Preview
IB DP Biology — Form and Function | SL | B1.2 Proteins
📋 What You Will Learn
- Identify the general structure of an amino acid and explain how the R group determines its properties
- Explain how condensation reactions link amino acids via peptide bonds to form polypeptides
- Describe the four levels of protein structure — primary, secondary, tertiary, and quaternary — and the bonds that stabilise each
- Analyse how the sequence of amino acids determines the three-dimensional shape and function of a protein
- Apply understanding of protein structure to explain how denaturation disrupts function
🔍 Guiding Question How does the sequence of amino acids in a polypeptide determine the structure and function of a protein?
🌍 Why This Matters Sickle cell disease is caused by a single amino acid substitution in haemoglobin — glutamic acid replaced by valine at position six of the beta chain. That one change alters the protein's shape, causes red blood cells to deform under low oxygen conditions, and leads to a cascade of clinical consequences. One amino acid. One disease. This is the power of protein structure.
The Building Blocks: Amino Acids
Before a cell can build an enzyme, a receptor, a structural fibre, or a signalling molecule, it must first assemble the right amino acids in the right order. Amino acids are the monomers of proteins — small, versatile molecules whose variety makes the enormous diversity of protein function possible.
Every amino acid shares the same backbone. At the centre sits a carbon atom — the alpha carbon — bonded to four groups: an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain. That side chain is everything. It is what makes one amino acid different from another.
📖 Key Definition — Amino Acid An amino acid is an organic molecule containing both an amino group (–NH₂) and a carboxyl group (–COOH) attached to the same alpha carbon, along with a variable R group (side chain) that determines the chemical properties of that amino acid.
There are 20 amino acids used in protein synthesis, each with a unique R group. Some R groups are non-polar and hydrophobic — they avoid water and tend to cluster in the interior of folded proteins. Others are polar or charged and hydrophilic — they interact readily with water and often appear on protein surfaces. A few carry acidic or basic R groups that can participate in chemical reactions. This chemical diversity in the R groups is the molecular basis for the extraordinary range of protein functions.
💡 Note — Essential vs Non-Essential Amino Acids Of the 20 amino acids, humans can synthesise 11 (non-essential). The remaining 9 (essential amino acids) must be obtained from the diet because our cells lack the metabolic pathways to produce them. This is why dietary protein quality matters — a protein that contains all 9 essential amino acids is called a complete protein.
Joining Amino Acids: The Peptide Bond
Here is the problem the cell had to solve: how do you link small, soluble monomers into a long, stable chain without using a separate fastener? The answer is elegant — use the amino acids themselves. The carboxyl group of one amino acid reacts with the amino