biological molecules



2 monosaccharides make a disaccharide:

  • Sucrose = α-glucose + fructose
  • Maltose = α-glucose + α-glucose
  • Lactose = α-glucose + galactose

Polymer: polysaccharide

Pentose sugar = 5 carbon atoms

Hexose sugar = 6 carbon atoms

Amylose and amylopectin: both polysaccharides

glycosidic bond holds two monosaccharides together

α-glucose is used in starch and glycogen, whereas β-glucose is used for cellulose

1 of 14


Made up of amylose and amylopectin:

  • Amylose is a continuous chain of a-glucose, and the hydrogen bonds causes it to coil. Linked by 1,4 glycosidic bonds
  • Amylopectin consists of branches, due to 1,6 glycosidic bonds. It contains 1,4 glycosidic bonds too. This enables starch to fold up compactly - therefore storing a large amount of energy
  • 1,4 and 1,6 glycosidic bonds means which carbon the bond is on

Used as fuel for humans, used as storage for plants so that bulbs and tubers can survive through the winter.

Especially suited for storage as it is insoluble and does not diffuse out of cells easily, and does not have any affects on water potential and thus osmosis

Starch + amylase = maltose. Maltose is a disaccharide and therefore can be hydrolysed into glucose

2 of 14


More branched than starch, contains more 1,6 glycosidic bonds, hence more compact

Stored in the muscles (so close to the site of respiration)

Liver has larger reserves of glycogen and continually breaks it down to maintain a stable blood glucose concentration

3 of 14


Monomer in cellulose: β-glucose

Cellulose chains: every other molecule of β-glucose is upside-down so the CH₂OH side-chains stick out alternatively on opposite sides. This makes it very straight. Very long, and line up together and become linked by hydrogen bonds.

Small bundles of cellulose molecules make very thin fibres, called microfibrils. They are able to withstand stretching like steel fibres of the same diameter. Groups of microfibrils are joined to make thicker, stronger fibres (just like string)

In cell walls, these fibres are criss-crossed, making the walls resistant to stretching in any direction

So well suited to its function that it is found throughout the plant kingdom. It's (probably) the most abundant carbohydrate.

Neither humans nor mammals can make an enzyme which breaks down cellulose, but cattle and rabbits carry bacteria in their guts which hydrolyse cellulose so they can make use of the energy

4 of 14


Saturated fatty acids: No double bonds between any carbon atoms. Higher melting point than unsaturated. E.g. butter or lard (solid at room temperature)

Unsaturated fatty acids: Have a double bond between carbon atoms (causes a kink or a bend)

Polyunsaturated fatty acids: Has many double bonds between carbon atoms. E.g. sunflower or olive oil liquid at room temperature)

Glycerol is a type of alcohol. Contains three -OH groups, which allow the molecule to join with three fatty acids to make a triglyceride

In animal cells there are usually 14-16 carbon atoms in the fatty acid chain

Ester bond is formed between glycerol and the fatty acid

5 of 14


Very similar to triglyceride, but only has two fatty acids and instead has a phosphate attached to the glycerol head

Hydrophilic head: the glycerol and phosphate

Hydrophobic tails: the fatty acids

When mixed in water, they arrange themselves in a double layer with their hydrophobic tails pointing inwards and their hydrophilic heads pointing outwards. This double layer is called the phospholipid bilayer and is used in cell membranes

6 of 14


Monomers: amino acids

Polymer: polypeptide

Two amino acids joined together is a dipeptide

There are 20 amino acids found in almost all living organisms. Can be arranged in a large range of different sequences

Human blood is red because it is made of haemoglobin, an iron-containing protein which is vital for transporting oxygen from the lungs to respiring cells

Enzymes are proteins, and antibodies are also proteins.

Bond between the amino acids is a peptide bond

7 of 14

Protein Structure

Primary structure:

  • The sequence of amino acids in a polypeptide chain

Secondary Structure:

  • The shape the polypeptide chain folds into, e.g. an alpha-helix or a beta-pleated sheet. The bonds between the amino acids can affect this, because they could be ionic bonds (forms between an amino acid with a positive charge and one with a negative charge. Not very strong), hydrogen bonds (which form between the R-groups of amino acids. Not very strong), and disulphide bridges (which form between amino acids that contain sulfur in their R-groups. Quite strong, less breakable than ionic or hydrogen bonding)

Tertiary Structure:

  • Further folding of the proteins

Quaternary Structure:

  • Multiple polypeptide chains joined together
8 of 14

Food Tests: Benedict’s Test (Reducing)

Tests for: reducing sugars


1) Place sample in Benedict's solution

2) Heat test sample

Why does it change colour?: Cu(II) ions in Benedict's solution get reduced to orange Cu(I) ions.

Colour change indicates: Red = Very high number of reducing sugars, Orange = High number of reducing sugars, Yellow = Moderate number of reducing sugars, Green = low amount of reducing sugars, Blue = no reducing sugars

9 of 14

Food Tests: Benedict's Test (Non-Reducing)

Tests for: non-reducing sugars


1) Check there is no reducing sugar present by heating part of the sample with Benedict's solution

2) Hydrolyse the rest of the sample by heating with dilute hydrochloric acid

3) Neutralise by adding sodium hydrogencarbonate

4) Follow the test steps for reducing sugars

10 of 14

Food Tests: The Iodine Test

Tests for: starch (amylopectin)


1) Add iodine solution to the sample

Colour change: orange to blue-black

11 of 14

Food Tests: The Emulsion Test

Tests for: lipids


1) Dissolve the test sample by shaking with ethanol

2) Pour the resulting sample into water in a test tube

You can see lipids present if there is a milky white emulsion on the water

12 of 14

Food Tests: Biuret's Test

Testing for: Proteins


1) Add sodum hydroxide to the test sample

2) Add a few drops of dilute copper sulfate solution

Colour Change: blue to mauve/purple

13 of 14

Bonds and Reactions

Condensation Reaction: When one monomer joins to another and a water molecule is removed

Hydrolysis Reaction: When water molecules are added in the process of breaking bonds between molecules, for example when breaking a polymer into monomers

Hydrogen bond: covalent bond between two molecules, also called an ester bond (in fats), phosphodiester bond (in DNA and RNA), glycosidic bond (in sugars) or peptide bond (in proteins)

Organic molecules always have a carbon atom (therefore water is inorganic)

14 of 14


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Biological molecules resources »