Overview
This lecture covers the main types of biological molecules—carbohydrates, lipids, and proteins—their structure, function, and the chemical tests used to detect them in food samples.
Biological Molecules: Importance and Elements
- Biological molecules are the essential building blocks of cells and living organisms.
- The three most important biological molecules are carbohydrates, lipids, and proteins.
- Carbohydrates and lipids both contain carbon, hydrogen, and oxygen; proteins also contain nitrogen.
Carbohydrates
- Carbohydrates provide energy for respiration and are mainly obtained as starch from foods like potatoes, rice, and pasta.
- Plants store carbohydrates as insoluble starch; animals store them as glycogen.
- Enzymes break down starch into glucose, the single sugar molecule used in respiration.
Lipids
- Lipids include fats and oils, found in animal products (meat, butter, cheese, eggs) and plant oils (olive, sunflower, rapeseed).
- Functions: making cell membranes, insulation, organ protection, and long-term energy storage.
- Lipids are made of glycerol and three fatty acid chains; digestion breaks them into these components.
Proteins
- Proteins are vital for growth, repair, and cell structure, and are made from amino acids.
- High-protein foods: meat, fish, cheese, eggs, beans, peas, and nuts.
- Proteins are complex, folded molecules formed by chains of up to thousands of amino acids.
- Enzymes and many hormones are proteins.
Food Tests for Biological Molecules
- Starch: add iodine solution (yellow-brown); turns blue-black if starch is present.
- Glucose: add Benedict's solution (blue) and heat; turns green, orange, or brick red if glucose is present (intensity indicates amount).
- Protein: add Biuret solution (blue); turns lilac/purple if protein is present.
- Lipids: add ethanol, shake, then add water; a cloudy white emulsion indicates lipids are present.
Key Terms & Definitions
- Carbohydrate — molecule made of carbon, hydrogen, and oxygen, used for energy.
- Lipid — fat or oil molecule made of glycerol and fatty acids, used for energy storage and cell structure.
- Protein — large molecule made of amino acids, necessary for growth, repair, enzymes, and hormones.
- Amino acid — building block of proteins.
- Enzyme — protein that catalyzes chemical reactions.
- Glycogen — storage form of carbohydrate in animals.
- Starch — storage form of carbohydrate in plants.
Action Items / Next Steps
- Learn the structure and function of carbohydrates, lipids, and proteins.
- Memorize the food tests for starch, glucose, protein, and lipids and how to perform them.
- Review summary tables for chemical food tests.