Classification of Carbohydrates


As you know, carbohydrates or glucids are polyhydroxylated aldehydes or ketones, their derivatives and polymers. Glucose is a typical example of carbohydrates.

Observe that glucose has an aldehyde group (drawed in red) and five hydroxyl groups.

Most of carbohydrates are present with a cyclic structure in nature, as a consequence of internal linkages between the carbonyl carbon (of the aldehyde or ketone group) with one of the hydroxyl groups in the same molecule. This graphic represents glucose in a cyclic form:

Considering the polymerization degree (PD) of carbohydrates, they can be classified in Monosaccharides, Oligosaccharides and Polysaccharides.


Monosaccharides are formed by a single molecule. It means that when hydrolyzed they can not release simpler molecules. Examples of this group of carbohydrates are glucose, ribose and fructose, among others.

Monosaccharides can be subclassified according to different criteria, for example:

According to the main (carbonyl) function:

If the carbonyl group belongs to an aldehyde function, the monosaccharide is classified as an aldose. Glucose is a typical aldose. If the main function is a ketone, then the monosaccharide is classified as a ketose. 

Fructose (see structure below) is a ketose, since it is a polyhydroxylated ketone.

According to the number of carbons:

Monosaccharides can be classified in trioses, tetroses, pentoses, hexoses, heptoses and octoses, according to the number of carbons in the molecule.

According to the steric series:

According to the type of esteroisomers, monosaccharides can be classified as L or D (most of the carbohydrates in the animal kingdom belongs to D series)


According to the kind or anomer:

According to the position of the anomeric hydroxyl, monosaccharides can be classified as Alpha or Beta.



Usually, these criteria are combined for describing a monosaccharide, e.g. a compound can be described as a Beta-D-aldohexose.



 They are formed by 2-9 monomers linked through glycosidic linkages; in other words, when hydrolyzed these compounds release 2 to 9 monosaccharides (some texts say up to 20; in fact, oligosaccharides release “a few” monosaccharides).

According to the number of monosaccharides in the oligosaccharide, oligosaccharides can be dissacharides, trisaccharides, tetrasaccharides, etc. Disaccharides, formed by just 2 monosaccharides, are the most important subgroup of oligosaccharides. Disaccharides that appear in nature are lactose, or milk sugar (formed by galactose and glucose), and sucrose or table sugar (formed by fructose and glucose). Other important disaccharides are produced as result of starch digestion: maltose and isomaltose. These disaccharides are ,both of them, formed by two molecules of glucose, but linked in different ways. Cellobiose is a third dissacharide formed also by two molecules of glucose, but linked in such a way that animals can not break, unless animals have in the digestive system specific microorganisms that hydrolyze these linkages, as herbivors have (Cellobiose is formed as result of the digestion of cellulose).



Polysaccharides are carbohydrates formed by more than 9 monosaccharides (some texts say more than 10 monosaccharides, other texts say more than 20…in fact, they usually are formed by a lot of monosaccharides!). When the polysaccharides are formed by the same type of monosaccharides, they are called homopolysaccharides.




The molecules that form starch, glycogen and cellulose are formed by hundreds of molecules of just one type of monosaccharides (glucose, in this cases), linked through glycosidic linkages. These polysaccharides are typical examples of homopolysaccharides.

If the polysaccharide is formed by different types of monosaccharides, then it is called a heteropolysaccharide. Hyaluronic acid, formed by thousands of alternant units of glucuronic acid and N-acetylglucosamine, is an example of heteropolysaccharides.

Hyaluronic acid is an important component of the extracellular matrix in the skin and the conective tissue. This compound has received lately a lot of attention from the media as an antiaging agent.


Which enzyme is it anyway?





After the introduction of sugar in his diet, a baby begins to present vomiting, jaundice and hemorrhage. He arrives to your hospital in a comatose state, and you detect hypoglicemia with fructosemia. Your presuntive diagnosis is a Hereditary Fructose Intolerance, a genetic disease caused by a lack of:


a) Aldolase A


b ) Aldolase B


c) Fructokinase


d) Fructose 1,6 bisphosphatase


e) Fructose 6 phosphatase


f) Glycogen phosphorylase a


g) Glycogen phosphorylase b


h) Glycogen Synthase a


j) Glycogen Synthase b


k) Glucokinase


l) Hexokinase


m) Phosphofructokinase I


n) Phosphofructokinase II



Q: About GSD and Glycogen structure (C-09)


Andersen’s Disease is a rare disease caused by the deficit of the Branching enzyme, responsible for the formation of branches in glycogen structure. This results in an abnormal glycogen with few branching points and long peripheral chains. Clinically, hepatoesplenomegaly, cirrhosis of the liver and hepatic failure are major concerns. As the problem is located in the branching points, it is obvious that there is a defect in the formation of:


a)     Alpha 1,4 -O-glycosidic linkages


b)     Alpha 1,6 -O-glycosidic linkages


c)      Alpha 1 ,Beta 2 -O-glycosidic linkages


d)     Beta1,3 -O-glycosidic linkages


e)     Beta1,4 -O-glycosidic linkages


f)       Beta1,6 -O-glycosidic linkages



(You can find in this post the answers to the Questions C-06, C-o7 and C-O8)


As discussed in a former post Polysaccharides are carbohydrates formed by more than 9 monosaccharides linked by glycosidic bonds.


 When they are formed by the same kind of monosaccharides, they are called homopolysaccharides, like starch, glycogen and cellulose, formed each of them by hundreds of molecules of glucose linked by glycosidic linkages.

If the polysaccharides molecules are formed by different kinds of monosaccharides, they are considered heteropolysaccharides. Hyaluronic acid, formed by thousands of alternative units of N-acetyl glucosamine and glucuronic acid, is an example of heteropolysaccharide.  






Cellulose is a linear polymer of D-glucose residues bonded by b(1, 4)-O-glycosidic linkages. It is the most abundant carbohydrate in nature.


It is formed by glucose units, linked by Beta-1, 4 O-glycosidic linkages. We can say then that, if we consider the kind of linkage, the repeating unit in cellulose is cellobiose, the disaccharide formed by two molecules of glucose linked by Beta-D-O glycosidic bonds, (that is why some text books say that the monomer in cellulose is cellobiose).


The long fibers of cellulose are held together by intermolecular hydrogen bonds. Hydrogen bonding continues in the same plane with other chains as well as in planes above and below this plane to form strong, fibrous bundles. It made cellulose very appropriate for its structural function in plants


Since cellulose is formed by glucose molecules, it can be a source of energy for certain species. The lack in human beings of appropriate enzymes for digesting cellulose make this polysaccharide unsuitable for human nutrition (Have you though about how hunger in the world could disappear if we had enzymes for digesting cellulose?). Cellulose and derivatives are used as a component of laxatives for humans.





Starch is the second most abundant carbohydrate in nature.

The biological functions include, in plants, the main way of storage of sugar, and consequently, of energetic sources; in humans, the first supply of glucose on diet (Answer to C-O7)

Starch is not really a molecule, but a grain formed by two different kinds of molecules:  Amylose and Amylopectin




Amylose is a linear molecule formed by glucose units linked by alpha-1, 4 O glycosidic linkages. Taking in account the kind of linkage we can say that the repeating unit in amylose is maltose. (It explains that some books indicate that the monomeric unit in amylose is maltose).


Amylose molecule is helicoidal




Amylopectin is the second type of molecule that forms starch. It is a branched molecule, formed also by glucose. Amylopectin contains D-glucose residues bonded together by a(1, 4)-O-glycosidic linkages with branching through a(1      6)-O-glycosidic linkages.

The disaccharides that can be obtained from the digestion of amylopectin are maltose and isomaltose.


Amylopectin shows a branch each 24-30 units of glucose,




The structure of glycogen is very similar to amylopectin but more branched, with one branch every 8 to 12 glucose unit


Glycogen is the way in which glucose is stored in animals. Glycogen is stored mainly in liver (to release glucose to blood when necessary) and in muscle, where it is used as a reserve of energy for muscular contraction (Answer to C-o8)




Heteropolysaccarides contain two or more different kind of monosaccharides. Usually they provide extracellular support for organisms of all kingdoms:  the bacteria cell envelope, or the matrix that holds individual cells together in animal tissues, and provides protection, shape and support to cells, tissues and organs.


Heteropolysaccharides provide extracellular support to very different organisms, from bacteria to humans; together with fibrous proteins, like collagen, elastin, fibronectin, laminin and others, heteropolysaccharides are the most important components of the extracellular matrix.  Hyaluronic acid, condroitin sulfates and dermatan sulfates are important heteropolysaccharides in the extracellular matrix. These heteropolysaccharides usually are formed by the repetition of a disaccharide unit of an aminosugar and an acid sugar. 


A typical example


Other common constituents are sulfate groups linked to certain monosaccharides. Usually heteropolysaccharides are associated with proteins forming proteoglycans, glycosaminoglycans or mucopolysaccharides (since they are abundant in mucous secretions).  As a group, they perform diverse functions: structural, water metabolism regulation (as a reservoir of water), cellular cement, biological sieve, biological lubricant, docking sites for growth factors, among other functions.


Established specific functions of some glycosaminoglycans are:


Hyaluronic Acid (Hyaluronate): It is a lubricant in the synovial fluid of joints,

give consistency to vitreous humor, contributes to tensile strength and elasticity of cartilages and tendons (Answer to C-O6)


Chondroitin Sulfates: contributes to tensile strength and elasticity of cartilages, tendons, ligaments and walls of aorta.


Dermatan sulfate (former chondroitin sulfate B) is found mainly in skin, but also is in vessels, heart, lungs. It may be related to coagulation and vascular diseases and other conditions.


Keratan sulfate: Present in cornea, cartilage bone and a variety of other structures as nails and hair.




It is a potent natural anticoagulant produced in the Mast Cells that causes antithrombin bind to thrombin and produce inhibition of blood coagulation.


Glycosaminoglycans are synthesized in the ER and Golgi. They are degraded by lysosomal hydrolases. A deficiency of one of the hydrolases results in a mucopolysaccharidosis. These are hereditary disorders in which glycosaminoglycans accumulate in tissues, causing symptoms such as skeletal and extracellular matrix deformities, and mental retardation.

Examples of these genetic diseases are Hunter and Hurler syndromes.

These diseases, caused by different enzyme deficits, are characterized by physical deformities, mental retardation and disturbances in the degradation of heparan sulfate and dermatan sulfate.


Carbohydrate Extended Matching Questions



Questions C-06 to C-08 








Another format you can expect to see in your Multiple Choice exams:





Choose, among the following polysaccharides, which correspond to each of the questions that appear below:



a)     Cellulose



b)     Glycogen



c)      Starch



d)     Hyaluronic acid



e)     Heparin



f)       Chondroitin Sulfate



(C-06) This carbohydrate acts as lubricant of synovial fluid and contributes to

            tensile strength and elasticity of cartilages and tendons. It is also an

            important component of skin.





(C-07) It is the most important polysaccharide in human diet:





(C-08 ) It is the form in which glucose is stored in muscle and liver for ulterior use as fuel:





Another Carbohydrate Puzzle


A New Carbohydrate Crossword!

                              net efekt photostream in Flickr 

The Biochemical Puzzle page has been updated.
It contains a new puzzle (a crossword) about carbohydrates structure and functions.
Have a good time solving it!

A pending Solution to the Tiles Problem about biological catalysts have been posted also.

Answer to Carbohydrate Question (C-04)


Original Question C-04 







Answer: (d)


Heparin is a heteropolysaccharide (a polysaccharide formed by different kinds of monosaccharides).


In fact, Heparin is a family of molecules that are usually composed by the repetition of a sulfated amino sugar and an acid sugar. The most abundant pair of monosaccharides whose repetition forms heparin is this one                                               



The physiological function of heparin is subject of discussion:  it is related to the inflammation process and not to coagulation in physiological conditions.


Heparin inhibits the coagulation process by inhibiting, indirectly, the action of Thrombin.


Normally, during the coagulation process, Thrombin (factor II), a proteolytic enzyme, (as many coagulation factors), catalyzes the conversion of fibrinogen to fibrin, activates factors V, VIII and XI and also promotes platelet activation.


Antithrombin is an antiprotease protein with an important role in the regulation of normal coagulation. It inhibits mainly the proteases of the intrinsic pathway of coagulation, but also affects other factors or the extrinsic and the complement pathways.


Heparin increases the inhibitory action of antithrombin in thousands of times. Heparin can act through two mechanisms:


1.- An allosteric mechanism, in which Heparin provokes conformational changes in antithrombin that increases its ability to inhibit some of the coagulation factors,


 2.- By forming ternary complexes Heparin-antithrombin-Thrombin.


Since it is necessary that the heparin molecule be big enough to bind properly to antithrombin and thrombin for forming the ternary complex (Heparin molecule should have more than 18 monosaccharides for allowing the simultaneous binding), small molecules of Heparin have no effect on Thrombin but maintain anticoagulant activity affecting other factors, mainly factor X. Because of this fact and that natural Heparin molecules are very heterogeneous, Low Molecular Weight Heparins (LMWHs), that show better pharmacokinetics properties, have been developed in order to achieve a better medical regulation of the anticoagulant therapy.


The medical uses and indications of Heparins and LMWH are discussed in detail in this article of the American Heart Association.


AHA Scientific Statement


Guide to Anticoagulant Therapy: Heparin: a Statement for Healthcare Profession