Hello Future Biologists! Welcome to Biological Molecules

Ready to unlock the secrets of what makes up every living thing—including you? This chapter, Biological Molecules, is foundational. These tiny chemical components are the building blocks, the fuel, and the workers inside every cell. Understanding them is like learning the alphabet before reading a whole book!

Don't worry if some of the names look complicated. We will break them down step-by-step using simple analogies. Let's get started!


The Essential Ingredients: Water

Before we look at the big organic molecules, we must remember the most important one: Water (\(H_2O\)). Your body is about 60-70% water, and it is vital for almost every process.

Key Roles of Water in the Human Body:

  • Solvent: Water is an excellent solvent, meaning it can dissolve many substances (like salts and glucose). This is crucial for transport—everything in your blood (nutrients, waste) is dissolved in water.
  • Transport: It forms the main component of blood plasma, allowing substances to be moved around the body.
  • Temperature Regulation: When you sweat, the evaporation of water from your skin absorbs a large amount of heat, cooling you down.
  • Chemical Reactions: Many reactions in the body (like digestion) require water to occur.

Quick Takeaway: Water is the body's universal solvent and temperature regulator.


The Big Three Organic Molecules (Macromolecules)

These large molecules all contain Carbon (C), Hydrogen (H), and Oxygen (O). Proteins also contain Nitrogen (N).

1. Carbohydrates (The Fast Fuel)

Think of carbohydrates as the body’s fast, easy-to-access energy source, like kindling for a fire.

Structure and Types:

Carbohydrates are built from smaller units called sugars.

  • Monosaccharides (Single Sugars): The simplest form. The most important one is Glucose. This is the main fuel used by cells for respiration.
  • Polysaccharides (Complex Sugars): Large molecules made by joining many monosaccharides together.
    • In plants, energy is stored as Starch.
    • In humans and animals, energy is stored as Glycogen, found mainly in the liver and muscles.
Function in the Human Body:

The primary function is Energy Supply. Glucose is broken down during respiration to release energy (ATP).

Quick Review Box: Carbohydrates

Building Block: Glucose (monosaccharide)
Human Storage Form: Glycogen
Primary Role: Fast energy source


2. Lipids (The Long-Term Battery)

Lipids include fats and oils. They are less dense and harder to break down than carbs, making them excellent for long-term storage—like the body’s deep reserve battery.

Structure:

Most lipids (fats) are formed from two main components:

  • One molecule of Glycerol
  • Three molecules of Fatty Acids

Did you know? Lipids contain proportionally much less oxygen than carbohydrates do. This packed structure allows them to store more energy per gram.

Functions in the Human Body:
  • Energy Storage: They store huge amounts of energy for later use.
  • Insulation: The layer of fat under the skin helps keep you warm (thermal insulation).
  • Protection: They provide padding around delicate organs, like kidneys.
  • Cell Membranes: Lipids form the basic structure of all cell membranes.

Quick Takeaway: Lipids are for long-term energy storage, insulation, and structure.


3. Proteins (The Workers and Builders)

Proteins are the most diverse molecules in the body. They are the enzymes, the antibodies, and the structural materials—they do most of the "work."

Structure and Assembly:

The building blocks of proteins are small molecules called Amino Acids.

Analogy: Think of amino acids as different coloured Lego bricks. You can link them in countless sequences to build different structures (proteins).

Step-by-Step Protein Formation:

  1. Amino acids link together in long chains.
  2. The bond between two amino acids is called a Peptide Bond.
  3. A chain of amino acids is called a Polypeptide.
  4. This polypeptide chain then folds up precisely into a specific 3D shape. The final 3D shape determines the protein's function!
Functions in the Human Body:

Their functions are incredibly varied:

  • Enzymes: They act as biological catalysts, speeding up chemical reactions (e.g., digestion).
  • Structural: They provide strength (e.g., Collagen in skin, bone, and cartilage).
  • Transport: They carry molecules around the body (e.g., Haemoglobin transports oxygen).
  • Immunity: They form Antibodies to fight disease.

Memory Trick (The Four N's of Protein): Proteins contain Nitrogen, are built from Numerous amino acids, and form eNzymes and aNtibodies.


Investigating Biological Molecules: Food Tests

In the lab, we use chemical tests to identify which biological molecules are present in a food sample. Make sure you know the reagent used and the specific colour change for a positive result!

1. Testing for Starch (A Complex Carbohydrate)

Reagent: Iodine Solution

  • Negative Result: Remains brown/orange-yellow.
  • Positive Result: Changes from brown/orange-yellow to Blue-Black.

2. Testing for Reducing Sugars (Simple Carbohydrates, e.g., Glucose)

Note: Sucrose is not a reducing sugar, but glucose and maltose are.

Reagent: Benedict's Solution (which is originally blue)

Process:

  1. Add Benedict's solution to the food sample.
  2. Heat the mixture gently in a water bath (about \(80^\circ C\)) for a few minutes.
  • Negative Result: Solution remains blue.
  • Positive Result: Solution changes colour, eventually forming a precipitate that can be green, yellow, orange, or Brick Red (the highest concentration of sugar).

Common Mistake Alert: Students often forget that the Benedict's test requires heating!

3. Testing for Protein

Reagent: Biuret Reagent (made of copper sulfate and sodium hydroxide, which is originally blue)

Process:

  1. Add a few drops of sodium hydroxide solution (alkali).
  2. Add a few drops of copper sulfate solution.
  • Negative Result: Solution remains blue.
  • Positive Result: Solution changes from blue to Lilac or Purple.

4. Testing for Lipids (Fats and Oils) - The Emulsion Test

Reagent: Ethanol (Alcohol)

Process:

  1. Mix the food sample with ethanol and shake well.
  2. Pour the ethanol mixture into a tube of cold water.
  • Negative Result: Water remains clear.
  • Positive Result: A cloudy, white layer or Emulsion forms in the water. (This happens because the fat dissolves in the alcohol but is insoluble in water, causing tiny droplets to suspend, making it look cloudy.)

Summary of Food Tests (Quick Reference)

Molecule Reagent Positive Result (Colour Change) Heating Required?
Starch Iodine Blue-Black No
Reducing Sugar Benedict's Brick Red Precipitate Yes
Protein Biuret Lilac/Purple No
Lipid Ethanol + Water Cloudy Emulsion No

Congratulations! You now understand the basic building blocks of life. These molecules are essential for everything from running a marathon (carbs) to fighting off a cold (proteins)! Keep up the great work!