Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Biological molecules are primarily composed of carbon (), hydrogen (), and oxygen (). Proteins also contain nitrogen () and sometimes sulfur ().
Carbohydrates include simple sugars (monosaccharides like glucose, ) and complex polymers (polysaccharides like starch, glycogen, and cellulose).
Lipids are made of one glycerol molecule joined to three fatty acid chains. They are used for long-term energy storage and insulation.
Proteins are long chains of amino acids. The sequence of amino acids determines the shape and function of the protein (e.g., enzymes, antibodies).
Starch and glycogen are storage molecules made of glucose, while cellulose forms the plant cell wall.
Chemical tests: Iodine solution tests for starch (color change: orange-brown to blue-black); Benedict's solution tests for reducing sugars (blue to brick-red precipitate); Biuret reagent tests for proteins (blue to violet); Ethanol emulsion tests for lipids (formation of a cloudy white emulsion).
📐Formulae
💡Examples
Problem 1:
A student adds Benedict's solution to a solution of unknown sugar and heats it in a water bath. The color changes from blue to green and finally to brick-red. Identify the substance and explain the result.
Solution:
The substance is a reducing sugar (e.g., glucose, ).
Explanation:
Benedict's reagent contains copper(II) ions (). When heated with a reducing sugar, the sugar reduces the copper ions to copper(I) oxide (), which forms a brick-red precipitate. The color intensity indicates the concentration of the sugar.
Problem 2:
Contrast the elemental composition of a triglyceride and a typical protein.
Solution:
Triglycerides contain and , while proteins contain and often .
Explanation:
Both molecules share and . However, the amino group () in amino acids introduces nitrogen () into proteins, and certain R-groups (like in cysteine) introduce sulfur ().