Revision:Edexcel biology unit 1 molecules and cells - molecules


Metabolism is a term to describe all reaction which is taking place within a cell and is separated into two types which are

  • anabolic = compounds being built up
  • catabolic = compounds being broken down


The water molecule is a molecule which is made up from 2 hydrogen atoms and 1 oxygen atoms. It is bounded by 2 covalent bonds and has is polar.

The reason why water molecules are important for living organisms are because :

  • They are important solvents since they are polar
  • Important coolants for excessive metabolic reactions
  • Important for insulation since they have high specific heat capacities
  • It is a very good mechanism for transport since they are highly cohesive
  • Important reagents for chemical reactions ( such as hydrolysis and photosynthesis )


Separated into 3 types which are monosaccharides, disaccharides and polysaccharides


  • they are three main types of monosaccharides which you have to know in this level which are

Pentoses - Such as ribose (with formula C5H10O5) and fructose

Hexose - Such as alpha and beta glucoses

  • All members of monosaccharides are reducing since they have the aldehyde groups, therefore they will cause a brick red persipitate to form when tested with benedict`s solution.


  • You would need to know 3 types of dissacharides in this level which is
    • Lactose - Formed between glucose and galactose and is bounded by 1,4 glycosidic bonds (found in milk)
    • Maltose - Formed by 2 molecules of glucose and is bounded by 1,4 glycosidic bonds (found in starch after being hydrolysed)
    • Sucrose – Formed by a molecule of glucose and a molecule of fructose bounded by 1,2 glycosidic bonds (Found in most plant since it is used to transport sugary compounds)
  • The formation of bonds between 2 monosaccharides is called condensation and the breakage of these bonds are called hydrolysis. Hydrolysis of these bonds can be brought about by adding dilute HCl.
  • Both lactose and maltose are reducing but sucrose is not a reducing sugar. Therefore disaccharides can or cannot be reducing. It is not always a reducing agent.


  • There are three types of polyssacharides that we need to know at this level which are


  • A polymer of alpha glucose and is bounded by 30% amylose (1,4 glycosidic bonds) and 70% amylopectin (both 1,4 and 1,6 glycosidic bonds)
  • It is used as a food storage for plants and is built from the amyloplasts in the cytoplasm of plant cells.
  • Starch is an important storage molecule since it is compact, insoluble in water, inert to chemical reactions and is easily hydrolysed when required.


  • A polymer of alpha glucose with 1,4 glycosidic bonds but even more 1,6 glycosidic bonds compared to starch since it is even more branched..
  • It is used as an energy store in animals and is also found in bacterias
  • Glycogen is an important storage molecule since it is compact, insoluble in water, inert to chemical reactions and is easily hydrolysed when required.


  • A polymer of beta glucose and is by only 1,4-glycosidic bonds. Therefore it is not branched but arranged in a matrix which is bounded by hydrogen bonds
  • It is the constituent of cell walls and can be hydrolysed by ruminant animals by the action of cellulase enzymes..

All polysaccharides are non reducing sugars.


  • Lipids are basically fats and oild that contain elements of carbon, hydrogen and oxygen
  • An example of a lipid would be the triglyceride which is formed by glycerol and fatty acids .The bond formed is catalysed by the the process of condensation between the –OH groups of the glycerol and the fatty acids. The bonds formed between them are the ester bonds. Since triglyceride itself is an ester.
  • There are two types of fatty acids which are unsaturated and saturated fatty acids. An example of a saturated fatty acid would be stearic acid and unsaturated would be the oleic acid. The difference between saturated and unsaturated fatty acids are the double bonds and the amount of hydrogen present at where saturated acids have more hydrogen and no double bond present.
  • The usages of triglycerides are important energy stores, compact, insoluble in water, releases energy twice the amount of carbohydrates, and provides buoyancy for aquatic animals.
  • The usages of fatty acids include the formation of triglycerides and is also required for the formation of phospholipids.


  • Formed between fatty acids, glycerol and a phosphate group.
  • The head is hydrophilic (soluble in water) but the tail is hydrophobic (insoluble in water).
  • Therefore phospholipids are used as the main constituents of cell membranes


Primary structure

  • It is the number , type and sequence of amino acids which makes up a polypeptide chain

8Each amino acid is binded to another acid by peptide bonds by the process of condensation. Amino acids are amphoteric since it has a zwitterion shapes.

Secondary structure

  • It is the shape of molecule formed when polypeptide chains are either coiled by hydrogen bonds into a spiral springs ,the alpha helix or linked to form beta pleated sheets
  • The alpha helix coils the polypeptide chains and are help in place by hydrogen bonds. An example of this would be keratin.
  • The beta pleated sheets are formed by hydrogen bonds taking place between the CO and NH groups .It is very strong but supple. An example would be collagen since it is fibrous.
  • Insoluble in water.

Tertiary structure

  • It is the folding of a polypeptide to form a globular shape.
  • R groups of these proteins are bounded by ionic bonds, disulphide bridges and hydrogen bonds.
  • An example would be enzymes.

Quaternary structure

  • A complex protein molecule which has more than one polypeptide chains bounded to it.
  • An example would be haemoglobin.

Experimental tests

  • Bear in mind that most of the unit 1 tests has at least 1 biological test question which can yield up to 5 points and these tests should not be neglected.

Benedict test

  • Test to confirm reducing properties of sugars.
  • Add a few drops of benedict solution into the sugar and boil. If it shows a brickred precipitate, a reducing sugar is present. Since copper 2+ is reduced in copper +1 which shows are brick red precipitate

Biuret test

  • Test to confirm the contents of protein or the presence of protein.
  • Add an equal amount of protein and biuret reagent. A purple-violet color would confirm the presence of proteins. This can be used as a qualitative test since the color is gets more intense as the protein content increases.

Sudan III test

  • To test the for the presence of fat
  • Add equal amount of Sudan III and fat contents together. A suspension of red liquid and white precipitate on the top would confirm that there is fat present.


These notes are aimed at people studying Edexcel Biology A Level, but will be suitable for other exam boards too.

Originally written by MalaysianDude on TSR Forums.