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Theory Notes

 General Introduction

  • Carbohydrates are the most abundant compounds found in nature (cellulose: 100 billion tons annually)
  • They make up most of the organic matter on earth because of their extensive roles in all forms of life.
  • One of the four major classes of biomolecules along with proteins, nucleic acids and lipids.
  • The term carbohydrate is derived from the French  term : hydrate de carbone, they are hydrates of carbon
  • Compounds composed of C, H, and O
  • Empirical formula (CH2O)n , for example when n = 5 then C5H10O5
  • Not all carbohydrates have this empirical formula: e.g. sugar derivatives deoxysugars, amino sugars etc., do not follow this rule.

General characteristics

  • They have  large number of hydroxyl groups (poly hydroxy)- Figure-1-a
  • In addition they may contain-an aldehyde group (polyhydroxy aldehydes) or a keto group (polyhydroxy ketones)- Figure-1-b
  • Their derivatives may also contain nitrogen, phosphorus or sulfur- Figure-1-c

 Polyhydroxy compounds

Figure-1-a)  Carbohydrates are derivatives of polyhydroxy aldehyde or ketone compounds


b)  Aldehyde derivative  

Keto group

c) Ketone derivative

Functions of Carbohydrates

1) Sources of energy, especially for brain and red blood cells.

2) Intermediates in the biosynthesis of other basic biomolecules (lipids and proteins)

3) Associated with other entities such as glycosides, vitamins and antibiotics

4) Ribose and deoxyribose sugars form part of the structural framework of RNA and DNA.

4) Form structural tissues in plants and in microorganisms

5) Carbohydrates are linked to many proteins and lipids, where they play key roles in mediating interactions among cells and interactions between cells and other elements in the cellular environment.

Classification of carbohydrates

1) Monosaccharides (monoses or glycoses)
Trioses, Tetroses, Pentoses, Hexoses

2) Oligosaccharides
Di, tri, tetra, penta, up to 9 or 10
Most important are the disaccharides

3) Polysaccharides or glycans
a) Homo polysaccharides
b) Heteropolysaccharides


  • Also known as simple sugars
  • Cannot be hydrolyzed further
  • Classified  either by the number of carbon atoms or by the nature of functional group-aldoses or ketoses
  • Most  of the carbohydrates (99%) are straight chain compounds
  • D-glyceraldehyde is the simplest of the aldoses (aldotriose), all other sugars have the ending -ose (glucose, galactose, ribose, lactose, etc.)
  • Keto group containing monosaccharides have the ending-ulose (Xylulose, ribulose, erythrulose), except dihydroxyacetone

MONOSACCHARIDES – Classification

1-According to number of carbons they contain in their backbone structures-
 It is a variable prefix followed by the suffix (-ose)

  • Trioses=3 Carbon,
  • Tetroses=4C,
  • Pentoses=5C,
  • Hexoses=6C,
  • Heptoses=7C

 2- According to nature of reactive group – depending on the presence of Aldehyde or keto group.Aldose sugars e.g. glyceraldehyde (figure-2-a)

  • Aldose sugars e.g. glyceraldehyde (figure-2-a)
  • A ketose sugars e.g. dihydroxyacetone (figure-2-b)


Figure-2)-a) Glyceraldehyde-  An Aldotriose                          

Dihydroxy acetone

b) Dihydroxy acetone- A ketotriose

 Details of classification

Number of carbon atoms Aldose Ketose
3 (Trioses) Glyceraldehyde Dihydroxyacetone
4(Tetroses) Erythrose Erythrulose
5 (Pentoses) Ribose, Arabinose, Xylose Ribulose, Xylulose
6 (Hexoses) Glucose, Galactose, Mannose Fructose
7(Heptoses) Sedoheptulose

 Biological significance of monosaccharides

  • Monosaccharides are important fuel molecules as well as building blocks of nucleic acids.
  • Trioses- Glyceraldehyde and dihydroxyacetone, in their phosphorylated forms are intermediates of glycolysis.
  • Tetroses are intermediates of HMP (Hexose mono phosphate) pathway, which is an intermediate pathway of glucose utilization
  • Pentoses form the structural components of glycoproteins, nucleotides and nucleic acids. They also serve as intermediates in the HMP pathway.
  • Hexoses
    • Glucose is an important fuel molecule, preferred source of energy for the brain cells and the only source of energy for the red blood cells and the cells lacking mitochondria.
    • Fructose- component of table sugar(Sucrose), honey and source of energy for the spermatozoa
    • Galactose- An important component of milk sugar (Lactose)
    • Mannose- An important component of glycoproteins.
    • Heptoses- Sedoheptulose is an important intermediate of HMP pathway
    • Nonoses- Sialic acid is an important component of glycolipids.
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Dietary fiber is indigestible part of plant foods that makes stool soft and thus enables smooth bowel movements, prevents constipation, hemorrhoids and diverticulosis. 

It consists of all of the components of the cell walls of plants that are not broken down by the body’s digestive enzymes.

Dietary Fiber and Total Fiber

“Dietary fiber” as noted on the “Nutrition Facts Label” of commercial foods, is fiber originally present in the food. “Total fiber” consists of dietary fiber and added fiber – substances that are adding to original food to increase their fiber content or to change their physical properties. Pectin, for example, is added to jam to give it a gel form.

Dietary fiber can be grouped into two main categories, those that are soluble and those that are insoluble in water.

Soluble Fiber

Soluble fiber (viscous fiber) partly dissolves in water and forms gel with it. Foods rich in soluble fiber include beans and other legumes (peas, soy, and lentils), oats, barley, citrus fruits (oranges, grapefruit), psyllium husk and flax seedSubstances found in soluble fiber are gum, pectin, some hemicelluloses, mucilage and storage polysaccharides (starch and glycogen).

Beneficial Effects of Soluble Fiber

  • Soluble fiber may prevent both diarrhea and constipation. It absorbs water from the gut and thus makes stool soft, but not bulky.
  • Soluble fiber may help in weight loss. It slows down the speed of the passage of food through the gut thus giving a feeling of fullness.
  • Soluble fiber may help to prevent and treat diabetes type 2. It slows down absorption of glucose from the intestine into the blood thus preventing high spikes of glucose in the blood after a meal.
  • Soluble fiber may lower total and LDL cholesterol and thus helps to prevent ischemic heart disease and stroke. It binds bile acids in the bowel and removes them from the body and thus reduces their absorption into the blood. Lost bile acids are replaced by synthesis from blood cholesterol. This is one theory about how soluble fiber lowers blood cholesterol levels.
  • Soluble fiber may prevent bile salt diarrhea after a gallbladder removal.   

Unwanted Effects of Soluble Fiber

Soluble fiber, if ingested in excess may cause:

  • Abdominal bloating and flatulence
  • Dehydration, if ingested without water
  • Pectin may reduce absorption of cholesterol-lowering drugs, like Lovastatin

Soluble Fiber Supplements

Examples of soluble fiber supplements:

  • Metamucil (psyllium – isphagula)
  • Citrucel (methylcellulose)
  • Benefiber (wheat dextrin)
  • FiberChoise (Inulin)
  • Pectin

The soluble fibers such as pectin and true plant gums are mucilaginous and are digestible.

Pectins are predominantly polygalacturonic acids with varying amounts of other hexose or pentose residues.

True plant gums are complex poly saccharides composed of primarily arabinose, fucose, galactose, mannose, rhamnose, and xylose. The gums are soluble in water and are digestible by the enzymes in the intestinal tract. Both pectins and gums are mucilaginous; they absorb water to form viscous gels in the stomach that decrease the rate of gastric emptying. The mucilaginous nature of the soluble fibers, pectins, and gums tends to decrease the rate at which carbohydrates are digested and absorbed, thus decreasing both the rise in blood glucose levels and the ensuing increase in insulin concentration.

Insoluble Fiber

Insoluble fiber can not be dissolved in water. Foods rich in insoluble fiber include whole wheat and other whole grains and most dark green leafy vegetables, like cabbage and cauliflower. Substances found in insoluble fiber include cellulose, hemicellulose and lignin.

Beneficial Effects of Insoluble Fiber

  • Insoluble fiber may help to prevent constipation, hemorrhoids and diverticulosis. It binds water and thus makes stool soft and bulky; it also speeds up the passage of food through the intestine.

Unwanted Effects of Insoluble Fiber

  • Ingesting foods with insoluble fiber containing sulphur (garlic, onions) may result in excessive gas.
  • Insoluble fiber eaten on an empty stomach may aggravate symptoms of irritable bowel syndrome.
  • Insoluble fiber ingested without water may result in severe constipation or even intestinal obstruction.
  • Certain types of insoluble fiber may trigger diarrhea in sensitive people.
  • Excessive ingestion of supplements containing insoluble fiber, especially in small children, may reduce absorption of calcium, magnesium, iron, copper and zinc.

1) Cellulose is a major structural component of plant cell walls. Cellulose is a long, linear polymer of glucose (β-D-glucopyranose) units that are joined by β(1→4) glycosidic bonds . Cellulose molecules have an extended, rigid structure that is stabilized by interchain hydrogen bonds.

Starch, the plant storage polysaccharide, which is also a polymer of glucose, differs in its structure in that the glucose monomer units are joined by α(1→4) glycosidic bonds .Starch is composed of two types of polymers, amylose, which has a nonbranched helical structure, and amylopectin, which is branched with α(1→6) glycosidic bonds joining the branches to the main polymer chain. Although starch is easily digested by salivary and pancreatic amylase and the disaccharidases present on the brush border of intestinal mucosal cells, cellulose cannot be hydrolyzed. The β(1→4) glycosidic bonds of the cellulose chain cannot be cleaved by the amylases present in the digestive tract.


 Figure -1- Showing the molecular structure of cellulose, indicating the repeating disaccharide unit, cellobiose.



Figure- 2-The molecular structure of starch, indicating the repeating disaccharide unit, maltose, as well as the α-1,6-glycosidic bond present in the branch points of amylopectin.

2) Hemicelluloses are also polysaccharides that are structural components of plant cell walls. However, unlike what their name implies, they are unrelated to cellulose. They are polymers that are made up of a variety of sugar monomers that include glucose, galactose, mannose, arabinose, and xylose, as well as acidic forms of these monosaccharides. Xylose is the monosaccharide that is most abundant. Hemicelluloses have a random, amorphous structure that is suitable for their location in the plant cell wall matrix. Depending on their molecular structure, hemicelluloses are partially digestible.

 3) Lignins are formed by the irreversible dehydration of sugars that result in aromatic structures. The remaining alcohol or phenol OH groups can react with each other and with aldehyde and ketone groups to form polymers. These polymers cannot be broken down by the digestive enzymes and, like cellulose and the indigestible portion of hemicelluloses, form the stool bulk.


Figure-3-  A lignin molecule in an early stage of condensation. The aromatic rings are a result of irreversible dehydration of sugar residues.

Although cellulose and hemicellulose are insoluble, they absorb water to swell and increase the stool bulk. This results in larger, softer stools. It has been shown that diets plentiful in insoluble fiber also increase the transit time of food in the digestive tract and decrease intracolonic pressure. Lignins, in addition to increasing stool bulk, also bind organic molecules such as cholesterol and many potential carcinogens. 

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Lyxose is a constituent of lyxoflavin isolated from human heart muscle. It is a monosaccharide containing 5 carbon atoms. D- Lyxose is present in nucleic acids of yeast, human heart muscle and Kanamycin antibiotic.


The compounds with the same molecular formula but differing in configuration around one carbon atom are called Epimers. Epimeric carbon is the asymmetric carbon atom other than carbon of aldehyde or Ketone group e.g . carbon number 2, 3 and 4 of glucose .

Isomers differing as a result of variations in configuration of the —OH and —H on carbon atoms 2, 3, and 4 of glucose are referred as epimers of Glucose. Biologically, the most important epimers of glucose are mannose and galactose, formed by epimerization at carbons 2 and 4, respectively.  Mannose and Galactose are not epimers of each other as they differ in configuration around 2 carbon atoms.

Figure-1 showing epimers of glucose

Similarly D- Xylulose is the C-3 epimer of D-Ribulose

Blood Group Antigens

The antigens which determine blood types belong to glycoproteins and glycolipids.  There are three types of blood-group antigens: O, A, and B. They differ only slightly in the composition of carbohydrates.

L –Fucose is a methyl pentose and is an important component of blood group antigens. (See figure).

L-Fucose is rare L sugar found of the oligosaccharide chains of N- and O-linked glycoproteins. Glycoproteins (also known as mucoproteins) are proteins containing branched or unbranched oligosaccharide chains; they occur in cell membranes and many other situations.

Figure- 2-showing Blood-group antigens.  

All humans contain enzymes which catalyze the synthesis of the O antigen.  Humans with A-type blood also contain an additional enzyme (called A-type enzyme here) which adds N-Acetyl galactosamine to the O antigen.  Humans with B-type blood contain another enzyme (called B-type enzyme here) which adds Galactose to the O antigen.  Humans with AB-type blood contain both A-type and B-type enzymes while humans with O-type blood lack both types of enzymes. N-Acetyl glucosamine and N-Acetyl galactosamine are acetylated amino sugars.


Trehalose is a non reducing sugar. Sugars containing free aldehyde or ketone group can reduce other reagents e.g. . They can reduce cupric ions of Fehling and Benedict’s reagents into cuprous ions :

Cupric ( blue ) + sugar ________> Cuprous ( red ) + oxidized sugar

These tests are one of the earliest tests to detect the presence of reducing sugar in urine.

Trehalose is a disaccharide containing two glucose residues linked together by alpha1-1 glycosidic linkage. Since both the functional groups are involved in the linkage and there is no free functional aldehyde or ketone group to carry out reduction, thus Trehalose is a non reducing sugar. On the same ground Sucrose is a non reducing sugar. Sucrose has no free reactive group because the anomeric carbons of both monosaccharides units are involved in the glycosidic bond. So, sucrose neither shows reducing nor mutarotation characters.

Figure- 3-showing structure of Trehalose- 2 glucose residues linked together by Alpha 1-1 glycosidic linkage

Tests to differentiate Glucose, Fructose and Mannose

Glucose and Fructose both give –

 1) Positive Molisch Test- since both are carbohydrates. Molisch test is a general test for all types of carbohydrates.(See the details below)

 2) Positive Benedict’s test, since both are reducing sugars.

 3) Osazone formation- Both sugars give similar shaped crystals. This test is used for the identification of sugars. It involves the reaction of monosaccharide with phenyl hydrazine, a crystalline compound. All reducing sugars form osazones with excess of phenyl hydrazine when kept at boiling temperature. Each sugar has a characteristic crystal form of osazones.  The reaction involved can be represented as follows-

Figure-4- Reaction showing formation of osazones

Three molecules of phenyl hydrazine are required, the reaction takes place at first two carbon atoms. The upper equation shows the general form of the osazone reaction, which affects an alpha-carbon oxidation with formation of a bis-phenylhydrazone, known as an osazone.

D-fructose and D-mannose give the same osazone as D-glucose. The difference in these sugars present on the first and second carbon atoms are masked when osazone crystals are formed. Hence these three sugars form similar needle-shaped crystals arranged like sheaves of corn or a broom.  It is seldom used for identification these days . HPLC or mass spectrometry is used for the identification of sugars present in the biological fluids.           


 Figure-5- showing formation of osazone crystals,

Figure-6- Needle shaped crystals of Glucose, Mannose and Fructose.

4) Seliwanoff test- Monosaccharides are normally stable to dilute acids, but are dehydrated by strong acids.

          D-ribose (Pentoses)when heated with concentrated HCl yields furfural (cyclic anhydride)

          D-glucose(Hexoses) under the same conditions yields 5-hydroxymethyl furfural

Practical Applications– The furfural derivatives can condense with phenolic compounds to give colored products. This forms the basis for Molisch test. This test is a sensitive test but it is nonspecifically given by all carbohydrates. Alpha nephthol is used in this test. A purple colored ring develops if carbohydrate is present.

Similar to this Seliwanoff Test is undertaken with Resorcinol, a cherry red color is produced if fructose is present. Sucrose also gives same reaction with Seliwanoff reagent.

The other tests are Anthrone test and Bial’s test etc.

Adenosine and Adenosine Mono Phosphate

The compound that consists of ribose linked by an N-glycosidic bond to N-9 of adenine is- Adenosine

Adenosine is a nucleoside. Nucleosides are derivatives of purines and pyrimidines that have a sugar linked to a ring nitrogen of a heterocycle called heterocyclic “base by N-Glycosidic linkage.

Adenosine contains

a)    Base –Adenine and

b)    sugar- Ribose, a pentose sugar.

Base and sugar are linked together by N- glycosidic linkage.

Figure- 7-Showing the structure of Adenosine

Mononucleotides are nucleosides with a phosphoryl group esterified to a hydroxyl group of the sugar. Additional phosphoryl groups linked by acid anhydride bonds to the phosphoryl group of a mononucleotide form nucleoside diphosphates and triphosphates

Figure-8- Showing the structure of Adenosine Mono Phosphate

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