📌 Chapter Overview
- Living organisms are made of organic molecules containing carbon
- Carbohydrates, proteins, and lipids are built from smaller units
- We can identify these molecules using specific chemical tests
- DNA is a double helix made of nucleotides held by hydrogen bonds
- Base pairing rules determine DNA structure (A–T and C–G)
1. Chemicals & Life: The Building Blocks
What Makes Something "Organic"?
An organic molecule is any molecule that contains carbon. This includes carbohydrates, proteins, and lipids — the three main types of molecules found in living things. The fourth type, DNA, is also organic because it contains carbon. All living organisms are built from these carbon-based molecules, which is why carbon is sometimes called "the element of life."
💡 Remember: Organic = contains carbon. It's not about where the molecule comes from (living or non-living), it's purely about whether carbon is in it.
The Big Idea: Large Molecules from Small Units
One of the most important ideas in this chapter is that large biological molecules are built by linking together many small units. Think of them like LEGO blocks — tiny bricks join together to make something much bigger. Here's how it works:
Large Molecule = Small Units Joined Together
Starch, proteins, and DNA are all made by linking together smaller building blocks with chemical bonds.
This "building from units" idea is the key to understanding why changing one small unit can change the entire molecule — which is exactly what happens in genetic mutations.
The Chemical Elements in Organic Molecules
Not all organic molecules contain the same elements. Here's what's in each type:
| Molecule Type |
Chemical Elements |
Example |
| Carbohydrate |
Carbon, Oxygen, Hydrogen |
Glucose, starch, cellulose |
| Protein |
C, H, O, Nitrogen + sometimes Sulphur |
Enzymes, antibodies, haemoglobin |
| Lipid (Fat) |
Carbon, Oxygen, Hydrogen |
Butter, olive oil, cholesterol |
🔍 Key Difference: Proteins are the only organic molecules that always contain nitrogen. If you see nitrogen in a biological molecule, it's almost certainly a protein. Some proteins also contain sulphur.
Carbohydrates: Simple Sugars to Complex Starch
Carbohydrates are the body's fuel molecules. They come in different sizes, and understanding the difference between them is crucial:
Monosaccharides (Single Sugar Units)
Glucose is the most important monosaccharide in biology. It's a simple sugar — a single molecule with the molecular formula C₆H₁₂O₆. Glucose is:
- Made directly by photosynthesis in plants
- Used immediately by cells for energy (respiration)
- The building block for starch and cellulose
Disaccharides (Two Sugar Units)
maltose
GLUCOSE + GLUCOSE → MALTOSE + WATER
(C₆H₁₂O₆) + (C₆H₁₂O₆) → (C₁₂H₂₂O₁₁) + (H₂O)
condensation reaction
Polysaccharides (Many Sugar Units)
many
| Polysaccharide |
Where It's Found |
What It Does |
|---|