Biology (0610) | Biological molecules

Welcome to Biological Molecules! (IGCSE Biology 0610 Topic 4)

Hello Biologists! This chapter is incredibly important because it covers the basic building blocks of all life on Earth—including you! We will look at the essential molecules that make up our food and our cells: carbohydrates, fats, proteins, and the amazing molecule of life, DNA. Understanding these foundations will make future topics like nutrition, enzymes, and genetics much easier. Let's get started!

Part 1: The Essential Ingredients (Chemical Elements)

Every living thing is built from just a few basic chemical elements. These elements combine to form the large biological molecules (also called macromolecules) that run our cells.

1.1 Elements in Biological Molecules (Core 4.1.1)

• Carbohydrates (like sugar and starch) are made up of: Carbon, Hydrogen, and Oxygen (C, H, O).
• Fats and Oils (Lipids) are also made up of: Carbon, Hydrogen, and Oxygen (C, H, O).
• Proteins are much more complex and always contain: Carbon, Hydrogen, Oxygen, and Nitrogen (C, H, O, N). They sometimes also contain Sulfur.

Part 2: The Building Blocks (Monomers and Polymers)

Think of large biological molecules like a long chain of Lego bricks. The individual bricks are the small molecules, and the completed chain is the large molecule.

Key Definitions:

• Monomer: The small unit molecule (like a single Lego brick).
• Polymer: The large molecule made by joining many monomers together (the long Lego chain).

2.1 Carbohydrates (Core 4.1.2a)

Carbohydrates are the main energy source for most organisms.

• The small molecule (monomer) is Glucose (a simple sugar).
• Glucose monomers join together to form large molecules (polymers) like:
  • Starch: Used as an energy store in plants (like potatoes).
  • Glycogen: Used as an energy store in animals and fungi (stored in our liver and muscles).
  • Cellulose: Used to build plant cell walls (provides structural support).

2.2 Proteins (Core 4.1.2b)

Proteins have many roles: making enzymes, hormones, antibodies, and building muscle.

• The small molecule (monomer) is the Amino Acid.
• Amino acids link together in long chains (polymers) to make Proteins.

2.3 Fats and Oils (Lipids) (Core 4.1.2c)

Fats and oils (lipids) are used for long-term energy storage, insulation, and forming cell membranes.

• The large molecule (lipid) is formed from two smaller components: Fatty Acids and Glycerol.

Don't worry, you don't need to know the complex names of the bonds that join these units, just the main molecules involved!

Part 3: The Blueprint of Life - DNA (Supplement 4.1.4)

The genetic material, DNA (Deoxyribonucleic acid), is a special biological molecule that carries the instructions for making proteins.

3.1 Structure of DNA

DNA has a very famous structure described as a double helix. Imagine a ladder that has been twisted up!

• Strands: The DNA molecule is made up of two strands.
• Coiling: These two strands are coiled together to form the double helix shape.
• Chemical Bases: Each strand contains specific chemicals called bases.
• Base Pairing: Bonds between pairs of bases hold the two strands together in the middle (the rungs of the ladder).

3.2 Complementary Base Pairing

The bases do not pair randomly. They follow a strict rule called Complementary Base Pairing:

• Always pairs with T (Adenine pairs with Thymine).
• Clways pairs with G (Cytosine pairs with Guanine).

Part 4: Testing for Biological Molecules (Food Tests) (Core 4.1.3)

In the lab, we use simple chemical tests to find out which biological molecules are present in a food sample. You need to know the reagent (the chemical added) and the resulting colour change for a positive result.

4.1 Test for Starch

• Reagent: Iodine Solution (usually dissolved in potassium iodide solution).
• Starting Colour: Brown or orange-yellow.
• Positive Result: The solution turns blue-black (or dark purple/black).
• Method Note: This test is usually done at room temperature.

4.2 Test for Reducing Sugars (e.g., Glucose)

Reducing sugars are simple sugars like glucose.

• Reagent: Benedict’s Solution.
• Starting Colour: Blue.
• Method Note: You must heat the mixture (usually in a hot water bath) for a few minutes.
• Positive Result: A colour change occurs, forming a precipitate. The colour sequence is Blue → Green → Yellow → Orange → Brick-Red. A brick-red colour indicates a high concentration of reducing sugar.

4.3 Test for Proteins

• Reagent: Biuret Test (often uses sodium hydroxide solution followed by copper sulfate solution, sometimes sold as a single Biuret reagent).
• Starting Colour: Blue.
• Positive Result: The solution turns lilac or purple.
• Method Note: This test is done at room temperature.

4.4 Test for Fats and Oils (Lipids)

• Test Name: Ethanol Emulsion Test.
• Method:
  1. Mix the food sample with ethanol (alcohol).
  2. Pour the ethanol solution into a test tube containing cold water.
• Positive Result: A cloudy white emulsion (a milky appearance) forms in the water.
• Analogy: If you stir a clear oil into clear water, it mixes. But if you dissolve the oil in alcohol first, when you add it to water, the lipid comes out of solution and creates a visible, cloudy layer, like milk!

4.5 Test for Vitamin C

Vitamin C (Ascorbic Acid) is an essential vitamin, but unlike the others, it is not a large macromolecule.

• Reagent: DCPIP (Dichlorophenolindophenol) solution.
• Starting Colour: Blue.
• Method: Add the food sample (usually a juice) drop-by-drop to a small volume of DCPIP solution.
• Positive Result: If Vitamin C is present, the blue DCPIP solution is reduced (chemically changed) and becomes colourless.
• Note: The more drops of juice needed to turn the DCPIP colourless, the less Vitamin C the food contains.