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.

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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. 

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.

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

Hydrolysis reactions

Definition

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

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. 

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. 

Iodine test

The iodine test can be used to detect starch. 

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. 

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.

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.

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