4.1 Biological Molecules

2026 📋 Syllabus Objectives

By the end of these notes, you should be able to:

  1. List the chemical elements that make up carbohydrates, lipids, proteins, and DNA.
  2. State that large molecules are built from smaller molecules — specifically: starch, cellulose and glycogen from glucose; proteins from amino acids; lipids from fatty acids and glycerol; and DNA from nucleotides.
  3. Describe and carry out chemical tests for starch (iodine solution), glucose and maltose (Benedict's solution), protein (biuret test), and lipids (ethanol emulsion test).

📖 Main Content

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Section 1 — Chemical Elements in Biological Molecules

Everything in your body is made of chemicals. The most important large molecules in living organisms are carbohydrates, lipids, proteins, and DNA. Each one is built from specific chemical elements (the basic building blocks of all matter).

💡 Think of chemical elements like letters of the alphabet — just a few letters can combine to make thousands of different words, just like a few elements can make thousands of different molecules.

(a) Carbohydrates

Carbohydrates are made of three elements:

  • Carbon (C)
  • Hydrogen (H)
  • Oxygen (O)

A simple way to remember this: C, H, O — "CHO". Examples of carbohydrates include sugars (like glucose), starch, and cellulose.

(b) Lipids (Fats and Oils)

Lipids are also made of three elements, the same three as carbohydrates:

  • Carbon (C)
  • Hydrogen (H)
  • Oxygen (O)

So both carbohydrates and lipids contain C, H, and O — but they are arranged very differently, which gives them completely different shapes and jobs in the body.

💡 Fats are lipids that are solid at room temperature (like butter). Oils are lipids that are liquid at room temperature (like sunflower oil). Both are still called lipids.

(c) Proteins

Proteins are more complex. They always contain four elements:

  • Carbon (C)
  • Hydrogen (H)
  • Oxygen (O)
  • Nitrogen (N)

Some proteins also contain small amounts of sulphur (S), but this is not always present — nitrogen is the key element that sets proteins apart from carbohydrates and lipids.

💡 Memory tip: Proteins = C, H, O, N (and sometimes S). The extra element is nitrogen — this is what makes proteins unique!

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