Exploring the Architectural Wonders of Proteins

Cover image. The many jobs of proteins: enzymes at work, collagen holding tissue together, haemoglobin carrying oxygen, antibodies defending the body, and receptors listening at the cell surface. Everything in this lesson is one of these jobs.

Level: GCSE foundations, A-level and introductory university extension. This lesson rebuilds a thin old list into something you can actually use: what proteins do — the seven jobs they carry out in every living thing, with real examples and the shape behind each job.

What do a spider's web, a blood clot and a bee sting have in common? The web is silk, a protein. The clot is fibrin, a protein. The sting hurts because of melittin, a protein in the venom that punches holes in cell membranes. Three completely different things — a rope, a plug and a weapon — all built from the same 20 amino acids. That is the wonder of proteins: one set of bricks, and a body builds enzymes, ropes, couriers, guards, messengers, motors and warehouses out of them. The four levels of protein structure are taught properly in the lesson on Understanding Structure for Functional Excellence — this lesson takes the structure as given and asks: what do proteins actually do all day?

One paragraph is all the background this lesson needs. A protein is a chain of amino acids joined by peptide bonds and folded into a three-dimensional shape. There are 20 standard amino acids, and the order they appear in — the sequence — decides how the chain folds. The fold decides the job: a pocket in the fold becomes an active site, a flat surface becomes a binding site, a long rope becomes a fibre. Change the shape and you change the job — which is why one changed amino acid in haemoglobin can bend a whole red blood cell into a sickle. The chemistry of the amino acids themselves is covered in the lesson on The Chemical Basis of Life; everything from here on is about the work.

Figure 1. The seven jobs proteins do: enzymes, transport, structural, defence, signalling, movement and storage. One protein, one job — and the shape decides which.

Job	What it does	Example to remember
Enzymes	Speed up a chemical reaction	Amylase breaks down starch
Transport	Carry something from one place to another	Haemoglobin carries oxygen
Structural	Give strength and shape	Collagen in skin and tendon
Defence	Protect against germs and injury	Antibodies tag invaders
Signalling	Carry messages between cells	Insulin lowers blood glucose
Movement	Make things move	Actin and myosin in muscle
Storage	Keep a useful substance until it is needed	Ferritin stores iron

Learn the table as seven jobs and one example each. The rest of the lesson is the detail behind each row — and the detail is where the exam marks are.

Job 1: Enzymes — the workers

Enzymes are the most numerous protein job, and the one students meet first. An enzyme is a protein that speeds up a chemical reaction without being used up. Every enzyme has an active site — a pocket in its folded shape that fits one substrate and no other. Amylase fits starch; pepsin fits protein; lactase fits lactose. The fit is the whole trick: the active site holds the substrate in exactly the right position for the reaction to happen, and holds it in a strained shape that makes the reaction easier.

The old picture of the active site was lock and key: a rigid pocket and a substrate that fits it exactly, like a key in a lock. Daniel Koshland improved on that in 1958 with the induced-fit model: the active site is flexible, and it changes shape slightly when the substrate binds, closing around it. The enzyme is not a passive lock; it grips. After the reaction the products leave, the enzyme returns to its original shape, and it is free to work again. A single enzyme molecule can do this thousands or millions of times a second — which is why a pinch of enzyme can process a vat of substrate.

Figure 2. How an enzyme works: the old lock-and-key picture and the induced-fit picture. The active site fits one substrate; after the reaction the enzyme is unchanged and can work again.

Three enzymes are worth knowing by name and by job. Amylase, in saliva and the small intestine, breaks starch into maltose. Pepsin, in the stomach, starts protein digestion by cutting long polypeptide chains into shorter pieces — it works in acid, which is why the stomach is acid. Catalase, in nearly every cell, splits hydrogen peroxide — a harmful by-product of metabolism — into water and oxygen. And one more: DNA polymerase III, the bacterial enzyme that copies DNA during cell division, is a team of about ten subunits working together, each doing one part of the copying job.

Worked example: how fast is an enzyme?

The figures below are practice figures, invented for this lesson — not real data from any experiment. Catalase is often quoted as one of the fastest enzymes known, at around 40 million reactions a second for one molecule. Take a practice sample of 5 catalase molecules, each working at that rate. The total is 5 × 40,000,000 = 200,000,000 hydrogen peroxide molecules broken down every second. The arithmetic is trivial; the point is the scale. An enzyme is not used up, so five molecules can process two hundred million substrate molecules a second — and that is why a cell needs only a tiny amount of each enzyme.

Job 2: Transport — the couriers

Some proteins carry things. The most famous is haemoglobin, the transport protein of the blood: four subunits, each with one haem group, carrying four oxygen molecules from the lungs, where oxygen is plentiful, to the tissues, where it is scarce. The loading and unloading are driven by concentration — oxygen binds where there is plenty of it and lets go where there is little. That is all it takes, and it is enough to keep every cell in the body supplied.

Figure 3. Haemoglobin carries oxygen: it loads four oxygen molecules in the lungs, where oxygen is plentiful, and unloads them in the tissues, where oxygen is scarce. Myoglobin, one chain with one haem, stores a single oxygen in muscle.

Haemoglobin's quiet partner is myoglobin, the oxygen store in muscle: a single chain with one haem group, holding one oxygen molecule until the muscle needs it. The two are easy to confuse and the exam likes to test the difference — haemoglobin carries, myoglobin stores, and the difference is the number of chains. Two other couriers matter. Albumin, the main protein of blood plasma, carries fatty acids, hormones and some drugs around the body, and helps hold water in the blood. Transferrin carries iron in the blood to the cells that need it — the bone marrow, where red blood cells are made, takes up more iron than any other tissue. And transport happens inside cells too: membrane proteins such as the glucose transporter carry glucose into cells, and the sodium-potassium pump moves sodium out and potassium in, using ATP — a transport protein that is also a pump.

Worked example: how much oxygen does the blood carry?

This example uses real textbook figures, not invented ones. Each gram of haemoglobin binds about 1.34 millilitres of oxygen — a figure called Hüfner's constant. A typical adult has about 5 litres of blood, with about 150 grams of haemoglobin per litre. That is 5 × 150 = 750 grams of haemoglobin in total. Multiply by Hüfner's constant: 750 × 1.34 = 1,005 millilitres — about one litre of oxygen, carried in the blood at any moment. The number comes from the structure: four oxygen molecules per haemoglobin, four haem groups, four subunits. A transport protein's capacity is written in its shape.

Job 3: Structural — the ropes and scaffolds

Some proteins are not workers or couriers; they are building material. Collagen is the main structural protein of skin, tendons, ligaments and bone — about a quarter to a third of all the protein in the body is collagen. Its shape is a triple helix, three chains twisted into a rope, with the small amino acid glycine every third residue so the chains can pack tightly. A rope is the right shape for a job that is pure strength: collagen has no active site, because it has nothing to catalyse. Keratin, the protein of hair, nails and the outer layer of skin, is rich in alpha-helix and heavily cross-linked, which is why hair is strong and hard to stretch. Elastin is the opposite of a rope: a stretchy, cross-linked protein in artery walls, lungs and skin that widens with each heartbeat and springs back. And silk — fibroin, mostly beta-sheet — is a rope so strong that, weight for weight, it rivals steel. The same building material, four different shapes, four different jobs.

Structure works inside the cell as well. Tubulin builds microtubules, the hollow tubes that form the cell's internal skeleton, hold the cell's shape, and serve as tracks along which other proteins carry cargo. A structural protein's job is to be there and hold its shape — and the exam rewards the student who can say why a triple helix is a good rope and a bad enzyme.

Job 4: Defence — the guards

The immune system is mostly protein. Antibodies — also called immunoglobulins — are Y-shaped proteins with two identical binding sites at the tips of the arms. Each antibody fits one specific antigen, the way a key fits one lock, and by binding it marks the invader for destruction. The antibody does not kill the germ itself; phagocytes and the complement system do the killing, and the antibody is the tag that says "this one". That distinction matters, and the exam tests it.

Defence has other proteins too. Fibrinogen, a soluble protein in the plasma, is converted into fibrin — insoluble threads that form the mesh of a blood clot and trap blood cells when a vessel is cut. Lysozyme, an enzyme in tears, saliva and mucus, breaks bacterial cell walls — defence done by an enzyme, which is a reminder that the seven jobs are a map, not seven sealed boxes. And interferons are signalling proteins released by infected cells that warn neighbouring cells to arm themselves against a virus — defence and signalling in the same molecule.

Job 5: Signalling — the messengers

Some proteins carry messages. Insulin is a protein hormone made in the pancreas: after a meal it signals liver and muscle cells to take in glucose and store it as glycogen, so blood glucose falls. Glucagon, made in the same organ, does the opposite — it signals the liver to break down glycogen and release glucose, so blood glucose rises. The two work as a pair, pulling blood glucose in opposite directions, and diabetes is what happens when the insulin side of that pair fails.

A hormone needs a receiver, and the receiver is also a protein: the receptor, usually sitting in the cell membrane, that the hormone binds to. When insulin binds its receptor, the receptor changes shape and switches on a chain of events inside the cell. The hormone is the message, the receptor is the ear, and both are proteins. Growth hormone, which signals the body to grow, works the same way — one protein carrying the message, another protein receiving it. The confusion to avoid is enzyme versus hormone: an enzyme catalyses a reaction, a hormone carries a message. Insulin does not break anything down; it tells cells what to do.

Job 6: Movement — the motors

Muscles move because two proteins slide past each other. Actin forms thin filaments; myosin forms thick filaments with heads like small hooks. The myosin heads grip the actin, pull, let go, and grip again — and each pull shortens the sarcomere, the repeating unit of a muscle fibre. The filaments themselves do not shorten: they slide. That is the sliding filament theory, worked out by Hugh Huxley and colleagues in 1954, and it is still the accepted account of how muscle works.

Figure 4. Muscle movement: actin and myosin filaments slide past each other. In a relaxed muscle the sarcomere is long and the filaments overlap only slightly; in a contracted muscle the sarcomere is short and the overlap is deep. The filaments themselves do not shorten — each myosin pull uses one ATP.

Each pull uses one molecule of ATP. ATP also lets the myosin head release from actin after the pull, ready to grip again — which is why a muscle that has run out of ATP cannot relax, and its heads stay gripped. That is rigor mortis: the stiffness of a body after death is myosin heads locked onto actin with no ATP left to release them. Movement is not confined to muscle, either. Inside every cell, the motor proteins dynein and kinesin walk along microtubule tracks carrying vesicles and organelles, and the same sliding mechanism drives the beating of cilia and flagella. A protein motor is a shape that grips, pulls and lets go — over and over, millions of times.

Job 7: Storage — the warehouses

The last job is the quietest: keeping something until it is needed. Ferritin is a hollow protein cage inside cells that stores iron safely — up to about 4,500 iron atoms per molecule — and releases it when the cell needs it. Free iron would be toxic, damaging the cell that holds it, so the cage matters as much as the store. Casein, the main protein of milk, stores amino acids for the calf — about 80% of the protein in cow's milk is casein. Ovalbumin, the main protein of egg white, stores amino acids for the developing chick. And in seeds, storage proteins such as gluten — the gliadin and glutenin of wheat — hold amino acids for the seedling until germination.

One protein sits on the border between storage and transport, and it is worth naming: myoglobin. It stores oxygen in muscle, one molecule per chain, and hands it over when the muscle works hard. Storage or transport? Both, in a sense — but the exam calls it a storage protein, because it holds oxygen in place rather than carrying it around the body. Borderline cases like this are exactly what examiners use to test whether a student understands the job or has only memorised the list.

One protein, several jobs?

The seven jobs are a map, not seven sealed boxes, and a few proteins do more than one job. Lysozyme is an enzyme and a defence protein: it catalyses a reaction — breaking bacterial cell walls — and that reaction is its defence. Albumin is a transport protein and a buffer: it carries fatty acids and hormones, and it also helps hold water in the blood by osmosis. Interferon is a defence protein and a signalling protein: it carries a warning from one cell to the next. But most proteins do one job well, and the exam question is usually "which job does this protein do?" — so learn the main job for each example, and note the second job only where it is genuine.

Five pairs that get confused

Enzyme vs hormone. An enzyme catalyses a reaction; a hormone carries a message. Insulin is a hormone, not an enzyme — it does not break anything down, it tells cells what to do.

Antibody vs antigen. The antibody is the body's protein, Y-shaped, with two binding sites. The antigen is the foreign molecule it binds to. The antibody tags; the antigen is tagged.

Haemoglobin vs myoglobin. Haemoglobin carries oxygen in the blood — four chains, four oxygens. Myoglobin stores oxygen in muscle — one chain, one oxygen. Same haem group, different job.

Receptor vs hormone. The hormone is the message; the receptor is the protein that receives it. Insulin is the hormone; the insulin receptor is the ear on the cell surface.

Transport vs storage. A transport protein moves something from place to place — haemoglobin, transferrin. A storage protein holds something where it is until it is needed — ferritin, casein. Myoglobin sits on the border: it stores oxygen in muscle.

Five mistakes that sound like biology

1. "All proteins are enzymes." They are not. Enzymes are one job out of seven. Collagen catalyses nothing; ferritin catalyses nothing; an antibody catalyses nothing.

2. "Antibodies kill germs." They do not. An antibody binds its antigen and marks it. Phagocytes and the complement system do the killing.

3. "Haemoglobin and myoglobin do the same job." They do not. Haemoglobin carries oxygen in the blood; myoglobin stores it in muscle. One is a courier, the other a warehouse.

4. "Enzymes are used up in reactions." They are not. An enzyme is unchanged by the reaction it catalyses and can work again — that is what makes it a catalyst.

5. "Collagen is a globular protein." It is not. Collagen is fibrous — a long triple-helix rope. Globular proteins are compact and usually soluble; collagen is neither.

How to answer an exam question on this topic

Use the sequence job → protein → shape feature → what it does → what goes wrong if it fails. Name the job, name the protein, say which feature of its shape does the work, say what the job achieves, and say what happens when the protein is missing or broken.

Model paragraph: Haemoglobin's job is transport: it carries oxygen from the lungs to the tissues. It is built from four subunits, each carrying one haem group with an iron atom, so one haemoglobin molecule carries four oxygen molecules. Oxygen binds where it is plentiful, in the lungs, and is released where it is scarce, in the tissues, because binding follows concentration. If haemoglobin is missing or faulty — as in anaemia, or in sickle-cell anaemia, where one changed amino acid alters the shape — the tissues receive less oxygen, and the person becomes tired and breathless. That paragraph names a job, a protein, a shape feature, a mechanism and a failure — which is what the marks are for.

Frequently asked questions

Why can 20 amino acids do so many jobs?
Because the sequence decides the fold, and the fold decides the job. Twenty different R groups, arranged in thousands of different orders, give an almost unlimited number of shapes — pockets, ropes, cages, hooks and keys.

Can a protein change jobs?
Almost never. A protein's shape is fixed by its sequence, so its job is fixed too. What changes is how much of the protein a cell makes, or whether it is switched on or off — the protein itself does not retrain.

Why is insulin a protein but some hormones are not?
Hormones come in different chemical kinds. Peptide hormones such as insulin are proteins; steroid hormones such as testosterone are lipids. The message is the same kind of job, but the molecule doing it is different.

Can a protein be both structural and an enzyme?
Rarely, and the exam treats them as separate jobs. Lysozyme is the honest exception: it is an enzyme whose reaction is defence. Most proteins do one job, and the question is which one.

Why does a fever make you feel weak?
Partly because heat denatures proteins: the bonds holding their folds break, enzymes slow down, and muscle proteins work less well. A high fever is dangerous precisely because proteins lose their shape — and their shape is their job.

What happens to the protein in food?
It is digested, not absorbed whole. Stomach acid denatures it and pepsin cuts it into shorter chains; enzymes in the small intestine finish the job, and only amino acids cross into the blood. The body then builds its own proteins from those amino acids.

Continue learning

• Understanding Structure for Functional Excellence — the four levels of protein structure, the bonds at each level, and denaturation: the shape behind every job in this lesson.
• The Chemical Basis of Life — the chemistry behind the bonds in this lesson: amino and carboxyl groups, condensation and hydrolysis, and the monomer-to-polymer table.
• Carbohydrates: Classification and Types — the other great polymer lesson: how monosaccharides join by condensation, the same reaction that joins amino acids.
• Lipids: Classification and Types — the third biological molecule: why lipids are defined by solubility, not by a chain of monomers.
• Biological Method — the method behind the measurements in this lesson: counting, taking means, and repeating observations.
• Microscopy and Cell Size — why a protein is far too small to see with a light microscope, and how scale bars work.

Sources and further reading

These notes were written from the sources below and from the site's own linked lessons. The counting figures in the worked examples are invented for teaching and are labelled as such; the oxygen-capacity figures use the textbook Hüfner constant.

• OpenStax, Biology 2e, Chapter 3: Biological Macromolecules — amino acids, protein structure and protein functions.
• Khan Academy, Biology: Macromolecules — proteins, enzymes and protein function.
• BBC Bitesize, GCSE Biology: Proteins — enzymes, protein structure and denaturation.
• Biology LibreTexts, Biology — protein functions, enzymes, transport and structural proteins.
• Sanger, F., 1955 — the amino acid sequence of insulin, the first protein sequenced.
• Pauling, L. et al., Science, 1949 — sickle-cell anaemia as a molecular disease.
• Koshland, D. E., 1958 — the induced-fit model of enzyme action.
• Huxley, H. E. and Niedergerke, R., 1954 — the sliding filament theory of muscle contraction.
• PreachBio, Understanding Structure for Functional Excellence — the four levels of protein structure, for the shape behind the jobs in this lesson.

20 Explained MCQs — answers hidden until opened

Open each question to reveal its answer and explanation. The interactive quiz below uses a different set of questions.

1. What decides what job a protein can do?

A. Its three-dimensional shape   B. Where it was first discovered   C. The colour of the protein   D. The number of amino acids only
Answer: A. A protein's job is decided by its folded shape: the shape makes the active site, the binding site or the rope. The same 20 amino acids can build silk, insulin and haemoglobin — what differs is the sequence and the fold.

2. Which protein digests starch in the mouth and small intestine?

A. Lactase   B. Pepsin   C. Catalase   D. Amylase
Answer: D. Amylase is the enzyme that breaks starch down into maltose. It works in the mouth and the small intestine. Pepsin digests protein, lactase digests lactose, catalase breaks down hydrogen peroxide.

3. What does catalase do?

A. It carries oxygen in the blood   B. It splits hydrogen peroxide into water and oxygen   C. It builds collagen fibres   D. It breaks down starch into maltose
Answer: B. Catalase splits hydrogen peroxide — a harmful by-product of metabolism — into water and oxygen. It is one of the fastest enzymes known, often quoted at around 40 million reactions a second per molecule.

4. Which enzyme works in the stomach and starts digesting protein?

A. Amylase   B. Lipase   C. Pepsin   D. Lactase
Answer: C. Pepsin is the stomach enzyme that starts protein digestion, cutting long polypeptide chains into shorter pieces. It works best in the acid of the stomach. Amylase digests starch, lactase digests lactose, lipase digests fat.

5. What does the induced-fit model say about the active site?

A. The substrate changes the enzyme's amino acid sequence   B. The enzyme changes shape slightly when the substrate binds   C. The enzyme is used up in the reaction   D. The active site is rigid and never changes shape
Answer: B. In the induced-fit model (Koshland, 1958), the enzyme's active site changes shape slightly when the substrate binds, closing around it. The old lock-and-key picture — a rigid pocket and a perfect fit — is only a first approximation.

6. What happens to an enzyme after it catalyses a reaction?

A. It is broken down into amino acids   B. It turns into the substrate   C. It is unchanged and can catalyse another reaction   D. It becomes part of the product
Answer: C. An enzyme is a catalyst: it speeds up a reaction without being used up. After the products leave, the enzyme is unchanged and free to catalyse another reaction — which is why a small amount of enzyme can process a large amount of substrate.

7. Which protein carries oxygen in the blood?

A. Ferritin   B. Haemoglobin   C. Myoglobin   D. Albumin
Answer: B. Haemoglobin is the transport protein of the blood: four subunits, each with one haem group, carry four oxygen molecules from the lungs to the tissues. Myoglobin stores oxygen in muscle, albumin carries fatty acids, ferritin stores iron.

8. How does myoglobin differ from haemoglobin?

A. Myoglobin is one chain storing one oxygen in muscle; haemoglobin is four chains carrying four oxygens in blood   B. Myoglobin works in the blood and haemoglobin in muscle   C. They are the same protein with different names   D. Myoglobin carries four oxygens and haemoglobin carries one
Answer: A. Myoglobin is a single chain with one haem group, so it stores one oxygen molecule in muscle. Haemoglobin is four chains working together, carrying four oxygen molecules in the blood. Same haem group, different job — the difference is the number of chains.

9. What does albumin carry in the blood?

A. Iron atoms in a cage   B. Glucose into cells   C. Fatty acids, hormones and some drugs   D. Oxygen
Answer: C. Albumin, the main protein of blood plasma, carries fatty acids, hormones and some drugs around the body, and helps hold water in the blood by osmosis. Oxygen is carried by haemoglobin; iron is stored by ferritin.

10. What is the main structural protein of skin, tendons and bone?

A. Collagen   B. Fibrin   C. Elastin   D. Keratin
Answer: A. Collagen is the main structural protein of skin, tendons, ligaments and bone — about a quarter to a third of all the protein in the body is collagen. Its triple-helix rope shape is what makes it strong.

11. Which protein makes hair and nails tough?

A. Actin   B. Elastin   C. Keratin   D. Collagen
Answer: C. Keratin is the tough fibrous protein of hair, nails, claws and the outer layer of skin. It is rich in alpha-helix and heavily cross-linked, which is why hair is strong and hard to stretch.

12. Which protein lets skin and artery walls stretch and spring back?

A. Keratin   B. Myosin   C. Collagen   D. Elastin
Answer: D. Elastin is the stretchy protein of artery walls, lungs and skin. Its coiled, cross-linked chains stretch under pressure and spring back — which is why an artery can widen with each heartbeat and narrow again.

13. What do antibodies do?

A. Kill bacteria by digesting them   B. Break down starch   C. Bind to a specific antigen and mark it for destruction   D. Carry oxygen to tissues
Answer: C. An antibody binds to one specific antigen — its shape fits that antigen and no other — and marks it for destruction by phagocytes or the complement system. Antibodies do not kill germs themselves; they tag them.

14. What is fibrinogen's job?

A. It digests protein in the stomach   B. It carries iron in the blood   C. It stores amino acids in seeds   D. It is converted into fibrin to form a blood clot
Answer: D. Fibrinogen is a soluble plasma protein. When a blood vessel is cut, a chain of reactions converts fibrinogen into fibrin — insoluble threads that form the mesh of a blood clot and trap blood cells.

15. Which protein is a hormone that lowers blood glucose?

A. Adrenaline   B. Insulin   C. Growth hormone   D. Glucagon
Answer: B. Insulin is a protein hormone made in the pancreas. After a meal it signals liver and muscle cells to take in glucose and store it as glycogen, so blood glucose falls. Glucagon does the opposite.

16. What does glucagon do?

A. It breaks down hydrogen peroxide   B. It lowers blood glucose   C. It carries oxygen in the blood   D. It raises blood glucose by releasing glucose from the liver
Answer: D. Glucagon, also made in the pancreas, raises blood glucose: it signals the liver to break down glycogen and release glucose into the blood. Insulin and glucagon work as a pair, pulling blood glucose in opposite directions.

17. Which two proteins slide past each other to make a muscle contract?

A. Actin and myosin   B. Keratin and fibrin   C. Tubulin and ferritin   D. Collagen and elastin
Answer: A. Muscle contraction is actin and myosin sliding past each other: myosin heads grip the thin actin filaments, pull, let go and grip again, shortening the sarcomere. Collagen and elastin are structural proteins, not motors.

18. What does each myosin pull use?

A. One oxygen molecule   B. One glucose molecule   C. One amino acid   D. One ATP molecule
Answer: D. Each myosin pull uses one ATP molecule: ATP binds to the myosin head, lets it release from actin, and is split to cock the head ready for the next pull. Without ATP the heads stay gripped — which is why muscles stiffen after death.

19. Which protein stores iron safely inside cells?

A. Haemoglobin   B. Albumin   C. Ferritin   D. Transferrin
Answer: C. Ferritin is a hollow protein cage inside cells that stores iron safely — up to about 4,500 iron atoms per molecule — and releases it when the cell needs it. Free iron would be toxic, so the cage matters. Transferrin carries iron in the blood; it does not store it.

20. Which claim stays inside the evidence of the practice figures in this lesson?

A. Five catalase molecules at about 40 million reactions a second each break down about 200 million hydrogen peroxide molecules a second   B. Catalase works faster in every cell of the body   C. Every enzyme works at 40 million reactions a second   D. One catalase molecule breaks down all the hydrogen peroxide in the body
Answer: A. Only the first claim repeats the practice figures: five catalase molecules at about 40 million reactions a second each give 5 × 40 million = 200 million reactions a second. The others turn one practice calculation into a universal rule.

Separate Interactive Quiz — 20 different questions

These quiz questions are different from the MCQs above. Select an answer for each question. You will not see whether an answer is correct until you submit all 20 questions. Use Reset to try again.