- Carbohydrates may be defined as polyhydroxy aldehyde or ketones or compounds which produce them on hydrolysis.
- They serves as the major source of chemical energy for living organism (e.g. sugar and starch), as well as important constituents of supporting tissue (e.g. cellulose).
- It is the most abundant organic molecules composed of carbon, hydrogen and oxygen.
- Beside these elements carbohydrate may contain other elements like nitrogen, phosphorous, Sulphur etc.
- Carbohydrate literally obtain from “hydration of carbon”.
- The empirical formula of carbohydrate is (C.H2O)n.
- Carbohydrate contributes a major role in our food source and they are abundantly or sufficiently found both animal and plant source.
Functions of carbohydrates
- Energy supply for body function.
- Carbohydrates are precursors for many organic compounds (fats, amino acid).
- Participate cellular function of cell growth and fertilization
- Carbohydrate also serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body.
- Some carbohydrate molecules are used to inactivate drugs or toxic substances. Example: Glucoronic acid.
- Carbohydrates also participate in the formation of large molecules like lipids and amino acid.
- Some carbohydrates act as catalyst or promotors for biochemical reaction.
Classification of carbohydrates
- Monosaccharide (One sugar molecule) [Glucose, Fructose, Galactose]
- Disaccharide (Two sugar molecules) [Sucrose, Lactose, Maltose]
- Oligosaccharide (2-10 sugar molecules) [Raffinose, Stachyose]
- Polysaccharide (10 or more sugar molecules) [Starch, Glycogen, Cellulose]
- Derived carbohydrates
- Oxidation and reduction products
- Amino and deoxy sugar
- Monosaccharides (having one saccharide or sugar unit) are the simplest group of carbohydrates and are often referred to as simple sugars.
- Once they attempted hydrolysis is not cleaved to smaller carbohydrate e.g. glucose
- General formula Cn (H2O) n , Ratio 1:2:1.
- Monosaccharides are classified according to:
- The number of carbon atom present in the molecule.
- Whether they contain an aldehyde or ketone group.
- It is crystalline solids.
- Very soluble in water.
- Most have sweet in taste.
- When functional group in monosaccharide is aldehyde called aldose.
- Example: Glyceraldehyde, Glucose.
- When functional group is a keto group are called ketoses.
- Example: Dihydroxyacetone, Fructose
Classification of monosaccharides
|Monosaccharide (empirical formula)||Aldose||Ketose|
|Trioses (C3H6O3) |
- The aldehyde group present in monosaccharide always attached to the any of the terminal carbon atoms whereas the functional group of ketone always present at any carbon atom except the terminal carbon atoms.
Structure of monosaccharide
- When some molecular formula represent two or more compounds simultaneously those are differ from one another by their physical and chemical properties then such compounds are called as isomers and the phenomenon is called as isomerism.
- To show the property of isomerism a carbohydrate compound must contain one or more asymmetric carbon atom or chiral carbon atom.
- An asymmetric carbon atom is one which is attached to four different groups or atoms.
- The number of isomers that can be formed by a carbohydrate compound can be calculated by the formula 2n, where n = number of isomers as it contains four asymmetric carbon atoms. Out of these 16 isomers. 8 are of D-form and 8 are of L-form.
- Carbohydrate can show two types of isomerism;
- Stereo isomerism (Geometrical isomerism)
- Optical isomerism
- When same molecular formula represents two or more compound simultaneously which are differ in their spatial arrangements then such compounds are called as stereo isomerism.
- The carbohydrates starting from triose can show stereo isomerism as they contain asymmetric carbon atom.
- Here, the glyceraldehyde compound contain one asymmetric carbon atom cam form two stereo isomers namely D-Glyceraldehyde (-OH group on right hand side) and L-Glyceraldehyde (-OH group on left hand side).
- Both these isomers are mirror images of one another.
- When a beam of plane polarized light is passed through a solution exhibiting optical activity, the beam of light may rotates towards right or left.
- It turns towards right, the monosaccharide taken in the solution is said to be D-form or assigned by positive sign (D-form/+), on the other hand if the beam of light rotates towards left then the monosaccharide taken in the solution L-form or assigned as negative sign (L-form/-).
- When a solution contain equal quantity of D and L form of isomerism than its optical activity would be zero. Such mixture called as racemic mixture.
Ring structure of D-Glucose
- In open chain D-Glucose the carbon atom containing aldehyde group can be condensed with any of the carbon atom containing hydroxyl group of the same molecule and form a ring containing oxygen.
- Here two changes occur in D-Glucose.
- An additional hydroxyl group is formed in the carbon atom containing aldehyde group.
- A ring contain oxygen is formed in between the aldehyde carbon and the carbon atom whose one of the hydrogen atom was migrated to form new hydroxyl group.
- The alpha and beta form of the D-glucose are called alpha-anomer and beta-anomer respectively.
- The ring structure of D-glucose the first carbon atom becomes asymmetric and ensures two different anomers (isomer).
- Hence, the first carbon atom is called as anomeric carbon atom.
- In course of time, the optical rotation may change and attains an equilibrium condition. This change in optical rotation is called as Mutarotation.
- Example: alpha-D-Glucose (+1120), D-glucose (52.50), beta-D-Glucose (+190)
- Freshly prepared alpha-D-glucose solution has an optical rotation of +1120 which gradually decreases and attains an equilibrium with optical rotation 52.50. On the other hand freshly prepared beta-D-glucose solution has optical rotation +190 which graducally increases and attains an equilibrium with optical rotation 52.50.
- From the above example it is clear that in equilibrium condition D-glucose may remain in three forms that are alpha-D-glucose, bet-D-glucose and open chain D-glucose.
Chemical properties of monosaccharide
- Reaction due to aldehyde or ketone
- Formation of osazone
- Formation of cynohydrin
- Reaction due to hydroxyl (-OH) group
- Formation of ester
- Formation of glycoside
- Formation of furfural
Reaction due to aldehyde or ketone group
- When a sugar solution is heated with phenyl hydrazine in boiling water bath in presence of sodium acetate the corresponding osazone is formed.
- Example: Glucose on treatment with phenyl hydrazine gives rise to glucosazone.
Formation of osazone:
- This an identification test performed to known whether a carbohydrate compound is reducing or non-reducing.
Formation of cyanohydrin
- When a monosaccharide is treated with hydrogen cyanide the corresponding cyanide forms.
- A cyanohydrin on hydrolysis gives two sugar acids. These sugar acids on reduction gives two sugar molecules each containing one extra carbon atom.
- Under high pressure monosaccharide gets reduced to sugar alcohols.
- Depending upon the types of oxidizing agent taken monosaccharides undergoes oxidation to give different product.
- When bromine water is taken as oxidizing agent: the aldehyde group of monosaccharide is oxidizing to give the corresponding carboxylic acid.
- Example: Glucose on oxidation in presence of bromine water gives gluconic acid.
- When hydrogen peroxide is taken as oxidizing agent: the primary alcohol group of monosaccharide is oxidized to carboxylic acid.
- It is a stereo isomerism when two molecular formula exist in dynamic equilibrium. So that the amount of one substance is changed.
- On treatment with dilute solution of an alkali the monosaccharides undergoes a molecular change.
- Here, one of the hydrogen atoms migrates from one carbon atom to another to form a mixture of enolic compound
Reactions due to hydroxyl group
Formation of esters
- Monosaccharide on treatment with phosphoric acid in presence of magnesium as the catalyst the alcohol group is esterified to give the corresponding ester.
- The reaction may be enzymatic or non-enzymatic in nature.
Formation of glycosides
- When a sugar molecule is heated with on alcohol in presence of HCl as the catalyst, the corresponding glycoside is formed.
- The new bond so formed is known as the glyosidic bond.
- So, a glycoside contain a sugar portion and a non-sugar portion in the same molecule. Here the non-sugar portion is called as aglycon.
- Example: when D-Glucose is heated with methyl alcohol in presence of HCl as the catalyst then the corresponding glycoside is formed.
Formation of Furfural (Dehydration)
- When monosaccharide is treated with concentrated sulphuric acid, it undergoes a dehydration reaction with the elimination of three water molecules.
- Furfural on treatment with phenolic compound like alpha-napthol get condense into a colored compound.
- This test is used as a common identification test for carbohydrate which is popularly known as Molish’s test.
- The carbohydrate compound containing 2-10 sugar units are called as oligosaccharide.
- These are the carbohydrate molecules that gives two sugar units on hydrolysis.
- The disaccharide the two sugar unit are held with each other by glycosidic bond.
- The glycosidic bond in a disaccharide is generally formed in between first carbon atom of one monosaccharide to the second carbon atom of another monosaccharide.
- If the bond is formed in between reducing groups of both monosaccharide units then the resultant disaccharide is non-reducing in nature, because both the reducing property. Hence, it can’t form osazone also.
- Example: Sucrose
- On the other hand if the aldehyde or ketone group of any of the monosaccharide is free (not involved in bond formation) then, the resultant disaccharide can show reduction and it can form osazone also.
- Example: Lactose and maltose.
- The reducing property of sugar is generally inversely proportional to the numbers of monosaccharide unit they have.
- Generally the disaccharides are crystalline water soluble substance having sweet in taste.
Sucrose (Can sugar or table sugar)
- Sucrose contains one glucose and one fructose molecules which are held with each other by (α1, β2) glycosidic linkage.
- As the aldehyde group of α-D-Glucose and ketone group of β-D-Fructose both are involved in bond formation, so no any reducing group is left free. Hence sucrose can’t show reduction and it can’t form osazone.
- Sucrose is the photosynthetic product of plant generally stored in root, tuber, stem etc.
- It is used as sweeting agent in pharmaceutical industry or food industry.
- Sucrose is dextrorotary in nature which on hydrolysis gives one glucose molecules (Levorotatory) and one fructose molecule (Dextrorotatory)
- Induction of levorotation in presence of fructose (during the hydrolysis of sucrose) is called as inversion of sugar. So, here glucose and fructose are called as inverted sugar.
- Maltose is composed of two α-D-Glucose units which are linked with one another by α(1-4) glycosidic linkage
- In maltose the glycosidic bond formed is in between first carbon atom of α-D-Glucose to the fourth carbon atom of another α-D-Glucose. So, the reducing group (aldehyde group of second α-D-Glucose is free). Therefore, this reducing group ensures the reducing property of maltose. Hence maltose is a reducing sugar and it can form osazone also.
- It is used in food industry as a flavoring agent.
- Starch on treatment of enzyme amylase at breakdown into glucose units. So, maltose is also called as the intermediate product of starch digestion.
- Is popularly known as milk sugar which is composed of one β-D-Galactose and α-D-Glucose unit. Both these monosaccharide are held with one another by 1-4 glycosidic linkage. Therefore, the reducing group of glucose unit is free for reduction and hence it is a reducing sugar.
- It is used as the principal element of carbohydrate nutrition
- Polysaccharides are polymers of monosaccharide units with high molecular weight.
- They are usually tasteless (Non-sugars) and form colloids with water.
- Polysaccharide are up two types Homopolysaccharide and Heteropolysaccharide.
- The polysaccharide which on hydrolysis gives one type of monosaccharide units is called Homopolysaccharide.
- Example: Starch, Cellulose, Glycogen, Insulin, Dextrin, Chitin
- The polysaccharides which gives more than one types of monosaccharide units on hydrolysis are called Heteropolysaccharide.
- Example: Heparin, Glucoronic acid.
Example of Homopolysaccharide
- It is also called as plant carbohydrate which form dietary source of other living organisms.
- It is a homopolymer of D-glucose unit held with each other by glycosidic bond.
- Again starch contain two polysaccharides units
- Amylose contribute about 15-20% of total starch molecules, whereas amylopectin contributes about 80-85% of total starch molecule.
- It is a long straight chain polymer having 200-1000 units of D-glucose which are held with each other by α-(1-6) glycosidic linkage.
- Amylose are usually used for the identification test for carbohydrate.
- It is a branch chain polysaccharide with α-(1-6) glycosidic bond at the branch point, whereas α-(1-4) glycosidic bond are present at straight chain.
- Amylopectin gives radish violet color with iodine solution which is used for the identification test for starch.
- It is a Homopolysaccharide generally present in fibrous part of the plant
- It is predominant part of cell wall.
- Cellulose composed of β-D-glucose units linked by β-(1-4) glycosidic linkage.
- Cellulose are not sweet in taste and does not shows hydrolysis.
- Usually cellulose cannot be digest by human body because the enzyme responsible for its digestion is absent. So patient suffering from constipation are generally prescribed cellulose like Isobgul, Wheat, Barry.
- It is also known as animal carbohydrate as it us the storage form of carbohydrate in animals.
- It is a high molecular weight compound and it sufficiently found in liver, muscle, brain etc.
- It is a homopolymer of glucose having up to 25000 of glucose unit.
- Linkage consist of α-(1-4) glycosidic linkage (Main point) and α-(1-6) glycosidic linkage (branch point).
- Dextrins are the breakdown products of starch by the enzyme amylase or dilute acids.
- Linkage α-(1-4) and α-(1-6) glycosidic bond.
- Dextrins are used as plasma substituent in case of severe blood loss (retain water in the circulation for a longer period of time).
- Inulin is a polymer of fructose i.e., fructosan.
- Found in dahlia bulbs, garlic, onion etc.
- It has low molecular weight (about 5000) polysaccharide.
- Chitin is composed of N-acetyl D-glucosamine units held together by β (1-4) glycosidic bond.
- It is found exoskeleton of some invertebrates e.g. insects, crustacean.
- When different types of sugar units and their derivatives are composed to form polysaccharide they are referred to as Heteropolysaccharide or heteroglycan.
- It is a heteroglycan made up of several units of amino sugar and uronic acid.
- These are commonly known as glycosaminoglycan.
- Some Mucopolysaccharide is composed by combination of protein and form mucoproteins or mucoids or proteoglycans
- Mucoproteins may contain up to 95% carbohydrate and 5o% protein.
- It an essential component of tissues structure (connective tissue cartilage, blood vessels, skin, tendons), consist of collagen and elastin.
- It is a Heteropolysaccharide containing repeating units of N-acetyl D-glucosamine and D-glucoronic acid.
- Both these units are held with α-(1-3) glycosidic linkage.
- Hyaluronic acid is sufficiently found in connective tissue like cartilage, umbilical cord, synovial fluid etc.
- It acts as lubricant in joints.
- It is also a Heteropolysaccharide containing repeating units of N-sulphated glucosamine along with D-glucoronic acid which are held with one another by α-(1-4) glycosidic linkage.
- It is sufficiently found in liver, lungs, spleen and walls of large arteries.
- It is an anticoagulant and helps in releasing enzyme lipoprotein lipase.
Chondroitin sulfate (condones-cartilage)
- A Mucopolysaccharide containing repeating units of N-acetylgalactosamine along with D-glucoronic acid which are held with one another by α-(1-3) glycosidic linkage.
- Generally human body contains three types of chondroitin sulfate.
- Chondroitin sulphate “A” – cartilage, tendon, bone
- Chondroitin sulfate “B” – skin
- Chondroitin sulfate “C” – cartilage, tendons.
Disease related to carbohydrate
- Diabetic mellitus
- a) Hyperglycemia
- It is a chronic condition that characterized by increase blood glucose level due to abnormal metabolism of carbohydrate.
- Polyphagia – increase hunger or appetite
- Polydipsia – increase thrust
- Polyuria – increase urine output.
- Absorption of glucose from intestine is increased.
- Oxidation and utilization of glucose is decrease.
- Formation of glycogen in liver is decrease.
- Increase glycogen break down in liver.
- Deficiency of insulin which regulate the carbohydrate metabolism.
- Insulin is synthesized from β-cell of islet of Langerhans and responsible for absorption of glucose in blood.
- Neuropathy (damage or dysfunction of one or more nerve)
- Retinopathy (damage to the retina of the eyes)
- Nephropathy (kidney damage)
- Weight loss
- Intake of low carbohydrate diet.
- Adequate physical exercise.
- Taking oral anti-diabetic medication.
- Insulin injection if necessary.
- b) Hypoglycemia
- Decrease sugar level in blood due to abnormal metabolism of carbohydrate.
- Increase insulin production (may be due to tumor in pancreases)
- Decrease secretion of thyroid and adrenal gland hormone.
- Taking low carbohydrate containing food.
- Hunger and headache – lack of concentration
- Sweetening (increase heart rate) – convulsion (involuntary movement of muscle).
- Weight loss
- Provide carbohydrate diet.
- Conversion of glucose from glycogen called glycogenesis.
- Pancreases – Glucagon – Glycogen to Glucose – Increase in BSL
- Conversion of glucose to glycogen called glycogenesis.
- Pancreases – Insulin – Glucose to Glycogen – Decrease in BSL
- a) Hyperglycemia
- A condition of presence of glucose or sugar in urine.
- Elevated blood glucose levels (Diabetic mellitus).
- Renal glycosuria (problem with reabsorption of glucose in kidney tubules).
- Hyper glycaemia glycosuria
- Abnormal high or low level of glucose in body.
- Renal glycosuria
- It is caused due to defect in tubular reabsorption.
- Elementary glycosuria
- It is caused due to excessive intake of sugar or carbohydrate (Temporary condition).
- Hyper glycaemia glycosuria
- Lactose intolerance
- It is caused by deficiency of lactose enzyme in the body.
- Fructose intolerance
- It occurs when intestinal cells unable to break down fructose efficiently.
- It occurs when enzyme galactose – 1 – phosphate uridyltransferase (GALT) is missing or not functioning.
- It is rare genetic metabolic disorder that affect metabolism of sugar galactose properly.