Lesson 5: Carbohydrates

Question 1

Carbohydrates

Answer 1

Organic molecules commonly referred to as sugars.

Made up of carbon, hydrogen and oxygen. The general formula is Cx(H2O)y

There are two main types of carbohydrates

Monosaccharides

Polysaccharides

Question 2

Monosaccharides

Answer 2

Monosaccharides are the simplest form of carbohydrates.

Contain a carbonyl group and at least two hydroxyl groups.

Empirical formula is CH2O.

Examples include glucose, fructose and galactose.

Question 3

Hexose Sugars

Answer 3

All contain six carbon atoms and are
called hexoses.

Have low molar mass, are sweet,
soluble in water and have a crystalline
structure.

Monosaccharides with aldehyde groups
are called reducing sugars since they
are strong reducers (easily oxidized)

All hexose sugars have the molecular
formula C6H12O6. However, there are
many isomers.

Question 4

Straight Chain and Cyclic Structure

Answer 4

The previous slide contained the
straight chain structure of three
monosaccharides.

However, in aqueous solution
monosaccharides adopt a ring
structure.

This occurs as a result of an
intermolecular reaction between
the aldehyde group on C1 and
the OH group on C5
The intermolecular reaction that causes the ring structure
to form in aqueous solution produces an asymmetric
carbon at C1.

Therefore, there are two ring structure isomers of glucose
called α-glucose and β-glucose.

The difference has an effect on the properties of their
polymer

Question 5

Condensation
Reactions

Answer 5

Monosaccharides undergo condensation reactions in order
to form disaccharides and polysaccharides.

Hydroxide groups on adjacent monosaccharides react to
form a bond, known as a glycosidic link, and a molecule of
water.

Two molecules of α-glucose condense to form a 1-4
glycosidic link which produces the disaccharide maltose.

Question 6

Disaccharides

Answer 6

Disaccharides are composed of two monosaccharides joined together.

Combining different monosaccharides produces different disaccharides.

Disaccharides are soluble molecules that can be broken down into their
component monosaccharides through acid hydrolysis or by enzyme
catalysed reactions (digestion)

Question 7

Polysaccharides

Answer 7

Polysaccharides are long chains of monosaccharides held together by
glycosidic bonds.

Have large molar mass, are not sweet, are insoluble or slightly soluble in
water and are non-reducing.

Polysaccharides differ in the nature of their recurring monosaccharide, their
bonds, the length of their chains and the degree of branching.

Question 8

Glucose Based Polysaccharides

Answer 8

There are three common glucose based polysaccharides.

Starch

Glycogen

Cellulose

Question 9

Starch

Answer 9

Starch is a polymer of α-glucose and is the
main form of carbohydrate storage in
plants.

Starch can form a compact spiral structure
and is stored in the starch granules of
plants.

There are two forms of starch

Amylose – a straight chain polymer of
α-glucose with 1-4 glycosidic bonds.

Amylopectin – a branched polymer of
α-glucose with both 1-4 and 1-6
glycosidic bonds.

Question 10

Glycogen

Answer 10

A polymer of α-glucose.

Main form of carbohydrate storage
in animals, found in the liver and
muscle tissue.

Similar to amylopectin but with
more 1-6 glycosidic branches.

Question 11

Cellulose

Answer 11

A polymer of β-glucose.

It is the structural material in the cell
walls of plants

A linear polymer with 1-4 β-glycosidic
linkages

This forms an uncoiled structure with
alternate glucose molecules upside
down.

This allows cellulose to form cables
known as microfibrils that give a rigid
structure.

Question 12

Digestion

Answer 12

Polysaccharides are broken down into their monosaccharide units during
the digestive process.

The human body has enzymes that digest starch and glycogen through a
series of enzyme controlled reactions (hydrolysis) during which the
glycosidic links are broken.

The human body does not produce enzymes that work on cellulose.

Question 13

Dietary Fibre

Answer 13

Due to the fact that the human body does not produce cellulase, cellulose
will pass through the digestive system chemically intact.

Cellulose and other substances that pass through the digestive system
intact are called dietary fibre.

Dietary fibre is helpful in preventing conditions such as constipation and
hemorrhoids and IBS (irritable bowel syndrome).

The Fibrils abrade the walls of the digestive tract causing it to produce more
mucus which helps smooth the passage of undigested food through the gut.