Biological Compounds

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Carbohydrates are made up of monosaccharides – which are with relatively small molecules used as building blocks. As well as monosaccarides other monomers like amino acids and nucleotides are also used as building-blocks for the larger polymer structures. Polymers are the larger structures, built from monomers, and include carbohydrates, proteins and nucleic acids.

The diagram below and to the left shows α-glucose (alpha glucose) which differs structurally to β-glucose (beta glucose) with the right-hand side hydrogen being trapped ‘up’ for α-glucose and ‘down’ for β-glucose. These glucose molecules are known as isomers because they made from the same components but are different in how they are attached to one another.


Many sugars are produced and used within chemical reactions within cells. An example of this would be glyceraldehyde, a triose three-carbon sugar used in early photosynthesis. It has the molecular formula C3H6O3 and is shown below in a simplified version.


As well as triose sugars there are also pentose sugars and, hence the name, contain five carbons. An example of a pentose sugar would be ribose (C5H10O5 and shown in the diagram below) and deoxyribose (C5H10O4) would be another example.


Glucose (C6H12O6) and fructose (C6H12O6) are also sugars but contain six carbons and so are called hexose sugars. Both glucose and fructose have the same empirical formula, but differ in structure.


Disaccharides consist of two monosaccharide sugars linked together by a glycosidic bond with the elimination of water in a condensation reaction.

The diagram above shows two alpha glucose molecules which have bonded in a condensation reaction, releasing water. The resulting disaccharide formed is maltose (see table below).

MonosaccharidesDisaccarideWhere are they found?
Glucose + Glucose= MaltoseIn Malt sugar
Glucose + Fructose= SucroseIn Cane sugar
Glucose + Galactose= LactoseIn Milk sugar


A polysaccharide is a large and complex molecules which are built from large numbers of monosaccharides linked by glycosidic bonds. An example of a polysaccaride would be starch (for plants) and glycogen (for animals). They are both used for storage because they are very effective at what they do, glucose can be added or taken away from them very easily.


Starch is formed from many alpha glucose polymers and because of the type of bonds form a helix. It also contains many hydrogen bonds. It works well as an energy store within in plants because its molecules are compact and insoluble but are easily hydrolysed. Within starch there are 2 similar molecules. These are Amylose and Amylopectin.

  • Amylose

Amylose is a large unbranched chain of α-glucose forming 1,4-glycosidic bonds (bonds between Carbon 1 in the first molecule and Carbon 4 in the bonding molecule.) Every 6 risidules it completes 1 coil, which is held by weak hydrogen bonds, because of this structure it is very compact and takes up little room which is ideal for storage. On average 30% of all starch samples are Amylose.

  • Amylopectin

Amylopectin is the remaining 70%. Whereas amylose is straight chains and doesn’t branch; Amylopectin branches every 20 residules as well as coiling every 6. The branches are 1,6-glycosidic bonds and result in an even more compact molecule.

  • Starch will not dissolve in water.
  • It has no osmotic effects.
  • Is completely inert.
  • Is easily hydrolised when required for energy.


Glycogen is the main long-term storage molecule for energy in animals and humans. It too is made of long chains of α-glucose with 1,4-glycosidic bonds but branches every 10 risidules which makes it even more compact. It is mainly made in the liver but is stored in muscle tissue where it is readily availible to be used quickly in respiration.

Lipids (Fats)

The most common type of lipids that you’re expected to know about are triglycerides. Lipids are usually known as fats and oils.

Lipids like carbohydrates contain carbon, hydrogen and oxygen but are different because they only contain a small amount of oxygen. They are formed by condensation reactions between glycerol and fatty acids, this is a similar reaction to the one shown in the diagram for disaccharides.

The chemical properties of lipids:

  • Insoluble in water.
  • May be solid or liquid at room temperature depending on their structure and composition.


The diagram above shows the structure of a protein, they are different from carbohydrates and lipids by containing an amino group which contains nitrogen. Most proteins are very large molecules formed from long chains of amino acids. Other than the amino group they have a carboxyl group usually containing a carbon, two oxygen and a hydrogen (COOH).

As well as the amino and carboxyl group, proteins also have an R-group. This R-group is where each protein differs from one another.

Structure of a Protein

Proteins are made up of amino acids in a linear sequence, the amino group of one amino acid reacts with the carboxyl group of another amino acid in a condensation reaction releasing water. This reaction is called a peptide bond and the result of this reaction would be a dipeptide. If there are more than two amino acids bonded in this way it’s called a polypeptide.