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Biological Molecules - Carbohydrates, lipids and proteins

Grade 11A LevelBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Biological molecules are primarily composed of carbon (CC), hydrogen (HH), and oxygen (OO). Proteins also contain nitrogen (NN) and sometimes sulfur (SS).

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Carbohydrates include simple sugars (monosaccharides like glucose, C6H12O6C_6H_{12}O_6) and complex polymers (polysaccharides like starch, glycogen, and cellulose).

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Lipids are made of one glycerol molecule joined to three fatty acid chains. They are used for long-term energy storage and insulation.

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Proteins are long chains of amino acids. The sequence of amino acids determines the 3D3D shape and function of the protein (e.g., enzymes, antibodies).

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Starch and glycogen are storage molecules made of glucose, while cellulose forms the plant cell wall.

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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

C6H12O6 (Glucose)C_6H_{12}O_6 \text{ (Glucose)}

Cn(H2O)n (General formula for carbohydrates)C_n(H_2O)_n \text{ (General formula for carbohydrates)}

2C6H12O6→C12H22O11+H2O (Maltose formation via condensation)2C_6H_{12}O_6 \rightarrow C_{12}H_{22}O_{11} + H_2O \text{ (Maltose formation via condensation)}

Glycerol+3×Fatty Acids→Triglyceride+3H2O\text{Glycerol} + 3 \times \text{Fatty Acids} \rightarrow \text{Triglyceride} + 3H_2O

💡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, C6H12O6C_6H_{12}O_6).

Explanation:

Benedict's reagent contains copper(II) ions (Cu2+Cu^{2+}). When heated with a reducing sugar, the sugar reduces the copper ions to copper(I) oxide (Cu2OCu_2O), 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 C,H,C, H, and OO, while proteins contain C,H,O,N,C, H, O, N, and often SS.

Explanation:

Both molecules share C,H,C, H, and OO. However, the amino group (−NH2-NH_2) in amino acids introduces nitrogen (NN) into proteins, and certain R-groups (like in cysteine) introduce sulfur (SS).