Heavily summarised notes for biological molecules (carbohydrates)

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- Introduction

- Structure of carbohydrates

- Functions of carbohydrates

- Tests for carbohydrates


- Carbohydrates are substances that contain the elements carbon, hydrogen and oxygen.

- They have the general formula Cx(H2O)y, where x and y are variable numbers.


- Carbohydrates are divided into three main classes:

- Monosaccharides

- Disaccharides

- Polysaccharides

1. Monosaccharides

- Defined as a carbohydrate which cannot be hydrolysed to simpler carbohydrate.

- The general formula is (CH2O)n

- Common examples are glucose, fructose and galactose (hexose sugars)

2. Disaccharides

- A disaccharide is formed by a condensation reaction between two monosaccharides - one molecule of water is removed from the pair of monosaccharides (Fig. 1).

Common examples are:

- maltose = glucose + glucose

- lactose = glucose + galactose

- sucrose = glucose + fructose

- The bond formed between the 2 monosaccharides is called the glycosidic bond.


3. Polysaccharides

- Polysaccharides are polymers of monosaccharides.

Common examples are:

- Starch (storage polysaccharide)

- Glycogen (storage polysaccharide)

- Cellulose (structural polysaccharide)

* Structure of starch

- Starch is a polymer of glucose

- It has two components: amylose and amylopectin.

i. Amylose

- It has a straight chain structure.

- Consists of several thousand glucose residues joined by α(1-4) glycosidic bonds.

- These bonds cause the chain to join helically into a more compact shape

ii. Amylopectin

- It is also compact but has a highly branched structure.

- The branches are formed by α(1-6) glycosidic bonds.

- It has twice as many glucose residues as amylose


1. Monosaccharides are for synthesis of nucleic acids (ribose is a constituent of RNA, deoxyribose is a constituent of DNA) and synthesis of disaccharides and polysaccharides.

2. As a source of...