Sciences - Co-ordinated (Double) (0654) | Biological molecules

🧬 Chapter B4: Biological Molecules – The Building Blocks of Life

Hello future biologist! This chapter is all about the essential chemicals that make up every living thing, from tiny bacteria to massive trees, and even you! These vital chemicals are called Biological Molecules.
Understanding them is crucial because they govern how we get energy (carbohydrates), how we grow and repair (proteins), and how we store long-term energy (fats). Let's dive into the chemistry that makes life possible!

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1. The Chemical Ingredients (Elements)

All biological molecules are organic molecules, meaning they are primarily built around chains of carbon atoms.

What elements are they made of? (Core 1)

• Carbohydrates (like sugar and starch): Consist of Carbon (C), Hydrogen (H), and Oxygen (O). (Think of it as CHO).
• Fats and Oils (Lipids): Also consist of Carbon (C), Hydrogen (H), and Oxygen (O), but with much less oxygen relative to hydrogen.
• Proteins: Consist of Carbon (C), Hydrogen (H), Oxygen (O), and critically, Nitrogen (N). (Often include Sulfur, but stick to the required CHON list).

Memory Aid: Remember the main elements in proteins using the acronym CHON (Carbon, Hydrogen, Oxygen, Nitrogen).

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2. Large Molecules from Small Ones

In Biology, we deal with giant molecules (polymers) made up of many tiny, repeating units (monomers). Think of it like building a huge LEGO model (the polymer) from many identical small LEGO bricks (the monomers).

2.1 Carbohydrates (Core 2a)

The basic building block (monomer) for most carbohydrates is the sugar glucose.

• Monomer: Glucose (a simple, reducing sugar).
• Polymers: Glucose molecules link together to form large, complex molecules (polysaccharides):
  • Starch: Used for energy storage in plants (like potatoes).
  • Glycogen: Used for energy storage in animals (stored primarily in the liver and muscles).
  • Cellulose: Used to build plant cell walls (provides structural support).

2.2 Proteins (Core 2b)

Proteins are the 'workers' of the cell, responsible for everything from transport to fighting infection (antibodies).

• Monomer: Amino acids.
• Polymer: Proteins.

There are 20 different types of amino acids, and they link up in long, complex chains that then fold into specific 3D shapes. This shape is essential for the protein's function!

2.3 Fats and Oils (Lipids) (Core 2c)

Fats (solid at room temperature) and oils (liquid at room temperature) are lipids. They are crucial for long-term energy storage and insulation.

• Building Blocks: Unlike carbohydrates and proteins, fats are assembled from two different types of smaller molecules: fatty acids and glycerol. (Typically, one glycerol molecule joins with three fatty acid molecules).

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3. Testing for Biological Molecules (Food Tests)

How do we prove a food contains starch, sugar, protein, or fat? We use chemical tests that result in a specific, noticeable colour change (qualitative analysis).

3.1 Test for Starch (Core 3a)

This is the simplest test. We use Iodine Solution.

Procedure:

1. Add a few drops of iodine solution directly onto the food sample (or into a suspension of the food).

Results:

• Negative (No starch): Solution remains the colour of iodine (yellow-brown/orange).
• Positive (Starch present): Solution changes instantly to blue-black.

3.2 Test for Reducing Sugars (Core 3b)

This test detects simple sugars like glucose (but not complex sugars like sucrose, unless they are boiled first). The key reagent is Benedict's Solution, and it requires heat.

Procedure:

1. Place about 2 cm³ of the food solution into a test tube.
2. Add about 2 cm³ of Benedict's solution (which is blue).
3. Place the test tube in a beaker of boiling water (a boiling water bath) for 2 to 5 minutes.

Results:

• Negative (No reducing sugar): Solution remains blue.
• Positive (Reducing sugar present): The blue solution changes colour and forms a coloured solid (precipitate). The colour progression depends on the amount of sugar:
  • Low concentration: Blue $\to$ Green $\to$ Yellow
  • High concentration: Orange $\to$ Brick-red

Common Mistake Alert: Always remember Benedict's test requires heating! If you forget to heat it, you will get a false negative.

3.3 Test for Proteins (Core 3c)

This test uses Biuret reagent (often supplied as two solutions: sodium hydroxide and copper sulfate, or sometimes ready-mixed).

Procedure:

1. Add sodium hydroxide solution to the food solution.
2. Add a few drops of copper(II) sulfate solution.
3. Shake gently.

Results:

• Negative (No protein): Solution remains blue.
• Positive (Protein present): Solution changes to purple or lilac.

3.4 Test for Fats and Oils (The Ethanol Emulsion Test) (Core 3d)

Fats and oils (lipids) do not dissolve in water, but they do dissolve in organic solvents like ethanol. This principle is used for this test.

Procedure:

1. Chop up the food sample and shake it thoroughly with about 2 cm³ of ethanol in a clean test tube. (This dissolves any fat present).
2. Pour the ethanol solution into a separate test tube containing an equal amount of cold water.

Results:

• Negative (No fat/oil): The water remains clear.
• Positive (Fat/Oil present): The lipid, which was dissolved in the ethanol, cannot dissolve in the water and precipitates out, forming a visible white cloudiness called a milky emulsion.

You've completed the building blocks! These molecules are the foundation for the next chapters on enzymes and nutrition. Great work!