Why are disaccharides less reducing than monosaccharides?

My teacher told me the statement, but if both monosaccharide and disaccharides have a single free active group, how is this possible?

What Is The Biological Function Of Monosaccharides, Disaccharides, And Polysaccharides?

Monosaccharides are a basic form of carbohydrate. It is a basic sugar and it is very important for your body. They are the building blocks of all the other forms of chemicals that you mentioned. Monosaccharides are called glucose when it is dissolved into the blood stream. They are biologically important because they are basically what give you all your energy, so they are in simple terms your body’s fuel.

Disaccharides is a double molecule of monosaccharide that is broken down by the body into the simpler form of a monosaccharide. They are biologically important because again, they provide energy for the body to function and live off.

Polysaccharides are multiples of either monosaccharides or disaccharides that are joined together by lots of glycosidic bonds. They can be made up of the same or different types of saccharides. They are sometimes more commonly known as starch, and again these chains of molecules are broken down by the body to create energy and sustenance for itself. The more active throughout the day you are, the more of these are broken down for your body to use.

Starch is found in foods such as potatoes, rice and pasta. Starch is a very important element for our body to function. As part of a balanced diet, you should eat at least one portion of starchy foods per day to ensure that your body has enough energy. There are, of course, other foods that can provide energy for your body, but these are often more fast burning energies.

Starch is a great energy source because it keeps releasing energy throughout the day. Another good source of starch is in whole grain foods, which release energy slowly throughout the day. It is important that you keep your energy levels up by eating regularly throughout the day.

Why are disaccharides less reducing than monosaccharides? - Biology


The term carbohydrate was originally used to describe compounds that were literally "hydrates of carbon" because they had the empirical formula CH2O. In recent years, carbohydrates have been classified on the basis of their structures, not their formulas. They are now defined as polyhydroxy aldehydes and ketones. Among the compounds that belong to this family are cellulose, starch, glycogen, and most sugars.

There are three classes of carbohydrates: monosaccharides, disaccharides, and polysaccharides. The monosaccharides are white, crystalline solids that contain a single aldehyde or ketone functional group. They are subdivided into two classes aldoses and ketoses on the basis of whether they are aldehydes or ketones. They are also classified as a triose, tetrose, pentose, hexose, or heptose on the basis of whether they contain three, four, five, six, or seven carbon atoms.

With only one exception, the monosaccharides are optically active compounds. Although both D and L isomers are possible, most of the monosaccharides found in nature are in the D configuration. Structures for the D and L isomer of the simplest aldose, glyceraldehyde, are shown below.

The structures of many monosaccharides were first determined by Emil Fischer in the 1880s and 1890s and are still written according to a convention he developed. The Fischer projection represents what the molecule would look like if its three-dimensional structure were projected onto a piece of paper. By convention, Fischer projections are written vertically, with the aldehyde or ketone at the top. The -OH group on the second-to-last carbon atom is written on the right side of the skeleton structure for the D isomer and on the left for the L isomer. Fischer projections for the two isomers of glyceraldehyde are shown below.

These Fischer projections can be obtained from the skeleton structures shown above by imaging what would happen if you placed a model of each isomer on an overhead projector so that the CHO and CH2OH groups rested on the glass and then looked at the images of these models that would be projected on a screen.

Fischer projections for some of the more common monosaccharides are given in the figure below.

Glucose and fructose have the same formula: C6H12O6. Glucose is the sugar with the highest concentration in the bloodstream fructose is found in fruit and honey. Use the Fischer projections in the figure of common monosaccharides to explain the difference between the structures of these compounds. Predict what an enzyme would have to do to convert glucose into fructose, or vice versa.

If the carbon chain is long enough, the alcohol at one end of a monosaccharide can attack the carbonyl group at the other end to form a cyclic compound. When a six-membered ring is formed, the product of this reaction is called a pyranose, shown in the figure below.

When a five-membered ring is formed, it is called a furanose, shown in the figure below.

There are two possible structures for the pyranose and furanose forms of a monosaccharide, which are called the a - and b -anomers.

The reactions that lead to the formation of a pyranose or a furanose are reversible. Thus, it doesn't matter whether we start with a pure sample of a -D-glucopyranose or b -D-glucopyranose. Within minutes, these anomers are interconverted to give an equilibrium mixture that is 63.6% of the b -anomer and 36.4% of the a -anomer. The 2:1 preference for the b -anomer can be understood by comparing the structures of these molecules shown previously. In the b -anomer, all of the bulky -OH or -CH2OH substituents lie more or less within the plane of the six-membered ring. In the a -anomer, one of the -OH groups is perpendicular to the plane of the six-membered ring, in a region where it feels strong repulsive forces from the hydrogen atoms that lie in similar positions around the ring. As a result, the b -anomer is slightly more stable than the a -anomer.

Disaccharides are formed by condensing a pair of monosaccharides. The structures of three important disaccharides with the formula C12H22O11 are shown in the figure below.

Maltose, or malt sugar, which forms when starch breaks down, is an important component of the barley malt used to brew beer. Lactose, or milk sugar, is a disaccharide found in milk. Very young children have a special enzyme known as lactase that helps digest lactose. As they grow older, many people lose the ability to digest lactose and cannot tolerate milk or milk products. Because human milk has twice as much lactose as milk from cows, young children who develop lactose intolerance while they are being breast-fed are switched to cows' milk or a synthetic formula based on sucrose.

The substance most people refer to as "sugar" is the disaccharide sucrose, which is extracted from either sugar cane or beets. Sucrose is the sweetest of the disaccharides. It is roughly three times as sweet as maltose and six times as sweet as lactose. In recent years, sucrose has been replaced in many commercial products by corn syrup, which is obtained when the polysaccharides in cornstarch are broken down. Corn syrup is primarily glucose, which is only about 70% as sweet as sucrose. Fructose, however, is about two and a half times as sweet as glucose. A commercial process has therefore been developed that uses an isomerase enzyme to convert about half of the glucose in corn syrup into fructose (see Practice Problem 4). This high-fructose corn sweetener is just as sweet as sucrose and has found extensive use in soft drinks.

The monosaccharides and disaccharides represent only a small fraction of the total amount of carbohydrates in the natural world. The great bulk of the carbohydrates in nature are present as polysaccharides, which have relatively large molecular weights. The polysaccharides serve two principal functions. They are used by both plants and animals to store glucose as a source of future food energy and they provide some of the mechanical structure of cells.

Very few forms of life receive a constant supply of energy from their environment. In order to survive, plant and animal cells have had to develop a way of storing energy during times of plenty in order to survive the times of shortage that follow. Plants store food energy as polysaccharides known as starch. There are two basic kinds of starch: amylose and amylopectin. Amylose is found in algae and other lower forms of plants. It is a linear polymer of approximately 600 glucose residues whose structure can be predicted by adding a -D-glucopyranose rings to the structure of maltose. Amylopectin is the dominant form of starch in the higher plants. It is a branched polymer of about 6000 glucose residues with branches on 1 in every 24 glucose rings. A small portion of the structure of amylopectin is shown in the figure below.

The polysaccharide that animals use for the short-term storage of food energy is known as glycogen. Glycogen has almost the same structure as amylopectin, with two minor differences. The glycogen molecule is roughly twice as large as amylopectin, and it has roughly twice as many branches.

There is an advantage to branched polysaccharides such as amylopectin and glycogen. During times of shortage, enzymes attack one end of the polymer chain and cut off glucose molecules, one at a time. The more branches, the more points at which the enzyme attacks the polysaccharide. Thus, a highly branched polysaccharide is better suited for the rapid release of glucose than a linear polymer.

Polysaccharides are also used to form the walls of plant and bacterial cells. Cells that do not have a cell wall often break open in solutions whose salt concentrations are either too low (hypotonic) or too high (hypertonic). If the ionic strength of the solution is much smaller than the cell, osmotic pressure forces water into the cell to bring the system into balance, which causes the cell to burst. If the ionic strength of the solution is too high, osmotic pressure forces water out of the cell, and the cell breaks open as it shrinks. The cell wall provides the mechanical strength that helps protect plant cells that live in fresh-water ponds (too little salt) or seawater (too much salt) from osmotic shock. The cell wall also provides the mechanical strength that allows plant cells to support the weight of other cells.

The most abundant structural polysaccharide is cellulose. There is so much cellulose in the cell walls of plants that it is the most abundant of all biological molecules. Cellulose is a linear polymer of glucose residues, with a structure that resembles amylose more closely than amylopectin, as shown in the figure below. The difference between cellulose and amylose can be seen by comparing the figures of amylose and cellulose. Cellulose is formed by linking b -glucopyranose rings, instead of the a -glucopyranose rings in starch and glycogen.

The -OH substituent that serves as the primary link between -glucopyranose rings in starch and glycogen is perpendicular to the plane of the six-membered ring. As a result, the glucopyranose rings in these carbohydrates form a structure that resembles the stairs of a staircase. The -OH substituent that links the b -glucopyranose rings in cellulose lies in the plane of the six-membered ring. This molecule therefore stretches out in a linear fashion. This makes it easier for strong hydrogen bonds to form between the -OH groups of adjacent molecules. This, in turn gives cellulose the rigidity required for it to serve as a source of the mechanical structure of plant cells.

Cellulose and starch provide an excellent example of the link between the structure and function of biomolecules. At the turn of the century, Emil Fischer suggested that the structure of an enzyme is matched to the substance on which it acts, in much the same way that a lock and key are matched. Thus, the amylase enzymes in saliva that break down the a -linkages between glucose molecules in starch cannot act on the b -linkages in cellulose.

Most animals cannot digest cellulose because they don't have an enzyme that can cleave b -linkages between glucose molecules. Cellulose in their diet therefore serves only as fiber, or roughage. The digestive tracts of some animals, such as cows, horses, sheep, and goats contain bacteria that have enzymes that cleave these b -linkages, so these animals can digest cellulose.

Termites provide an example of the symbiotic relationship between bacteria and higher organisms. Termites cannot digest the cellulose in the wood they eat, but their digestive tracts are infested with bacteria that can. Propose a simple way of ridding a house from termites, without killing other insects that might be beneficial.

For many years, biochemists considered carbohydrates to be dull, inert compounds that filled the space between the exciting molecules in the cell the proteins. Carbohydrates were impurities to be removed when "purifying" a protein. Biochemists now recognize that most proteins are actually glycoproteins, in which carbohydrates are covalently linked to the protein chain. Glycoproteins play a particularly important role in the formation of the rigid cell walls that surround bacterial cells.

What is a Disaccharide

Disaccharides are sugar molecules composed of two monosaccharides. Therefore every disaccharide is composed of two chemical rings. The bond between two monosaccharides is called a glycosidic bond. Disaccharides are also simple sugars. Disaccharides are classified into two groups according to their reducing strength.

  • Reducing sugars – can act as a reducing agent
  • Non-reducing sugars – cannot act as a reducing agent

Figure 03: Structure of a Disaccharide

Therefore, some disaccharides are reducing sugars and some are not. All disaccharides are water soluble and colorless when dissolved in water. Some disaccharides are sweet tasting but some are not.

16.5: Properties of Monosaccharides

Monosaccharides such as glucose and fructose are crystalline solids at room temperature, but they are quite soluble in water, each molecule having several OH groups that readily engage in hydrogen bonding. The chemical behavior of these monosaccharides is likewise determined by their functional groups.

An important reaction of monosaccharides is the oxidation of the aldehyde group, one of the most easily oxidized organic functional groups. Aldehyde oxidation can be accomplished with any mild oxidizing agent, such as Tollens&rsquo reagent or Benedict&rsquos reagent. With the latter, complexed copper(II) ions are reduced to copper(I) ions that form a brick-red precipitate [copper(I) oxide Figure (PageIndex<1>)].

Any carbohydrate capable of reducing either Tollens&rsquo or Benedict&rsquos reagents without first undergoing hydrolysis is said to be a reducing sugar. Because both the Tollens&rsquo and Benedict&rsquos reagents are basic solutions, ketoses (such as fructose) also give positive tests due to an equilibrium that exists between ketoses and aldoses in a reaction known as tautomerism.

Figure (PageIndex<1>): Benedict&rsquos Test. Benedict&rsquos test was performed on three carbohydrates, depicted from left to right: fructose, glucose, and sucrose. The solution containing sucrose remains blue because sucrose is a nonreducing sugar.

These reactions have been used as simple and rapid diagnostic tests for the presence of glucose in blood or urine. For example, Clinitest tablets, which are used to test for sugar in the urine, contain copper(II) ions and are based on Benedict&rsquos test. A green color indicates very little sugar, whereas a brick-red color indicates sugar in excess of 2 g/100 mL of urine.

Molecular Structures

Carbohydrates can be represented by the 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.

Figure 1. Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. Aldoses have a carbonyl group (indicated in green) at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively.

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.

Monosaccharides can exist as a linear chain or as ring-shaped molecules in aqueous solutions they are usually found in ring forms.


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 (Figure 2).

Figure 2. Sucrose is produced from the chemical reaction between two simple sugars called glucose and fructose.

Common disaccharides include lactose, maltose, and sucrose. 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 covalent bonds is known as a polysaccharide (poly– = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. Polysaccharides may be very large molecules. Starch, glycogen, cellulose, and chitin are examples of polysaccharides.

Starch is the stored form of sugars in plants and is made up of amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose, and the excess glucose is stored as starch in different plant parts, including roots and seeds. The starch that is consumed by animals is broken down into smaller molecules, such as glucose. The cells can then absorb the glucose.

Figure 3. Amylose and amylopectin are two different forms of starch. Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose.

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 glucose levels decrease, glycogen is broken down to release glucose.

Cellulose is one of the most abundant natural biopolymers. The cell walls of plants are mostly made of cellulose, which 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 bonds between particular carbon atoms in the glucose molecule.

Every other glucose monomer in cellulose is flipped over and packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. Cellulose passing through our digestive system is called dietary fiber. While the glucose-glucose bonds in cellulose cannot be broken down by human digestive enzymes, herbivores such as cows, buffalos, and horses are able to digest grass that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase. The appendix also contains bacteria that break down 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.

Figure 4. In cellulose, glucose monomers are linked in unbranched chains by β 1-4 glycosidic linkages. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure.

Figure 5. Insects have a hard outer exoskeleton made of chitin, a type of polysaccharide.

As shown in Figure 4, 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.

Carbohydrates serve other functions in different animals. Arthropods, such as insects, spiders, and crabs, have an outer skeleton, called the exoskeleton, which protects their internal body parts. This exoskeleton is made of the biological macromolecule chitin, which is a nitrogenous carbohydrate. It is made of repeating units of a modified sugar containing nitrogen.

Registered Dietitian

Figure 6. Registered Dietitian Nutritionist (RDN)Chef Brenda Thompson works with foodservice staff to assemble her breakfast burrito recipe during the chef designed school taste testing in Idaho. Thanks to a U.S. Department of Agriculture (USDA) Team Nutrition grant RDN Chef Brenda Thompson, developed recipes for the Chef Designed School Lunch cookbook.

Obesity is a worldwide health concern, and many diseases, such as diabetes and heart disease, are becoming more prevalent because of obesity. This is one of the reasons why registered dietitians are increasingly sought after for advice. Registered dietitians help plan food andnutrition programs for individuals in various settings. They often work with patients in health-care facilities, designing nutrition plans to prevent and treat diseases. For example, dietitians may teach a patient with diabetes how to manage blood-sugar levels by eating the correct types and amounts of carbohydrates. Dietitians may also work in nursing homes, schools, and private practices.

To become a registered dietitian, one needs to earn at least a bachelor’s degree in dietetics, nutrition, food technology, or a related field. In addition, registered dietitians must complete a supervised internship program and pass a national exam. Those who pursue careers in dietetics take courses in nutrition, chemistry, biochemistry, biology, microbiology, and human physiology. Dietitians must become experts in the chemistry and functions of food (proteins, carbohydrates, and fats).

In Summary: Structure and Function of Carbohydrates

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.

Difference Between Disaccharide and Monosaccharide

Carbohydrates are a group of compounds which are defined as “polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones.” Carbohydrates are the most abundant type of organic molecules on earth. They are the source of chemical energy for living organisms. Not only this, they serve as important constituents of tissues. Carbohydrates are synthesized in plants and some microorganisms by photosynthesis. Carbohydrates got its’ name because it has the formula C x (H 2 O) x , and this looked like hydrates of carbon. Carbohydrate can be again categorized into three as monosaccharide, disaccharides and polysaccharides. Disaccharides and monosaccharides are readily soluble in water, and they are sweet in taste. They can be crystallized. As like there are some similarities between these two, there are a number of differences too.


Monosaccharides are the simplest carbohydrate type. Monosaccharide has the formula of C x (H 2 O) x . These cannot be hydrolyzed into simpler carbohydrates. They are sweet in taste. All monosaccharides are reducing sugars. Therefore, they give positive results with benedicts’ or Fehling’s reagents. Monosaccharides are classified according to,

  • The number of carbon atoms present in the molecule
  • Whether they contain an aldehyde or keto group

Therefore, a monosaccharide with six carbon atoms is called a hexose. If there are five carbon atoms, then it is a pentose. Further, if the monosaccharide has an aldehyde group, it is called as aldose. A monosaccharide with a keto group is called a ketose. Among these, the simplest monosaccharides are glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose). Glucose is another common example for a monosaccharide. For monosaccharides, we can draw a linear or a cyclic structure. In solution, majority of the molecules are in the cyclic structure. For example, when a cyclic structure is forming in glucose, the -OH on carbon 5 is converted into the ether linkage to close the ring with carbon 1. This forms a six member ring structure. The ring is also called a hemiacetal ring, due to the presence of carbon that has both an ether oxygen and an alcohol group.


Disaccharide is the combination of two monosaccharides. When two monosaccharides are joined together, an ester bond is formed between any two –OH groups. Commonly this happens between the 1 st and 4 th –OH groups in two monosaccharides. The bond formed between the two monomers is known as a glycosidic bond. During this reaction, a water molecule is removed. Hence, this is a condensation reaction. Sometimes, both the monomers in a disaccharide are the same and sometimes they are different. For example, to produce maltose, two glucose molecules are participating. Fructose is made by the condensation reaction between a glucose and fructose whereas lactose is made from glucose and galactose. Disaccharides are also common in nature. For example, sucrose is found in fruits and vegetables. Disaccharides can be hydrolyzed and produce the relevant monomers back. They are sweet in taste and can be crystallized. Most of the disaccharides can be hydrolyzed except sucrose.

What is the difference between Monosaccharide and Disaccharide?

• Monosaccharides are the simplest carbohydrates.

• Disaccharides are made from the combination of monosaccharides.

• Monosaccharides have a lower molecular weight than disaccharides.

• Disaccharides can be hydrolyzed whereas monosaccharides cannot.

• All the monosaccharides are reducing sugars. But all the disaccharides are not.

Carbohydrates serve 2 major functions: energy and structure. As energy, they can be simple for fast utilization or complex for storage. Simple sugars are monomers called monosaccharides. These are readily taken into cells and used immediately for energy. The most important monosaccharide is glucose (C6H12O6), since it is the preferred energy source for cells. The conversion of this chemical into cellular energy can be described by the equation below:

Long polymers of carbohydrates are called polysaccharides and are not readily taken into cells for use as energy. These are used often for energy storage. Examples of energy storage molecules are amylose, or starch, (plants) and glycogen (animals). Some polysaccharides are so long and complex that they are used for structures like cellulose in the cell walls of plants. Cellulose is very large and practically indigestible, making it unsuitable as a readily available energy source for cells.

Carbohydrates: Carbohydrates are composed of sugar units referred to as -saccharides.

Many monosaccharides such as glucose and fructose are reducing sugars,meaning that they possess free aldehyde or ketone groups that reduce weak oxidizing agents such as the copper in Benedict&rsquos reagent. The double bond in the carbonyl group is a source of electrons that can be donated to something else. That is to say, those electrons can be &ldquolost&rdquo by the sugar and &ldquogained&rdquo by another chemical. Benedict&rsquos reagent contains cupric (copper) ion complexed with citrate in alkaline solution. Benedict&rsquos test identifies reducing sugars based on their ability to reduce the cupric (Cu 2+ ) ions to cuprous oxide (Cu + ) at basic (high) pH. Cuprous oxide is green to reddish-orange. Roughly speaking, reduction is a type of chemical reaction that is paired with oxidation. In oxidation/reduction reactions (RedOx), some chemical loses electrons (oxidized) to another chemical that gains them (reduced). We remember whether a compound is reduced or gained by using the pneumonic: LEO goes GER or Loss of Electrons is Oxidation & Gain of Electrons is Reduction.

Monosaccharides contain a carbonyl group. The carbonyl is a source of electrons (the double bond on the oxygen). These electrons can be donated (or lost and oxidized) to reduce another compound (that gains those electrons).

Glucose is the preferred carbohydrate of cells. In solution, it can change from a linear chain to a ring.

Monosaccharides are capable of isomerizing. This means they alternate in structure from a linear chain to a ring form in solution. In the chain form, the aldehyde is free to donate (lose) electrons to reduce another compound. When monosaccharides undergo dehydration synthesis to form polymers, they can no longer isomerize into chains with free aldehydes and are unable to act as reducing sugars. Green color indicates a small amount of reducing sugars, and reddish-orange color indicates an abundance of reducing sugars. Non-reducing sugars produce no change in color (i.e., the solution remains blue).

Note: Cu 2+ has fewer electrons than Cu + .

When monosaccharides undergo dehydration synthesis to form polymers, they can no longer isomerize into chains with free aldehydes and are unable to act as reducing sugars. Green color indicates a small amount of reducing sugars and reddish-orange color indicates an abundance of reducing sugars. Non-reducing sugars produce no change in color (i.e., the solution remains blue).

Structural Carbohydrates

In food, more complex carbohydrates are derived from larger polysaccharides. These larger carbohydrates are fairly insoluble in water. Dietary fiber is the name given to indigestible materials in food most often derived from the complex carbohydrates from vegetable material. Some of this material serves the plants as a structural component of the cells and is completely insoluble. Cellulose is the major structural carbohydrate found in plant cell walls. Similarly, animals and fungi have structural carbohydrates that are composed of the indigestible compound called chitin. We will not be testing for these items.

Cellulose is a complex carbohydrate of glucose molecules. It is the major structural component of plant cell walls. It&rsquo structural durability is enhanced by intramolecular hydrogen bonds.

Chitin is a structural carbohydrate found in animal shells or fungi cell walls. The polymer contains amide groups that differentiate it from other carbohydrates composed of glucose.

Formation and Breakdown of Disaccharides

When disaccharides are formed from monosaccharides, an -OH (hydroxyl) group is removed from one molecule and an H (hydrogen) is removed from the other. Glycosidic bonds are formed to join the molecules these are covalent bonds between a carbohydrate molecule and another group (which does not necessarily need to be another carbohydrate). The H and -OH that were removed from the two monosaccharides join together to form a water molecule, H2O. For this reason, the process of forming a disaccharide from two monosaccharides is called a dehydration reaction or condensation reaction.

When disaccharides are broken down into their monosaccharide components via enzymes, a water molecule is added. This process is called hydrolysis. It should not be confused with the process of dissolution, which happens when sugar is dissolved in water, for example. The sugar molecules themselves do not change structure when they are dissolved. The solid sugar simply turns into liquid and becomes a solute, or a dissolved component of a solution.

Why are disaccharides less reducing than monosaccharides? - Biology

Simple sugars are far and away the predominant carbohydrate absorbed in the digestive tract, and in many animals the most important source of energy. Monosaccharides, however, are only rarely found in normal diets. Rather, they are derived by enzymatic digestion of more complex carbohydrates within the digestive tube.

Particularly important dietary carbohydrates include starch and disaccharides such as lactose and sucrose. None of these molecules can be absorbed for the simple reason that they cannot cross cell membranes unaided and, unlike the situation for monosaccharides, there are no transporters to carry them across.

This section will focus on understanding the processes involved in assimilation of three important carbohydrates: starch, lactose and sucrose. The key concepts involved in all three cases are that:

  • the final enzymatic digestion that liberates monosaccharides is conducted by enzymes that are tethered in the lumenal plasma membrane of absorptive enterocytes (so-called "brush border hydrolyases").
  • glucose generated by digestion of starch or lactose is absorbed in the small intestine only by cotransport with sodium, a fact that has exceptionally important implications in medicine.

Brush Border Hydrolases Generate Monosaccharides

Polysaccharides and disaccharides must be digested to monosaccharides prior to absorption and the key players in these processes are the brush border hydrolases, which include maltase, lactase and sucrase. Dietary lactose and sucrose are "ready" for digestion by their respective brush border enzymes. Starch, as discussed previously, is first digested to maltose by amylase in pancreatic secretions and, in some species, saliva.

Dietary lactose and sucrose, and maltose derived from digestion of starch, diffuse in the small intestinal lumen and come in contact with the surface of absorptive epithelial cells covering the villi where they engage with brush border hydrolases:

  • maltase cleaves maltose into two molecules of glucose
  • lactase cleaves lactose into a glucose and a galactose
  • sucrase cleaves sucrose into a glucose and a fructose

At long last, we're ready to actually absorb these monosaccharides. Glucose and galactose are taken into the enterocyte by cotransport with sodium using the same transporter. Fructose enters the cell from the intestinal lumen via facilitated diffusion through another transporter.

Absorption of Glucose and Other Monosaccharides: Transport Across the Intestinal Epithelium

Absorption of glucose entails transport from the intestinal lumen, across the epithelium and into blood. The transporter that carries glucose and galactose into the enterocyte is the sodium-dependent hexose transporter, known more formally as SGLUT-1. As the name indicates, this molecule transports both glucose and sodium ion into the cell and in fact, will not transport either alone.

The essence of transport by the sodium-dependent hexose transporter involves a series of conformational changes induced by binding and release of sodium and glucose, and can be summarized as follows:

  1. the transporter is initially oriented facing into the lumen - at this point it is capable of binding sodium, but not glucose
  2. sodium binds, inducing a conformational change that opens the glucose-binding pocket
  3. glucose binds and the transporter reorients in the membrane such that the pockets holding sodium and glucose are moved inside the cell
  4. sodium dissociates into the cytoplasm, causing glucose binding to destabilize
  5. glucose dissociates into the cytoplasm and the unloaded transporter reorients back to its original, outward-facing position

Fructose is not co-transported with sodium. Rather it enters the enterocyte by another hexose transporter (GLUT5).

Once inside the enterocyte, glucose and sodium must be exported from the cell into blood. We've seen previously how sodium is rapidly shuttled out in exchange for potassium by the battery of sodium pumps on the basolateral membrane, and how that process maintains the electrochemical gradient across the epithelium. The energy stored in this gradient is actually what is driving glucose entry through the sodium-dependent hexose transporter described above. Recall also how the massive transport of sodium out of the cell establishes the osmotic gradient responsible for absorption of water.

Glucose, galactose and fructose are tranported out of the enterocyte through another hexose transporter (called GLUT-2) in the basolateral membrane. These monosaccharides then diffuse "down" a concentration gradient into capillary blood within the villus.

Absorption of Water and Electrolytes

Absorption of Amino Acids and Peptides

Section Summary

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.

Watch the video: Ζάχαρη. Efi Koloverou Dietitian (January 2022).