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3.1.1 Monomers and polymers
3.1.1 Monomers and polymers
Monomers
These are molecules or smaller units from which the larger molecules are made by joining the monomers together. This is called a condensation reaction because a molecule of water is produced in the process. Two molecules can be joined together to form a glycosidic bond between them in a condensation reaction. Two sugar molecules that are already joined can have their bond broken in a hydrolysis reaction this requires the use of a water molecule. This is the reverse of condensation.
3.1.2 Carbohydrates
Carbohydrates are a family of organic molecules made up of carbon, hydrogen, and oxygen atoms. Some are small, simple molecules, while others form long polymers. Carbohydrates are made up of carbon (C), hydrogen (H) and oxygen (O). The ratio of hydrogen to oxygen is 2 to 1.
Carbohydrates have the general formula (CH 2O)x.
Simple carbohydrates are generally called sugars. The most common arrangements found in sugars are:
- Pentose, a five sided sugar, e.g. ribose and deoxyribose.
- Hexose, a six sided sugar, e.g. glucose and fructose.
- In solution, these naturally form rings rather than straight chain structures
Monomers
Monosaccharides are used as a primary energy source for fueling cellular respiration.
Monosaccharides are single-sugar molecules. They include:
- glucose (grape sugar and blood sugar).
- fructose (honey and fruit juices).
- galactose (another isomer found in avacado's and sugar beet).
Monosaccharides generally contain between three and seven carbon atoms in their carbon chains. The 6 carbon hexose sugars occur most frequently.
All monosaccharides are reducing sugars, so they can participate in reduction reactions. They can therefore be identified by the reducing sugar test using Benedict's solution.
Alpha and beta glucose
There are two important isomers of the glucose molecule which are only slightly different from each other. You will have to remember their structure for AS Biology.
Notice that in alpha glucose the H and OH are the same way around but in beta glucose H and OH on the right hand side are reversed.
The α and β glucose molecules, (above), are examples of structural isomers. They contain the same atoms but the hydroxyl groups are linked to the C1 atom differently.
Disaccharides
When larger molecules are made from smaller monomer units the type of reaction is described as a condensation reaction that results in one of the products being water, hence the term condensation. When 2 single sugar units bond they form a double sugar or disaccharide as shown below for 2 alpha glucose molecules.
The disaccharide produced by condensation of 2 alpha glucose molecules is maltose where the monomers are linked by a type of covalent bond called a glycosidic bond.
The important disaccharides and their monomers are:
Sucrose
Components: glucose + fructose
Source: A simple sugar found in plant sap.
Maltose
Components: glucose + glucose
Source: Maltose is a product of starch hydrolysis and is found in germinating grains.
Lactose
Components: glucose + galactose
Source: Milk
All the above disaccharides are reducing sugars except for sucrose which has to be hydrolysed then tested with benedicts solution.
Polysaccharides
Starch
Starch is a polymer of glucose, made up of long chains of α-glucose molecules.
Starch contains a mixture of two different types of molecules:
25-30% amylose: long unbranched chains of many hundreds of glucose molecules linked by 1-4 glycosidic bonds forming a helical structure.
70-75% amylopectin: branched chains with 1-4 and 1-6 glycosidic bonds every 23-30 glucose units. The carbon atoms in alpha glucose are numbered as shown in the diagram below:
Alpha glucose Amylose
Glycogen
Glycogen is chemically similar to amylopectin, but is more extensively branched.
It is composed of α-glucose molecules, but there are more 1,6 glycosidic links mixed with the 1,4 glycosidic links.
Glycogen is the energy storage compound found in animal tissues and in many fungi.
It is present in tissues such as the liver and muscle cells, which are both centers of high metabolic activity. Due to the highly branched structure enzymes are able to breakdown the molecule quickly to release glucose for energy when required. The reaction is a hydrolysis reaction and requires water.
Cellulose
Cellulose is made of beta glucose molecules in which alternating molecules are rotated 180 degrees. This results in a straight chain molecule and when many of these molecules are arranged side by side (parallel) hydrogen bonds form along their length. These structures are strong and are called microfibrils which from a criss-cross network resulting in a material called cellulose. This is the most abundant polysaccharide on Earth. It is strong and makes up the cell wall of plant cells, allowing them to become turgid without bursting.
Exercise 1
1.What are organic compounds?
2.Carbohydrates are organic compounds. State:
a.Which elements they contain
b.The general formula for a simple sugar.
3.State the meaning of the following terms:
a.Monosaccharide
b.Disaccharide
c.Polysaccharide
4.The following monosaccharides are made up of the same atoms. State how they are the same and different from each other:
- Glucose
- Fructose
- Galactose
5.What are the monosaccharide units for the following disaccharides?
a.Sucrose
b.Lactose
c.Maltose
6.State two differences between monosaccharides and polysaccharides.
7.Glucose is an important molecule in both plants and animals. Show, using a diagram, how two glucose molecules can combine to form a disaccharide. Show on your diagram the position of the glycosidic bond.
8.State why it is important for glucose in both animals and plants to be converted to a polysaccharide for storage.
9.For the following polysaccharides describe their structure and state how this is important to their function:
a.Starch
b.Glycogen
c.Cellulose
Lipids
Lipids are a group of organic compounds with an oily, greasy, or waxy consistency and can be solids or liquids. Like carbohydrates, lipids contain carbon, hydrogen, and oxygen, but in lipids, the proportion of oxygen is much smaller. They are relatively insoluble in water and tend to be hydrophobic (water repellent). Lipids are however soluble in organic solvents such as ethanol and ether.
Triglycerides
Triglycerides are made up of a glycerol molecule bonded to 3 fatty acid molecules. Glycerol is an alcohol, as the name suggests, but has 3 alcohol groups (-OH). The fatty acid molecules are quite long about 14 - 20 carbon atoms in length if saturated, but can be up to 24 carbon atoms long if unsaturated. Fatty acids are essentially carboxylic acids that have the carboxyl group (-COOH) at the end of the molecule.
