Physics Made Easy

Biological Molecules

Monomers and Polymers: Monomers are small molecules that combine to form long chain molecules known as polymers (see Chemistry section for further details).

Condensation is the process by which two monomers bond to each other; a small molecule (usually water) is lost from the resulting molecule.

Hydrolysis is the process by which polymers are split into monomers; a molecule of water is used to split each bond between monomers.


Carbohydrates are biological molecules made up of carbon, hydrogen and oxygen. They function as a major source of energy for plants and animals and also a structural molecule in plants.

Monosaccharides (sugars): are the basic monomer units of carbohydrates. Monosaccharide molecules usually have the formula (CH2O)n. Monosaccharides are sweet, soluble and crystalline solids such as glucose (C6H12O6), fructose (C6H12O6) and ribose (C5H10O5).

Disaccharides consist of two monosaccharides linked by a glycosidic bond. Disaccharides are sweet, soluble and crystalline solids such as sucrose (glucose+fructose) and maltose (glucose+glucose).


Polysaccharides consist of thousands of monosaccharides linked by glycosidic bonds to form long chains. Polysaccharides are not sweet or crystalline. Three important polysaccharides are starch, cellulose and glycogen.

Structure of glucose: glucose has two ring structures, known as a -glucose and b -glucose.


Biochemical test for sugars

  • Reducing sugars: (monosaccharides e.g. glucose and fructose). Heat with Benedict’s reagent. The blue Cu2+ ions in Benedict’s reagent are reduced to form a brick-red precipitate of copper (I) oxide if reducing sugars are present. This is observed as a gradual colour change from blue ® green ® yellow ® orange ® red; the greater the concentration of reducing sugar, the more extreme the colour change.
  • Non-reducing sugars: (disaccharides e.g. sucrose). Heat with dilute HCl to hydrolyse into its component monosaccharides (which are, of course, reducing sugars). Neutralise the sample with NaHCO3 (sodium hydrogencarbonate or sodium bicarbonate), then test as in part 1.

Structure of starch: starch is actually two different polysaccharide molecules- amylose and amylopectin. Amylose is a polysaccharide formed from a -glucose molecules linked by 1-4 carbon linkages (carbon no. 1 of one molecule links to carbon no. 4 of the next molecule). This makes for a straight-chain molecule. Amylopectin is much the same, except that it has a few 1-6 carbon linkages as well as 1-4 linkages, making the molecules slightly branched. Starch is used as an energy source by plants and animals, and it also serves as a structural molecule in plants.

Test for starch: add a few drops of iodine in potassium iodide solution to the sample. A blue-black precipitate is formed if starch is present.

Structure of glycogen: glycogen consists of α-glucose molecules linked by 1-6 carbon linkages, making it a highly branched molecule. It is used by animals as an energy store, and is found in the liver and muscles.

Structure of cellulose: cellulose consists of β-glucose molecules linked by 1-4 carbon linkages. Because b -glucose molecules are used, the glycosidic bonds alternate between the top and bottom of the molecule from one monomer to the next. This means cellulose molecules can form hydrogen bonds with each other (cross-linking). This makes cellulose resistant to hydrolysis, and most animals are unable to digest it. Cellulose is found in plant cell walls.

Test for cellulose: add Schultze’s solution to the sample. If cellulose is present, the solution turns purple.

Proteins are biological molecules containing carbon, hydrogen, oxygen and sometimes sulphur. They are used in the growth and repair of cells. Proteins can be divided into two types- fibrous proteins such as keratin and collagen are structural molecules, whilst globular proteins make up enzymes, hormones and antibodies.

Biuret test for proteins: First, add KOH solution to the sample to clarify it. Then, add a few drops of copper sulphate. If protein is present a blue ring appears, and the solution turns purple on shaking. Note: this is a test for peptide bonds (see below).

Amino acids are the basic monomer units of proteins. There are about 20 naturally occurring amino acids. They have the general structure


where ‘R’ represents any of a number of functional groups, from a simple hydrogen atom to a long carbon-hydrogen chain (for more on the chemistry of amino acids, see “Chemistry” section).

Polypeptides are long chains of amino acids linked by peptide bonds between the carboxylic acid group of one molecule and the amino group of the next-


Structure of proteins: classed as follows-

The primary structure is the sequence of amino acids in the polypeptide chain. This sequence determines the secondary and tertiary structures of the protein.

The secondary structure is the shape of the polypeptide chain. It either coils up to form an a -helix or a b pleated sheet. Fibrous proteins have a well developed secondary structure.

The tertiary structure is the overall 3D shape of the molecule formed by irregular folding of the a -helix. Globular proteins have a well-developed tertiary structure. Secondary and tertiary structures are held together by hydrogen bonds, sulphur bridges and ionic bonds between different amino acids in the polypeptide chain (e.g. amino acid number 7 might form a hydrogen bond with amino acid no 12).

The quaternary structure describes the combination of two or more polypeptide chains to form a biologically active molecule such as an enzyme.

Lipids are biological molecules composed of carbon, hydrogen and oxygen (proportionally less oxygen as compared to carbohydrates). There are three main types:

Simple lipids e.g. fats, oils and waxes, which are used as energy stores.

Compound lipids e.g. phospholipids and glycolipids, which are, for example, found in cell membranes

Steroids e.g. cholesterol, vitamin D and sex hormones.

Triglycerides are lipid molecules containing three fatty acids linked to one molecule of glycerol by ester bonds.


The R represents any one of a large number of different carbon-hydrogen chains.

Saturated and unsaturated fatty acids: saturated fatty acids contain only C-C single bonds between carbon molecules (all bonds occupied). Unsaturated fatty acids have some C=C double bonds.

Phospholipids are triglycerides where one fatty acid group is replaced with a phosphate group. They are discussed in more detail in Cell Structure and Function.

Test for lipids: shake a sample of test substance with ethanol and add to an equal volume of water. If lipid is present a cloudy white emulsion will appear; the cloudier the emulsion, the higher the concentration of lipid within the sample.

Chromatography: a technique for separating and identifying a mixture of chemicals. The method is as below:


The chromatogram may have to be placed in an iodine tank or sprayed with a developing agent such as ninhydrin in order to develop the spots (i.e. make them visible). This is discussed further in the Chemistry section.

Rf (retention factor) values: are used to identify chemicals on a chromatogram:

Rf= distance travelled by solute/distance travelled by solvent

Each chemical has a particular Rf value for a particular solvent; if you know the solvent used on a chromatogram you can identify unknowns by determining their Rf value and comparing with results tables.

Electrophoresis: a technique used to separate charged particles, e.g. DNA fragments. Method as below:


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