Variety of polypeptides at Higher Level: The same statement as SL. At HL the sequence you learn about here becomes the primary structure (2.2.7), and the R-groups along it decide how the chain folds.
Practise this as you read
- Describe how an almost infinite variety of polypeptides is possible.
- Outline the roles of proteins, with named examples.
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The big idea: All proteins are built from the same 20 amino acids, yet proteins carry out an enormous variety of jobs — more than any other type of molecule in a living thing.
Why so many jobs?
- The reason is shape
- The order of amino acids folds each protein into a specific 3-D shape
- That shape decides exactly what the protein can do
- So 'protein' is not one job — it is a whole toolkit: catalysts, building materials, carriers, messengers, defenders and more
Proteins do many different jobs. Each role category is shown with a named example — the same kind of molecule, doing very different work.
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- Protein
- A molecule made of one or more chains of amino acids folded into a specific 3-D shape. The shape determines the protein's function.
- Function (of a protein)
- The specific job a protein does — for example catalysing a reaction, carrying a substance, or signalling between cells.
- Functional diversity
- The very wide range of different jobs that proteins, as a group, are able to carry out.
- Deficiency
- A shortage of, or a fault in, a particular protein, so that the job it normally does cannot be carried out properly.
Why so many jobs from one kind of molecule?: Because the 20 amino acids can be ordered in countless ways, an almost unlimited number of different shapes is possible.
Each different shape gives a different function — so proteins can specialise to do almost any task the cell needs.
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Learn the main role categories with one clear example each.
How to read the table
- A named role linked to a named protein earns the mark, not a vague 'proteins are important'
- Read each row as a pair: the role, and a named example that does it
| Protein role | What it does | A named example |
|---|---|---|
| Enzyme (catalyst) | Speeds up a specific chemical reaction in the cell | Amylase — breaks down starch into sugar |
| Structural | Gives strength and support to tissues | Collagen — strengthens skin, tendons and bone |
| Transport | Carries a specific substance from one place to another | Haemoglobin — carries oxygen in red blood cells |
| Hormone | A chemical message that travels in the blood | Insulin — signals cells to take up glucose |
| Antibody (defence) | Binds to a specific pathogen to help destroy it | Antibodies — recognise and bind invading microbes |
| Movement (contractile) | Generates force so structures can move | Actin and myosin — make muscle contract |
| Receptor | Detects a specific signal and triggers a cell response | Membrane receptors — bind a hormone such as insulin |
| Pigment | Absorbs light to drive a light-dependent process | Rhodopsin — absorbs light in the rod cells of the eye |
A few examples worth memorising: Enzymes are catalysts — e.g. amylase digests starch.
Haemoglobin is a transport protein — it carries oxygen in red blood cells.
Collagen is structural — it strengthens skin, tendons and bone.
Insulin is a hormone — it signals cells to take up glucose.
Rhodopsin is a pigment in the eye — it absorbs light so we can see.
The exam often turns this around. Instead of asking 'what does protein X do?', it asks: 'protein X is missing or faulty — what goes wrong?'
The logic is always the same. Find the job that protein did, then say that job can no longer happen.
How to reason about a protein deficiency: This 'job → lost job → consequence' chain is exactly what scores the marks.
Three steps
Name the protein's job
What does this protein normally do?
Remove it
If the protein is missing or non-functional, that job is not carried out.
State the consequence
Name the specific process or structure that now fails (e.g. no rhodopsin → light is not detected → vision is impaired).
job → lost job → consequence
| If this protein is missing or faulty… | …this is the job it did… | …so this fails |
|---|---|---|
| Haemoglobin | carries oxygen in the blood | Oxygen delivery to tissues falls — tiredness, breathlessness |
| Insulin | signals cells to take up glucose | Blood glucose stays high — it cannot be controlled |
| Collagen | gives skin, tendons and bone strength | Tissues become weak and fragile |
| Rhodopsin (a pigment) | absorbs light in the rod cells of the retina | The eye cannot detect light well — vision (especially in dim light) is impaired |
| Antibodies | bind pathogens so they are destroyed | Infections are harder to fight off |
Protein present (works)
- The protein folds into its correct shape
- It can do its specific job (carry, catalyse, signal…)
- The physiological process runs normally
- e.g. rhodopsin present → the eye detects light
Protein deficient (job lost)
- The protein is missing or non-functional
- Its specific job is not carried out
- The process that relied on it fails
- e.g. rhodopsin missing → the eye cannot detect light well (vision impaired)
An easy way to remember: 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.
The big idea: Three facts multiply together: 20 kinds of amino acid, any order, any length. The result is far more possible polypeptides than any organism could ever make.
The points to remember
- 20 amino acids are coded for in the genetic code, so each position in a chain has 20 options.
- Amino acids can join in any order: every extra position multiplies the possibilities by 20.
- Chains can be any length, from a few amino acids to many thousands.
- So the number of possible polypeptides is almost infinite, and each sequence folds into its own shape.
Remember it as: 20 kinds, any order, any length.
Real example: titin: Titin, the protein that acts like a spring in muscle, is a single chain of over 30,000 amino acids, the longest polypeptide known in humans.
See how examiners mark answers
Answer exam-style questions with model answers. Learn exactly what earns marks and what doesn't.
How this is tested: The skill that earns the marks is linking a specific protein to a specific job, and then reasoning what is lost if that protein is absent.
| Paper | What you are asked | What to give |
|---|---|---|
| Paper 1A (multiple choice) | Given a protein (or its job), identify which deficiency would impair a specific body function | For example, which missing protein would most affect vision: the light-absorbing pigment rhodopsin |
| Paper 2 | Outline the functional diversity of proteins | Several distinct roles, each with a named example |
Exam-style question — a protein deficiency and a lost function
A person produces very little of the pigment protein found in the rod cells of the retina. Suggest, with a reason, which physiological function is most likely to be impaired. [2]
How to score both marks
- Identify the function impaired. The most likely impaired function is vision (in particular, seeing in dim light).
- Give the reason — link protein to job. The pigment protein in rod cells (rhodopsin) normally absorbs light to start a nerve signal; with very little of it, the rod cells cannot detect light properly, so vision is impaired. (Mark 1: vision / sight is affected. Mark 2: because the pigment normally absorbs light to allow detection, and it is missing.)
Final answer
Vision is impaired, because the rod-cell pigment (rhodopsin) that normally absorbs light to trigger a nerve signal is in short supply, so light is not detected properly.
Why this scores full marks: It does two things: names the function lost (vision) and explains it by the protein's job (absorbing light).
A deficiency answer needs the link — 'the protein normally does X, so without it X fails' — not just the name of the disease or symptom.
| If this protein is missing or faulty… | …this is the job it did… | …so this fails |
|---|---|---|
| Haemoglobin | carries oxygen in the blood | Oxygen delivery to tissues falls — tiredness, breathlessness |
| Insulin | signals cells to take up glucose | Blood glucose stays high — it cannot be controlled |
| Collagen | gives skin, tendons and bone strength | Tissues become weak and fragile |
| Rhodopsin (a pigment) | absorbs light in the rod cells of the retina | The eye cannot detect light well — vision (especially in dim light) is impaired |
| Antibodies | bind pathogens so they are destroyed | Infections are harder to fight off |
A two-mark Describe in the Paper 2 style:
Describe how an infinite variety of different polypeptide chains is possible.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.