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Biology for Advanced Level Secondary Schools

including mammals. Carbohydrates contain polyhydroxy ketones with a ketone group
three elements, namely carbon, hydrogen, (C=O), such as dihydroxyacetone, ribulose,
and oxygen in which hydrogen and oxygen and fructose sugars. Carbohydrates can
are in the ratio of 2:1 (two hydrogen atom be oxidised to yield energy. For example,
and one oxygen atom). This ratio or during respiration glucose is oxidised to
proportion of hydrogen to oxygen is the produce energy.
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same as in water, hence the name hydrate
of carbon. The general empirical formula C H O + 6O 6 6CO + 6H O + Energy
2
6
2
6
12
for carbohydrates is C (H O) , where by
x
y
2
x and y are variables. Aldoses and ketoses are reducing
compounds, as they have a tendency of
Properties of carbohydrates reducing Copper (II) in Benedict’s solution
They are either simple sugars or compound into Copper (I), which precipitates as a red
sugars. The latter are formed by condensation solid substance of Copper (I) oxide.
of the former. For example; starch is
formed by condensation of several glucose Classes of carbohydrates
units. They have one hydrated carbon Carbohydrate can be classified according
(CH O), hence the name carbohydrates. to the number of basic sugars or saccharide
2
The ratio of hydrogen to oxygen in a units present in a molecule. These classes
carbohydrate molecule is always 2:1. They include the following:
are derivatives of polyhydroxyl alcohols (a) Monosaccharides
and can be polyhydroxy aldehydes with (b) Disaccharides
-
an aldehyde group H-C=O, examples (c) Polysaccharides
are glyceraldehyde, ribose, glucose,
and galactose sugars. They can also be

Table 1.2: Major types of carbohydrates and their sources

Type Examples Sources
Monosaccharides Fructose Sweet fruits and honey
Glucose Fruits, such as grapes, coconut water, and sweet potatoes

Galactose Milk and dairy products
Disaccharides Sucrose Sugar cane and carrots

Lactose Milk
Maltose Malt, such as radicles of germinating cereals
Polysaccharides Starch Yam, irish potatoes, sweet potatoes, and green banana

Cellulose Cell walls of various plant cells
Chitin Cell walls of fungal hyphae, and the exoskeleton of
arthropods such as insects, crabs, and prawns

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