Summary of "IGCSE Biology - Biological Molecules (4.1)"

IGCSE Biology — Topic 4.1: Biological molecules

Four major classes of biological molecules: carbohydrates, lipids (fats/oils), proteins and nucleic acids. Carbon is the primary element common to all four classes.
Polymers and monomers: many biological molecules are long chains (polymers) built from repeating subunits (monomers).

Elements: carbon, hydrogen and oxygen.
Monomer example: glucose (a simple soluble sugar).
Polysaccharides formed from glucose:
- Starch: plant storage molecule; large, often branched and relatively insoluble so it does not pass freely through membranes.
- Glycogen: animal storage molecule; highly branched and insoluble.
- Cellulose: long chains of glucose arranged into microscopic fibrous layers that make up the plant cell wall.

Elements: carbon, hydrogen and oxygen (different proportions and structure compared with carbohydrates).
Typical structure (triglyceride): three fatty acid molecules joined to one glycerol molecule.
Properties and functions:
- Triglycerides are insoluble in water and serve as energy storage.
- Fats are components of cell membranes and internal membranes.

Polymers of amino acids. There are 20 different amino acids; each protein has a specific amino-acid sequence.
All amino acids contain carbon, hydrogen and oxygen and nitrogen; some also contain sulfur and phosphorus.
Roles of proteins:
- Enzymes (biological catalysts)
- Hormones (signalling molecules)
- Hemoglobin (oxygen transport)
- Structural components of organelles and membranes

Starch (iodine test)
- Procedure:
  - Make a starch suspension (mix starch powder with warm water) in a test tube.
  - Add a few drops of iodine solution.
- Positive result: iodine changes color from brown to blue–black.
Reducing sugars (Benedict’s test; e.g., glucose, fructose)
- Procedure:
  - Mix equal volumes of the sugar solution and Benedict’s solution in a test tube.
  - Heat the mixture.
- Result: color change depends on concentration — blue → green → yellow → brick-red (red/brown precipitate of copper oxide) as reducing sugar amount increases.
Protein (Biuret test)
- Procedure:
  - Mix equal volumes of the protein solution (e.g., albumin from egg white), dilute sodium hydroxide (NaOH) and copper sulfate (CuSO4) solution.
- Positive result: Biuret reagent changes from pale blue to purple.
Lipids (ethanol emulsion test)
- Procedure:
  - Add a few drops of oil to ethanol in a test tube and shake until the fat dissolves.
  - Add water and observe.
- Positive result: formation of a milky-white emulsion.
Vitamin C (DCPIP test)
- Procedure:
  - Place a weak solution of DCPIP (blue dye) in a test tube.
  - Add fresh lemon juice dropwise using a syringe.
- Positive result: DCPIP is decolorized (blue → colorless). More drops required for lower vitamin C concentrations (e.g., orange juice vs lemon).

DNA (deoxyribonucleic acid) is made of two long strands of nucleotides coiled into a double helix.
Each nucleotide consists of:
- a five-carbon sugar (deoxyribose),
- a phosphate group, and
- an inorganic base.
Four bases: adenine (A), thymine (T), cytosine (C) and guanine (G).
The two strands are held together by chemical bonds between bases; base pairing is specific: A pairs with T, and C pairs with G. This specific pairing is essential for DNA replication and function.