Digestion: from a plate of pap to a molecule in the blood
By the end of this lesson you will be able to
- Outline why food must be digested: the large molecules in food, the small molecules that cells can use, and the difference between them.
- Describe the structure of the alimentary canal organ by organ, and the function of each organ, including the pancreas and liver.
- Distinguish mechanical from chemical digestion, and explain why both are needed.
- Explain how amylase, protease and lipase work: substrate, product, where each is made and the pH and temperature it needs.
- Explain how the small intestine is adapted for absorption — surface area, thin walls, blood supply — and distinguish egestion from excretion.
The puzzle
A plate of seswaa and pap contains beef muscle, maize starch and a little fat. By tomorrow, some of that beef protein is part of your muscle — but cow muscle protein and human muscle protein are different molecules, built to different plans. Somewhere between the plate and your biceps the beef stopped being beef. And the organ that took it apart, your stomach, is itself made of protein. Why does it digest the beef and not itself?
Chew a piece of plain bread for a full minute without swallowing. Predict what will happen to its taste, and explain what that tells you about where digestion starts. Then predict: if you swallowed a marble, how long would it take to come out — and would it be digested at all?
Compare your answer
The bread turns noticeably sweet. Saliva contains an enzyme, amylase, which starts breaking the starch (not sweet) into the sugar maltose (sweet) — so chemical digestion begins in the mouth, before anything reaches the stomach. The marble would pass through in one to three days and come out unchanged: digestion is a chemical process that works only on food molecules, with specific enzymes for each type. Glass has no enzyme, so it is simply pushed along by the muscles of the gut wall and egested. Digestion is not grinding; it is taking molecules apart.
The science — structured notes
1.1 Why digest at all — the requisites of life
Every cell in your body needs a supply of small molecules: glucose to respire for energy, amino acids to build proteins, fatty acids and glycerol for membranes and energy stores, plus water, minerals and vitamins. Food does not arrive in that form. Starch is a chain of thousands of glucose units; a protein is a folded chain of hundreds of amino acids; a fat molecule is three fatty acids joined to a glycerol. These molecules are far too large to pass through the wall of the gut into the blood, and even if they could, a cow’s protein is useless to a human cell. Digestion is the breaking down of large, insoluble food molecules into small, soluble ones that can be absorbed into the blood and used by cells.
| Nutrient in food | Large molecule | Digested to | What cells use it for |
|---|---|---|---|
| Carbohydrate (pap, bread, rice, sugar) | Starch — a chain of glucose units | Glucose (simple sugar) | Respiration: releasing energy |
| Protein (beef, beans, eggs, milk) | Protein — a chain of amino acids | Amino acids (about 20 kinds) | Building new proteins: muscle, enzymes, antibodies, growth and repair |
| Fat / lipid (oil, butter, fat on meat) | Triglyceride — glycerol plus three fatty acids | Fatty acids and glycerol | Cell membranes, insulation, long-term energy store |
| Vitamins, minerals, water | Already small | Not digested — absorbed as they are | Enzymes, bones, blood, every reaction in the cell |
| Fibre (cellulose in vegetables, bran) | Chains of glucose humans cannot break | Not digested | Gives the gut muscles something to push against; feeds gut bacteria |
1.2 The alimentary canal, organ by organ
The alimentary canal is the tube itself, mouth to anus. Food moves along it by peristalsis: rings of muscle in the wall contract behind the food and relax in front of it, squeezing it along like toothpaste in a tube — which is why you can swallow lying down, and why astronauts can eat.
| Organ | Structure | What happens there | Time food spends |
|---|---|---|---|
| Mouth | Teeth, tongue, three pairs of salivary glands | Teeth cut and grind (mechanical); saliva moistens the food and its amylase begins starch → maltose (chemical); the tongue rolls a bolus | Under a minute |
| Oesophagus | Muscular tube, 25 cm | Peristalsis carries the bolus to the stomach; no digestion | About 10 s |
| Stomach | Muscular bag, wall lined with glands and mucus; a ring of muscle (sphincter) at each end | Churns food with gastric juice: hydrochloric acid (pH 1–2) kills microbes and gives the right pH for protease (pepsin), which begins protein → shorter chains | 2–4 hours |
| Small intestine — duodenum then ileum | About 7 m long, 2.5 cm wide, lined with villi | Bile from the liver and pancreatic juice enter the duodenum; enzymes finish digestion of starch, protein and fat; the ileum absorbs the small molecules into the blood | 3–5 hours |
| Large intestine (colon) | About 1.5 m, wider, no villi | Absorbs water and some minerals; bacteria feed on fibre and make vitamin K; the remains become faeces | 10–40 hours |
| Rectum and anus | Storage; two sphincters | Faeces stored, then egested | — |
Three organs are part of the system but not part of the tube — food never enters them. The salivary glands make saliva. The pancreas makes pancreatic juice, containing amylase, protease and lipase and an alkali (sodium hydrogencarbonate) that neutralises the stomach acid. The liver makes bile, stored in the gall bladder and released into the duodenum when fat arrives.
A person has their gall bladder removed after it becomes blocked with gallstones. The surgeon says they can eat normally but should avoid very fatty meals for a while.
(a) State what the gall bladder does and which organ makes the substance it stores.
(b) Explain why a large fatty meal is the problem, and not a plate of pap.
(c) Suggest why a small fatty meal can still be digested.
Reveal answer
(a) The gall bladder stores and concentrates bile, which is made by the liver; it squeezes the bile into the duodenum when fat arrives.
(b) Bile breaks large fat droplets into tiny ones (emulsification), giving lipase a much larger surface to work on; without a stored supply, a big fatty meal gets only the trickle of bile the liver makes at that moment, so much of the fat is not digested and passes on to the large intestine, causing discomfort. Pap is starch and needs amylase, not bile, so it is unaffected.
(c) The liver continues to make bile continuously and it drips straight into the duodenum; a small quantity of fat can be emulsified by that steady supply, so digestion is slower but complete.
1.3 Mechanical and chemical digestion
A student cuts a 2 cm cube of potato into eight 1 cm cubes and then into sixty-four 0.5 cm cubes. Calculate the total surface area at each stage and comment on what happens to the volume.
One 2 cm cube: 6 faces × (2 × 2) cm² = 24 cm²; volume 8 cm³.
Eight 1 cm cubes: 8 × 6 × (1 × 1) = 48 cm²; volume still 8 × 1 = 8 cm³.
Sixty-four 0.5 cm cubes: 64 × 6 × (0.5 × 0.5) = 96 cm²; volume still 8 cm³.
Each halving of the side doubles the surface area while the volume — and the mass of food — stays the same. The surface area per cm³ of food goes from 248 = 3 cm² per cm³ to 968 = 12 cm² per cm³. That ratio is what chewing, churning and bile are for.
“Bile is an enzyme that digests fat.”
Reveal answer
False. Bile contains no enzymes. It does two things: its bile salts emulsify fat — break large drops into tiny droplets, a physical change that increases surface area — and it is alkaline, so it helps neutralise the acid arriving from the stomach. The enzyme that digests fat is lipase, made in the pancreas. Bile makes lipase faster; it does not do its job.
1.4 Enzymes — the tools that take molecules apart
An enzyme is a protein that speeds up a chemical reaction without being used up: a biological catalyst. Each enzyme has an active site whose shape fits one kind of molecule, its substrate, the way a key fits a lock. Amylase fits starch and nothing else; protease fits proteins; lipase fits fats. The substrate binds, the bond is broken, the products leave, and the enzyme is ready for the next molecule — thousands of times a second.
| Enzyme | Substrate → products | Made in | Works in | Best pH |
|---|---|---|---|---|
| Amylase | Starch → maltose (then maltase: maltose → glucose) | Salivary glands; pancreas | Mouth; small intestine | About 7 |
| Protease (pepsin in the stomach, trypsin from the pancreas) | Protein → shorter chains → amino acids | Stomach wall; pancreas | Stomach; small intestine | Pepsin 2; trypsin 8 |
| Lipase | Fat → fatty acids + glycerol | Pancreas | Small intestine | About 8 |
Because the active site is a shape, anything that changes the shape stops the enzyme. Two things do:
A class mixed 5 cm³ of starch solution with 1 cm³ of amylase in water baths at different temperatures and timed how long the mixture took to stop turning iodine blue-black (starch gone). Results: 20 °C — 240 s; 30 °C — 120 s; 40 °C — 65 s; 50 °C — 150 s; 60 °C — no change after 600 s. Calculate a rate for each and describe the pattern.
Rate = 1time, in s⁻¹. At 20 °C: 1240 = 0.0042 s⁻¹; 30 °C: 0.0083; 40 °C: 0.015; 50 °C: 0.0067; 60 °C: 0 (no starch was digested in 600 s).
The rate roughly doubles from 20 to 30 °C (0.0042 to 0.0083 s⁻¹) and nearly doubles again to 40 °C, the highest value; above 40 °C it falls, to less than half at 50 °C, and at 60 °C the enzyme is denatured, so the reaction stops. Enzyme activity against temperature is a hill with a peak near body temperature, not a straight line.
Precision: times were read to the nearest second on a stopwatch, so rates are quoted to two significant figures — the raw data do not support more.
Biological washing powders contain protease and lipase and are sold with the instruction “wash at 30–40 °C”. Explain what the enzymes do to a gravy stain, why the temperature instruction is given, and why a 90 °C wash with the same powder gets the stain out less well.
Reveal answer
Gravy is protein and fat: the protease breaks the protein into soluble amino acids and the lipase breaks the fat into fatty acids and glycerol, which dissolve in the water and rinse away instead of clinging to the fibres. At 30–40 °C the enzymes are near their optimum temperature: molecules collide often enough for the reaction to be fast, and the enzyme keeps its shape. At 90 °C the enzymes are denatured — the protein chain unfolds and the active sites lose their shape, so no substrate can bind — and the powder becomes an ordinary detergent working on a stain that has, in addition, been cooked onto the cloth.
1.5 Absorption — why the small intestine is seven metres of folds
By the end of the duodenum, digestion has produced glucose, amino acids, fatty acids and glycerol. The ileum’s job is to get them into the blood, and it is built for one thing: surface area. Its inner wall is folded, the folds carry millions of finger-like villi about 1 mm tall, and each cell on a villus has its own microscopic folds, microvilli. Together they turn a tube whose smooth inside would be about half a square metre into an absorbing surface of roughly 30 m² — the floor of a classroom.
| Feature of the small intestine | How it helps absorption |
|---|---|
| Very long (about 7 m) and folded | Food spends hours in contact with the wall; every metre adds surface |
| Villi and microvilli | Multiply the surface area hundreds of times: more molecules cross per second |
| Wall of a villus one cell thick | Short distance for molecules to diffuse from the gut to the blood |
| Dense network of capillaries in each villus | Blood carries absorbed glucose and amino acids away at once, keeping the concentration in the blood low so diffusion continues; the blood goes first to the liver |
| A lacteal (lymph vessel) in each villus | Absorbs fatty acids and glycerol, which re-form as fat and enter the lymph before the blood |
| Moist, with mucus | Molecules must be dissolved to cross a membrane |
| Cells with many mitochondria | Some molecules are absorbed by active transport, against the concentration gradient, which needs energy from respiration |
Most absorption is by diffusion: the digested molecules are concentrated inside the gut and dilute in the blood, so they move down the concentration gradient through the thin wall. But the gut does not leave the last of the glucose behind; cells in the villi pump it into the blood by active transport even when the gradient is against them, spending energy to do so. Water is absorbed in the small intestine too, and the large intestine recovers most of what is left.
In coeliac disease the immune system reacts to gluten, a wheat protein, and the villi become flattened — the lining of the small intestine looks almost smooth. Predict three consequences for the person, and explain each with the structure of a villus.
Reveal answer
Weight loss and tiredness: with the villi flattened the surface area for absorption falls dramatically, so much of the glucose and amino acids from digested food is never absorbed and passes on — less energy and fewer building materials reach the cells, whatever the person eats. Anaemia: iron and vitamins are absorbed across the same wall, so the shortage of surface causes shortages of these too, and fewer red blood cells are made. Diarrhoea and fatty stools: unabsorbed food and fat reach the large intestine, drawing water with them and feeding bacteria. Remove gluten from the diet and the villi regrow within months, because the lining renews itself constantly — which is why the treatment is a diet, not a drug.
1.6 Egestion — and why it is not excretion
What is left after the ileum — fibre, some water, dead cells from the gut lining, bacteria and undigested remains — moves into the large intestine, where water is absorbed until the remains are semi-solid faeces. Their removal through the anus is egestion. It is easy to confuse with excretion, and examiners set the trap every year. Excretion is the removal of waste products made by the cells’ own chemical reactions: carbon dioxide from respiration, breathed out at the lungs; urea from the breakdown of amino acids, removed by the kidneys (Lesson 1.4). Faeces were never inside a cell; they are food that was never absorbed. Egestion is the gut emptying its tube; excretion is the body disposing of its own chemistry.
A student writes: “Enzymes in the stomach digest the fat in the meal, and the products are absorbed through the stomach wall. The waste is then excreted as faeces.” Find three errors and correct them.
Reveal answer
Fat is not digested in the stomach — lipase works in the small intestine at pH 8, after bile from the liver has emulsified the fat; the stomach’s enzyme is a protease. Almost nothing is absorbed through the stomach wall (a little water and alcohol); the products of digestion are absorbed in the ileum, through the villi. Faeces are egested, not excreted: they are undigested food that was never part of a cell, whereas excretion removes the chemical waste of cells, such as urea and carbon dioxide.
Try it — the enzyme and the temperature it likes
Catalase is an enzyme in every cell — potato cells are full of it — that destroys hydrogen peroxide, a poison the cell makes by accident. Pour hydrogen peroxide on cut potato and the oxygen it releases makes foam. The foam height in 60 seconds is your measure of enzyme activity. Set the temperature with the water bath, run 10 °C to 70 °C, three trials each, and plot foam height against temperature. Then change the pH and the mass of potato and see what each does to the peak.
Suppose your mean foam heights were: 10 °C → 9 mm, 20 → 18, 30 → 37, 40 → 59, 50 → 39, 60 → 1, 70 → 1 mm.
(a) Describe the relationship, quoting values, and identify the optimum temperature from this data.
(b) Explain the shape of the graph at the level of particles and enzyme molecules.
(c) The pH slider is set at 7. Predict what changing it to 5 would do to the whole curve, and why.
Reveal answer
(a) Foam height rises with temperature from 9 mm at 10 °C to a peak of 59 mm at 40 °C, roughly doubling for each 10 °C step (9, 18, 37), then falls steeply — to 39 mm at 50 °C and only 1 mm at 60 °C, with no further fall at 70 °C because by 60 °C almost every enzyme molecule is already denatured. The optimum from this data is about 40 °C; with 10 °C steps we cannot say whether the true peak is at 37 or 42 °C, so an improvement would be to test 35, 40 and 45 °C.
(b) Below the optimum, warmer molecules move faster, so hydrogen peroxide molecules collide with the active site of catalase more often and more of the collisions have enough energy to react: the rate rises. Above about 45 °C the vibrations of the enzyme molecule become violent enough to break the weak bonds holding its shape, the active site deforms and the substrate no longer fits — the enzyme is denatured, and the more molecules are denatured, the lower the rate, until by 60 °C almost none are working and 70 °C can make it no worse.
(c) Catalase works best near pH 7. At pH 5 the extra hydrogen ions alter the charges on the enzyme’s surface and distort the active site, so fewer substrate molecules fit: every point on the curve would be lower, the peak would still be near 40 °C but shorter, and the enzyme would be fully denatured at a lower temperature because its shape is already strained.
Common mistakes to avoid
- “Food is digested in the stomach” — starch starts in the mouth, protein in the stomach, everything finishes in the small intestine.
- “Bile is an enzyme” — bile emulsifies fat and neutralises acid; lipase digests fat.
- “The enzyme is killed by heat” — denatured: the active site changes shape.
- “Large surface area so more absorption” — give the mechanism: more molecules diffuse across per second; thin wall; capillaries keep the gradient.
- Egestion called excretion — faeces are undigested food; excretion removes cell waste (CO₂, urea).
- “Absorbed into the body” — absorbed into the blood (or the lacteal), through the wall of the ileum.
- Products muddled — starch → glucose; protein → amino acids; fat → fatty acids and glycerol. Learn the three pairs.
How we found out — a hole in a stomach and a dog that drooled
For two thousand years digestion was thought to be a kind of cooking, or a grinding. In 1780 the Italian priest and experimenter Lazzaro Spallanzani swallowed small linen bags of meat on strings, pulled them back up, and found the meat partly dissolved — and, by collecting his own gastric juice, showed that it digested meat in a warm glass just as well as in a stomach. Digestion was chemistry, and the chemical could be taken out of the body. Then, in 1822, a young fur trader named Alexis St Martin was shot in the stomach on Mackinac Island in North America. He survived, but the wound healed into a permanent opening. The army surgeon William Beaumont realised he had been handed a window into a living stomach. For a decade he tied pieces of food to strings, lowered them in, pulled them out at intervals, and recorded what had happened; he collected gastric juice and watched it work in a cup; he found that it contained hydrochloric acid, that it worked faster at body temperature, and that anger and fear slowed digestion. His 238 experiments, published in 1833, are the foundation of gastric physiology. In 1836 the German physiologist Theodor Schwann isolated the active ingredient of the juice and named it pepsin — the first enzyme ever discovered in an animal.
The nervous control of digestion was found by the Russian physiologist Ivan Pavlov in the 1890s. He fitted dogs with small tubes so that saliva and gastric juice could be collected, and measured how much flowed when food was seen, smelled, or eaten. The stomach, he showed, begins to secrete juice at the sight of food, before any food arrives — and, famously, a dog that repeatedly heard a bell before feeding would drool at the bell alone. Pavlov received the Nobel Prize in 1904 for the digestion work, not for the bell; the bell was what he noticed on the way. Beaumont measured what the stomach did; Pavlov showed that the nervous system (Lesson 1.4) tells it when to start.
Beaumont found that gastric juice digested meat in a warm cup but much more slowly in a cold one, and not at all if the juice was boiled first. Explain what each observation showed, and why the boiled juice was the most important result.
Reveal answer
The warm cup showed that digestion is a chemical process that can happen outside the body — it needs the juice, not the stomach’s movements or “vital spirit”. The cold cup showed that the process depends on temperature, as chemical reactions do: slower when cold. The boiled juice showed that whatever does the digesting is destroyed by heat and does not come back on cooling — which is not how acid behaves (acid survives boiling), so something else in the juice was responsible: a heat-sensitive substance we now call an enzyme, denatured by the boiling. That result pointed Schwann to pepsin three years later, and it is the same evidence you get when the 70 °C run of the lab makes no foam.
Where it is used now
- Oral rehydration in Botswana’s clinics. Diarrhoea kills by dehydration, and the small intestine absorbs water fastest when glucose and sodium are absorbed with it. The oral rehydration solution handed out at every clinic — sugar, salt, water in exact proportions — uses the villi’s own transport to pull water back into the blood. It has saved more children’s lives than any drug.
- Fermented foods. Ting (fermented sorghum porridge) and sour milk are partly pre-digested by bacteria, whose enzymes break down starch and protein before you eat — easier to digest, longer to keep, and the bacteria produce acids that stop others growing.
- Lactose intolerance. Most adults worldwide, including a majority in southern Africa, stop making the enzyme lactase after childhood; undigested milk sugar passes to the large intestine, where bacteria ferment it. Lactose-free milk is milk treated with lactase in the dairy — an enzyme doing in a tank what the gut no longer does.
- Ulcers cured with antibiotics. Since the discovery that Helicobacter pylori causes most stomach ulcers, a week of tablets has replaced a lifetime of antacids and, often, surgery.
- Nutrition and stunting. A child who eats enough but absorbs poorly — because of repeated gut infections — grows less; programmes that treat the infections and deworm children are digestion science applied to a whole population.
Science now — and the scientist you need to be
What is happening in 2026
The digestive system has turned out to be less a tube than an ecosystem. The trillions of bacteria in your large intestine — the microbiome — digest the fibre you cannot, make vitamins, train your immune system and send chemical signals to your brain, and sequencing their DNA from a single stool sample is now routine in research hospitals. The gut also turns out to be an organ of hormones: a hormone released by the intestine after a meal, GLP-1, tells the brain you are full and slows the stomach, and drugs copying it are now among the most widely prescribed treatments for obesity and type 2 diabetes in high-income countries — though cheap, long-established tablets such as metformin still treat far more people with type 2 diabetes worldwide, and how quickly the newer drugs reach southern Africa at an affordable price is still uncertain. A camera the size of a vitamin pill can now be swallowed to photograph the whole small intestine as peristalsis carries it along, and software trained on millions of images helps endoscopists spot the small growths in the colon that become cancer if missed. And enzymes themselves are being designed: lactase for lactose-free milk, proteases in washing powders that work at 20 °C to save energy, and engineered enzymes that digest plastic bottles. Every one of them relies on the same lock-and-key idea that Beaumont’s boiled juice first hinted at.
The mindshift
What you need to be able to do
Criterion A questions in this unit give you an unfamiliar situation — a gall bladder removed, a washing powder, an enzyme from a hot spring — and expect you to reach for the mechanism: substrate and active site, denaturing, surface area and diffusion. Name the organ, name the enzyme, give the substrate and the product, and explain with a mechanism rather than a label. The Think task below is the rehearsal.
See an example
The washing powder listed “protease, amylase, lipase, mannanase” — one enzyme for each kind of stain. The lactose-free milk said “lactase added”. The formula listed “partially hydrolysed whey protein” — protein already cut into short chains, easier for a baby’s gut. Fresh pineapple contains bromelain, a protease; after ten minutes the skin on my hand tingled and the jelly (which is gelatin, a protein) had turned to liquid where the pineapple touched it. The AI answered that canning heats the pineapple above 60 °C, denaturing the bromelain, so tinned pineapple cannot digest gelatin — and that is why jelly recipes say not to use fresh pineapple.
Key terms
| Digestion | The breakdown of large insoluble food molecules into small soluble ones that can be absorbed into the blood. |
| Alimentary canal | The tube from mouth to anus: mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus. |
| Peristalsis | Waves of muscle contraction in the gut wall that push food along. |
| Mechanical digestion | Breaking food into smaller pieces without changing its molecules — teeth, churning, emulsification — to increase surface area. |
| Chemical digestion | Breaking the bonds within food molecules using enzymes: starch → glucose, protein → amino acids, fat → fatty acids and glycerol. |
| Enzyme | A protein that catalyses one reaction; its active site fits its substrate; denatured by heat or the wrong pH. |
| Amylase / protease / lipase | The enzymes that digest starch / protein / fat. |
| Bile | Made by the liver, stored in the gall bladder; emulsifies fat and neutralises stomach acid. Not an enzyme. |
| Denatured | An enzyme whose active site has lost its shape, so the substrate no longer fits; permanent. |
| Villus (plural villi) | A finger-like projection of the small intestine wall: large surface area, one cell thick, capillaries and a lacteal. |
| Absorption | Movement of digested molecules through the gut wall into the blood or lymph, by diffusion and active transport. |
| Egestion | Removal of undigested food as faeces; not excretion. |
Think like a scientist
Criterion A Read the passage, then answer every part.
Bacteria living in hot springs at 80 °C make a protease that works best at 75 °C and keeps working at 90 °C. A company sells this enzyme to a tannery in Francistown, where hides are soaked in it to strip the remaining flesh and hair before the leather is treated. The tannery finds the enzyme works well in warm tanks but that at 25 °C it is slow, and that if the tank water is acidified to pH 3 to control microbes the enzyme stops working. Meanwhile a doctor treating a patient whose pancreas has stopped working prescribes capsules containing amylase, protease and lipase, to be swallowed with every meal, and warns that the capsules are useless if they are opened and the powder is sprinkled on food.
(a) Explain how the hot-spring protease removes flesh from a hide, naming the substrate and the products. [3]
(b) Explain, using particles and the active site, why the enzyme is slow at 25 °C and why it can still work at 90 °C when human protease cannot. [4]
(c) Explain why acidifying the tank to pH 3 stops this enzyme, even though the human stomach protease works at pH 2. [2]
(d) Explain why the patient whose pancreas has failed needs all three enzymes, and describe one symptom you would expect without them. [3]
(e) Suggest why the capsules must be swallowed whole, and where in the alimentary canal they should release their contents. [3]
Model answer
(a) Flesh is mostly protein, the enzyme’s substrate. Protein molecules bind to the protease’s active site, which breaks the bonds between amino acids, producing short chains and then free amino acids. These small molecules are soluble and dissolve into the tank water, so the flesh disappears from the hide rather than being scraped off.
(b) At 25 °C the protein and enzyme molecules move slowly, so they collide with the active site less often and with less energy, and fewer collisions lead to a bond being broken — the rate is low, though the enzyme is undamaged. The hot-spring enzyme has evolved with stronger bonds holding its folded shape, so at 90 °C its active site keeps its shape and substrate still fits; a human protease, built for 37 °C, vibrates itself out of shape above about 45 °C — its active site deforms, substrate cannot bind, and it is denatured permanently.
(c) Each enzyme has its own optimum pH, set by the charges on the amino acids that hold its shape and line its active site. The hot-spring enzyme is built for the neutral or slightly alkaline water of its spring; at pH 3 the extra hydrogen ions change those charges, the active site is distorted and the enzyme is denatured. Pepsin is a different protein, shaped so that pH 2 is what holds its active site together.
(d) The pancreas supplies the enzymes that complete the digestion of all three nutrients in the small intestine: amylase for starch, protease for protein and lipase for fat. Without them, only the starch begun in the mouth and the protein begun in the stomach are partly digested, and fat is not digested at all; the large undigested molecules cannot be absorbed and pass on to the large intestine. The patient loses weight despite eating, and has pale, greasy, foul-smelling stools because of the undigested fat.
(e) The enzymes are proteins. Sprinkled on food and swallowed, they meet the stomach’s acid (pH 1–2) and its protease, pepsin, which denatures and digests them before they can act. A capsule with an acid-resistant coating passes through the stomach intact and dissolves in the duodenum, where the pancreatic juice would normally arrive, the pH is about 8 (the optimum for these enzymes) and the food is about to be absorbed in the ileum.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Name the enzyme that digests starch, state where it is made, and give the product of its action.
Reveal answer
Amylase [1]; made in the salivary glands and the pancreas [1]; breaks starch into maltose, a sugar (then maltase in the small intestine wall converts maltose to glucose) [1].
Explain why the pancreas releases sodium hydrogencarbonate into the duodenum along with its enzymes.
Reveal answer
Food arriving from the stomach is acidic, at about pH 2 [1]; the pancreatic enzymes (trypsin, lipase, amylase) have an optimum near pH 8 and would be denatured — their active sites distorted — in acid [1]; the alkali neutralises the acid so the enzymes keep their shape and work at their optimum [1].
A student writes: “Digestion happens in the stomach, where enzymes are killed if the food is too hot. Faeces are then excreted.” Identify three errors.
Reveal answer
Digestion happens in the mouth, stomach and small intestine, not only the stomach [1]; enzymes are denatured, not killed — they are molecules, and hot food is cooled by the mouth long before it could affect them [1]; faeces are egested, not excreted, because they are undigested food rather than the waste of cell chemistry [1].
Describe two ways in which a villus is adapted for absorption and, for each, explain how the adaptation increases the rate at which glucose enters the blood.
Reveal answer
Any two of: the villi (and microvilli) give a very large surface area [1], so many more glucose molecules can cross the wall each second [1]; the wall is one cell thick [1], so the diffusion distance from the gut to the blood is short and molecules cross quickly [1]; a network of capillaries carries glucose away [1], keeping its concentration in the blood low so the gradient stays steep and diffusion continues [1]. Maximum 4.
True or false, with a reason: “Chewing food is a form of chemical digestion because it makes the food smaller.”
Reveal answer
False [1]: chewing is mechanical digestion — it breaks food into smaller pieces, increasing surface area, but the molecules are unchanged; chemical digestion breaks bonds within molecules using enzymes, and in the mouth that is the amylase in saliva, not the teeth [1].
A person takes a drug that blocks the production of stomach acid. Predict two effects on digestion and one on the risk of infection, with reasons.
Reveal answer
Pepsin needs pH 2 to be activated and to keep its active-site shape, so protein digestion in the stomach slows and more reaches the small intestine undigested [1]; the food arriving in the duodenum is less acidic, so less alkali is needed and pancreatic digestion continues normally — overall protein digestion is delayed rather than prevented [1]; the acid normally kills most bacteria swallowed with food, so without it more survive into the intestine and the risk of gut infections such as food poisoning rises [1].
Summary
- Cells need small soluble molecules: glucose, amino acids, fatty acids and glycerol. Digestion breaks the large molecules in food into these; absorption moves them into the blood.
- Mouth (amylase, teeth) → oesophagus (peristalsis) → stomach (acid, protease, churning) → small intestine (bile, pancreatic enzymes, absorption through villi) → large intestine (water) → egestion.
- Enzymes are protein catalysts with a specific active site; activity rises with temperature to an optimum near 37 °C and then collapses as the enzyme is denatured; each has its own optimum pH.
- Villi: large surface area, one-cell-thick wall, capillaries and lacteals — more molecules cross per second, over a short distance, down a gradient the blood keeps steep.
- Egestion (undigested food) is not excretion (the waste of cell chemistry).
Next time → Breathing and the lungs: how the air you breathe reaches the glucose you have just absorbed, and what exercise and smoking do to the exchange.