Sugars (Carbohydrate) are biological macromolecules of C, H, O three elements can be divided into monosaccharides, disaccharides, and polysaccharides. Sugar

Carbohydrates are an important class of organic compounds widely distributed in nature. The sucrose consumed daily, the starch in the grain, the cellulose in the plant body, the glucose in the human blood, and the like are all sugars.


Carbohydrates play an important role in life activities and are the main source of energy needed to sustain life activities in all living things. The most important sugars in plants are starch and cellulose, and the most important polysaccharide in animal cells is glycogen.

Development History

Dry screaming, sugar. The word “sugar” appeared in the Six Dynasties. Li Shizhen’s  ”Compendium of Materia Medica” contains: “The sugar method is exported to the Western Regions. Tang Taizong sent people to pass the law into China, and the cane was divided into the Qing Dynasty by the cane. It was sugarcane. The sand was condensed with sand. As stone urn cause frost ice who is Shimi, as icing for sugar. “sugar “and commonly known as the” sugar “different,” sugar “means sugar, refers to all the sugar has a sweet taste, such as glucose, maltose, and the most important sucrose, while sugars include all monosaccharides, disaccharides and polysaccharides, and not only sweet substances.

Sugar Structure

It is mainly composed of carbon, hydrogen, and oxygen. It is a general term for polyhydroxy aldehydes or polyhydroxy ketones and their polycondensates and certain derivatives.

Carbohydrates include monosaccharides, polymers and derivatives of monosaccharides . Glucose is a monosaccharide. Maltose, sucrose, and lactose are disaccharides.

Monosaccharides are polyaldols or polyhydroxy ketones and their cyclic hemiacetals or derivatives, aldehydes or ketones with multiple hydroxyl groups. Polysaccharides are monosaccharide condensed polymers.

Structural formula

In the past, all chemicals whose molecular formula could be written as C m (H2O) n were called “carbohydrates”. According to this definition, some scientists think that formaldehyde (CH2O) is the simplest sugar, but others think that It is glycolaldehyde (C2H4O2).


However, sugars other than one or two carbon atoms are biochemically understood.

Natural sugars are usually composed of a simple carbohydrate: Monosaccharide, which has the formula (CH2O)n, (n≥ 3). A monosaccharide having a typical H-(CHOH) X (C = O) – (CHOH) Y -H structure, i.e. polyhydroxy group aldehydes or polyhydroxy ketones.

Like: glucose, fructose, glyceraldehyde are monosaccharides.

However, some biological substances like uronic acid and deoxy sugar do not conform to this formula, and there are many substances whose molecular formula conforms to this formula but it is not sugar (eg formaldehyde (CH2O) and inositol (CH2O6).

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The linear form of the monosaccharide is usually present in combination with a monocyclic sugar in the form of a ring, which is formed by the reaction of a carbonyl group (C=O) on the aldehyde/ketone with a hydroxyl group (-OH) to form a hemiacetal and form a New COC key bridge.

Monosaccharides can be linked together in various ways to form polysaccharides (or oligosaccharides, also known as oligosaccharides ). Many saccharides contain one or more modified monosaccharide units which may be substituted or removed by one or more groups.

For example, a component of DNA deoxyribose is to be ribose is modified sugar; chitin is a repeated N- acetylglucosamine sugars consisting of (one nitrogen atom glucose) fragments.

Chemical properties

Starch identification

1.  Take 2 clean tubes and use a marker to number (such as A and B) in the upper part of the tube for later use.

2.  Weigh 2 g of sucrose and starch with a balance, put them into 100 ml of clear water, dissolve them, and set aside.

3.  Take 3 ml of each of the sucrose solution and the starch solution, and add the same amount of dilute iodine solution, observe and record the color change of the solution.


Identification of sugars

Test for reducing sugar

The sugar is classified into a reducing sugar and a non-reducing sugar depending on whether it has a reducing property. Reducing sugars such as monosaccharides, maltose, and lactose react with the Feilin reagent to produce a brick red precipitate.

Therefore, Felin reagents are commonly used in experiments to detect the presence of reducing sugars.

 1.  Take 3 clean tubes and number them for use.

 2.  Take 3 ml of each of the sucrose solution and the starch solution, and inject two tubes into them, and then add 1 ml of water.

 3.  Add 3 ml of the starch solution to the third tube, and then add 1 ml of diluted saliva.

4.  Add 2 ml of Feilin reagent to each of the 3 tubes, heat for 2 min in water, observe and record the color change of the solution.

It is recommended to consider that the Feilin reagent is mainly prepared by mixing a NaOH solution with a mass concentration of 0.1 g/ml and a CuSO4 solution having a mass concentration of 0.05 g/ml.

Anthrone Test

Under the action of concentrated sulfuric acid, sugars can be dehydrated to form furfural or hydroxymethylfurfural, which can react with anthrone to form a blue-green furfural derivative. After the sample was identified as a saccharide by the above reaction, a different test of monosaccharide, disaccharide, aldose or ketose was carried out.

Identification of Monosaccharides and Polysaccharides: Buff Test

In the acidic solution, the reduction rates of monosaccharides and reduced disaccharides are significantly different. Buff reagent (1% dilute acetic acid solution containing 5% copper acetate) is weakly acidic.


It can oxidize monosaccharides in 2 minutes to form brick red cuprous oxide. It has an orange or orange-red precipitate, showing the presence of monosaccharides. Since the orange precipitate is suspended in the blue copper acetate solution, the green color sometimes occurs.

Identification of ketose: Seliwanoff test

The principle of this test is to convert ketose into concentrated hydroxymethylfurfural with concentrated hydrochloric acid, and then condense with resorcinol (Seliwanoff test) to form a red product.

An equal volume of concentrated hydrochloric acid and a few drops of Seliwanoff reagent were added to the water-soluble sample, and the resulting mixture was heated just to boiling. If the solution appears red in 2 minutes, a dark black precipitate is formed, indicating the presence of ketone. Long-term placement or prolonged heating time, aldose also color reaction, but the color is slightly lighter and generally no precipitation.



The simplest class of monosaccharide -saccharide species, the monosaccharide molecule contains many hydrophilic groups, is easily soluble in water, and is insoluble in organic solvents such as ether and acetone.

Simple monosaccharides generally contain 3-7 carbon atoms. The polyhydroxy aldehyde or polyhydroxy ketone, the constituent elements of which are C, H, O glucose, fructose, galactose and the like. Glucose is the main energy source of life activities, ribose is a constituent of RNA, and deoxyribose is a constituent of DNA. The molecular formula of glucose and fructose is C6H12O6. They are isomers.


There are many kinds of monosaccharides in the living body, such as ribose and deoxyribose, which are monosaccharides containing 5 carbon atoms, and glucose, fructose, and galactose are monosaccharides containing 6 carbon atoms.

Monosaccharides are the smallest of these types because they cannot be hydrolyzed into smaller carbohydrates. These are some of which have two or more hydroxyl groups is an aldehyde or ketone type. The unmodified monosaccharide formula can be expressed as (CH 2 O) n, which is called a “carbohydrate” because it is a multiple of carbon and water molecules.

Monosaccharides are an important fuel molecule and a structural fragment of nucleic acids. n = 3 in the smallest monosaccharide, namely: dihydroxyacetone or D- and L- glyceraldehyde.

Triose sugar, for example, Glyceraldehyde

Pentose, five-carbon sugars such as ribose, deoxyribose

Hexoses such as glucose, fructose (the chemical formula is C 6 H 12 O 6)

Monosaccharides can be classified by three different characteristic segments: the position of the carbonyl group, the number of carbon atoms in the molecule and its chiral configuration. If the carbonyl group in the carbon chain genus ends of the molecule aldehyde class, said monosaccharides: aldose; if the carbonyl carbon chain in the case of intermediate molecular -one class, called monosaccharides: ketose.

Monosaccharides containing three carbon atoms are called: triose; four carbon atoms are called butyrate; five are called pentose, six are called hexoses, and so on.

Except for the first and last carbon atoms in the carbon chain of a sugar molecule, each carbon atom carries a hydroxyl group (-OH) and has an asymmetry, so that their chiral centers can be either R or S. Because of this asymmetry, the molecular formula of a certain sugar can exist as multiple isomers.

For example, aldose D-glucose has the formula (CH2O) 6, of which six carbon atoms are chiral, so D-glucose is one of 2 = 16 possible stereoisomers. As another example, glyceraldehyde is propionaldehyde having one possible stereoisomer and is also an enantiomer and an epimer.

The keto sugar molecule corresponding to 1,3-dihydroxyacetone and aldose-propionose is a symmetric molecule without a chiral center. The D or L configuration is determined by the orientation of the asymmetric carbon atom furthest from the carbonyl group: in the standard Fischer projection, the molecule is a D-type sugar if the hydroxyl group is on the right side and an L-type sugar on the left side.


It should be noted here that the “D-” and “L-” prefixes cannot be confused with “d-” and “l-“, which refers to the rotation of polarized light in the plane of the sugar molecule. “d-” and “l-” is now less used in sugar chemistry.

The aldehyde or ketone group of the linear monosaccharide irreversibly reacts with another carbon atom to form a hemiacetal or hemiketal to give a heterocyclic ring with an oxygen bridge connecting the two carbon atoms. Rings composed of five or six atoms are called furanose and pyranose, respectively, and these cyclic sugars are chemically balanced with the sugars in the linear form.

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In the process of forming a cyclic sugar from a linear sugar, a carbon atom containing a carbonyl oxygen atom is referred to as an anomeric carbon. The carbon atom to a chiral center in the molecule after the ring has two possible configurations: If a chiral isomer of the oxygen atoms may be above or below the plane, called the thus obtained: anomeric Things. If the -OH substituent on the anomeric carbon is in trans configuration with the exocyclic CH 2 OH group (ie, not on the ringside), it is called: α an anomeric; in another case, both are in the ring. On the same side, in a cis configuration, it is called: beta anis. Since the cyclic sugar and the linear sugar itself are converted into each other, there is a balance between the two anomers. In the Fisher projection, the alpha anomer is expressed as the anomeric hydroxyl group and the CH 2 OH exhibit a transform, and the β an anomeric object is a cis form.


A saccharide consisting of two monosaccharides joined together, called a Disaccharide.

Disaccharide is composed of two monosaccharide units by a dehydration reaction to form called a glycosidic bond is a covalent bond connected to each other. During the dehydration process, one molecule of the monosaccharide removes the hydrogen atom while the other molecule of the monosaccharide removes the hydroxyl group.


The unmodified disaccharide formula can be expressed as C 12 H 22 O 11. Although there are many types of disaccharides, most are not common.

Maltose, sucrose, lactose, etc. are common disaccharides. One molecule of maltose is hydrolyzed to produce two molecules of glucose; one molecule of sucrose is hydrolyzed to produce one molecule of glucose and one molecule of fructose, and one molecule of lactose is hydrolyzed to produce one molecule of glucose and one molecule of galactose. It can be seen that the disaccharide is composed of two molecules of monosaccharides.

Sucrose is the most abundant disaccharide, which is the most important sugar in plants. Brown sugar, white sugar, rock sugar, etc. are all made from sucrose. Sucrose consists of a D-glucose molecule and a D-fructose molecule, and its system is named: O -α-D-glucopyranosyl-(1→2)-D-fructosefuranoside, which consists of glucose and fructose. composition. Glucose is pyranose; fructose is furanose. The two monosaccharides are connected by an oxygen atom on the first carbon (C1) of D-glucose to the carbon No. 2 (C2) of D-furanose. The suffix  glycoside indicates that two monosaccharide anomeric carbons are involved in the formation of glycosidic bonds.

Lactose is widely found in natural products such as mammalian breast milk.

Maltose (two D-glucose linked to alpha sugar by 1, 4 carbon atoms) and cellophane (two D-glucose linked to a beta sugar via a 1, 4 carbon atom).

Disaccharides can also be classified as reducing disaccharides and non-reducing disaccharides, which are linked to each other by removing one molecule of water from the hemiacetal (ketone) hydroxyl groups of two monosaccharide molecules. There is no hemiacetal (ketone) hydroxyl group present in such a disaccharide molecule, and thus any one of the monosaccharide moieties can no longer be converted from a cyclic form to an aldehyde (ketone) form. This disaccharide does not have a spinning phenomenon and a reducing property, nor can it produce glycocalyx, so it is called a non-reducing disaccharide.

Further, the trisaccharide is a monosaccharide which is hydrolyzed to form three molecules. Such as raffinose. Starch is a saving substance, cellulose is a cell wall, and a sugar element is an energy storage substance.


The chemical formula is (C6H10O5)n Including starch, cellulose, glycogen, and xylose.

Compound sugar

A product of the reducing end of a saccharide and a protein or lipid. It is widely distributed in living things and has many important functions. Cell identification, characterization, and immunity are all related to it. Sugars and proteins combine with protein-based glycoproteins, such as most proteins in the blood; there are also sugar-based ones, such as proteoglycans, which are important components of animal connective tissue.

In combination with lipids, such as lipopolysaccharide is present in the outer membrane of bacteria, the components are mainly polysaccharides; in addition, there are called glycolipids, which are mainly composed of lipids, and are mostly linked to the membrane of cells.

Glycolipids can be derived from sphingosine or glycerol, but they are the most widely distributed in nature, and the most studied to date is glycosphingolipids(see sphingolipids).

Asymmetry of complex carbohydrates: Glycolipids and Glycoproteins are only distributed on the outer surface of cells.

Oligosaccharides and polysaccharides

Both oligosaccharides and polysaccharides are long-chain molecules composed of monosaccharide units through glycosidic bonds. The difference between the two is the number of monosaccharide units on the chain: oligosaccharides usually contain 3-10 monosaccharide units, while polysaccharides have more than 10 monosaccharide units.

In practical applications, the classification of sugars is more inclined to personal judgment. For example, the above-mentioned disaccharides can be regarded as oligosaccharides, and also include trisaccharide- raffinose and tetrasaccharide-salose.

Oligosaccharides (oligosaccharides) – polymerized from 2-10 monosaccharide molecules, which can form monosaccharides after hydrolysis, including disaccharides, trisaccharides, tetrasaccharides, and the like.

Polysaccharides – polymerized from more than 10 monosaccharide molecules. After hydrolysis, a plurality of monosaccharides or oligosaccharides can be produced. According to whether the composition of the monosaccharide formed after hydrolysis is the same, it can be divided into:

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Isopolysaccharide: The homopolysaccharide consists of a monosaccharide that hydrolyzes to form the same monosaccharide. Such as gum Arabic, sugar, starch, cellulose and so on. The expressions for both starch and cellulose are (C 6 H 10 O 5n. But they are not isomers because their number of n is different. Wherein starch n < cellulose n.

Heteropolysaccharides: Heteropolysaccharides are composed of various monosaccharides, which are hydrolyzed to form different kinds of monosaccharides. Such as mucopolysaccharide, hemicellulose, and the like.

Related information

Biological function

Sugars mainly include starch without sweetness and maltose with sweetness. They are the most important energy substances in the human body and play an important role in the human body.

As a bioenergy source, for example, muscle contraction and nerve conduction.

As a carbon source for the biosynthesis of other substances.

As a structural substance of a living body.

Glycoproteins, glycolipids and the like have various physiologically active functions such as cell recognition and immunological activity.

Used as a material for storing nutrients (such as starch and glycogen ) or as a cell wall for animal exoskeletons and plant cells (eg, chitin and cellulose).

Ribonose, which is a five-carbon aldose, is an essential component of various cofactors (such as ATP, FAD, and NAD) and is also the backbone of some genetic material molecules (such as RNA).

It has a great correlation with the immune system, fertilization, disease prevention, blood coagulation, and growth.

Relative sweetness

Fructose 175 (the sweetest sugar)

Sucrose 100

Glucose 74

Maltose 32

The scientific method of consumption

Most sugars, such as monosaccharides and disaccharides, should be taken in a quantitative manner, and should not be excessive. Especially for diabetics, it may have adverse effects.

Cellulose, relative to other sugars, can be eaten in large quantities, which cannot be hydrolyzed in the human body, but can help digestion, prevent constipation, hemorrhoids and rectal cancer, lower cholesterol, prevent and treat diabetes.

Sugar is the main source of energy needed by the human body. When the body is not enough sugar, it will consume fat, so it is not recommended to diet to lose weight.

Sugar nutrition

Carbohydrates are both important institutional substances for living organisms and a major source of life-sustaining activities for living organisms. In addition, sugars can bind to proteins to form glycoproteins and play an important role in life activities.

A variety of foods are rich in sugars, including fruit, soft drinks, bread, pasta, beans, potatoes, rice bran, rice, and wheat. Sugar is a common source of energy in living things, but it is not essential for humans. Sugar is not an integral part of any other molecule, and the body can also extract energy from proteins and fats. The brain and cranial nerves generally cannot burn fat to gain energy but can be replaced with glucose or ketose.

From human gluconeogenesis process, with specific amino acids, triglycerides are the glycerol backbone, some or synthetic fatty acid glucose. Sugar contains 15.8 kilojoules per gram (ie 3.75 kilocalories) and 16.8 kilojoules (4 kilocalories) per gram of protein, while 37.8 kilojoules (9 kilocalories) per gram of fat.


Biology generally cannot convert all sugars into energy, of which glucose is the most common source of energy, especially since the brain can only be powered by glucose (due to the low permeability of the blood-brain barrier). Many organisms have the ability to metabolize other monosaccharides and disaccharides into energy, but glucose is preferred and most digestible.

For example, in E. coli, when lactose is encountered, the lactose operon releases the enzyme to digest lactose, but if both lactose and glucose are present, the lactose operon is repressed and glucose is first digested.

Polysaccharides are also a common source of energy. Many organisms can break down starch into glucose, but most organisms cannot digest cellulose, chitin and other polysaccharides. These sugars can only be digested by certain bacteria and protists.

For example, ruminants and termites use microbes to treat cellulose. Although these complex sugars cannot be easily digested, they are an important part of human nutrition, called dietary fiber. They can also be made into other types of sugars by industrial means, such as chitosan (processed from chitin).

The biggest benefit of dietary fiber for humans is that it promotes gastrointestinal motility and makes the digestive system work better. The US Drugs Organization recommends that 45–65% of the calories in food for each US and Canadian come from sugar to reduce the risk of heart disease and obesity.

The Food and Agriculture Organization of the United Nations and the World Health Organization have also jointly recommended that each country develop nutrition guidelines, with 55–75% of the total food energy per person coming from sugar and up to 90% directly from sugar.

Carbohydrate classification

Another name for carbohydrates, “carbohydrate”, is the result of biochemists who previously discovered that certain sugars can be written as C n (H 2 O) m, so the sugar is a compound of carbon and water, but later the findings demonstrate that many sugars do not conform to their molecular formula, such as rhamnose (C 6 H 12 O 5). Some substances conform to the above formula but are not sugars such as formaldehyde (CH 2 O). Carbohydrates are just the most common form of sugar. We narrowly understand sugar as carbohydrate.

Historically, nutritionists have only classified sugars into simple and complex, but this classification is inevitably ambiguous. Today’s “simple sugars” generally refer to monosaccharides and disaccharides, while “complex sugars” refer to polysaccharides (including oligosaccharides).


However, “complex sugars” were first seen in the US Senate Human Nutrition Needs Committee publication “American Nutritional Goals” (1977), which means different meanings, referring to “fruits, vegetables, and whole grains.” Nutritionists use the term “complex sugars” to refer to any digestible sugar in foods containing fiber, vitamins, and minerals, relative to digested sugars that provide less other nutrients.

Many people (and even nutritionists) believe that complex sugars (polysaccharides, such as starch) are slower to digest than simple sugars (such as monosaccharides) and are therefore healthier.

In fact, the effects of simple sugars and complex sugars on blood sugar levels are similar. Some simple carbohydrates are digested very slowly (eg sugar), while some complex carbohydrates, especially after treatment, can rapidly increase blood sugar levels (such as starch). From this, it can be seen that the rate of digestion depends on a number of factors, including other nutrients that are fed together, food preparation methods, differences in the rate of metabolism in the individual, and the chemical structure of the carbohydrate.

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Nutritionally, the concept of the glycemic index (GI) and glycemic load (GL) is used to reflect the effects of food on the human body after digestion. The glycemic index measures the rate at which the body absorbs glucose from the food, while the blood glucose load measures the total amount of glucose that can be absorbed in the food.

Among the two indices, the highest represents the food with the highest sugar content and the highest blood sugar level. The insulin index is a similar, updated calculation that measures the effect of food on blood insulin levels, taking into account the amount of glucose (or starch) and certain amino acids in the food.

Dietary guidelines generally recommend the consumption of complex carbohydrates (starch) and nutrient-rich simple carbohydrates such as fruits, vegetables, and dairy products to compensate for the consumption of large amounts of carbohydrates.

Excessive consumption of highly processed carbohydrate sources such as corn or potato chips, candies, sugary drinks, pastries, and white rice is generally considered unhealthy. The US Department of Agriculture ‘s 2005 American Dietary Guidelines no longer uses simple/complex classifications to recommend foods rich in cellulose and whole grains.

Glucose metabolism

Glucose metabolism can be divided into two aspects: decomposition and synthesis. Decomposition includes glycolysis and tricarboxylic acid cycle, synthesis includes heterogeneity of sugar, synthesis of glycogen and structural polysaccharide, etc. Intermediate metabolism also has a pentose phosphate pathway and uronic acid pathway. Wait.

Glucose metabolism is regulated by nerves, hormones, and enzymes. The metabolism of different tissues in the same organism varies greatly. The brain tissue always breaks down the sugar at the same speed. The myocardial and skeletal muscles degrade normally under normal conditions but can reach a high speed when the myocardial hypoxia and skeletal muscle spasm. The synthesis of glucose is mainly carried out in the liver. The sugar metabolism of different tissues reflects their different functions.

(A) Digestion

The energy required by the organism is mainly provided by the catabolism of the sugar. The use of sugar as a source of energy requires that the more complex sugar molecules be metabolized (ie, digested) into monosaccharides before they can be absorbed and metabolized. The enzyme that hydrolyzes sugars is a carbohydrate.

Carbohydrase is divided into two types: polysaccharides and glycosidase. Polysaccharides hydrolyze polysaccharides, which catalyze the hydrolysis of simple nucleosides and disaccharides. There are many types of polysaccharide enzymes, such as amylase, cellulase, xylanase, pectinase and the like.

The sugar in human food is generally based on starch. An enzyme that hydrolyzes starch and glycogen is called Amylase. Amylases are available in both alpha-amylase and beta-amylase.

(B) Absorption

The small intestine of humans and animals can directly absorb monosaccharides and enter the blood circulation through capillaries. The indigestible disaccharides, oligosaccharides, and polysaccharides are not absorbed, are decomposed by intestinal bacteria, and are released or participate in metabolism in the form of CO2, methane, acid and H 2. The absorption rates of various monosaccharides are different, D-galactose>D-glucose>D-fructose>D-mannose>D-xylose>arabinose.

The absorption mechanism of monosaccharides includes passive transport and active transport, both of which are transported across the membrane by a specific protein. Passive transport, ie solute transported by the concentration gradient, slow absorption, no energy consumption; active transport, ie solute reverse concentration gradient transport, fast absorption, energy consumption.

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