carbohydrates

Question 1

definition of carbohydrates

Answer 1

is a poly hyroxy aldehyde or polyhydroxy ketone or a compound that yields such derivatives upon hydrolysis

Question 2

function of carbohydrates

Answer 2

-provide energy through oxidation
-supply carbon for the synthesis of cell components
-serve carbon as a stored form of chemical energy
-form part of the structural elements of some cells and tissues

Question 3

classification of carbohydrates

Answer 3

• monosaccharides
• disaccharides
• oligosaccharides
• polysaccharides

Question 4

monosaccharides definition

Answer 4

A monosaccharide is a carbohydrate that contains a single polyhydroxy aldehyde or polyhydroxy ketone unit.
Monosaccharides cannot be hydrolyzed further into simpler sugars.
Naturally occurring monosaccharides have from three to seven carbon atoms; five- and six-carbon species are especially common

Question 5

mono classification

Answer 5

Monosaccharides are further classified according to the number of carbon atoms they contain:
3- trios
4- tetrose
5- pentose
6- hexose

Question 6

examples of monosaccharides

Answer 6

-No. of Carbon
-Type of Sugar
-Aldoses
-Ketoses
3
Trioses (C3H6O3)
Glyceraldehyde (Aldotriose)
Dihydroxyacetone (Ketotriose)
4
Tetroses (C4H8O4)
Erythrose (Aldotetrose)
Erythrulose (Ketotetrose)
5
Pentoses (C5H10O5)
Ribose (Aldopentose)
Ribulose (Ketopentose)
6
Hexoses (C6H12O6)
Glucose, Galactose (Aldohexose)
Fructose (Ketohexose)
7
Heptoses (C7H14O7)
Glucoheptose
Sedoheptulose

Question 7

d-ribose and deoxyribose (mono)

Answer 7

-Two pentoses, ribose and deoxyribose, are extremely important because they are used in the synthesis of nucleic acids (DNA and RNA).
-D-Ribose is a component of a variety of complex molecules, including ribonucleic acids (RNAs) and energy-rich compounds such as adenosine triphosphate (ATP).
-2-Deoxy-D-ribose (along with phosphate groups) forms the long chains of deoxyribonucleic acid (DNA).
2-deoxyribose differs from ribose by the absence of one oxygen atom, that in the –OH group at C2.
Both ribose and 2-deoxyribose exist in the usual mixture of open-chain and cyclic hemiacetal forms.

Question 8

D-glucose (m)

Answer 8

-hexose glucose is the most important nutritionally and the most abundant in nature.
-Glucose is present in honey and fruits such as grapes, figs, and dates.
-Ripe fruits, particularly ripe grapes (20%–30% glucose by mass), are a good source of glucose, which is often referred to as grape sugar.
-Glucose is also known as blood sugar because it is the sugar transported by the blood to body tissues to satisfy energy requirements.
-the normal concentration of glucose in human blood is in the range of 70 –100 mg/dL.
-All tissues use glucose as a primary source of energy.
Erythrocytes and brains cells utilize glucose solely for energy.

Question 9

Oxidation to Produce Acidic Sugars

Answer 9

Monosaccharide oxidation can yield three different types of acidic sugars The oxidizing agent used determines the product:
-Aldonic acids: Oxidation of the aldehyde end of an aldose with mild oxidizing agent such as bromine gives an aldonic acid. the oxidation of the aldehyde end of D-glucose with bromine produces D-gluconic acid.

-Aldaric acids: Oxidation of both ends of an aldose (the aldehyde and the terminal primary alcohol group) with Strong oxidizing agent such as nitric acid produces a dicarboxylic acid.
Such polyhydroxy dicarboxylic acids are known as aldaric or saccharic acid. For glucose, this oxidation produces glucaric acid.

-Alduronic acid: In biochemical systems, enzymes can oxidize the primary alcohol end of an aldose such as glucose, without oxidation of the aldehyde group, to produce an alduronic acid.
For D-glucose, such an oxidation produces D-glucuronic acid.

Question 9

D-galactose (m)

Answer 9

-D-Galactose is seldom encountered as a free monosaccharide.
I-n the human body, galactose is synthesized from glucose in the mammary glands to produce lactose (milk sugar).
-D-Galactose is sometimes called brain sugar because it is a component of glycoproteins (protein–carbohydrate compounds) found in brain and nerve tissue.
-Like glucose, galactose is an aldohexose; it differs from glucose only in the spatial orientation of the – OH group at carbon 4.

Question 10

D-fructose

Answer 10

-D-Fructose, often called levulose or fruit sugar, occurs in honey and many fruits.
-Aqueous solutions of naturally occurring D-fructose rotate plane-polarized light to the left; hence the name levulose.
-D-fructose is present in honey in equal amounts with glucose.
-Like glucose and galactose, fructose is a 6-carbon sugar.
-However, it is a ketohexose rather than an aldohexose.
-In solution, fructose forms five-membered rings:
-Fructose is sweeter than sucrose and is an ingredient in many sweetened beverages and prepared foods.
-As a phosphate, it is an intermediate in glucose metabolism.
-Seminal fluid is rich in fructose and sperms utilize fructose for energy.

Question 11

reduction to produce sugar alcohols (M)

Answer 11

The carbonyl group present in a monosaccharide (either an aldose or a ketose) can be reduced to a hydroxyl group, using hydrogen as the reducing agent.

The resulting compound is one of the polyhydroxy alcohols known as alditols or sugar alcohols.
-For example, the reduction of D-glucose gives D-glucitol.
-D-Glucitol, also known by the common name D-sorbitol, is used as the sweetener.
-D-Sorbitol accumulation in the eye is a major factor in the formation of cataracts due to diabetes.

Question 11

test for the presence of an aldehyde group in a carbohydrate.

Answer 11

-Oxidation reactions are often used as a test for the presence of an aldehyde group in a carbohydrate.
-Benedict’s test and Fehling’s test both employ a solution of Cu2+ ions in aqueous base.
-When the carbohydrate is oxidized, the blue Cu2+ ion is reduced to Cu2O, which forms a brick red precipitate.

Question 12

sugar alcohols formed from mannose, fructose and galactose are:

Answer 12

D-Mannose- D-Mannitol
D-Fructose- D-Mannitol + D-Sorbitol
D-Galactose- D-Dulcitol
D-Xylose- D-Xylitol
Mannitol is frequently used medically as an osmotic diuretic to reduce cerebral edema.

Question 13

glycoscide formation (m)

Answer 13

-Cyclic monosaccharides (hemiacetals and hemiketals) readily react with alcohols in the presence of acid solution to form acetals and ketals, which are called glycosides.
-A glycoside produced from glucose is called a glucoside, that from galactose is called a galactoside, and so on.
-For example, glucose reacts with methanol to produce methyl glucoside.
-The bond between the anomeric carbon atom of the monosaccharide and the oxygen atom of the –OR group is called a glycosidic bond.
-Disaccharides and polysaccharides form as a result of glycosidic bonds between monosaccharide units.

Question 14

phosphate ester formation

Answer 14

-The hydroxyl groups of a monosaccharide can react with acids and derivatives of acids to form esters.
-The phosphate esters are particularly important because they are the usual intermediates in the breakdown of carbohydrates to provide energy.
-Phosphate esters are frequently formed by transfer of a phosphate group from ATP to give the phosphorylated sugar and ADP.
-For example, specific enzymes in the human body catalyze the esterification of the hemiacetal group (carbon 1) and the primary alcohol group (carbon 6) in glucose to produce the compounds glucose 1-phosphate and glucose 6-phosphate, respectively.

Question 15

Disaccharides

Answer 15

Disaccharides are sugars composed of two monosaccharide units linked together by the acetal or ketal linkages.
They can be hydrolyzed to yield their monosaccharide building blocks by boiling with dilute acid or reacting them with appropriate enzymes.
In disaccharide formation, one of the monosaccharide reactants functions as a hemiacetal, and the other functions as an alcohol.

Question 16

important disaccharides

Answer 16

Three important naturally occurring disaccharides are:
-Maltose
-Lactose
-Sucrose
They illustrate the three different ways monosaccharides are linked:
-By a glycosidic bond in the α orientation (maltose),
-A glycosidic bond in the β orientation (lactose), or
-a bond that connects two anomeric carbon atoms (sucrose

Question 17

occurance of Maltose (d)

Answer 17

Maltose, often called malt sugar, is present in fermenting grains and can be prepared by enzyme-catalyzed degradation of starch.

In the body, it is produced during starch digestion by α-amylase in the small intestine and then hydrolyzed to glucose by a second enzyme, maltase.

Question 18

chemistry of maltose

Answer 18

Chemically, maltose consists of two D-glucose units, one of which must be α-D-glucose.

The glycosidic linkage between the two glucose units is called an (1 → 4) linkage.

Maltose has one free hemiacetal group. Consequently, maltose exists in three forms:
-α-maltose,
-β-maltose, and
-open-chain form.
In solution, maltose exists as an equilibrium mixture of all the three forms.
In the solid state, the β form is dominant.

Question 19

lactose intolerance

Answer 19

-The genetic condition lactose intolerance is an inability of the human digestive system to hydrolyze lactose.
-In lactose-intolerant individuals, lactose remains in the intestines undigested rather than being absorbed, causing fullness, discomfort, cramping, nausea, and diarrhea.

Question 20

chemistry of lactose

Answer 20

-Chemically, lactose is made up of β-D-galactose unit and a D-glucose.
-The linkage between two sugar units is β (1→4) glycosidic linkage.

Question 20

properties of maltose

Answer 20

-Maltose is a white crystalline solid, with a melting point 160–165 °C.
-It is soluble in water and is dextrorotatory.
-Maltose is a reducing sugar because the hemiacetal group on the right unit of D-glucose is in equilibrium with the free aldehyde and can be oxidized to a carboxylic acid.
-It is also capable of exhibiting mutarotation.
-Hydrolysis of D-maltose produces two molecules of D-glucose.
-Acidic conditions or the enzyme maltase is needed for the hydrolysis to occur.

Question 20

occurance of lactose

Answer 20

-Lactose, or milk sugar, is the principal carbohydrate in milk.
-Human mother’s milk obtained by nursing infants contains 7%–8% lactose, almost double the 4%–5% lactose found in cow’s milk.
-Lactose is an important ingredient in commercially produced infant formulas that are designed to simulate mother’s milk.

Question 21

occurance of sucrose

Answer 21

-Sucrose is common table sugar, which is principally extracted from sugarcane and sugar beets.
-Sugar cane contains up to 20% by mass sucrose, and sugar beets contain up to 17% by mass sucrose.

Question 22

properties of lactose

Answer 22

-Lactose is a white, crystalline solid with a melting point 203°C and is also dextrorotatory.
-The equilibrium mixture has a specific rotation +52.5°.
-It is less soluble in water and much less sweet than sucrose.
-The presence of a hemiacetal group in the glucose unit makes lactose a reducing sugar.
-Lactose can be hydrolyzed by acid or by the enzyme lactase, forming an equimolar mixture of galactose and glucose.
-In the human body, the galactose so produced is then converted to glucose by other enzymes.

Question 22

invert sugar

Answer 22

-The term invert sugar comes from the observation that the direction of rotation of plane-polarized light changes from positive (clockwise) to negative (counterclockwise) when sucrose is hydrolyzed to invert sugar.
-The rotation is +66o for sucrose. The net rotation for the invert sugar mixture of fructose (-92o) and glucose (+52 o)is -40o.

Question 23

chemistry of sucrose

Answer 23

-The monosaccharide units that make up sucrose are α-D-glucose and β-D-fructose.
-The α C-1 carbon of the glucose is linked to the β C-2 carbon of the fructose in a glycosidic linkage that has the notation α,β(1 → 2).

Question 24

proporties of sucrose

Answer 24

-Sucrose is a white crystalline solid, soluble in water and with a melting point 180°C.
-When heated above its melting point, it forms a brown substance known as caramel.
-It is dextrorotatory and has a specific rotation of + 66.7°.
-Sucrose does not exhibit mutarotation.
-Sucrose is not a reducing sugar because both anomeric groups are involved in the glycosidic linkage.
-Sucrase, the enzyme needed to break the α, β(1 → 2) linkage in sucrose, is present in the human body. Hence sucrose is an easily digested substance.
-Sucrose hydrolysis (digestion) produces an equimolar mixture of D-glucose and D-fructose called invert sugar.
-Honeybees and many other insects possess an enzyme called invertase that hydrolyzes sucrose to invert sugar.
-Thus honey is predominantly a mixture of D-glucose and D-fructose with some unhydrolyzed sucrose.

Question 25

polysaccharides

Answer 25

A polysaccharide is a polymer that contains many monosaccharide units bonded to each other by glycosidic linkages.

Polysaccharides are often also called glycans.

Glycan is an alternate name for a polysaccharide.

Question 25

homopolysaccharides

Answer 25

A homopolysaccharide is a polysaccharide in which only one type of monosaccharide monomer is present.
The most important Homopolysaccharides are:
-Starch
-Glycogen
-Cellulose
Starch and glycogen are examples of storage polysaccharides, cellulose and chitin are structural polysaccharides.

Question 26

occurance of starch

Answer 26

-Starch is energy-storage polysaccharide in plants.
-It occur in plant cells usually as starch granules in the cytosol.
-Starch is fully digestible and is an essential part of the human diet.
-The major sources of starch are beans, the grains wheat and rice, and potatoes.

Question 27

chemistry of starch

Answer 27

Starch is a homopolysaccharide containing only α-D-glucose monosaccharide units.
Two different polyglucose polysaccharides can be isolated from most starches:
a- Amylose
b- Amylopectin.

Question 28

amylose

Answer 28

-Amylose, a linear glucose polymer, usually accounts for 15%–20% of the starch.
-In amylose’s non-branched structure, the glucose units are connected by (α1→4) glycosidic linkages.
-The number of glucose units present in an amylose chain depends on the source of the starch; 300–500 monomer units are usually present.

Question 29

classification of polysaccharides is based upon

Answer 29

1-The identity of the monosaccharide repeating unit(s) in the polymer chain:
-The more abundant polysaccharides in nature contain only one type of monosaccharide repeating unit.
-Such polysaccharides, including starch, glycogen, cellulose, and chitin, are examples of homopolysaccharides.
-Polysaccharides whose structures contain two or more types of monosaccharide monomers, including hyaluronic acid and heparin, are called heteropolysaccharides.

2-The length of the polymer chain:
-Polysaccharide chain length can vary from less than a hundred monomer units to up to a million monomer units.

3-The type of glycosidic linkage between monomer units:
-As with disaccharides, several different types of glycosidic linkages are encountered in polysaccharide structures.

4-The degree of branching of the polymer chain:
-The ability to form branched-chain structures distinguishes polysaccharides from the other two major types of biochemical polymers: proteins and nucleic acids, which occur only as linear (unbranched) polymers.

Question 31

glycogen overview

Answer 31

Glycogen is the glucose storage polysaccharide in humans and animals.
Glycogen, sometimes called animal starch, serves the same energy storage role in animals that starch serves in plants.
The largest amounts of glycogen are stored in the liver and muscles.
The total amount of glycogen in the body of a well-nourished adult human is about 350 g

Question 31

chemitsry of glycogen

Answer 31

Glycogen is a branched-chain polymer of α-D-glucose, and in this respect it is similar to the amylopectin fraction of starch.
Like amylopectin, glycogen consists of a chain of α(1→4) linkages with α(1→6) linkages at the branch points.
Branch points occur about every 8-12 residues in glycogen.
Glycogen is about three times more highly branched than amylopectin, and is much larger—up to one million glucose units per molecule.

Question 32

chemistry of cellulose

Answer 32

It is a linear homopolysaccharide of β-D-glucose, and all residues are linked in β(1→4) glycosidic bonds.
Typically, cellulose chains contain about 5000 glucose units.

Question 32

breakdown of cellulose

Answer 32

Humans and other animals lack the enzymes capable of catalyzing the hydrolysis of β-linkages in cellulose.
However, the intestinal tracts of grazing animals such as horses, cows, and sheep contain bacteria that produce cellulase, an enzyme that can hydrolyze β(1→4) linkages and produce free glucose from cellulose.
Despite its nondigestibility, cellulose is still an important component of a balanced diet.
It serves as dietary fiber.
Dietary fiber provides the digestive tract with “bulk” that helps move food through the intestinal tract and facilitates the excretion of solid wastes.

Question 33

cellulose overview

Answer 33

Cellulose is the major structural component of plants, especially of wood and plant fibers.
Cotton is almost pure cellulose(95%), and wood is about 50% cellulose.

Question 34

amylopectin

Answer 34

-Amylopectin, a branched glucose polymer, accounts for the remaining 80%–85% of the starch.
-Amylopectin has a high degree of branching in its polyglucose structure.
-The branch points involve α(1→6) glycosidic linkages.
-There are usually 24 to 30 D-glucose units, all connected by α(1→4) linkages, between each branch point of amylopectin.
-because of the branching, amylopectin has a larger average molecular mass than the linear amylose.
-Up to 100,000 glucose units may be present in an amylopectin polymer chain.

Question 35

breakdown of starch

Answer 35

-All of the glycosidic linkages in starch (both amylose and amylopectin) are of the α type.
-In amylose, they are all α(1→4); in amylopectin, both α(1→4) and α(1→6) linkages are present.
-Because starch molecules are digested mainly in the small intestine by α-amylase, which catalyzes hydrolysis of the α(1→4) linkages, starch has nutritional value for humans.
-Complete hydrolysis of both amylose and amylopectin yields only D-glucose.
-The most usual conformation of amylose is a helix with six residues per turn.
-Iodine molecules can fit inside the helix to form a starch–iodine complex, which has a characteristic dark-blue color.
-The formation of this complex is a well-known test for the presence of starch.

Question 36

heteropolysaccharides

Answer 36

A heteropolysaccharide is a polysaccharide in which more than one (usually two) type of monosaccharide monomer is present.
They are also called heteroglycans.

Question 37

Glycosaminoglycans (GAGs) or
Mucopolysaccharides

Answer 37

a GAG is an unbranched heteropolysaccharide, made up of repeating disaccharides.
The repeating disaccharide unit is generally composed of:
-An amino sugar which is almost always either an
N-acetylglucosamine or
N-acetylgalactosamine.
-A uronic acid residue which usually is D-glucuronic acid or in some cases it may be L-Iduronic acid.
In some glycosaminoglycans, one or more of the hydroxyls of amino sugar are esterified with sulphate, hence they are called sulphur-containing mucopolysaccharides.
Glycosaminoglycans whose sugar units are not sulphated are called sulphur-free mucopolysaccharides.
They are also called mucopolysacchride because they were first extracted from mucus.
The presence of sulphate and carboxyl groups contributes to acidity of molecules, making them acidic mucopolysaccharides

Question 38

types Glycosaminoglycans (GAGs) or
Mucopolysaccharides

Answer 38

The important mucopolysaccharides or GAGs include:
-Hyaluronic acid
-Chondroitin sulfate
-Dermatan sulfate
-Heparin
-Keratan sulfate

Question 39

occurance and composition of Hyaluronic Acid

Answer 39

Occurrence:
Hyaluronic acid is an important GAG found in the ground substance of synovial fluid of joints, and vitreous humour of eyes.
It is also present in the umbilical cord and forms the gel around ovum.

Composition:
Hyaluronic acid is composed of repeating units of D-glucuronic acid and N-acetylglucosamine linked together by β(1→3) glycosidic linkages.
It contains up to 25,000 repeats of basic disaccharide units held by β(1→4) linkages.

Question 40

hydrolysis of hyaluronic acid

Answer 40

It yields equimolecular quantities of D-glucosamine, D-glucuronic acid, and acetic acid.
Hyaluronidase, an enzyme secreted by some pathogenic bacteria, can hydrolyze the glycosidic linkage of hyaluronic acid, rendering tissues more susceptible to bacterial invasion.
Hyaluronidase of semen hydrolyzes the outer polysaccharides coat of the ovum allowing a better penetration of sperm into the ovum.

Question 41

Chondroitin Sulphate occurance and compoisiton

Answer 41

-occurrence:
It is present in ground substance of connective tissues widely distributed in cartilage, bone, tendons, cornea and skin.

-Composition:
Chondroitin sulphate contains repeating units of D-glucuronic acid and N-acetylgalactosamine linked together by β(1→3) glycosidic linkages.
The repeating units in turn are joined together by β(1→4) linkages.

Question 41

fuction of hyluronic acid

Answer 41

Hyaluronic acid forms clear, highly viscous solution that serves as lubricants in the synovial fluid of joints and give the vitreous humour of eye its jelly-like consistency.

Question 42

types of chondroitin sulphate

Answer 42

Types:
There are two types, one sulphated on the C-4 of the N-acetylgalactosamine residue and other at C-6.
-Chondroitin A is Chondroitin-4-Sulphate.
-Chondroitin C is Chondroitin-6-Sulphate.

Question 43

function of chondroitin sulphate

Answer 43

Function:
They are required for the synthesis of osteocytes and cartilage.
It is required for the transparency of cornea.

Question 44

Dermatan Sulphate occurance and composition

Answer 44

Occurrence
Dermatan sulphate is found in skin, blood vessels, heart valves and tendons.
The dermatan sulphate is also known as chondroitin sulphate B.
Composition
It is structurally related to chondroitin 4-sulphate.
The only difference is that there is an inversion in the configuration around C5 of D-glucuronic acid to form L-Iduronic acid.

Question 45

keratan sulphate occurance and composition

Answer 45

Occurrence:
It is present in cornea, cartilage and intervertebral disc.
Composition:
Keratan sulphate is composed of repeating units of D-galactose and N-acetylglucosamine-6-sulphate linked by β(1→4)linkages.
The repeating units are in turn joined together by β(1→3) linkages.
It is the only GAG, which doesn’t contain any uronic acid.

Question 46

heparain occurance and compostion

Answer 46

Occurrence:
Heparin is present in liver, lungs, spleen and monocytes.
Chemistry:
It contains repeating units of sulphated glucosamine and D-glucuronic acid joined together by α(1→4) glycosidic linkages.
Heparin is a small polysaccharide with only 15–90 disaccharide residues per chain.

Question 47

heparin function

Answer 47

Function:
The best known of heparin’s biochemical functions is that of an anticoagulant; it helps prevent blood clots.
It binds strongly to a protein involved in terminating the process of blood clotting, thus inhibiting blood clotting.