Science - Combined (0653) | Biological molecules

🔬 B4 Biological Molecules: The Building Blocks of Life

Hello there! This chapter might seem like chemistry, but it’s actually the foundation of all biology. Biological molecules are the essential substances that make up every single living cell—from the food you eat to the muscles you use.

Understanding these molecules—carbohydrates, proteins, and fats (lipids)—is key to understanding nutrition, energy, and growth. Don't worry if the names sound complicated; we’ll break them down into simple, easy-todigest pieces!

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Part 1: The Essential Ingredients

The Chemical Elements

Everything organic contains a few basic elements. You must remember which elements form the major biological molecules:

• Carbohydrates: Contain Carbon (C), Hydrogen (H), and Oxygen (O).
• Fats (Lipids): Contain Carbon (C), Hydrogen (H), and Oxygen (O). (They typically have much less oxygen compared to carbohydrates).
• Proteins: Contain Carbon (C), Hydrogen (H), Oxygen (O), and most importantly, Nitrogen (N). Some also contain Sulfur (S).

Memory Aid: All three have C, H, O. Protein is special because it needs N (for Nitrogen) to build things like muscles!

Big Molecules from Small Parts (Monomers and Polymers)

Think of large biological molecules like a huge model built from Lego bricks.

• The small, single bricks are called Monomers.
• When many monomers join together, they form a large chain called a Polymer (or macromolecule).

The syllabus requires you to know the monomers (small molecules) that make up the large molecules (polymers):

Large Molecule (Polymer)	Made from Small Molecules (Monomer)	Examples of Polymers
Carbohydrates	Glucose (a simple sugar)	Starch, Glycogen, Cellulose
Proteins	Amino acids	Enzymes, Antibodies, Haemoglobin
Fats/Oils (Lipids)	Fatty acids and Glycerol	Fats stored under the skin

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Part 2: Carbohydrates

Carbohydrates are your body's main source of quick energy.

Structure and Storage

• The monomer is glucose (a simple sugar). Glucose is vital because it is the fuel used in respiration (B12) to release energy.
• Plants link many glucose molecules together to form starch for storage. (This is why potatoes and rice are starchy.)
• Animals (including you!) store glucose mainly as glycogen, especially in the liver and muscles.
• Cellulose is also made from glucose monomers, but it is arranged differently. It forms strong fibres used to make plant cell walls.

Did you know? Cellulose is what we call dietary fibre (roughage). Humans cannot digest it, but it is essential for keeping our digestive system healthy!

Testing for Carbohydrates (Food Tests)

We use chemical tests to identify whether food contains starch or sugars.

1. Test for Starch (Using Iodine Solution)

This is the simplest test!

1. Add a few drops of iodine solution to the food sample (often a liquid or scraped surface).
2. Result:
   • Positive: If starch is present, the colour changes from yellow/brown to blue-black.
   • Negative: If starch is absent, the solution remains yellow/brown.

2. Test for Reducing Sugars (Using Benedict's Solution)

Reducing sugars include simple sugars like glucose, fructose, and maltose. Sucrose (table sugar) is *not* a reducing sugar.

1. Mix the food sample with Benedict's solution (which is bright blue).
2. Place the test tube in a beaker of hot water (a water bath) and heat gently for several minutes.
3. Result:
   • Positive: If a reducing sugar is present, the colour changes from blue through green, yellow, orange, and finally, a dense brick-red precipitate (solid).
   • Negative: If no reducing sugar is present, the solution remains blue.

! Common Mistake Alert: Students often forget that the Benedict’s test requires heating. No heat, no colour change!

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Part 3: Proteins

Proteins are the workhorses of the body. They are complex molecules essential for growth, repair, and controlling reactions (like enzymes, which we cover in B5).

Structure and Function

• The monomers are amino acids. There are 20 different common types of amino acids, which can be linked together in millions of ways to make unique proteins.
• Proteins have a huge variety of functions, including: making up muscles, making antibodies (for immunity), and acting as hormones.

Testing for Protein (Using Biuret Reagent)

The Biuret test detects the chemical bonds (peptide bonds) between amino acids.

1. Mix the food sample with aqueous sodium hydroxide (an alkali, Solution A).
2. Add a few drops of aqueous copper(II) sulfate (Solution B). The mixture of A and B is the Biuret reagent.
3. Result:
   • Positive: If protein is present, the solution changes from blue to violet or purple.
   • Negative: If protein is absent, the solution remains blue.

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Part 4: Fats and Oils (Lipids)

Fats (lipids) are generally used for long-term energy storage, insulation, and protecting organs.

Structure and Storage

• Fats are made from two types of smaller molecules: Fatty acids and Glycerol.
• Fats are solids at room temperature (like butter), and oils are liquids (like cooking oil). They are both classified as lipids.
• Fats store twice as much energy per gram compared to carbohydrates, making them excellent energy reserves.

Testing for Fats and Oils (Using Ethanol Emulsion Test)

Fats do not dissolve in water, but they do dissolve in ethanol (alcohol).

1. Mix a small amount of the food sample with a few cm³ of ethanol (alcohol) in a test tube.
2. Shake the mixture vigorously to allow the fat to dissolve in the ethanol.
3. Carefully pour the resulting liquid (the ethanol/fat solution) into a test tube containing water.
4. Result:
   • Positive: If fat is present, a cloudy white layer called an emulsion forms in the water.
   • Negative: If fat is absent, the water remains clear.

Analogy: Imagine mixing clear alcohol (ethanol) with cooking oil. When you then pour this mixture into water, the oil can no longer stay dissolved and forms tiny droplets, making the water look milky white and cloudy!

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Final Review: Biological Molecules Summary

You have successfully mastered the main structural and testing concepts for the molecules of life. When revising, focus especially on the colour changes and specific steps needed for each food test!

Good luck with your revision!