4) Biological molecules


4) Biological molecules

A nutrient is a substance which is needed for growth, repair and metabolism. The three main nutrients are:

  • carbohydrates
  • proteins
  • lipids (fats and oils)

These nutrients are all examples of organic chemicals. This means that they all contain carbon atoms,

covalently bonded to the atoms of other elements.



  • May be simple, soluble sugars or complex materials like starch and cellulose
  • All contains C,H,O only
  • Glucose – C6H12O6


Monosaccharides – sugars with a single carbon ring, eg. glucose, fructose.

Disaccharide – sugars with two carbon rings, eg. maltose, sucrose.

Polysaccharide – many glucose molecules joined together, eg. glycogen (found in animals), starch (found in plants).

Cellulose – consists of even longer chains of glucose molecules.

Mono- and disaccharides are readily soluble in water.

Polysaccharides are not readily soluble in water.



Structural proteins: proteins contributing to the structure of the cells, eg. to the cell membranes, the mitochondria, ribosomes and chromosomes.

Enzymes: control the chemical reactions that keep the cell alive. Present in the membrane systems, in the mitochondria, in special vacuoles and in the fluid part of the cytoplasm.


Proteins are large molecules made from smaller units of amino acids.



  • Fats = solid
  • Lipids = liquid

Lipids are large molecules made from smaller units of fatty acids and glycerol.


Food tests:


Test for starch – Starch is detected using Iodine solution. This turns blue-black in the presence of starch.


Test for reducing sugar – Reducing sugars are detected using Benedict’s solution. The solution will

change from clear blue to cloudy green, then yellow and finally to a red precipitate of copper oxide.


Test for protein – Proteins are detected using Biuret reagent. This turns purple colour when mixed with protein.


Test for lipids – Lipids are detected using the Emulsion test.

  1. the test substance is mixed with 5 cm^3 of ethanol
  2. an equal volume of distilled water is added
  3. a milky-white emulsion forms if the test substance contains lipids


Test for vitamin C – Vit. C is detected using the DCIP test.

  1. Draw up 2 cm^3 of fresh lemon juice into a plastic syringe
  2. Add into a test tube with 2 cm^3 of a 0.1% solution of DCPIP (a blue dye). The DCPIP will suddenly become colourless as the juice is added. Note the amount of juice added


  1. Repeat the experiment but with orange juice in the syringe. If it takes more orange juice to decolourise the DCPIP, the orange juice must contain less vitamin C



  • There are about 20 different amino acids in animal protein, including alanine, leucine, valine, glutamine, cysteine, glycine and lysine.
  • A small protein molecule might be made up from a chain consisting of a hundred or so amino acids.
  • Each type of protein has its amino acids arranged in a particular sequence.
  • The chain of amino acids in a protein takes up a particular shape as a result of cross-linkage.
  • Cross-linkage form between amino acids that are not neighbours.


The shape and structure of a protein molecule has a very important effect on its reactions with substances.

  • Active site of enzymes
  • Binding site of Antibodies


Denature – when a protein is heated over 50’C, the cross-linkages in its molecules break down; the protein molecules lose their shape and will not usually regain it even when cooled. Eg. Egg white.


Structure of DNA

  • Made up of long chains of nucleotides, forming into two strands.
  • A Nucleotide is a 5-carbon sugar molecule joined to a phosphate group (-PO3) and an organic base
  • In DNA the sugar is deoxyribose
  • The organic base is either adenine (A), thymine (T), cytosine (C) or guanine (G).

The nucleotides are joined by their phosphate groups to form long chain, often thousands of nucleotides long. The phosphate and sugar molecules are the same all the way down the chain but the bases may be any one of the four listed.

The DNA in a chromosome consist of two strands (chains of nucleotides) held together by chemical bonds between the bases.

The size of the molecules ensures that A always pairs with T, and C pairs with G.

The double strand is twisted to form a helix.


  • Most cells contain about 75% water and will die if their water content falls much below this
  • Water is a good solvent and many substances move about the cells in a watery solution.
  • In green plants, water combines with CO2 to form sugar
  • In animals, water helps to break down and digest food molecules.
  • Blood is made up of cells and a liquid called plasma. Plasma is 92% water and acts as a transport medium for many dissolved substances. Blood cells are carried around the body in the plasma.
  • Water also acts as a transport medium in plants. Water passes up the plant from the roots to the leaves in xylem vessels and carries with it dissolved mineral ion. Phloem vessels transport sugars and amino acids in solution from leaves to their places of use or storage.
  • Water plays an important role in excretion in animals. It acts as a powerful solvent for excretory materials.
  • Water has a diluting effect, reducing the toxicity if the excretory materials.
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