1. Overview
Biological molecules are the fundamental chemical building blocks that make up all living organisms. Understanding their structure, composition, and how to identify them is essential for studying how organisms grow, repair tissues, and obtain energy.
Key Definitions
- Monomer: A small, single molecule that can be bonded to other identical molecules to form a polymer (e.g., glucose).
- Polymer: A large molecule made up of many repeating smaller units (monomers) joined together (e.g., starch).
- Reducing Sugar: A type of sugar (like glucose or maltose) that can donate electrons to another chemical, resulting in a color change during a chemical test.
- Double Helix: The structural shape of DNA, consisting of two strands coiled around each other.
Core Content
A. Chemical Elements in Biological Molecules
All living things are composed primarily of four elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).
- Carbohydrates: Contain Carbon, Hydrogen, and Oxygen.
- Fats (Lipids): Contain Carbon, Hydrogen, and Oxygen.
- Proteins: Contain Carbon, Hydrogen, Oxygen, and Nitrogen (some also contain Sulfur).
B. Large Molecules from Smaller Units
Large biological molecules (macromolecules) are synthesized from smaller subunits:
- Starch, Glycogen, and Cellulose: All are large polysaccharides made from glucose units.
- Starch: Energy storage in plants.
- Glycogen: Energy storage in animals/fungi.
- Cellulose: Structural component of plant cell walls.
- Proteins: Formed from long chains of amino acids.
- Fats and Oils (Lipids): Each lipid molecule is made of one glycerol molecule attached to three fatty acids.
A central glycerol molecule shown as a vertical block with three horizontal "tails" representing fatty acids, forming an 'E' shape.
C. Food Tests (Step-by-Step)
| Test | Substance | Reagent | Method | Positive Result |
|---|---|---|---|---|
| Starch | Starch | Iodine Solution | Add a few drops of iodine to the sample. | Blue-black |
| Reducing Sugars | Glucose/ Maltose | Benedict’s Solution | Add reagent and heat in a water bath (approx. 80°C). | Green $\rightarrow$ Yellow $\rightarrow$ Orange $\rightarrow$ Brick red |
| Proteins | Amino acids/ Protein | Biuret Reagent | Add Biuret solution to the sample. | Purple / Violet |
| Fats & Oils | Lipids | Ethanol Emulsion | Dissolve sample in ethanol, then pour into water. | Cloudy-white emulsion |
| Vitamin C | Ascorbic acid | DCPIP | Add sample drop-by-drop to a fixed volume of DCPIP. | Blue $\rightarrow$ Colorless |
Extended Content (Extended Only)
DNA Structure
Deoxyribonucleic acid (DNA) is the molecule that contains genetic information. Its structure is linked closely to its function of storing and copying data.
- Shape: It consists of two long strands coiled together to form a double helix.
- Bases: Each strand contains chemical groups called bases. There are four types: A, T, C, and G.
- Pairing: The two strands are held together by bonds between these bases.
- Complementary Base Pairing: Bases always pair specifically:
- A always pairs with T
- C always pairs with G
A twisted ladder (double helix) where the "rungs" are labeled A-T and C-G, showing the specific pairing across the two phosphate-sugar backbones.
Key Equations
While there are no complex mathematical formulas for this topic, you should understand the molecular ratio of a lipid:
- 1 Lipid Molecule = 1 Glycerol + 3 Fatty Acids
- Numerical Data: In experiments (like DCPIP titration), you may need to calculate the volume of juice required to decolorize the reagent.
- Example: If 5.0ml of Juice A decolorizes DCPIP, and 1.1ml of Juice B decolorizes it, Juice B has a higher concentration of Vitamin C.
Common Mistakes to Avoid
- ❌ Wrong: Forgetting to heat the Benedict's test.
- ✓ Right: Benedict’s reagent only reacts with reducing sugars if it is heated in a water bath.
- ❌ Wrong: Stating that proteins contain only C, H, and O.
- ✓ Right: You must mention Nitrogen; it is the distinguishing element for proteins.
- ❌ Wrong: Describing the Biuret result as "pink."
- ✓ Right: Use the terms purple or violet to ensure full marks.
- ❌ Wrong: Confusing the bases in DNA (e.g., pairing A with C).
- ✓ Right: Remember the rule: A-T and C-G.
Exam Tips
- Command Word "State": This is the most common command word for this topic (found 10x in recent papers). It requires a brief, one-word or one-sentence answer (e.g., "State the elements in protein" $\rightarrow$ "C, H, O, and N").
- Food Test Tables: You are frequently asked to identify an "unknown" solution based on a table of test results. Look for the color changes: Blue-black always means starch; Brick-red always means reducing sugar.
- Real-World Context: Questions often involve testing fruit juices for Vitamin C or identifying the contents of a specific food item like milk or bread.
- Typical Values: Be prepared to handle small decimal numbers like 1.1 or 5.0 in titration/Vitamin C tables; always include units (ml or $cm^3$) if they are not provided in the table header.