Download TOPIC I CARBOHYDRATES DEFINITION, CLASSIFICATION AND

TOPIC I
CARBOHYDRATES
DEFINITION, CLASSIFICATION AND FUNCTIONS
A carbohydrate is a chemical compound, mostly plants products in origin and made up of elements
carbon, hydrogen and oxygen. Carbohydrates are a major source of energy for us. They are formed by all
green plants through the process photosynthesis. They are either small single units’ molecules or larger
units consisting of two several units or still larger and more complex ones consisting of many units linked
together. Those that have special significance in nutrition are the simple sugars (monosaccharide), the
double sugars (disaccharides) and the more complex sugars (polysaccharides) and dietary fibre.
Carbohydrates, together with lipids, proteins and nucleic acids, are one of the four major classes of
biologically essential organic molecules found in all living organisms. Carbohydrates, all coming from
the process of photosynthesis, represent the major part of organic substance on Earth, are the most
abundant organic components in the major part of fruits, vegetables, legumes and cereal grains, carry out
many functions in all living organisms and are the major energy source for humans in Western diet of
Mediterranean type. Finally, they provide flavor and texture in many processed foods.
Chemical classification of carbohydrates
Carbohydrates, many of them, but not all, have general formula (CH 2O)n (only molecules with n>4 are
considered carbohydrates); some, in addition to carbon (C), oxygen (O) and hydrogen (H), include
nitrogen or sulfur.
On the basis of the number of forming units, three major classes of carbohydrates can be defined:
monosaccharides, oligosaccharides and polysaccharides. The term “saccharide” derives from the greek
word “sakcharon”, which means sugar.
a) Monosaccharide (simple sugars)
A monosaccharide is a single “sugar” unit. Monosaccharide cannot be broken down into simpler sugar
units. They include:
Glucose from corn syrup, fruits, vegetables and honey
Fructose from fruit, honey
Galactose from digestion of milk sugar
www.naarocom.com
Page 1
Characteristics of monosaccharide





White, water soluble solids that tend to dissolve in water.
They have a sweet taste
Caramelize(melt and harden) when heated
Can be broken down when combined with water and heated. this process is called hydrolization
Are referred to as empty calories because they provide the body with energy and no other
nutrient.
Monosaccharides are also classified based on the number of carbon atoms. A six carbon monosaccharide
is known as a hexose; a five carbon monosaccharide is known as a pentose four carbon-pentose, three
carbons -trioses. A monosaccharide that contains six carbon atoms and aldehyde functional group is
known as an aldohexose. Glucose is an aldohexose. Fructose is a ketohexose.
Monosaccharides are classified based on their chemical reactivity. Sugars that react with mild oxidizing
agents such as Cu2+ ions are known as reducing sugars. All aldoses and ketoses are reducing sugars.
b) Disaccharides
They contain two monosaccharide units joined together .Disaccharides are white, water soluble solids.
They include:
Sucrose -found in sugar cane and consists of (1 glucose + 1 fructose).
Lactose is found in milk and consists of (1 glucose + 1 galactose) units linked together.
Maltose is found in germinating grains and digestion of starch. It consists of two glucose units linked
together.
Energy
C H O+C H O
Glucose
fructose
C H O + H O
enzyme
sucrose
c) Polysaccharides
They are complex compounds containing many units of monosaccharide linked together.
They are formed by a combination of more than two molecules of a monosaccharide.
They are all made up of glucose molecules linked in different arrangements.
Monosaccharide + Monosaccharide → Disaccharide + H 2O
Forms of polysaccharides
i.
Starch: it’s the storage form of carbohydrate in plants and comprises the primary source of
energy in the diet. Cereals grains seeds , roots, tubers contain large quantities of starch.
www.naarocom.com
Page 2
ii.
Resistant starch: this includes all the starch that escapes digestion in the small intestines.
It is the sum of starch and the products of starch degradation not absorbed in the small
intestine of healthy individuals. There are three forms of resistant starch:
 Starch that is physically inaccessible to digestive enzymes owing to enclosure in
iii.
food structures such as intact cells or partially milled or whole grain seeds.
 Resistant type starch granules in occurring uncooked potatoes and green bananas.
 Retrograded amylase occurring on processed foods.
Glycogen: it’s an animal starch stored in all animals. Since it is rapidly broken down,
iv.
ordinarily the diet contains none.
Cellulose and hemicellulose: they are found in skins of fruits and as structural part of all
plants. They are important for nutrition since they make up a large portion of dietary fibre.
Although they are indigestible by the human digestive system they help in increasing the
bulk of intestinal content and stimulating peristaltic movement
Characteristics of polysaccharides
 They do not dissolve in water
 Have no sweet taste
 When subjected to dry heat they form a brown coloured compound dextrin through

Dextrinization
When subjected to moist heat they absorb the water, swell and burst open releasing the enclosed starch
granules in a process called gelatinization.
Functions of carbohydrates

They are a rich source of energy- 1gm =4kcal. Excess is changed into fat and stored in the
adipose tissue as
Starch and glycogen, respectively in plants and animals. The stored carbohydrates can in turn be
mobilized for energy production. Glucose can supply energy both fueling ATP synthesis (ATP, the
cell’s energy currency, has inside a phosphorylated sugar) and in the form of reducing power as
NADPH.
It should be noted that glucose, used as energy source, “burns” without yielding metabolic
This is a SAMPLE (Few pages have been extracted from the
complete notes:-It’s meant to show you the topics covered in the full
notes and as per the course outline
www.naarocom.com
Page 3
Download more at our websites:
www.naarocom.com
To get the complete notes either in softcopy
form or in Hardcopy (printed & Binded)
form, contact us on:
Call/text/whatsApp +254
719754141/734000520
Email:
[email protected]
[email protected]
[email protected]
Get news and updates by liking our page on
facebook and follow us on Twitter
Sample/preview is NOT FOR SALE
www.naarocom.com
Page 4