Flashcards · 5090 · Topic 4

Biological molecules flashcards

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Chemical elements in carbohydrates, lipids and proteins: Carbohydrates and lipids: carbon, hydrogen, oxygen. Proteins: carbon, hydrogen, oxygen, nitrogen (some also sulfur). Nitrogen marks out proteins.
Starch, glycogen and cellulose: what they are made from and their roles: All are made from glucose. Starch: energy store in plants. Glycogen: store in animals. Cellulose: strength in plant cell walls.
Lipid: One glycerol + three fatty acids. Lipids release more energy per gram than carbohydrates when respired.
Protein: A chain of amino acids folded into a specific shape. The sequence of amino acids determines the shape, and the shape determines the function, e.g. of enzymes and antibodies.
Test for starch: Add iodine solution: orange-brown → blue-black if starch is present.
Test for reducing sugars (glucose, maltose): Add Benedict's solution and heat in a water bath: blue → green → yellow → brick-red. The further along the sequence, the more reducing sugar.
Test for protein: Biuret test (or sodium hydroxide then a few drops of copper sulfate): blue → purple/lilac. No heating.
Test for lipids: Ethanol emulsion test: shake the food with ethanol, then add water. A white cloudy emulsion shows lipid.
Which of the four food tests needs heating?: Only Benedict's (in a water bath). Iodine, biuret and the emulsion test are done at room temperature.