There are 3 important biological molecules to know for IGCSE Biology: carbohydrates, proteins and lipids

Carbohydrates

• Carbohydrates are made of carbon, hydrogen and oxygen atoms.
• They store energy in our bodies or serve as structural components. 
• Carbohydrates can be classified as monosaccharides, disaccharides, or polysaccharides.

• Monosaccharide is a simple sugar that consists of only one unit. ("mono" means one)
• They serve as building blocks for more complex carbohydrates, such as disaccharides or polysaccharides.
• Examples: glucose, fructose, galactose

• Disaccharide forms when two monosaccharides are joined together. ("di" means two)
• Examples
  • Maltose is formed by joining two glucose molecules
  • Sucrose is formed by joining glucose and fructose
  • Lactose is formed by joining glucose and malactose

• Polysaccharide forms when many monosaccharides are joined into a chain ("poly" means many)
• Examples
  • Glycogen stores energy in animals
  • Starch stores energy in plants
  • Cellulose is the main component of cell walls in plants

Proteins

• Proteins consist of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur atoms.
• Proteins are one of the most important biological molecules in our body since they do most of the work in cells and are required for the structure, maintenance and function of tissues and organs.
• Proteins are made up of hundreds or thousands of smaller units called amino acids. 
• There 20 different kinds of naturally occurring amino acids, and the sequence of the amino acids determines the structure and function of each protein.

Lipids

• Lipids consist of carbon, hydrogen and oxygen atoms.
• Some functions of lipids include energy storage and insulation.
• Lipids are made of glycerol molecules and fatty acid chains.
• Triglyceride, which is a type of lipid that is most common in our bodies, consist of 1 glycerol molecule and 3 fatty acid chains.

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Frequently asked on IGCSE Biology exams is... 👇

Tests to Detect Carbohydrates, Proteins and Lipids

Test for Reducing Sugars (example: glucose, fructose, galactose, maltose, lactose)

• Add the sample solution into a test tube.
• Add drops of Benedict's solution to the test tube.
• Heat in a water bath for 5 minutes.
• If reducing sugar is not present in the sample solution, the solution will remain blue. 
• If reducing sugar is present in the sample solution, the solution will turn green (low concentration), yellow, or brick red (high concentration).

Test for Starch

• Add drops of iodine solution to the sample.
• If starch is present, the iodine solution turns blue-black.
• If starch is not present, the iodine solution will remain yellowish brown.

Test for Protein

• Add drops of Biuret reagent to the sample.
• If there is protein in the sample, the solution turns purple.
• If there is no protein in the sample, the solution will remain blue.

Test for Lipid

• Add 95% ethanol to the sample and then mix with an equal volume of distilled water.
• If there is lipid in the sample, a milky-white emulsion forms.
• If there is no lipid, the solution remains colorless.

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