In plants:Carbohydrates are synthesized from CO2 and H2O in

presence of light (photosynthesis).CO2 + H2O (CH2O)n + O2

Plants synthesize glucose, the most important carbohydrate and store it as “Starch” or use to build “Cellulose”, which is used in the plant’s structural framework

In animals:Animals can synthesize carbohydrates from non-

carbohydrate sources:Examples: lipid glycerol and amino acids

The process of synthesizing glucose from non-carbohydrate sources is called “Gluconeogenesis”.

However, most animal carbohydrates are derived from plants.

Light

Most dietary carbohydrate is absorbed into bloodstream as glucose

Other sugars are converted into glucose in the liver

Glucose is the major metabolic fuel of mammals (except ruminants) and a universal fuel of the fetus.

Glucose is the precursor for synthesis of other carbohydrates of biological importance:◦ Examples

Starch and Glycogen (the storage form of carbohydrates in plants and animals, respectively.)

Ribose and deoxyribose (used in nucleic acids synthesis) Galactose (a sugar used to make milk sugar, lactose, in

glycolipids and in combination with proteins in glycoproteins and proteoglycans)

Diseases associated with abnormal carbohydrate metabolism include “Diabetes mellitus” and “Lactose intolerance”.

Monosaccharides are the simplest forms of carbohydrate. They can’t be hydrolyzed into simpler forms.

Classification They are classified into 3-, 4-, 5-, 6- or 7-

carbon sugars called, trioses, tetroses, pentoses, hexoses and heptoses, respectively

They can also be classified as aldoses or ketoses, depending on whether they have an aldehyde or a ketone group.

Disaccharides, are condensation products of 2 monosaccharide units

ExamplesMaltoseSucroseLactose

Oligosaccharides are condensation products of 3-10 monosaccharide units

Polysaccharides are condensation products of more than 10 monosaccharide units

Examples◦Starch◦Cellulose◦Dextrins◦Glycogen

Structure of glucose can be represented in 3 ways:

1. Straight chain2. Cyclic or simple ring (Haworth projection)3. Chair form.

O=C-H | H-C-OH |HO-C-H | H-C-OH | H-C-OH | CH2OH

D-Glucose

1 2 3

Isomerism of carbohydrates◦Example

Glucose, for example, has 4 asymmetric carbons which can make 16 isomers (42). Different configurations of OH groups about these asymmetric carbon atoms give rise to different types of isomerism and isomers.

O=C-H | H-C-OH |HO-C-H | H-C-OH | H-C-OH | CH2OH

1

2

3

4

5

6

D and L isomerism◦D and L isomers are mirror images to each other◦ It is determined by the orientation of the OH

group (to the right is D and to the left is L) of the asymmetric carbon atom as compared to that of the parent sugar “Glycerose or Glyceraldehyde”

◦Most monosaccharides present in animals are of D- configuration

◦Example D-glucose is called dextrose in clinical practice

O=C-H | H-C-OH |HO-C-H | H-C-OH | H-C-OH | CH2OH

1

2

3

4

5

6

O=C-H | HO-C-H | CH2OH

1

2

3

O=C-H | H-C-OH | CH2OH

1

2

3

D-Glycerose L-Glycerose

D-Glucose

Pyranose (6-membered ring) or furanose (5-membered ring) ring structures are based on the pyran and furan ring structure.

Glucose(Pyranose)

Fructose(Furanose)

Pyran Furan

Alpha- and beta- anomers◦ It is the isomerism that occurs about the

carbonyl group (called anomeric carbon atom), position 1 in glucose or 2 in fructose, which forms alcohol in the ring structure, where the sugar is called alpha- if the OH group of its anomeric carbon is to the right (downward in ring structure) or beta- if it’s to the left (upward in ring structure).

-D-Glucose -D-Glucose

Epimers or epimerization◦ It is the isomerism that occurs about the

asymmetric carbon atoms other than the anomeric carbon and the last (which determine the D / L and alpha- / beta- isomerism), e.g. carbon # 2, 3 and 4 in glucose.

◦ It gives rise to different sugars, which are epimers to the parent sugar Examples

Mannose and galactose are epimers of glucose formed by epimerization of glucose at carbon 2 and 4, respectively

Epimers

Isomers of D-Aldoses

Isomers of D-Ketoses

Glycosides, ◦They are formed by condensation (removal of

water molecule) between the (-OH) group of the anomeric carbon of a monosaccharide and a second compound that may or may not be another monosaccharide, e.g. maltose is formed between 2 glucose units, lactose is a disaccharide between glucose and galactose, sucrose is formed between glucose and fructose units.

◦ If the participating group of the 2nd compound is (-OH), the bond is called O-glycosidic bond, while it’s called N-glycosidic bond if the 2nd group is an amine (-NH2).

◦ If the participating monosaccharide is glucose, then the resulting compound is called “Glucoside” and the formed linkage is glucosidic linkage. If this monosaccharide is galactose, the resulting compound is a galactoside, and so on.

Disaccharides are glycosides, ◦Since Glycosides are formed by a bond

between the (-OH) gp of the anomeric carbon and another compound, the anomeric carbon loses its capacity to convert to the acetal group (-C=O), and hence it loses it’s reducing properties.

◦ If the 2nd compound (another monosaccharide) participates with (-OH) group other than that of the anomeric carbon, the resulting disaccharide still possesses reducing properties due to presence of free anomeric carbon that can convert into (-C=O).

◦The formed bond would be called by the type and the location of the participating groups, e.g. the bond in maltose is -1-4 glucosidic bond because it’s formed between an (-OH) group of -glucose (carbon no. 1) and an (-OH) group of carbon no. 4 of a second glucose.

-D-Glucose -D-Glucose

+

-D-Maltose

-1-4 G-linkage

-D-Galactose -D-Glucose

+

-D-Lactose

-1-4 G-linkage

Sucrose-D-Glucose -D-Fructose

-1-2 G-linkage

Disaccharides are Glycosides

Disaccharides are Glycosides

Polysaccharides Serve Storage and Structural Functions, ◦Starch is a homopolymer (same consisting

monosaccharides) of glucose. It’s the most abundant dietary carbohydrate. It’s the storage form of carbohydrate in plants. Two main constituents constitute starch, Amylose (15-20%), which has a non-branching helical

structure and consists of about 250-300 glucose units linked together with -1-4 glucosidic linkages; AND

Amylopectin (80-85%), which consists of branched chains composed of 24-30 glucose units linked together with -1-4 glucosidic linkages in the chain and -1-6 at the branch points.

◦Glycogen is the storage polysaccharide in animals. It’s more highly branched than amylopectin, with chains of 12 to 14 linked with -1-4 linkages in the chain and -1-6 at the branching points.

Polysaccharides Serve Storage and Structural Functions, ◦Chitin is a structural polysaccharide in the

exoskeleton of insects and crustaceans as well as in mushroom. It consists of a homopolymer of N-acetyl glucoseamine units joined by -1-4 linkages.

◦Cellulose is the main constituent of plant structure. It’s an insoluble homopolymer of glucose, linked with -1-4 linkages. Cellulose can’t be digested by mammals because of absence of cellulase enzyme that catalyzes the cleavage of the -1-4 linkages. Microorganisms in the gut of ruminants and other herbivores secrete cellulase enzyme that hydrolyze the -1-4 linkages.

HN-CO-CH3

-D-N-acetyl glucoseamine

Glycoproteins, ◦Also called Mucoproteins, occur in many fluids

and tissues, including cell membranes. They are proteins containing branched or unbranched oligosaccharide chains. Carbohydrate derivatives are linked to proteins through O-glycosidic or N-glycosidic linkages with (-OH) or (-NH2) containing amino acid side chains, respectively.

◦The human ABO blood groups are examples of glycoproteins or glycosides that have same oligosaccharide foundation (called O or more commonly, H antigen). Different blood groups result from the addition of one extra monosaccharide unit, either N-acetylgalactosamine (for Blood group A) or just galactose (for blood group B) through an -1-3 linkage to a galactose moiety of the O-antigen.