⚡ Challenge Paper Preparation

Challenge Prep: Human Nutrition

IGCSE Biology 0610 — Topic 7 — Extended

Topic 7 looks like a memory topic and is examined as a precision topic. Almost nothing here is hard to understand — the difficulty is that a dozen pairs of words mean genuinely different things and the mark scheme only accepts one of each pair. Bile emulsifies, it does not digest. Nutrients are absorbed into the blood and assimilated by cells. Undigested food is egested, not excreted. Physical digestion changes the pieces, chemical digestion changes the molecules. Amylase makes maltose, not glucose. An enzyme secreted by the pancreas acts somewhere else. Twelve traps, six walkthroughs, six lookalike pairs, a concept map and ten full practice questions below — every one of them aimed at a place where a sensible-sounding sentence earns nothing.

⚠️ Common Traps & Misconceptions

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Twelve traps that cost marks on Topic 7 challenge papers. Every one is an answer that sounds right and that mark schemes refuse.

⚠️ TRAP

Trap 1: Writing that bile digests fat

The Trap“Bile is released into the duodenum where it digests the fat.” It is the most common sentence written about bile and it is refused every year. It survives because bile clearly does something to fat, and “digests” is the verb the rest of the topic has trained you to use.

The TruthBile contains no enzymes at all. It emulsifies fats and oils — breaking large droplets into many small droplets and so increasing the surface area for chemical digestion. The fat molecules themselves are completely unchanged. The digestion is done by lipase, from the pancreas.

Why It MattersEmulsification is physical digestion — it belongs with chewing and churning, not with the enzymes. Getting that wrong destroys the physical-versus-chemical distinction the whole topic is built on, so one word can cost marks in three different questions.

Example Question“Outline the role of bile in the digestion of fats and oils. [3]”

⚠️ TRAP

Trap 2: Using absorption and assimilation as if they were the same word

The Trap“The glucose is assimilated into the blood and carried to the cells.” Fluent, confident, and it has used the wrong one of two words that a paper will very often ask you to define side by side.

The TruthAbsorption is the movement of nutrients from the intestines into the blood. Assimilation is the uptake and use of nutrients by cells. Absorption gets a molecule into the transport system; assimilation gets it into a cell and puts it to work.

Why It MattersThe two definitions are worth a mark each and they appear together constantly. They also fix the order of the five processes: a cell cannot use a nutrient that has not yet reached it, so assimilation can never come before absorption.

Example Question“Labelled amino acids appear first in a villus capillary, then inside a liver cell built into a plasma protein. Name the process at each stage. [2]”

⚠️ TRAP

Trap 3: Calling egestion excretion

The Trap“Faeces are the solid waste that the body excretes.” It sounds like a definition and it comes from everyday English, where any material leaving the body is called waste.

The TruthExcretion removes the waste products of metabolism — urea, carbon dioxide — substances made by the body’s own chemical reactions. Egestion removes undigested food that was never absorbed and never took part in metabolism at all. What decides the word is where the material came from, not what state it is in or which opening it leaves by.

Why It MattersThis is also the argument that explains why the alimentary canal is described as a tunnel through the body rather than part of it. Once you see that, questions about unabsorbed calcium, unabsorbed iron and flattened villi all answer themselves.

Example Question“Explain why the removal of undigested cellulose is egestion and not excretion. [2]”

⚠️ TRAP

Trap 4: Writing “the food is broken down” without naming any molecule

The Trap“In the small intestine, enzymes break down the food so it can be absorbed.” Nothing in that sentence is false. Nothing in it is worth a mark either.

The TruthEvery digestion mark scheme is written in molecules: starch → maltose by amylase, maltose → glucose by maltase, protein → amino acids by proteases, fats and oils → fatty acids and glycerol by lipase. Name the substrate, name the products, name the enzyme.

Why It MattersVague breakdown answers are the reason strong students score 4 out of 8 on a question they understood perfectly. The information was in their head; it was not on the paper.

Example Question“Describe the chemical digestion of a cheese sandwich, naming all the enzymes and products involved. [6]”

⚠️ TRAP

Trap 5: Saying amylase produces glucose

The Trap“Amylase breaks starch down into glucose, which is then absorbed.” It gets to the right final molecule by the wrong route and misses an entire syllabus statement.

The TruthStarch digestion takes two steps. Amylase produces maltose. Maltase then produces glucose, and it does so on the membranes of the epithelium lining the small intestine — not free in the fluid of the gut, but at the exact surface the glucose is about to be absorbed through.

Why It MattersIt also protects you in food-test questions: Benedict’s solution detects reducing sugars in general and cannot tell maltose from glucose, so a brick-red precipitate never proves that glucose has been made.

Example Question“A student concludes from a positive Benedict’s test that saliva converts starch to glucose. Evaluate this conclusion. [3]”

⚠️ TRAP

Trap 6: Assuming an enzyme acts where it is secreted

The Trap“Lipase is secreted by the pancreas and acts in the pancreas.” The two questions look like one question, so one answer gets used for both.

The TruthCambridge asks for where each enzyme is secreted and where it acts as separate points, precisely because for the pancreas they are different. Pancreatic juice is emptied into the duodenum, so pancreatic amylase, trypsin and lipase all act in the small intestine. Nothing is digested inside the pancreas. Salivary amylase is secreted by the salivary glands and acts in the mouth; pepsin is secreted by the stomach wall and acts in the stomach.

Why It MattersAmylase is the enzyme where this matters most: it is secreted in two places and acts in two places, which is why a starchy meal is still fully digested even though the salivary enzyme is destroyed by stomach acid on the way through.

Example Question“Name the two places where amylase acts, the organ that secretes it in each case, and explain why starch digestion has to begin again after the stomach. [3]”

⚠️ TRAP

Trap 7: Saying physical digestion makes the molecules smaller

The Trap“Chewing breaks the food down into small molecules so that they can be absorbed.” The word “breaks down” has slipped from pieces to molecules, and with it the entire distinction between the two kinds of digestion.

The TruthPhysical digestion is the breakdown of food into smaller pieces without any chemical change to the food molecules. A chewed piece of bread is smaller and is still starch — test it with iodine and it still goes blue-black. Its value is that it increases the surface area for the enzymes that do change the molecules.

Why It MattersIf physical digestion made molecules absorbable, there would be no reason for enzymes to exist. Getting this right is also what makes the bile question answerable, because emulsification is the same idea applied to droplets.

Example Question“Chewed bread still gives a blue-black colour with iodine solution. Explain what this shows. [2]”

⚠️ TRAP

Trap 8: Claiming that hydrochloric acid digests protein

The Trap“The stomach acid breaks down the protein in the food.” Acid feels destructive, the stomach is where protein digestion starts, and the two facts fuse.

The TruthHydrochloric acid has exactly two functions in the syllabus: it kills harmful microorganisms in the food, and it provides the acidic pH at which pepsin works best, around pH 2. The digestion is done by pepsin. The acid supplies conditions, not catalysis.

Why It MattersCandidates who give the acid the enzyme’s job cannot then explain why the stomach makes pepsin at all, and they get antacid questions backwards. Neutralising stomach acid reduces protein digestion — not because the acid was digesting, but because pepsin is now far from its optimum.

Example Question“Suggest one side effect of a drug that neutralises stomach acid, and explain it. [2]”

⚠️ TRAP

Trap 9: Using “osmosis” for nutrients at the villus

The Trap“Glucose passes into the blood by osmosis through the thin wall of the villus.” It appears in a large minority of answers about absorption and scores nothing every time.

The TruthOsmosis is the movement of water only. Glucose, amino acids and mineral ions move by diffusion when the gradient allows it, and by active transport — using protein carriers and energy from respiration — when they have to move against it.

Why It MattersThe clue for which process is wanted is always the direction. If the concentration is higher in the blood than in the gut and absorption still happens, it cannot be diffusion. That is also the reason the epithelial cells of a villus are packed with mitochondria.

Example Question“Glucose is 4 units in the lumen and 9 units in the capillary blood, and is still absorbed. Explain how. [3]”

⚠️ TRAP

Trap 10: Sending everything into the blood capillaries

The Trap“All the digested food is absorbed into the blood capillaries of the villus.” It is right for most nutrients, which is exactly why it is so easy to write — and it makes the lacteal impossible to explain.

The TruthTwo vessels, two cargoes. Blood capillaries take glucose and amino acids (and water-soluble vitamins and mineral ions). The lacteal takes fatty acids and glycerol into the lymphatic system, which eventually empties into the blood.

Why It MattersThere is a piece of evidence that makes this unforgettable: after a fatty meal the fluid in the lacteal turns milky while the capillary blood does not. Challenge papers use exactly that observation as a data stem.

Example Question“Explain why a villus contains both a capillary network and a lacteal. [2]”

⚠️ TRAP

Trap 11: Predicting the wrong direction after damage to absorption

The Trap“If the villi are flattened, less glucose will be found in the contents of the ileum.” The reasoning is “something is damaged, so there will be less of everything”, and it reverses the answer.

The TruthAbsorption is what removes nutrients from the lumen. Damage it and more is left behind, not less — and that surplus leaves in the faeces by egestion. The patient loses body mass and may become anaemic, precisely because the nutrients are still in the tube instead of in the blood.

Why It MattersAlmost every coeliac disease, gut surgery or intestinal parasite stem is marked on this single direction. Say out loud which process removes the substance from the lumen before you predict which way a number moves.

Example Question“Predict the effect of flattened villi on the glucose concentration in the lower ileum, and on the patient. [3]”

⚠️ TRAP

Trap 12: Answering “prevents a disease” when asked for the importance of a nutrient

The Trap“Vitamin C is important because it prevents scurvy. Vitamin D is important because it prevents rickets.” Both are true and neither describes what the nutrient does.

The TruthGive the job. Vitamin C is needed to form the connective tissue that holds cells together. Vitamin D is needed for the absorption of calcium. Calcium hardens bones and teeth and is needed for blood clotting. Iron is needed to make haemoglobin.

Why It MattersOnly the mechanism answers the harder question: why a child on a calcium-rich diet can still develop rickets. The answer is that without vitamin D the calcium is never absorbed — and there is no route to that sentence from “vitamin D prevents rickets”.

Example Question“A child with a calcium-rich diet, kept indoors, develops rickets. Explain fully. [3]”

🔍 Step-by-Step Walkthroughs

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Six challenge-level questions worked through in the order you should actually think about them. Try each part before revealing the next step.

Walkthrough 1 — Four Tubes That Prove Bile Is Not an EnzymeCream was mixed with pancreatic lipase at 37 °C in four tubes and the fatty acid produced in 15 min was measured. Tube 1 cream + lipase, droplet diameter 46 µm, 11 mg. Tube 2 cream + lipase + bile, 4 µm, 58 mg. Tube 3 cream + bile, 4 µm, 0 mg. Tube 4 cream + boiled lipase + bile, 4 µm, 0 mg. (a) Explain the difference between tubes 1 and 2. [3] (b) Explain what tubes 3 and 4 each show. [3] (c) State the general principle. [1]

The extra column is the whole explanation

Tube 1 has droplets of 46 µm; every tube containing bile has droplets of 4 µm. That is emulsification, and it is a purely physical change — the same fat, in smaller lumps. Many small droplets have a far larger total surface area than a few large ones, so lipase can reach much more of the fat, more enzyme–substrate complexes form per second, and the product rises from 11 to 58 mg. Quote both pairs of numbers; data questions reserve a mark for using the data.

Emulsified, and nothing produced

Tube 3 has droplets just as small as tube 2 and produces zero fatty acid. So emulsification on its own digests nothing. Bile contains no enzyme, and this single row is stronger evidence than any definition. If you have ever written “bile digests fat”, this is the result to remember.

Each control removes a different alternative explanation

Tube 4 contains bile and a denatured lipase, and again produces nothing. That rules out the possibility that something else in the mixture — the cream, the warmth, the bile — produced the fatty acid in tube 2. Together, tubes 3 and 4 show the result needs both bile and active lipase.

Physical versus chemical, one more time

Bile changes the size of the droplets; lipase changes the molecules. Emulsification is physical digestion and belongs with chewing and churning. Write that sentence and the mark is automatic.

Full Mark-Scheme Answer(a) Bile emulsifies the fat, reducing droplet diameter from 46 to 4 µm [1]; small droplets give a much larger total surface area [1]; more fat is exposed to lipase so more product is formed, 58 against 11 mg [1]. (b) Tube 3 produced nothing, showing bile alone cannot digest fat because it contains no enzyme [1]; tube 4 produced nothing, showing the product in tube 2 required active lipase [1]; each control eliminates a different alternative explanation [1]. (c) Bile changes the size of the droplets, not the molecules — emulsification is physical digestion [1].

Walkthrough 2 — Two Proteases and a Question About UnitsThe activity of two human proteases was measured at 37 °C, each expressed as a percentage of its own maximum. Enzyme P: pH 1, 74 %; pH 2, 100 %; pH 3, 58 %; pH 5, 9 %; pH 7 and above, 0 %. Enzyme Q: pH 5, 6 %; pH 7, 61 %; pH 8, 100 %; pH 9, 47 %; pH 11, 0 %. (a) Identify each enzyme and the region in which it acts. [3] (b) Explain why P shows zero activity at pH 8. [3] (c) A student says the data show Q is a better enzyme than P. Evaluate. [3]

The pH of the region tells you the name of the enzyme

P peaks at about pH 2, which is the stomach: P is pepsin, secreted by the stomach wall and acting in the stomach. Q peaks at about pH 8, which is the small intestine after bile has neutralised the acid: Q is trypsin, secreted by the pancreas and acting in the small intestine. The check that catches a swap: an enzyme placed in the wrong organ would be denatured in the only place it is ever found.

Term, mechanism, consequence

At pH 8, pepsin is far from its optimum and is denatured [term]; the shape of the active site changes so protein is no longer complementary to it [mechanism]; no enzyme–substrate complexes form, so no amino acids are produced [consequence]. Never write “killed” — an enzyme is a protein molecule and was never alive.

Both columns are percentages of different maxima

100 % for P and 100 % for Q may be wildly different real rates — the table gives no absolute values at all, so the two columns cannot be compared with each other. The claim is unsupported, and the strongest answer also notes that “better” is meaningless without stating the pH: each enzyme is best in its own conditions.

Read the headings before the numbers

Percentage of a maximum, arbitrary units, dry mass, time rather than rate — each of these changes what a conclusion is allowed to say. Thirty seconds spent on the column headings is the single highest-value habit in a challenge paper.

Full Mark-Scheme Answer(a) Optima about pH 2 and pH 8 [1]; P is pepsin, acting in the stomach [1]; Q is trypsin, acting in the small intestine [1]. (b) At pH 8 pepsin is denatured [1]; the active site changes shape and is no longer complementary to the substrate [1]; no enzyme–substrate complexes form so no product [1]. (c) Both columns are percentages of each enzyme’s own maximum [1]; no absolute rates are given so the enzymes cannot be compared [1]; each is best in its own conditions, so “better” has no meaning without stating the pH [1].

Walkthrough 3 — Where Does the Mass Actually Go?A meal of dry mass 320 g was followed through the alimentary canal. Dry mass remaining: leaving the stomach 318 g; leaving the duodenum 301 g; leaving the ileum 64 g; leaving the colon 58 g. (a) Calculate the percentage absorbed by the end of the ileum. [2] (b) Explain the difference between the small fall in the first two regions and the steep fall along the ileum. [3] (c) Explain why the colon removes only 6 g. [3] (d) Name the process removing the final 58 g and justify it. [2]

Answer the question that was asked

Absorbed = 320 − 64 = 256 g. As a percentage of the original: 256 ÷ 320 × 100 = 80 %. Answering 20 % gives the fraction still in the gut — correct arithmetic attached to the wrong question, which is an expensive way to lose two marks.

Breaking a molecule in half removes no mass

The stomach and duodenum are regions of digestion. Chopping starch into maltose changes the molecules but leaves every atom inside the tube, so the mass barely moves. The ileum is the region of absorption, where nutrients actually cross the wall and leave the canal — which is why the mass collapses there. This table separates the two processes better than any diagram.

The colon is busy, and the measurement cannot see it

The colon absorbs mainly water, and water contributes nothing to a dry mass. That is why a region doing a great deal of work shows almost no change on this table. Most of the remaining 58 g is fibre, which has no enzyme to digest it and so can never be absorbed.

Egestion, and why it is not excretion

The 58 g was never absorbed into the blood or into any cell, so it is not a waste product of metabolism. Its removal is egestion. Naming the process earns one mark; the justification earns the second, and it is the half most often left out.

Full Mark-Scheme Answer(a) (320 − 64) ÷ 320 × 100 [1] = 80 % [1]. (b) Stomach and duodenum are regions of digestion [1]; digestion changes molecules but does not remove mass from the canal [1]; the ileum is the main absorbing region, where nutrients cross into the blood [1]. (c) The colon absorbs mainly water [1]; water does not contribute to dry mass [1]; the remainder is largely undigested fibre, which cannot be absorbed [1]. (d) Egestion [1]; the material was never absorbed, so it is not a waste product of metabolism and its removal is not excretion [1].

Walkthrough 4 — A Method With Four Things Wrong With ItA student writes: “To show that chewing helps digestion, put a whole piece of bread into one tube of protease solution and a chewed piece into another. Leave them on the bench overnight. Whichever tube looks emptier in the morning shows chewing helps.” (a) Identify four faults and explain the effect of each. [4] (b) Rewrite the method properly. [3] (c) State and explain the prediction. [3]

The fault everybody misses

Bread is mainly starch, and the method uses a protease. Because of enzyme specificity, starch is not complementary to the active site of a protease, so almost nothing happens in either tube and the experiment cannot show anything at all. Always check substrate against enzyme before looking at anything else — candidates read “digestion” and stop checking.

Two more faults, in the order a mark scheme lists them

The masses of bread are not matched, so a difference could be caused by quantity rather than by surface area — the very variable being investigated. And the temperature is uncontrolled with no measured time; “overnight on a bench” differs from day to day and from tube to tube.

“Looks emptier” is not data

It is subjective, produces no numbers, and cannot be repeated or compared. Replace it with a quantitative end point: the time for iodine solution to stop giving a blue-black colour. Then the two tubes give two numbers that can actually be set against each other.

Say “pieces”, not “molecules”

The broken-up bread loses its blue-black colour sooner. Breaking it up is physical digestion — smaller pieces, unchanged starch molecules — and the greater surface area exposes more starch to the amylase, so more enzyme–substrate complexes form per second. An answer that says chewing “breaks the starch down” contradicts the definition it is being tested on.

Full Mark-Scheme Answer(a) Wrong enzyme — bread is starch so amylase is needed [1]; masses of bread not matched, so quantity is confounded with surface area [1]; temperature and time not controlled [1]; measurement subjective and not quantitative [1]. (b) Equal measured masses, one whole and one broken up, same volume and concentration of amylase [1]; both in a water bath at 37 °C, timing started together [1]; measure the time for iodine to stop giving blue-black, repeat three times and take a mean [1]. (c) The broken-up bread is digested in a shorter time [1]; breaking it up is physical digestion, so the pieces are smaller but the molecules are unchanged [1]; the greater surface area gives more enzyme–substrate complexes per second [1].

Walkthrough 5 — Reading a Villus From the DataGlucose in the gut contents, and internal surface area, were measured along the canal two hours after a starchy meal. Stomach 31 units, 0.1 m²; duodenum 44 units, 2.0 m²; upper ileum 26 units, 110 m²; lower ileum 4 units, 95 m²; colon 1 unit, 0.4 m². (a) Describe the pattern and explain the rise and the fall. [3] (b) Use the surface area column. [3] (c) In the lower ileum, glucose is 4 units in the lumen and 9 in the capillary blood, and is still absorbed. Explain. [3]

Rise, peak, fall — with the numbers attached

Glucose rises from 31 in the stomach to a peak of 44 in the duodenum, then falls steadily to 1 in the colon. Quoting two figures is very often a mark by itself, and it forces you to notice the peak rather than describing the table as “going down”.

Digestion makes it, absorption removes it

The rise is continued chemical digestion — pancreatic amylase and then maltase releasing more glucose. The fall is absorption through the wall of the ileum into the blood. The commonest wrong answer is “the glucose is used up”: nothing in the lumen respires, so glucose leaves by crossing the wall, not by disappearing.

110 m² against 2.0 m²

The ileum has by far the largest internal surface area, produced by folding of the lining, villi and microvilli. More surface means more molecules absorbed per second, which is exactly why the glucose figure falls most steeply there. Two columns are given so that they can be read together; using only one answers half the question.

Uphill movement cannot be diffusion

4 in the lumen against 9 in the blood is movement against the concentration gradient, so it must be active transport, using protein carriers and energy from respiration. That is why villus epithelial cells are packed with mitochondria. Osmosis is the wrong word whatever the numbers — osmosis moves water.

Full Mark-Scheme Answer(a) Rises from 31 to a peak of 44 in the duodenum, then falls to 1 in the colon [1]; the rise is continued chemical digestion of starch [1]; the fall is absorption into the blood [1]. (b) The ileum has much the largest surface area, 110 and 95 against 2.0 m² [1]; produced by folds, villi and microvilli [1]; more surface allows more absorption per second, matching the steep fall [1]. (c) Movement is against the concentration gradient [1]; therefore active transport using protein carriers [1]; requiring energy from respiration, hence the many mitochondria [1].

Walkthrough 6 — Two Children, One Condition, Opposite CausesFour children of the same age and mass were studied. Child 1: calcium 800 mg, vitamin D 10 µg, healthy. Child 2: calcium 820 mg, vitamin D 1 µg, soft bending bones. Child 3: calcium 190 mg, vitamin D 11 µg, soft bending bones. Child 4: calcium 790 mg, vitamin D 9 µg, healthy. (a) Name the condition. [1] (b) Explain why children 2 and 3 have it for opposite reasons. [4] (c) Child 2 is given a calcium supplement and does not improve. Explain. [3]

Two failures, two different columns

The condition is rickets. Child 2 has plenty of calcium and almost no vitamin D; child 3 has plenty of vitamin D and almost no calcium. Any answer naming a single nutrient as “the cause” has read only half the table — which is exactly what the design is testing.

Material versus the means of getting it

Child 3 has nothing to build with: bones cannot be hardened with calcium that was never eaten. Child 2 has the material but cannot absorb it, because vitamin D is needed for the absorption of calcium from the small intestine. Both end up with too little calcium in the blood and therefore soft, bending bones.

Where the extra calcium ends up

Without vitamin D the additional calcium is still not absorbed. It stays in the lumen of the alimentary canal and leaves in the faeces — it is egested, not excreted, because it never entered a cell. The fix is vitamin D, from oily fish, egg yolk, fortified foods, or sunlight on the skin.

The healthy rows are the control

Children 1 and 4 have both nutrients and are healthy, which shows the condition is not caused by age, mass or anything else the study held constant. Pointing that out is worth doing in an evaluation question — a table without a normal row proves much less than one with two.

Full Mark-Scheme Answer(a) Rickets [1]. (b) Child 3 has a very low calcium intake, so there is too little calcium to lay down in bone [2]; child 2 has enough calcium but almost no vitamin D, and vitamin D is needed for the absorption of calcium from the small intestine, so the calcium is not absorbed [2]. (c) Without vitamin D the extra calcium is still not absorbed [1]; it remains in the alimentary canal and is egested in the faeces [1]; the child needs vitamin D, from oily fish, egg yolk or sunlight on the skin [1].

🔍 Spot the Difference

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Six pairs that look almost identical and have different answers. The distinction is where the marks live.

Question A

A meal of fat is placed in a tube with bile only. What is produced?

Nothing. The droplets get smaller — that is emulsification — but bile contains no enzyme, so no fatty acids or glycerol are formed.

Question B

A meal of fat is placed in a tube with lipase only. What is produced?

Fatty acids and glycerol, but slowly. The fat is in a few large droplets, so the surface area available to lipase is small.

Key DifferenceBile changes the droplets; lipase changes the molecules. Only one of those is digestion. Put them together and the rate multiplies, because a large surface area plus a catalyst is far more than the sum of the two.

Question A

Glucose moves from the lumen of the ileum into a villus capillary. Name the process.

Absorption. A nutrient has moved from the intestine into the blood — the syllabus definition word for word.

Question B

That glucose is taken into a muscle cell and respired. Name the process.

Assimilation. A nutrient has been taken up by a cell and used.

Key DifferenceAbsorption ends in the blood; assimilation ends in a cell. They are consecutive events happening to the same molecule, which is why a paper can ask for both in one question and expect two different answers.

Question A

Undigested cellulose leaves in the faeces. Egestion or excretion?

Egestion. The cellulose was never absorbed, never entered a cell, and took no part in metabolism.

Question B

Bile pigment, made by the liver from old red blood cells, leaves in the faeces. Egestion or excretion?

Excretion. It is a waste product of the body’s own reactions — it just happens to leave by the same route.

Key DifferenceThe route out is irrelevant; the origin of the material decides the word. This pair is worth remembering because it proves that “anything in faeces is egested” is not a rule.

Question A

Amylase is secreted by the salivary glands. Where does it act?

In the mouth — secreted and used in the same place, and destroyed shortly afterwards by stomach acid.

Question B

Amylase is secreted by the pancreas. Where does it act?

In the small intestine. Pancreatic juice is emptied into the duodenum — nothing is digested inside the pancreas.

Key DifferenceSame enzyme, two sources, two sites of action. This is why Cambridge lists “where secreted” and “where it acts” as separate statements, and why starch digestion has to restart after the stomach.

Question A

A child eats 190 mg of calcium and 11 µg of vitamin D per day and develops rickets. Why?

There is almost no calcium to absorb. Absorption works perfectly; there is simply nothing to lay down in bone.

Question B

A child eats 820 mg of calcium and 1 µg of vitamin D per day and develops rickets. Why?

Plenty of calcium, but without vitamin D it is not absorbed — it passes straight through and is egested.

Key DifferenceSame disease, opposite causes: missing material versus missing the means of absorbing it. Any answer that names one nutrient as “the cause of rickets” can only explain one of these children.

Question A

Chewed bread is tested with iodine solution immediately. What colour?

Blue-black. Chewing is physical digestion — smaller pieces, and the starch molecules are completely unchanged.

Question B

Bread held in the mouth for five minutes is tested with iodine. What colour?

Orange-brown, or nearly so. Salivary amylase has had time to work — but that is chemical digestion, not the chewing.

Key DifferenceThe same mouth is doing two completely different jobs at once. Marks are lost when the sweetness that develops after a few minutes is credited to the teeth rather than to the enzyme.

🔗 Human Nutrition Concept Map

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Click each node to see how the sub-topics connect into one story: a meal, a tube, and a cell at the end of it.

⭐ CORE FRAMEWORK 1

Diet → what each nutrient is for → what happens when it is missing

Balanced Means Matched, Not Equal ▶

A balanced diet contains all the required nutrients in the correct amounts and proportions for that person.

The requirement changes with age, sex, activity level, body size and pregnancy — which is why the second clause is a separate mark.

You need about 250 g of carbohydrate and about 15 mg of iron a day. “Equal masses” is not a near miss; it is out by a factor of tens of thousands.

Every Nutrient Has a Job, and the Job Is the Mark ▶

Carbohydrate — the main energy source. Fat — energy store, insulation, cell membranes. Protein — amino acids for growth, repair and enzymes.

Vitamin C — connective tissue. Vitamin D — absorption of calcium. Calcium — bones and teeth, blood clotting. Iron — haemoglobin.

“Prevents scurvy” is not a job. Only the mechanism lets you answer the harder question about why a calcium supplement can fail.

The Two Nutrients That Do No Chemistry ▶

Fibre is cellulose and humans have no enzyme for it, so it is never digested and never absorbed. Its value is bulk: something for the gut muscles to grip.

Not digested → not absorbed → never respired. Any answer that has fibre releasing energy contradicts the definition of fibre.

Water needs no digestion because it is already tiny. It is the solvent for metabolism and transport, and it is absorbed directly.

⭐ CORE FRAMEWORK 2

One tube → five processes → the words that must not be swapped

The Canal, and the Organs That Are Not Part of It ▶

Alimentary canal: mouth → oesophagus → stomach → duodenum → ileum → colon → rectum → anus.

Associated organs: salivary glands, pancreas, liver, gall bladder. Food never passes through any of them — they deliver secretions into the canal.

The lumen of the canal is a tunnel through the body, not part of it. Nothing is truly inside you until it has been absorbed, and that one idea answers every egestion question.

Five Processes, in an Order That Cannot Be Rearranged ▶

Ingestion → digestion → absorption → assimilation → egestion.

Nothing can be absorbed until it is small and soluble; nothing can be assimilated until it has reached a cell. The biology forces the sequence.

Absorption = into the blood. Assimilation = into a cell, and used. Egestion = removal of material that was never absorbed.

Physical Digestion: the Pieces, Never the Molecules ▶

Teeth cut, tear and grind; the stomach’s muscular wall churns; bile emulsifies fat droplets. All three increase surface area and change nothing chemically.

Cutting a cube of side a into cubes of side b multiplies the surface area by a ÷ b, not by the number of pieces.

Tooth structure: enamel (hard, non-living, crown), dentine (living, bulk), pulp (nerves and blood vessels), cement (anchors the root in the jaw bone).

⭐ CORE FRAMEWORK 3

Enzymes → small soluble molecules → across the villus → into a cell

Three Enzymes, Four Facts Each ▶

Amylase: starch → maltose. Secreted by salivary glands (acts in the mouth) and by the pancreas (acts in the small intestine).

Protease: protein → amino acids. Pepsin from the stomach wall, acting at about pH 2; trypsin from the pancreas, acting at about pH 8.

Lipase: fats and oils → fatty acids and glycerol. Secreted by the pancreas, acting in the small intestine.

Maltase completes starch digestion — maltose → glucose, on the membranes of the epithelium lining the small intestine.

Two Chemicals That Set the Conditions ▶

Hydrochloric acid in gastric juice does exactly two things: kills harmful microorganisms, and gives pepsin its optimum pH. It digests nothing itself.

Bile is alkaline and neutralises the acidic mixture arriving from the stomach, providing a suitable pH for the pancreatic enzymes, which work best around pH 8.

So bile has two jobs — one physical (emulsification), one chemical (pH). Remember only one and you lose half the marks in any duodenum question.

The Villus Is the Same Argument as the Root Hair Cell ▶

Three adaptations, exactly as in Topic 3: large surface area (folds, villi, microvilli); short diffusion distance (wall one epithelial cell thick); maintained concentration gradient (rich blood supply carrying nutrients away).

Capillaries take glucose and amino acids. The lacteal takes fatty acids and glycerol into the lymphatic system — which is why lymph is milky after a fatty meal.

Movement is by diffusion when the gradient allows and by active transport when it does not. Never osmosis: osmosis moves water.

❌ “Why Is This Wrong?” Exercises

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Six real student answers. Find the fault before you reveal it.

Exercise 1: “Describe the role of bile in the digestion of fats and oils. [3]”

Student’s Answer“Bile is made in the gall bladder and released into the duodenum, where it digests the fat into fatty acids and glycerol.”

The FlawTwo errors in one sentence. Bile is made by the liver and only stored by the gall bladder. And bile contains no enzymes, so it cannot digest anything — the fatty acids and glycerol are produced by lipase.

Correct Answer“Bile is produced by the liver and stored in the gall bladder before release into the duodenum. It emulsifies fats and oils, breaking large droplets into many small droplets [1], which greatly increases the surface area [1] so that lipase can digest the fat to fatty acids and glycerol more rapidly [1].”

Key RuleBile changes the size of the droplets; lipase changes the molecules. If nothing chemical has happened, the process is physical, and the verb is emulsify.

Exercise 2: “Explain why a child on a calcium-rich diet who is kept indoors develops rickets. [3]”

Student’s Answer“Because vitamin D is needed to make bones strong, and being indoors means the child does not get enough vitamin D, so the bones stay soft.”

The FlawHalf of it is right — the sunlight link is there — but the mechanism is missing. Vitamin D does not become part of a bone. Without saying what it actually does, the answer cannot explain why a diet already rich in calcium made no difference, which is the whole point of the question.

Correct Answer“Vitamin D is made in the skin in sunlight, so a child kept indoors makes very little [1]. Vitamin D is needed for the absorption of calcium from the small intestine [1]. Without it the calcium in the diet is not absorbed, so it cannot be laid down in bone and the bones remain soft and bend under the child’s weight [1].”

Key RuleThe word that turns this into a full-mark answer is absorption. Eating a nutrient and having a nutrient are different things, and the step in between is where the marks are.

Exercise 3: “Describe what happens to the starch in a slice of bread from the mouth until glucose enters the blood. [4]”

Student’s Answer“The teeth chew the bread and break the starch down. Then amylase in the saliva turns the starch into glucose, which is absorbed into the blood in the small intestine.”

The FlawTwo mistakes. Chewing does not break the starch down — it breaks the bread down, and the starch molecules are unchanged. And amylase produces maltose, not glucose; the answer has skipped both pancreatic amylase and maltase.

Correct Answer“The teeth carry out physical digestion, increasing the surface area but leaving the starch molecules unchanged. Salivary amylase begins converting starch to maltose in the mouth [1]. The salivary enzyme is denatured by stomach acid, so pancreatic amylase continues the process in the small intestine [1]. Maltase, on the membranes of the epithelium lining the small intestine, converts maltose to glucose [1], which is absorbed into the capillaries of a villus [1].”

Key RuleStarch digestion is a two-step process in two places, and physical digestion never touches a molecule. Both of those points are separate syllabus statements.

Exercise 4: “Explain how the products of fat digestion leave the small intestine. [3]”

Student’s Answer“The fatty acids and glycerol pass by osmosis through the thin wall of the villus into the blood capillaries and are carried away to the cells.”

The FlawTwo errors again. Osmosis is the movement of water and cannot be used for a nutrient. And the products of fat digestion do not enter the blood capillaries — that is the route for glucose and amino acids.

Correct Answer“The fatty acids and glycerol pass through the epithelium of the villus, which is only one cell thick [1]; they enter the lacteal in the centre of the villus rather than a blood capillary [1]; the lacteal carries them into the lymphatic system, which eventually empties into the blood [1].”

Key RuleTwo vessels, two cargoes. The evidence is easy to remember: after a fatty meal the lacteal fluid turns milky and the capillary blood does not.

Exercise 5: “A patient’s villi are flattened by disease. Predict and explain the effect. [3]”

Student’s Answer“Less food would be digested, so there would be less glucose in the small intestine and the patient would put on weight because the food stays inside.”

The FlawVilli are an absorbing structure, not a digesting one, so digestion is largely unaffected. And the direction is reversed twice over: less absorption leaves more glucose in the lumen, and the patient loses mass because the nutrients leave in the faeces instead of entering the blood.

Correct Answer“The internal surface area of the ileum falls sharply, so much less is absorbed [1]. More glucose therefore remains in the contents of the lower ileum and colon [1]. The patient loses body mass and may become anaemic, because nutrients pass out in the faeces by egestion instead of entering the blood [1].”

Key RuleBefore predicting which way a number moves, say out loud which process removes the substance from the lumen. Damage that process and the number goes up.

Exercise 6: “State the two functions of hydrochloric acid in the stomach. [2]”

Student’s Answer“It kills bacteria in the food and it breaks down the protein into amino acids.”

The FlawThe first half scores. The second gives the acid the enzyme’s job. Acid does not catalyse anything — it creates the conditions in which pepsin works, and a candidate who believes otherwise cannot explain why the stomach makes pepsin at all.

Correct Answer“It kills harmful microorganisms taken in with the food [1], and it provides the acidic pH at which pepsin has its optimum activity, about pH 2 [1].”

Key RuleConditions versus catalysis. The same distinction explains antacid questions: neutralising the acid reduces protein digestion, not because the acid was digesting but because pepsin is now far from its optimum.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions. Write your answer first, then reveal the model answer and the examiner’s notes.

Question 1

[6 marks]

(a) Define a balanced diet and state two factors that change a person’s requirements. [3] (b) For vitamin C and for iron, state one dietary source and describe the importance of each. [3]

Model Answer(a) A diet containing all the nutrients required [1] in the correct amounts and proportions [1]; two factors from age, sex, activity level, body size, pregnancy [1].
(b) Vitamin C: citrus fruit or fresh green vegetables [1]; needed to form the connective tissue that holds cells together, for healthy gums and skin and for wound healing [1]. Iron: red meat, liver or dark green leafy vegetables; needed to make haemoglobin [1].

Examiner’s NotesSources must be foods. Importance must be a job, not a disease — “prevents scurvy” and “prevents anaemia” describe what happens without the nutrient and are not what the command word asks for.

Question 2

[7 marks]

(a) Name, in order, the four regions of the alimentary canal between the stomach and the anus. [4] (b) Define absorption, assimilation and egestion. [3]

Model Answer(a) Duodenum [1], ileum [1], colon [1], rectum [1].
(b) Absorption — the movement of nutrients from the intestines into the blood [1]. Assimilation — the uptake and use of nutrients by cells [1]. Egestion — the removal of undigested food from the body as faeces [1].

Examiner’s NotesCambridge names the parts of the small and large intestines separately, so “small intestine then large intestine” is worth two marks at most. The three definitions should be reproduced word for word — they are free marks that vanish the moment absorption and assimilation are swapped.

Question 3

[8 marks]

(a) Define physical digestion and explain why it is necessary. [2] (b) Describe how the shape of an incisor and of a molar each suits its function. [4] (c) Describe the part the stomach plays in physical digestion. [2]

Model Answer(a) The breakdown of food into smaller pieces without chemical change to the food molecules [1]; it increases the surface area for the action of enzymes in chemical digestion [1].
(b) An incisor is chisel-shaped with a sharp straight edge [1], used for biting and cutting pieces off the food [1]; a molar is broad with cusps or ridges [1], used for grinding and chewing food into small pieces [1].
(c) The muscular wall contracts, churning and mixing the contents [1]; this breaks the food into smaller pieces and mixes it with gastric juice, increasing the surface area [1].

Examiner’s NotesPart (b) needs structure and function joined for each tooth — naming the tooth and writing “for chewing” is half an answer. In (c), pepsin and hydrochloric acid earn nothing: the question says physical.

Question 4

[8 marks]

(a) Define chemical digestion and state its role. [2] (b) For amylase, protease and lipase, state the substrate and the products. [6]

Model Answer(a) The breakdown of large insoluble molecules into small soluble molecules [1]; only small soluble molecules can be absorbed into the blood [1].
(b) Amylase: starch [1] → maltose [1]. Protease: protein [1] → amino acids [1]. Lipase: fats and oils [1] → fatty acids and glycerol [1].

Examiner’s NotesAmylase gives maltose, not glucose — that last step is maltase’s, on the epithelial membranes of the small intestine. And lipase has two products; half of the pair earns nothing, because the mark scheme reads them as one point.

Question 5

[8 marks]

The table shows the pH along the alimentary canal: mouth 6.8, stomach 2.0, duodenum 7.8, ileum 7.6. (a) Name the protease acting in the stomach and the one acting in the small intestine, giving the organ that secretes each. [4] (b) Explain how the duodenum comes to be alkaline and why that matters. [4]

Model Answer(a) Pepsin [1], secreted by the stomach wall [1]; trypsin [1], secreted by the pancreas [1].
(b) Bile is an alkaline mixture released into the duodenum [1]; it neutralises the acidic mixture of food and gastric juices arriving from the stomach [1]; this provides a suitable pH for enzyme action [1]; the pancreatic enzymes have optima around pH 8 and would otherwise be denatured by the arriving acid [1].

Examiner’s NotesThis is the Supplement statement about bile that candidates forget because they have learned only the emulsification half. Bile does two things, and a duodenum question can ask for either.

Question 6

[8 marks]

(a) Describe the structure of a villus. [4] (b) Explain the significance of villi and microvilli. [2] (c) State the roles of the capillaries and of the lacteal. [2]

Model Answer(a) A finger-like projection of the lining of the small intestine, about 1 mm long and present in very large numbers [1]; its wall is a single layer of epithelial cells bearing microvilli [1]; it contains a network of blood capillaries [1]; and a lacteal in the centre [1].
(b) Villi and microvilli, with the folding of the lining, greatly increase the internal surface area of the small intestine [1], so many more molecules can be absorbed per second [1].
(c) Capillaries carry away glucose and amino acids [1]; the lacteal carries away fatty acids and glycerol into the lymphatic system [1].

Examiner’s NotesPart (a) says describe the structure, so “it has a large surface area” belongs in (b), not (a). Matching the answer to the command word matters as much as knowing the content.

Question 7

[8 marks]

A student sets up visking tubing containing starch and amylase, tied at both ends, rinsed on the outside and suspended in water at 37 °C. (a) State what the tubing and the surrounding water represent. [2] (b) Describe the results of testing the surrounding water with iodine solution and with Benedict’s solution after one hour, and what they show. [4] (c) Give two ways the model is unrealistic, with the consequence of each. [2]

Model Answer(a) The tubing represents the partially permeable wall of the small intestine [1]; the water represents the blood in the capillaries of the villi [1].
(b) Iodine solution stays orange-brown, showing starch has not passed through [1]; Benedict’s solution gives an orange or brick-red precipitate, showing reducing sugar has passed through [1]; so the starch has been digested into a small soluble molecule [1] which can then cross a partially permeable membrane, while the large insoluble molecule cannot [1].
(c) The tubing is smooth, with no folds, villi or microvilli, so the surface area is minute and absorption is far slower [1]; the water does not flow, unlike blood, so the concentration gradient falls instead of being maintained [1].

Examiner’s NotesThe negative test carries as much information as the positive one: starch appearing outside would mean the tubing was leaking and the experiment showed nothing. In (c), “it is not alive” is true of every model and scores nothing without a named consequence.

Question 8

[9 marks]

A student investigates the effect of temperature on lipase using milk and an alkaline indicator. Results: 10 °C, 640 s; 20 °C, 300 s; 37 °C, 110 s; 50 °C, 260 s; 70 °C, no change in 900 s. (a) Explain why the indicator changes colour and which product is responsible. [3] (b) Describe the control and the expected result. [2] (c) Explain the results at 70 °C and at 10 °C, making the difference between them clear. [4]

Model Answer(a) Lipase breaks fats and oils into fatty acids and glycerol [1]; the fatty acids are acidic so the pH falls [1]; the alkaline indicator changes colour as the mixture becomes acidic [1].
(b) An identical tube with boiled and cooled lipase, everything else the same [1]; no colour change, showing the change in the other tubes required active enzyme [1].
(c) At 70 °C the lipase is denatured: the shape of the active site has changed and is no longer complementary to the fat, so no complexes form [1] and the change is permanent [1]. At 10 °C the enzyme is undamaged but has little kinetic energy [1], so there are few effective collisions and the reaction is merely slow — warming the tube would restore it [1].

Examiner’s NotesGlycerol is neutral, so only the fatty acids can move the pH — check every measurement against the molecule it depends on. And never use the word denatured for the cold tube: zero activity at 10 °C and at 70 °C look identical on paper and have completely different causes.

Question 9

[9 marks]

A magazine claims: “Take a bile supplement with fatty meals — bile is the enzyme that digests fat.” (a) Evaluate this claim and describe accurately what bile does. [3] (b) A person has the gall bladder removed and is told they may still eat some fat but should avoid large fatty meals. Explain both halves of that advice. [3] (c) A drug prevents bile reaching the duodenum. Name the food that would be digested least effectively and explain why. [3]

Model Answer(a) The claim is wrong: bile is not an enzyme and contains none [1]; it emulsifies fats and oils, breaking large droplets into small ones and increasing the surface area [1]; the fat is actually digested by lipase, and bile also neutralises the acid arriving from the stomach [1].
(b) Bile is produced by the liver, not the gall bladder, so bile still reaches the duodenum and fat can still be digested [1]; but the gall bladder stores and concentrates bile and releases a large quantity when a meal arrives [1]; without that surge a large fat load is emulsified more slowly, so less surface area is available to lipase and some fat passes on undigested [1].
(c) Butter, or any largely fatty food [1]; without bile the fat stays as a few large droplets with a small surface area [1]; lipase can then reach only a small proportion of it, so digestion is very slow [1].

Examiner’s NotesPart (b) is really testing whether you know which organ makes bile and which merely stores it. Answers claiming that no bile at all reaches the duodenum lose both marks in one sentence.

Question 10

[9 marks]

A person eats a cheese sandwich containing starch, protein and fat. (a) Name the five processes of the digestive system in order, and define the two most often confused. [3] (b) Describe the complete chemical digestion of the protein, naming both enzymes, both organs and the conditions in each. [4] (c) Some of the sandwich leaves in the faeces. Name the process and explain why it differs from the removal of urea in urine. [2]

Model Answer(a) Ingestion, digestion, absorption, assimilation, egestion [1]; absorption is the movement of nutrients from the intestines into the blood [1]; assimilation is the uptake and use of nutrients by cells [1].
(b) Pepsin, secreted by the stomach wall, digests protein in the acidic conditions of the stomach at about pH 2 [1]; the hydrochloric acid provides that pH and kills harmful microorganisms [1]; trypsin, secreted by the pancreas, continues digestion in the alkaline conditions of the small intestine [1]; the products are amino acids, which are small and soluble and can therefore be absorbed [1].
(c) Egestion — the removal of undigested food as faeces [1]; urea is a waste product of metabolism made inside the body’s cells so its removal is excretion, whereas the undigested food was never absorbed and never took part in metabolism [1].

Examiner’s NotesPart (b) needs both proteases, each tied to its own organ and pH. Giving trypsin the stomach places an enzyme with an optimum near pH 8 into a solution of pH 2, where it would be denatured at once — and examiners notice when two halves of an answer contradict each other.