Explain the terms monomer and polymer.
	Explain the terms condensation and hydrolysis.
	Describe the structure and functions of carbohydrates.
	Describe the structure and functions of proteins.
	Describe the structure and functions of lipids.
	Describe tests for biological compounds.

Biological Molecules

Almost all material in a cell that is not water is made up of organic molecules. Organic compounds all contain carbon and hydrogen atoms. Most organic molecules also contain oxygen atoms, and some include nitrogen, sulphur and phosphorus. Many biological molecules are long chains or polymers made up from lots of small units called monomers.

The man and child can join up with other very similar monomer units to form a long chain polymer.

Carbohydrates

The main job of carbohydrates is to provide an organism with energy. Carbohydrates are also involved in storing energy and in plants they form an important part of the cell wall.

The most common carbohydrates are sugars and starches. Our common ‘sugar’ is sucrose. Glucose is the energy provider in sports drinks and starch is found in flour and potatoes. However, there are far more carbohydrates.

The basic structure of all carbohydrates is the same. They are made up from carbon, hydrogen and oxygen. There are three types of carbohydrates depending on the complexity of the molecules – monosaccharides, disaccharides and polysaccharides.

Monosaccharides – single sugars

In these simple sugars there is one oxygen atom and two hydrogen atoms for each carbon atom present in the molecule. A general formula for these is: (CH₂O)_n

Monosaccharides are made of only one sugar unit. There are triose sugars (n=3) like glyceraldehyde, pentose sugars (n=5) like ribose and deoxyribose.

The most common sugars are hexose sugars (n=6) such as glucose, sucrose and fructose and these often have a ring structure. The carbon atoms in the ring are numbered 1-6. Two types of glucose molecule exist and are shown below. What is the difference between the two?

Disaccharides – double sugars

The two molecules join together in a condensation reaction where a molecule of water is removed (this is sometimes called a dehydration synthesis reaction). The bond that forms between two sugar molecules is called a glycosidic bond. The bond is named after the number of the two carbon atoms that are joined together. For example, in the diagram below the number 1 carbon is joined to the number 4 carbon, so this is a 1-4 glycosidic bond.

Click here to see how a condensation reaction happens.

Polysaccharides

Polysaccharides are long chain polymers made up of sugar molecules linked by glycosidic bonds. Polysaccharides are most commonly made up of lots of glucose molecules linked in different ways. They are useful in biological systems for three reasons:

The three most common polysaccharides are starch, glycogen and cellulose.

Starch is particularly important as an energy store in plants. The sugars produced by photosynthesis are rapidly converted into starch. Storage organs, such as potatoes, are particularly rich in starch. Starch is made of long coiled chains of a-glucose, joined by 1-4 glycosidic bonds. The chains are branched – the more branches there are, the less easily the starch dissolves in water.

Glycogen is often described as the animal version of starch as it is the only carbohydrate energy store found in animals. It is very similar to starch because it is made up of lots of a- glucose units. However glycogen has some 1-6 glycosidic bonds as well as 1-4 links. Glycogen is found mainly in muscle tissue and particularly in liver tissue, which is very active and needs a readily available energy supply.

Cellulose is an important structural material in plants. It is the main constituent in plant cell walls. Like starch and glycogen it is made up of long chains of glucose, but in this case b-glucose linked by 1-4 bonds. The way in which the monomers are joined in cellulose means that the –OH groups protrude and hydrogen bonds can form between cellulose chains. This gives the cellulose considerable strength.

Proteins

Proteins make up 18% of the human body. As well as being the basis for things such as hair, nails and skin they play an even more vital role in the body. All enzymes are made of proteins. Enzymes control all the reactions which occur in our cells and also digest our food. Many hormones that control the functioning of our organs are also made of proteins. Proteins are very important molecules in all biological systems.

Proteins, like carbohydrates, are made up form carbon, oxygen and hydrogen but they also contain nitrogen. Many contain sulphur and some also have phosphorus and other elements.

Proteins are long chain polymers of small monomer units called amino acids.

There are about 20 naturally-occurring amino acids that can combine to form proteins which means that the variety of proteins that can be formed is huge.

Amino Acids

All amino acids have the same basic structure of an amino group (-NH₂) and a carboxyl group (-COOH) attached to the same carbon atom.

(R –) represents any group that is attached to the carbon atom. Here are some examples of real amino acids. Can you work out what the R - groups are in each of these molecules.

Amino acids can be linked together in long polymer chains to form protein molecules. They are joined together by peptide bonds.

You will notice from the diagram that a water (H₂0) molecule is formed when two amino acids are joined, what kind of reaction is this?

When two amino acids join they make up what is called a dipeptide. More and more amino acids can join together to form polypeptide chains, which may be from around ten to many thousands of amino acids long. A polypeptide can then fold or coil, or become associated with other polypeptides to form a protein.

Protein structure

When describing proteins four levels of description of their structure are used. These are the primary, secondary, tertiary and quaternary structures.

Primary structure

The secondary structure describes the 3-dimensional arrangement of the polypeptide chain. In many cases it can be a right-handed (alpha-) helix, which is a spiral coil. In other proteins the polypeptides can fold up into pleated sheets. Sometimes there is no regular secondary structure and the polypeptides form a random coil.

Some proteins are so large, with thousands of amino acids joined together, that they have very complicated structures. The tertiary structure describes how alpha-helices and pleated sheets are folded into these complex shapes. These 3-dimensional shapes are held together by hydrogen bonds, sulphur bridges and ionic bonds.

Some proteins, such as some enzymes and haemoglobin, are not made up of just one polypeptide chain but several. The quaternary structure describes the way in which these chains fit together.

Types of Proteins

Proteins fall into two main groups whose functions are determined by their tertiary structure.

Fibrous proteins have little or no tertiary structure. They are long polypeptide chains with occasional cross links, to other chains, making up fibres. They are insoluble in water and very tough which makes them ideal for their structural job within living things. They are found in connective tissue, tendons, muscle, the silk of spiders’ webs and as keratin making up hair, nails, horns and feathers.

Globular proteins have complex tertiary and sometimes quaternary structures. They are folded into spherical (globular) shapes. Antibodies in the blood are globular proteins. Other globular proteins are some hormones and the majority of enzymes. Because globular proteins have very complex shapes this makes them ideal for their role as enzymes because they can be highly specific. See 10.5.

Lipids

Another important group of organic chemicals which make up cells are lipids. Lipids include fat and cholesterol and so may not sound very good for us but they are essential for our lives.

Lipids are an important source of energy in the diet of many animals and the most efficient way for living things to store energy – they contain more energy per gram than carbohydrates and proteins. Many plants and animals convert spare food into fats and oils for use at a later date. Lipids are also essentially for making cell membranes and are also important in the nervous system.

Fats and Oils

One of the main groups of lipids are the fats and oils. They are chemically very similar but fats are solid at room temperature (e.g. butter, lard) whereas oils are liquid (e.g. olive oil, vegetable oil).

Fats and oils are made up of combinations of two kinds of organic molecules: fatty acids and glycerol.

There are a wide range of fatty acids. All fatty acids have a long hydrocarbon chain – a backbone of carbon atoms with hydrogen atoms attached – and a carboxyl group (– COOH) at one end. Fatty acids vary in two main ways. The length of the carbon chain can differ and more importantly the fatty acid may be saturated or unsaturated.

In a saturated fatty acid each carbon is joined to the next by a single bond.

In an unsaturated fatty acid the carbon chain contains one or more double carbon-carbon bonds. The presence of double bonds in the chain causes it to kink. Monounsaturated fatty acids have one double bond.

Polyunsaturated fatty acids have more than one double bond.

A fat or oil results when glycerol combines with one, two or three fatty acids to form a mono-, di- or triglyceride. A bond is formed between the carboxyl group (-COOH) of a fatty acid and one of the hydroxyl groups of the glycerol. The most common are triglycerides. Which fatty acid chains, on the diagram of the fat below, are unsaturated and which are saturated?

Another important kind of lipid which exist are phospholipids. These are similar in structure to triglycerides except that one of the fatty acid chains has been replaced with a phosphate group. The phosphate group has another molecule attached to its other end. Phospholipids are used in cells to make cell membranes (See 10.3).

Phospholipids are often represented as having a head (glycerol) and two tails (fatty acid chains).

Chemical Tests

Biochemical tests can be used to detect the presence of sugars, carbohydrates, proteins and lipids. The tests are summarised in the table below.

Biochemistry Links