Carbohydrates

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Table of Contents

Carbohydrates 4

2005, 2007, 2012 Q1.B 4

Disaccharides 6

Polysaccharides 9

Classification 10

Food source 10

Chemical Structure 10

Energy Value 10

NSP 11

Glycaemic Index 13

Carbohydrates

2005, 2007, 2012 Q1.B

Plants have the ability to make their own food by a process called photosynthesis.

  • H2O is absorbed via the roots.
  • CO2 (from air) is absorbed visa stomata in the leaves.
  • Sunlight (energy) is absorbed by chlorophyll in the leaves.

Result

  • The plant produces food (C6H12O6)
  • The plant food is also known as simple sugars called monosaccharides.
  • They are hexose sugars

Chemical Equation for Photosynthesis

6CO2 + 6H2O

sunlight

> C6H12O6 + CO2

chlorophyll

Elemental Composition of Carbohydrates

Carbo

hydrates

Water

C

H O

H:O = 2:1

The most common monosaccharide is Glucose. It has the following structure

Note: The only difference between monosaccharides is their molecular arrangement (i.e. the position of Carbon, Hydrogen and Oxygen differs with each hexose sugar.)

Monosaccharides are also known as simple sugars (they cannot be broken down any further and they are absorbed easily by the villi in the small intestine).

Disaccharides

In some plants the products of photosynthesis (i.e. monosaccharides) are arranged in pairs. Two monosaccharides link together.

As the chemical link is formed one molecule of water is eliminated in a condensation reaction (H from one monosaccharide and OH – hydroxyl group from the other monosaccharide.

Chemical formula for disaccharides C12H22O11 (Note: H:O=2:1)

C6H12O6 + C6H12O6 - H2O

C12H24O12

Example of Disaccharide

Formation

Source

Sucrose

Table Sugar

Lactose

Milk

Maltose

Germinating barley

= Glucose

= Galactose

= Fructose

Formation of Maltose

  • Two glucose units (monosaccharides) link together.
  • H2O (one molecule of water) is eliminated as H from one glucose joins with OH from the second glucose, i.e. condensation reaction.

H=Hydrogen

OH= Hydroxyl group.

Polysaccharides

In o...

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