Monomers & Polymers
Monomers (mono meaning one, think monobrow!)
Small, single units that act as the building blocks to create larger molecules.
Polymers (poly meaning more than two)
Made up of many monomers, usually thousands, chemically bonded together.
For monomers to bond together a chemical reaction occurs, this is a condensation reaction. Condensation reactions involve the removal of water. This removal of water from monomers enables a chemical bond to form between the monomers.
A hydrolysis reaction is the opposite of this - Hydro (water) lysis (to split). A water molecule is added between two bonded monomers (within a dimer or polymer) to break the chemical bond.
Carbohydrates are key biological molecules that store energy and can provide structural support to plant cells. Carbohydrates can be classified into three groups determined by how many units they are made of, as seen in the flow diagram below.
Larger carbohydrates, such as sucrose and starch, are made from monosaccharides. The monomers of carbohydrates are known as monosaccharides - glucose, galactose and fructose are three common examples. Monosaccharides are all sugars that are soluble in water. Their functions are either to provide energy or they are building blocks to create other molecules.
All carbohydrates contain three elements: carbon, hydrogen and oxygen (CHO). The general formula for a monosaccharide is CnH2nOn , where n = the number of carbon atoms it contains.
Glucose, C6H12O6, is a very important monosaccharide that can provide energy, be polymerised to form a structural support molecule (cellulose), or energy storage molecule (glycogen and starch). It contains 6 carbon atoms, which are labelled in red on diagram (a). You could be asked to draw glucose, but luckily only in as much detail as in diagram (b).
Glucose has two structural isomers (an isomer a compound that has the same formula, but the atoms are arranged differently). The diagrams above are of the isomer α glucose. β glucose is the second isomer. There is only one difference in the structural arrangement between these isomers, which can be seen on carbon atom 1 in the diagram below. The hydrogen (H) and hydroxyl group (OH) have swapped position. This small change has a significant impact on the bonding and final structure of the polymers that they form.
- α glucose and β glucose only differ structural on one of their carbon atoms. Which carbon atom is it that the H and OH swap position on.
- Your answer should include: One / 1
- Glucose is an abundant and important monosaccharide. What is it’s function?
- Your answer should include: Energy / Storage / Structural / Support
- Name three common monosaccharides:
- Your answer should include: Glucose / Galactose / Fructose
- Which type of carbohydrates are classed as sugars?
- Your answer should include: Monosaccharides / Disaccharides
- Formed from two monosaccharides
- Joined by a glycosidic bond
- Formed by a condensation reaction
There are three key disaccharides that you need to remember, and these are made from the three key monosaccharides you learnt.
- Glucose + Glucose –> Maltose
- Glucose + Galactose –> Lactose
- Glucose + Fructose –> Sucrose
The general formula for a disaccharide is:
(CnH2nOn) 2 – H20
Where n=the number of carbons the monosaccharide contains. This formula is simply x2 the monosaccharide formula and then minus water.
The diagram below demonstrates how a condensation reaction creates a disaccharide. A water molecule is being removed (highlight in red) from the hydroxyl group (OH) on carbon 1 and carbon 4 on the two monosaccharides. The bond that forms is known as a glycosidic bond (highlighted in blue). This diagram shows a 1,4, glycosidic bond because it is located between carbon 1 and carbon 4.
Disaccharides can be broken down back into monosaccharides via a hydrolysis reaction. Hydrolysis is when the water that was removed is added back again to break the glycosidic bond, as can be seen in the diagram below.
Reducing & Non-Reducing Sugars Test
Both monosaccharides and disaccharides and described as sugars because they are sweet and soluble. It is possible to conduct an experiment to test for the presence of these sugars. All of these sugars, except sucrose, are reducing sugars. Sucrose is a non-reducing sugar.
Reducing Sugar Test
To test for the presence of these sugars Benedict’s reagent is added. This is a bright blue liquid, due to it containing copper sulfate. The name reducing sugar is giving to sugars that can reduce Cu2+ ions in Benedict’s reagent to Cu+ ions in the form of copper (I) oxide, which forms a brick red precipitate.
The chemical procedure for this is:
- Add Benedict’s reagent to the sample you are testing
- If a colour change of blue to yellow/green/red is observed, then this is confirmation that a reducing sugar is present. The more red/brown the precipitate, the more sugar it contained.
Non-Reducing Sugar Test
Sucrose is called a non-reducing sugar because it cannot reduce Cu2+, this is because the chemical group needed for this reduction reaction is involved in the glycosidic bonds between the monosaccharides. To prove that sucrose is still a sugar, but it is just unable to reduce Cu2+ (a non-reducing sugar) the glycosidic bond must be hydrolysed to expose the reducing group. If a substance remains blue after the reducing sugars test, then the procedure to test to see if it is a non-reducing sugar is as follows:
- Mix sucrose with HCl and boil – this is acid hydrolysis and it breaks the glycosidic bond so that sucrose is hydrolysed back into glucose and fructose. Hint – to get the mark you must state BOIL, as below 100C there is not enough energy to break the glycosidic bond.
- Cool the solution and then add sodium hydroxide to make the solution alkaline. Benedict’s reagent only works in alkaline solutions, which is why this stage is essential. You must cool the solution first to prevent excessive, dangerous fizzing.
- Add a few drops on Benedict’s reagent and heat.
- If a colour change of blue to yellow/green/red is observed, then this is confirmation that a non-reducing sugar is present.