Biological Molecules: Lab 3

Biological Molecules in Living Systems

Introduction

Living organisms are composed of a diverse array of organic and inorganic molecules. These molecules are essential for the structure, function, and regulation of the body's tissues and organs. This section introduces the major classes of biological molecules and their roles in living systems.

• Organic molecules include carbohydrates, proteins, lipids, and nucleic acids.

• Inorganic molecules such as water and mineral ions are also vital for proper functioning.

• General laboratory analysis of these molecules is fundamental in biology and biochemistry.

Inorganic Ions in Living Systems

Role and Importance

Inorganic ions, though not as abundant as organic molecules, are crucial for many physiological processes. They are involved in nerve impulses, muscle contractions, and maintaining osmotic balance.

• Sodium (Na+): Major cation in extracellular fluid; essential for nerve function and fluid balance.

• Potassium (K+): Major cation in intracellular fluid; important for nerve impulses and muscle contraction.

• Calcium (Ca2+): Bone and teeth structure, blood coagulation, nerve impulse transmission.

• Magnesium (Mg2+): Enzyme activation, component of chlorophyll in plants.

• Iron (Fe2+, Fe3+): Component of hemoglobin, electron carrier molecules, transport of oxygen.

• Zinc (Zn2+): Enzyme cofactor, gene regulation.

• Cobalt (Co2+): Component of vitamin B12.

• Iodine (I-): Thyroid gland function.

Nonmetals such as H, C, N, O form the bulk of biological macromolecules.

Table: Major Inorganic Ions and Their Functions

Ion	Main Function
Na+	Extracellular fluid balance, nerve impulses
K+	Intracellular fluid balance, nerve impulses
Ca2+	Bone/teeth, blood clotting, muscle contraction
Mg2+	Enzyme cofactor, chlorophyll
Fe2+/Fe3+	Hemoglobin, electron transport
Zn2+	Enzyme cofactor, gene regulation
Co2+	Vitamin B12 component
I-	Thyroid hormones

Carbohydrates

Structure and Classification

Carbohydrates are molecules commonly known as sugars and starches. They serve as energy stores and structural molecules. Carbohydrates are classified based on the number of sugar units:

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides linked together (e.g., maltose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose).

Monosaccharides can be joined by glycosidic bonds to form disaccharides and polysaccharides.

Examples of Carbohydrate Structures

• Glucose (monosaccharide): C6H12O6

• Maltose (disaccharide): Two glucose units joined by an α(1→4) glycosidic bond

• Amylose (polysaccharide): Linear chain of glucose units

Benedict's Test for Reducing Sugars

Benedict's test is used to identify reducing sugars, which can donate electrons to other molecules. Reducing sugars have a free carbonyl group (aldehyde or ketone) that can reduce copper(II) ions to copper(I) oxide, resulting in a color change.

• Principle: Reducing sugars + Cu2+ (blue) → Cu2O (red precipitate)

• Color change: Blue → green → yellow → orange → red (depending on concentration)

Procedure:

1. Fill a beaker with boiling water.

2. Label test tubes and add 2 ml of different sugar solutions (e.g., glucose, sucrose, maltose, starch, etc.).

3. Add 2 ml of Benedict's reagent to each tube.

4. Place tubes in boiling water for two minutes.

5. Observe and record color changes.

IKI Test for Polysaccharides

The IKI (iodine-potassium iodide) test is used to detect polysaccharides, especially starch. Iodine interacts with the helical structure of amylose, producing a blue-black color.

• Positive result: Blue-black color indicates presence of starch.

• Negative result: No color change.

Procedure:

1. Label test tubes and add 2 ml of various solutions (distilled water, glucose, sucrose, glycogen, potato extract, etc.).

2. Add one drop of IKI solution to each tube and shake.

3. Observe and record results.

Lipids

Structure and Properties

Lipids are a diverse group of hydrophobic molecules, including fats, oils, waxes, and phospholipids. They are characterized by their insolubility in water and solubility in non-polar solvents.

• Fats and oils: Composed of glycerol and fatty acids (triglycerides).

• Phospholipids: Major component of cell membranes; contain a phosphate group.

• Steroids: Four fused carbon rings (e.g., cholesterol).

Tests for Lipids

• Grease Spot Test: Lipids leave a translucent spot on unglazed paper due to their non-polar nature.

• Sudan Black Test: Sudan Black dye stains lipids intensely because it is soluble in non-polar solvents.

Procedure for Sudan Black Test:

1. Add 3 ml of test solution (e.g., oil, water, potato extract) to test tubes.

2. Add 2 drops of Sudan Black to each tube, shake, and observe color change.

Discussion Questions

1. Explain the differences in reducing sugar test results for monosaccharides, disaccharides, and polysaccharides.

2. Why is it important not to heat samples for more than two minutes in Benedict's test?

3. Which component of starch (amylose/amylopectin) is primarily responsible for the blue color in the IKI test?

4. Describe the chemical linkages between monosaccharides in different polysaccharides.

5. What is the difference between a fat and a fatty acid?

6. Draw and compare the structures of triglycerides and phospholipids.

7. What properties of phospholipids make them suitable for forming membranes?

Additional info: The notes include both experimental procedures and theoretical background, suitable for a General Biology college course. The content covers essential biochemical tests and the structure-function relationship of major biomolecules.