Carbohydrates: types, structure and function

In a nutshell

Carbohydrates are polymers of monosaccharides. Glucose is the most common monosaccharide and makes up important storage molecules in plants and animals known as starch, glycogen and cellulose. The presence of carbohydrates in a sample can be detected using Benedict's test.

Carbohydrates

Carbohydrates are an important type of biological molecule made up of carbon $(C)$ , hydrogen $(H)$ and oxygen $(O)$ in the ratio of $1:2:1$ . The general formula can therefore be written as $(CH_2O)_n$ .

Most carbohydrates are polymers, meaning they are long chains of joined-up monomer units. The monomer units of carbohydrates are known as monosaccharides. Two monosaccharides joined together are known as disaccharides, and long chains of monosaccharides are called polysaccharides. Monosaccharides and disaccharides are often referred to as sugars.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — A carbohydrate: 1. Carbonyl group, 2. Hydroxyl group.

Monosaccharides

Definition

The monomer unit that makes up polysaccharides.

Examples

Glucose, fructose and galactose.

Glucose

Glucose is the most common monosaccharide. It is a hexose sugar as it has six carbon atoms and its formula can be written as $C_6H_{12}O_6$ . There are two types of glucose: alpha ( $\alpha$ ) and beta ( $\beta$ ). They are isomers of each other, meaning they have the same molecular formula but the atoms are arranged differently.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — Alpha and beta glucose: 1. OH is always above the ring, 2. In alpha glucose this OH is below the ring, 3. In beta glucose this OH is always above the ring.

Ribose and deoxyribose

Ribose and deoxyribose are very important monosaccharides. They are pentose sugars as they have five carbon atoms. They are found in nucleotides that make up RNA and DNA. They have very similar structures, but deoxyribose has one less oxygen atom at carbon number two.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — 1. Deoxyribose, 2. Ribose, 3. Deoxyribose is missing one oxygen atom.

Condensation reactions

Definition

A reaction that involves two monomers joining together to form a new chemical bond and the elimination of a water molecule.

Monosaccharides are joined together by condensation reactions and form glycosidic bonds.

Examples

Disaccharide	Monosaccharides
Maltose	Two $\alpha$ -glucose molecules are joined together via a glycosidic bond.
Sucrose	One molecule of glucose and one molecule of fructose are joined together via a glycosidic bond.
Lactose	One glucose molecule and one galactose molecule joined together via a glycosidic bond.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — The formation of maltose via a condensation reaction: 1. Water molecule is removed, 2. Maltose molecule, 3. Glycosidic bond

Hydrolysis reactions

Definition

The breaking down of polymers into monomers using a water molecule. Hydrolysis reactions are the opposite of condensation reactions.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — A general hydrolysis reaction: 1. Polymer, 2. Hydrolysis, 3. Monomer

Polysaccharides

Definition

A carbohydrate formed when two or more monosaccharides are joined together through condensation reactions.

There are three structurally significant polysaccharides you must know: starch, glycogen and cellulose.

Starch

Starch is the main storage material in plants. It is how plants store excess glucose so it can be used at a later time when it is required. Starch is a combination of two polysaccharides of $\alpha$ -glucose, known as amylose and amylopectin.

Amylose

Amylose is a long unbranched chain of $\alpha$ -glucose with a coiled structure that makes it very compact. This is suitable for storage as you can fit more amylose into a small space.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — The structure of amylose: 1. Glycosidic bond

Amylopectin

Amylopectin is a long, branched chain of $\alpha$ -glucose. It has many side branches that allow enzymes to quickly break down the glycosidic bonds and release glucose molecules.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — The structure of starch: 1. Amylose, 2. Amylopectin.

Iodine test

The iodine test can be used to detect starch.

1.	Add iodine dissolved in a potassium iodide solution to the test sample.
2.	If the sample changes from browny-orange to a dark, blue-black colour then starch is present.

Glycogen

Glycogen is the main storage material in animals. Similar to starch, it is a way for animals to store excess glucose and is made up of $\alpha$ -glucose.

Structurally, it is similar to amylopectin as it is highly branched, so glucose can be released quickly. It is also very compact and therefore ideal for storage.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — 1. The branched structure of glycogen.

Cellulose

Cellulose makes up cell walls in plants. Unlike glycogen and cellulose, it is made of $\beta$ -glucose monomers that bond to form straight cellulose chains. These chains are long and unbranched but are linked together via hydrogen bonds. This forms strong fibres known as microfibrils that provide cells with structural support.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — The structure of cellulose: 1. One cellulose molecule, 2. Weak hydrogen bonds, 3. One molecule of beta glucose.

Benedict's test

Benedict's test is a way of testing for sugars. All sugars can be classified as reducing or non-reducing based on their ability to donate electrons. Reducing sugars form coloured precipitates. Reducing sugars can donate electrons meaning the carbonyl group becomes oxidised and the sugar is acting as a reducing agent. Non-reducing sugars cannot donate electrons so they must first be broken down into monosaccharides before Benedict's test is performed.

Type of sugar	Examples
Reducing	Glucose, fructose and galactose.
Non-reducing	Sucrose and trehalose.

Procedure

1.	Add an excess of Benedict's reagent (blue) to the sample.
2.	Heat the sample in a water bath that has been brought to a boil.
3.	If the test is positive, it will form a coloured precipitate (see diagram).
4.	Benedict's test is semi-quantitative: the greater the concentration of reducing sugar the greater the colour change. Example The formation of a brick-red precipitate indicates a high amount of reducing sugar.
5.	If the result was negative (the mixture stayed blue), there may still be non-reducing sugar in the sample so we must break them down into monosaccharides.
6.	To a new sample, add dilute hydrochloric acid and place it into a water bath that has been brought to a boil.
7.	Neutralise the mixture with sodium hydrogencarbonate and repeat steps 1-4.
8.	If the solution remains blue, there is no sugar in the sample.

Biology; Biological molecules; KS5 Year 12; Carbohydrates: types, structure and function — 1. Increasing sugar concentration in sample. A. Blue, B. Green, C. Yellow, D. Orange, E. Brick-red.