharides composed of glucose molecules linked together.

Watch the video: A-level Biology POLYSACCHARIDES Biological Molecules-Learn these carbohydrates structure + function (May 2022).