Key Idea: Proteins are the cell's toolkit: catalysts, building materials, carriers, messengers and defenders. Every protein is a chain of amino acids joined by peptide bonds. That chain folds into a precise 3-D shape, and the shape decides the job — so a protein's whole story runs sequence → folding → function. This topic is a regular on Paper 1A (structure / bonding / element MCQs and a peptide-bond count) and on Paper 2 (drawing an amino acid, outlining the four levels of structure, and linking a protein — or a missing protein — to its function).
🧱 Amino acid structure
An amino acid is the monomer that proteins are built from. Every amino acid is built around one central carbon carrying four groups: an amino group (—NH₂), a carboxyl group (—COOH), a single hydrogen (—H) and a variable R group (side chain). Three of those four are identical in every amino acid — only the R group changes. Because there are 20 different R groups, there are 20 different amino acids. Amino acids contain carbon, hydrogen, oxygen AND nitrogen (N). The nitrogen comes from the amino group, and it is the element that carbohydrates and lipids lack — so nitrogen is the giveaway for a protein.
The generalized amino acid: one central carbon bonded to an amino group (—NH₂), a carboxyl group (—COOH), a hydrogen (—H) and a variable R group (highlighted) — only the R group differs between the 20 amino acids.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Part of the amino acid | Detail | Same or different between amino acids? |
|---|---|---|
| Central carbon | carries all four groups | same in all |
| Amino group (—NH₂) | contains the nitrogen; gives the 'amino' | same in all |
| Carboxyl group (—COOH) | the acid group; gives the 'acid' | same in all |
| Hydrogen (—H) | a single hydrogen | same in all |
| R group (side chain) | the variable side chain | DIFFERENT → 20 amino acids |
R = the 'Rest' = the changeable bit. The amino group, carboxyl group and hydrogen are the fixed core; the R group is what varies. And: nitrogen = protein. Carbohydrates and lipids have only C, H and O.
🔗 Peptide bonds & condensation
Two amino acids join by condensation: the carboxyl group (—COOH) of one reacts with the amino group (—NH₂) of the next. An —OH is removed from the carboxyl group and an —H from the amino group; together they leave as one water molecule (H₂O), and a new covalent peptide bond (CO—NH) forms. Two amino acids make a dipeptide; many make a polypeptide. The reverse reaction is hydrolysis — one water molecule is added to break a peptide bond (this is how protein is digested back into amino acids).
Condensation joins two amino acids: an —OH from one carboxyl group and an —H from the other amino group leave together as water, and a new peptide bond (CO—NH) forms between them.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Hydrolysis is the reverse: a water molecule is added across the peptide bond, splitting the dipeptide back into two separate amino acids (this is how dietary protein is digested).
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Feature | Condensation (joining) | Hydrolysis (breaking) |
|---|---|---|
| Effect on the peptide bond | forms it | breaks it |
| Water | released (—OH + —H out) | added (one H₂O in) |
| Groups / products | carboxyl + amino react | two separate amino acids |
| Where it matters | building a polypeptide | digesting protein |
Each peptide bond links one pair of amino acids, so a single chain has one fewer bond than amino acids: a chain of n amino acids has (n − 1) bonds. Across several chains, the quick rule is total amino acids − number of chains (e.g. 51 amino acids in 2 chains → 51 − 2 = 49 bonds).
🥗 Essential amino acids & diet
The body can make some of the 20 amino acids itself, but it cannot make the others — those must come from the food you eat. An amino acid the body cannot make is essential (it must be eaten). One the body can make is non-essential (it need not be eaten). Watch the trap: 'non-essential' does NOT mean unimportant — every amino acid is needed to build proteins; it only means not required in the diet.
| Feature | Essential amino acid | Non-essential amino acid |
|---|---|---|
| Can the body make it? | No — cannot be synthesised | Yes — can be synthesised |
| Where it must come from | the diet (food) only | the diet OR made in the body |
| Roughly how many in humans | about nine of the twenty | about eleven of the twenty |
| Answer wording | 'must be obtained from the diet' | 'can be synthesised by the body' |
Most animal proteins (meat, fish, eggs, dairy) supply all nine essential amino acids in a single food. Many single plant proteins are missing one or more essential amino acids, so a vegan must combine different plant proteins (e.g. rice + beans) so the gaps in one food are filled by another — together they supply all nine.
Essential = Eat it (the body can't make it). Plant eaters mix and match plant proteins so the combination is complete.
🌀 Protein structure & conformation
A protein does not stay as a straight chain — it folds into a precise 3-D shape called its conformation, and that shape is set by the amino acid sequence. Folding is described at four levels: primary (the sequence of amino acids) → secondary (α-helices and β-pleated sheets, held by hydrogen bonds) → tertiary (the whole single chain folds into one 3-D shape) → quaternary (two or more folded chains join, e.g. haemoglobin's four). Single-chain proteins have no quaternary structure.
The four levels of protein structure: primary (sequence) → secondary (α-helices and β-pleated sheets, held by hydrogen bonds) → tertiary (one chain folded into a 3D shape) → quaternary (two or more chains together). The amino acid sequence determines every higher level.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Level | What it is | What holds it together |
|---|---|---|
| Primary | the sequence (order) of amino acids | peptide bonds |
| Secondary | α-helices (coils) and β-pleated sheets | hydrogen bonds along the backbone |
| Tertiary | the whole single chain folded into a 3-D shape | bonds between R-groups |
| Quaternary | two or more folded chains (subunits) joined | the same R-group interactions, between subunits |
The folded shape is held mostly by weak bonds (hydrogen and ionic bonds). High temperature or extreme pH breaks these, so the chain unfolds and loses its 3-D shape — this is denaturation. A denatured protein stops working (e.g. an enzyme's active site changes so the substrate no longer fits). The sequence (peptide bonds) stays intact — only the conformation is lost, and the loss is usually permanent (like a cooked egg white).
Sequence decides shape, and shape decides function. Change the sequence and you change the folding; break the folding (denaturation) and you lose the function — but the sequence itself is unchanged.
🛠️ Protein functions
All proteins are built from the same 20 amino acids, yet as a group they do an enormous variety of jobs — more than any other type of molecule. The reason is shape: the order of amino acids folds each protein into a specific shape that decides what it can do. Learn the main role categories with one named example each — examiners want a named role linked to a named protein, not 'proteins are important'.
| Protein role | What it does | Named example |
|---|---|---|
| Enzyme (catalyst) | speeds up a specific reaction | amylase — digests starch |
| Structural | gives strength and support | collagen — strengthens skin, tendons, bone |
| Transport | carries a specific substance | haemoglobin — carries oxygen |
| Hormone | a chemical message in the blood | insulin — signals glucose uptake |
| Antibody (defence) | binds a pathogen to destroy it | antibodies — recognise microbes |
| Movement | generates force to move structures | actin & myosin — contract muscle |
| Pigment | absorbs light for a light-driven process | rhodopsin — absorbs light in the eye |
The exam often turns this around: 'protein X is missing — what goes wrong?' The logic is always job → lost job → consequence: 1. Name the job the protein does. 2. Remove it — that job is no longer carried out. 3. State the process that now fails (e.g. no rhodopsin → light is not detected → vision is impaired).
No protein, no job. To predict the effect of a deficiency, just ask 'what was this protein for?' — the answer that's lost is your consequence.
✍️ Worked examples
IB-style question — element composition
Two molecules are analysed. Molecule X contains carbon, hydrogen and oxygen only; molecule Y contains carbon, hydrogen, oxygen and nitrogen. State which molecule could be a protein, and explain how you can tell. [2]
How to score both marks:
Identify the protein. Molecule Y could be a protein (amino acid).
Explain using nitrogen. Proteins always contain nitrogen (in the amino group, —NH₂), whereas carbohydrates and lipids contain only carbon, hydrogen and oxygen. Y is the only molecule with nitrogen, so it is the one that could be a protein.
Molecule Y — it contains nitrogen, and proteins (unlike carbohydrates and lipids) always contain nitrogen.
IB-style question — atoms removed and the bond formed
Two amino acids react to form a dipeptide. Name the reaction, identify the atoms removed, and name the bond that forms. [3]
How to score all three marks:
Name the reaction. The amino acids join by condensation.
Identify the atoms removed. An —OH from the carboxyl group (—COOH) of one and an —H from the amino group (—NH₂) of the other leave together as one water molecule.
Name the bond. The new covalent bond joining them is a peptide bond (CO—NH).
Condensation: an —OH (carboxyl) and an —H (amino) leave as one water molecule, and a peptide bond forms.
IB-style question — count the peptide bonds
A protein is made of two polypeptide chains containing 21 and 30 amino acids. Calculate the total number of peptide bonds. [2]
Model answer:
Count each chain. Each chain has one fewer bond than amino acids: 21 − 1 = 20; 30 − 1 = 29.
Add them. Total = 20 + 29 = 49 peptide bonds. (Quick check: 51 amino acids − 2 chains = 49.)
20 + 29 = 49 peptide bonds (total amino acids 51 − 2 chains = 49).
IB-style question — outline protein structure
Outline the structure of proteins. [4]
How to score all four marks:
Primary. The sequence (order) of amino acids in the chain, joined by peptide bonds.
Secondary. Local folding into α-helices and β-pleated sheets, held by hydrogen bonds.
Tertiary. The whole single chain folds into a precise 3-D shape, held by bonds between R-groups.
Quaternary. Two or more folded chains (subunits) join into one functional protein. (1 mark per level.)
Primary = sequence; secondary = α-helices and β-pleated sheets (hydrogen bonds); tertiary = whole chain folded into a 3-D shape; quaternary = two or more chains joined.
IB-style question — a protein deficiency
A person produces very little of the pigment protein found in the rod cells of the retina. Suggest, with a reason, which function is most likely to be impaired. [2]
How to score both marks:
Name the function lost. Vision (especially seeing in dim light) is most likely impaired.
Give the reason — link protein to job. The rod-cell pigment (rhodopsin) normally absorbs light to start a nerve signal; with very little of it, light is not detected properly, so vision is impaired.
Vision is impaired, because the rod-cell pigment (rhodopsin) that normally absorbs light is in short supply, so light is not detected.
✅ Quick self-check
Tap each card to check yourself.
What four groups are on the central carbon of an amino acid, and which one varies? An amino group (—NH₂), a carboxyl group (—COOH), a hydrogen (—H) and a variable R group. Only the R group varies — 20 R groups give 20 amino acids. Amino acids contain C, H, O and nitrogen.
How do two amino acids join, and what is removed? By condensation — the carboxyl group of one reacts with the amino group of the other. An —OH and an —H are removed as one water molecule, and a peptide bond forms. Hydrolysis (adding water) breaks it.
What is the difference between essential and non-essential amino acids? Essential = the body cannot make it, so it must be eaten. Non-essential = the body can make it, so it need not be eaten. 'Non-essential' means not required in the diet, NOT unimportant.
Name the four levels of protein structure. Primary (sequence), secondary (α-helices and β-pleated sheets, hydrogen bonds), tertiary (whole chain folded into a 3-D shape) and quaternary (two or more chains joined). The sequence determines all of them.
What is denaturation, and what is preserved? Loss of a protein's folded shape from high temperature or extreme pH breaking the weak bonds. The protein unfolds and stops working, but the amino acid sequence (peptide bonds) stays intact.
Why are proteins so functionally diverse, and how do you predict a deficiency's effect? The 20 amino acids can be ordered countless ways, giving countless shapes — each shape gives a different job. For a deficiency, name the protein's job, then state that job is lost so the process that relied on it fails.
Exam Tips
- Amino acid = central carbon + four groups (amino, carboxyl, hydrogen, R); only the R group varies → 20 amino acids.
- Spotting a protein from its elements? Look for nitrogen — carbohydrates and lipids have only C, H and O.
- Two amino acids joining = condensation forms a peptide bond and releases one water (—OH + —H out) — name all three points.
- Peptide bonds = total amino acids minus the number of chains; one chain of n amino acids has n − 1 bonds, never n.
- 'Distinguish' essential vs non-essential needs BOTH sides: cannot make → must eat, versus can make → need not eat.
- A vegan must COMBINE plant proteins so all nine essential amino acids are supplied — that combining point is the marking line.
- 'Outline the structure of proteins [4]' = four distinct levels (primary, secondary, tertiary, quaternary), not one idea reworded.
- Denaturation loses the SHAPE (weak bonds break from heat/extreme pH), not the sequence — peptide bonds stay intact.
- For functions, link a named protein to a named job; for a deficiency, say 'protein normally does X, so without it X fails'.
Key Idea: Proteins are chains of amino acids — a central carbon with an amino group, a carboxyl group, a hydrogen and a variable R group (20 R groups → 20 amino acids; nitrogen marks a protein). They join by condensation (a peptide bond forms, water out) and split by hydrolysis (water in); a chain has (amino acids − 1) bonds. Essential amino acids must be eaten (a vegan combines plant proteins); non-essential ones the body makes. The chain folds through four levels — primary → secondary → tertiary → quaternary — set by the sequence, and denaturation (heat / extreme pH) unfolds it. Because each shape gives a job, proteins are the most functionally diverse molecules: enzymes, structural, transport, hormones, antibodies and pigments — and a deficiency means the job it did is lost.