Hi Tara. Before you start, here is the shape of Topic 7 and where the marks actually are. Sub-topic 7.1 is a list — seven nutrients, their sources, their jobs, and two deficiency diseases — and it is the easiest set of marks in the topic provided you learn a job for each nutrient rather than a disease. Sub-topic 7.2 is where most marks are quietly lost: the five processes (ingestion, digestion, absorption, assimilation, egestion) are used interchangeably by almost everyone and are worth learning as a strict sequence. Sub-topic 7.3 covers teeth and churning, and contains the Supplement point about bile emulsifying fats — write “bile digests fat” and you lose the mark every single time, however good the rest of the answer is. Sub-topic 7.4 is three enzymes, and for each one Cambridge wants four things: substrate, products, where it is secreted and where it acts — and for pancreatic enzymes those last two are different places. Sub-topic 7.5 is the villus, which is the same “large surface area, short diffusion distance, maintained concentration gradient” argument you already know from Topic 3, dressed in new vocabulary. Work through 7.1 to 7.5 in order, then use 7.6 as a checklist the night before a paper.
What “Balanced” Actually Means
Ask most people to define a balanced diet and they will say something about eating a bit of everything. That is not far off, but it is not the definition Cambridge pays for. Here it is:
A balanced diet contains all the nutrients required — carbohydrates, fats and oils, proteins, vitamins, mineral ions, fibre and water — in the correct amounts and proportions for that person.
Two halves, two marks. Everything is present, and it is present in the right quantities.
Notice the last three words: for that person. Balanced does not mean equal. A diet balanced for a sleeping infant would starve a footballer, and a diet balanced for a footballer would make the infant ill. The requirement changes with age, sex, activity level, body size, and whether the person is pregnant or breastfeeding. If a question asks you to define a balanced diet and gives it two marks, one of them is very often for saying that the requirement varies.
A favourite Paper 2 distractor is “a diet containing equal masses of each nutrient”. It sounds like what “balanced” ought to mean and it is completely wrong — you need roughly 250 g of carbohydrate a day and about 15 mg of iron, so you need about seventeen thousand times more carbohydrate than iron. The scales are being balanced against a person’s needs, not against each other.
The Seven Nutrients: Source, Job, Consequence
Learn these as a table with three columns, because that is how they are examined. The middle column — the job — is the one worth most, and the one most often replaced by a disease name. “Vitamin C prevents scurvy” tells the examiner what happens when it is missing; it does not say what the vitamin actually does, and questions that ask for the importance of a nutrient want the doing.
| Nutrient | Principal dietary sources | Importance in the body |
|---|---|---|
| Carbohydrates | Bread, rice, pasta, potatoes, cereals, sugar | The main and most readily available source of energy; glucose is respired directly by every cell |
| Fats and oils | Butter, cooking oils, cheese, nuts, oily fish | A concentrated energy store; thermal insulation under the skin; protection of organs; used in making cell membranes |
| Proteins | Meat, fish, eggs, milk, beans, pulses, nuts | Supply the amino acids used for growth and repair of tissues and for making enzymes and antibodies |
| Vitamin C | Citrus fruit, blackcurrants, peppers, fresh green vegetables | Needed to form the connective tissue that holds cells together — healthy skin, gums and blood vessel walls, and the healing of wounds |
| Vitamin D | Oily fish, egg yolk, liver, fortified milk — and made in the skin in sunlight | Needed for the absorption of calcium from the small intestine, and therefore for hardening bones and teeth |
| Calcium (mineral ion) | Milk, cheese and other dairy foods, green leafy vegetables, small fish with bones | Hardens bones and teeth; also needed for blood clotting and muscle contraction |
| Iron (mineral ion) | Red meat, liver, dark green leafy vegetables, pulses | Needed to make haemoglobin, the red pigment that carries oxygen in red blood cells |
| Fibre (roughage) | Wholegrain cereals, fruit, vegetables — it is the cellulose of plant cell walls | Provides bulk for the muscles of the gut wall to grip, keeping the contents moving and preventing constipation. Not digested and not absorbed |
| Water | Drinks, and most solid foods | The solvent in which the reactions of metabolism happen and in which substances are transported (blood plasma, digestive juices); also needed for temperature control by sweating |
“A source of vitamin C is vitamin C tablets” earns nothing, and neither does “a source of protein is protein.” Name a food. One food per nutrient is enough; two is safer if the mark is worth it.
The Two Nutrients That Do No Chemistry
Fibre and water are the odd pair, and they are examined precisely because they are odd. Everything else in the table is digested, absorbed and used inside cells. These two are not.
Fibre is never digested. Humans make no enzyme that can break cellulose down, so it passes the entire length of the alimentary canal unchanged and leaves in the faeces. That sounds like a reason to leave it out of the diet, and it is exactly the opposite. Fibre gives the muscular wall of the intestine something to grip and push against, so the contents move along at a healthy speed. Take the fibre away and the material moves slowly, the colon has longer to absorb water from it, and the result is hard, dry faeces — constipation.
If a molecule cannot be digested, it can never be made small and soluble; if it is never small and soluble, it can never be absorbed; and if it is never absorbed, it can never be respired. So any answer that has fibre “releasing energy slowly” contradicts the first fact you were given about it. Fibre’s value is mechanical.
Water is never digested either, but for the opposite reason: it is already a tiny molecule, so there is nothing to break. It is absorbed directly. About 60 % of your body is water and essentially every chemical reaction in you happens dissolved in it — which is why “water is the solvent for metabolism and transport” is a better answer than “water stops you being thirsty”.
Scurvy and Rickets — Two Diseases, Two Very Different Mechanisms
These are the only two deficiency diseases Cambridge names in this sub-topic, and they get swapped constantly. The way to stop swapping them is to learn the mechanism, not the pairing.
Vitamin C is needed to make the connective tissue that holds cells and tissues together. Without it, the tissue that is being repaired most often fails first: gums bleed and become swollen, teeth loosen, old wounds reopen, new wounds heal slowly, and the person bruises easily. The classic sufferer is a sailor on a long voyage with no fresh fruit or vegetables.
Vitamin D is needed for the absorption of calcium from the small intestine. Without it, calcium in the diet is not absorbed, so it cannot be laid down in bone. In a growing child the bones stay soft and bend under the body’s weight — bowed legs are the classic sign. Vitamin D is made in the skin in sunlight, so rickets appears in children kept indoors or heavily covered, even where the diet is rich in calcium.
That last sentence is the whole exam question. A child eating plenty of calcium can still get rickets, and explaining why requires the word absorption. Vitamin D does not become part of a bone. It makes the calcium available. Calcium that is not absorbed stays in the alimentary canal and leaves in the faeces — egested, never having been inside the body at all.
Cambridge does not name it in this sub-topic, but the third deficiency you will meet constantly in data questions is anaemia from a shortage of iron: too little haemoglobin, so less oxygen is carried, so the person is tired, pale and breathless on exertion. Learn it alongside the other two — papers use it in stems all the time.
The percentage is taken of the recommended value, so the recommendation goes on the bottom: 6.2 ÷ 14.8 × 100 = 41.9 %, which rounds to 42 %. If you had divided the other way you would have got 239 %, and a quick sanity check — she is eating less than recommended, so the answer must be under 100 — catches that instantly.
Eating a nutrient and having a nutrient are different things. The step in between is absorption, and for calcium that step needs vitamin D. So the answer is that vitamin D may also be lacking.
Without vitamin D the extra calcium is not absorbed into the blood. It stays in the lumen of the alimentary canal and leaves in the faeces — it is egested, not excreted, because it was never inside a cell. Three marks: vitamin D named; vitamin D needed for absorption of calcium; unabsorbed calcium egested rather than laid down in bone.
One Tube, and the Organs That Empty Into It
The alimentary canal is a single continuous tube running from the mouth to the anus. Food travels along it and nowhere else. That is worth pausing on, because it has a strange consequence: the inside of that tube is not really inside you. Anything that stays in the lumen has never crossed a cell membrane and never entered your body in any biological sense. It is a tunnel through you, and material only becomes part of you at the moment it is absorbed.
Cambridge splits the organs into two lists, and questions test the split directly.
Alimentary canal (food passes through): mouth → oesophagus → stomach → small intestine (duodenum, then ileum) → large intestine (colon, then rectum, then anus).
Associated organs (attached to the canal, food never passes through them): salivary glands, pancreas, liver, gall bladder.
The test for which list an organ belongs to: could a swallowed marble pass through it? If not, it is an associated organ.
What Each Organ Does
| Organ | What happens there |
|---|---|
| Mouth | Ingestion; physical digestion by the teeth; salivary amylase begins the digestion of starch; saliva lubricates the food for swallowing |
| Salivary glands | Secrete saliva, which contains amylase and mucus |
| Oesophagus | Carries food from the mouth to the stomach; muscular contractions of its wall push the food along. No digestion, no absorption |
| Stomach | Physical digestion by churning of its muscular wall; secretes gastric juice containing pepsin (a protease) and hydrochloric acid |
| Liver | Produces bile. It is also where many absorbed nutrients are processed — part of assimilation |
| Gall bladder | Stores bile and releases it into the duodenum through the bile duct when food arrives |
| Pancreas | Secretes pancreatic juice into the duodenum, containing amylase, protease (trypsin) and lipase |
| Duodenum | Where bile and pancreatic juice arrive; the region in which most chemical digestion is completed |
| Ileum | The main region of absorption; its lining is folded and covered in villi |
| Colon | Absorbs the remaining water and some mineral ions from the material passing through |
| Rectum | Stores faeces before they are removed |
| Anus | The opening through which faeces leave the body — egestion |
The Five Processes — Learn Them as a Sequence, Not a List
This is the part of Topic 7 that costs the most marks, and it costs them quietly. Five words, five definitions, and they are used interchangeably by almost everyone. The fix is to see them as one thing happening after another to one piece of food.
Not the state of the material. Not the opening it leaves by. What decides it is where the material came from. Urea is made inside your cells from surplus amino acids, so removing it is excretion. The cellulose in your faeces was swallowed, travelled the length of a tube, and left again without ever crossing a membrane, so removing it is egestion. Interestingly, bile pigment in faeces really is excreted — it was made by the liver — which shows that the same lump of material can contain both.
This is a nutrient moving from the intestine into the blood, which is the definition of absorption word for word. The justification is that the amino acids have crossed the wall of the intestine and are now in the blood.
Two things have happened: the amino acids have been taken up by a cell, and they have been used. That is assimilation. Building them into a plasma protein is the “use” half, and it is what makes this unambiguous.
Because in a normal exam answer these two words are used as if they meant the same thing. If you can say “absorbed into the blood, then assimilated by a cell” automatically, you will pick up marks in almost every Topic 7 paper.
The Definition, and the Word That Carries It
The breakdown of food into smaller pieces without any chemical change to the food molecules.
It matters because it increases the surface area of the food for the action of enzymes in chemical digestion.
Everything in this sub-topic hangs on the phrase without chemical change. Chew a piece of bread and you have smaller pieces of bread; the starch molecules inside them are exactly the same starch molecules they were before. Physical digestion changes the pieces. Chemical digestion changes the molecules. If you can keep those two sentences apart you will not lose a mark in this sub-topic.
Why does making the pieces smaller matter so much? Because enzymes work at surfaces. An enzyme molecule floating in the fluid of the gut can only act on a substrate molecule it can reach, and it can only reach the outside of a lump. Cut a cube of side 12 mm into cubes of side 3 mm and the volume has not changed at all, but the surface area has gone up by a factor of four — from 864 mm² to 3456 mm². Four times as much food is now exposed to the enzymes, so four times as many enzyme–substrate complexes can form per second.
For a cube of side a cut into cubes of side b, the surface area increases by a factor of a ÷ b, not by the number of pieces. Cutting 12 mm into 3 mm gives 64 pieces but only a fourfold increase in area. Answering “64” is the standard slip — you have counted the pieces, not the surface.
The Four Types of Tooth
Structure follows function all the way along the jaw. Each tooth shape does one mechanical job, and no shape can do another shape’s job — a point cannot make a clean straight cut, and a broad flat surface cannot grip and tear.
The Structure of a Tooth
Cambridge limits this to six named parts — enamel, dentine, pulp, nerves, blood vessels and cement — plus the fact that teeth are embedded in bone and the gums. That last point matters more than it looks: a tooth that was merely resting in soft tissue could not grind anything, because chewing generates forces of several hundred newtons.
| Part | What it is and what it does |
|---|---|
| Enamel | The hard, non-living outer layer of the crown — the hardest material in the body. Resists wear during chewing. Because it is non-living, a chip is never repaired |
| Dentine | Living tissue forming the bulk of the tooth beneath the enamel. Softer than enamel, and it transmits sensation towards the pulp |
| Pulp | The soft living core in the middle of the tooth, containing the nerves and blood vessels |
| Nerves and blood vessels | Nerves give sensation; blood vessels supply oxygen and nutrients to the living tissue of the tooth and remove waste |
| Cement | A thin layer covering the root, attaching it by fibres to the bone of the jaw so the tooth is held firmly in place |
The Stomach’s Mechanical Job
The stomach is a muscular bag. Its wall contracts rhythmically, churning the contents so the food is broken into smaller pieces and thoroughly mixed with gastric juice. That is its contribution to physical digestion, and it is all that a question about physical digestion in the stomach wants.
If the question says physical digestion in the stomach, the answer is churning by the muscular wall. Pepsin and hydrochloric acid are correct facts about the stomach and score nothing here, because they are chemical. A large fraction of students write everything they know about the stomach and are surprised to receive one mark out of two.
Supplement: Bile and Emulsification
Bile emulsifies fats and oils. It does not digest them.
Fat does not mix with water, so in the watery contents of the gut it collects into a few large droplets. A large droplet is terrible news for lipase, because lipase can only act on the surface of the droplet — and almost all of the fat is on the inside where the enzyme cannot reach it.
Bile, made by the liver and stored in the gall bladder, is released into the duodenum and breaks those large droplets into many small droplets. This is emulsification. The total volume of fat is exactly the same; the total surface area is enormously greater. Lipase can now reach far more of the fat, so digestion is much faster.
Bile contains no enzymes at all. It changes the size of the droplets, never the fat molecules. That is why emulsification is physical digestion, sitting alongside chewing and churning, and not chemical digestion.
The experiment that settles it: put cream and bile in a tube with no lipase. The droplets get smaller, and not one molecule of fatty acid is produced.
Write: “Bile emulsifies the fat, breaking large droplets into small ones and so increasing the surface area for lipase to act on.”
Never write: “Bile digests fat” or “bile breaks down fat”. Both are refused, and they are refused even when the rest of the answer is perfect.
The droplets in tube 3 are just as small as in tube 2, so emulsification has certainly happened. And the fatty acid produced is zero. Bile alone cannot digest fat, because it contains no enzyme.
Boiling has denatured the lipase, and again nothing is produced. So the fatty acid in tube 2 was made by active lipase and not by anything else in the mixture — not the bile, not the cream, not the warmth.
Each removes a different alternative explanation. Together they show that the large result in tube 2 needs both bile and working lipase, and that neither one on its own does anything useful. That is the logical structure of every good control experiment.
Why Food Has to Be Taken Apart
The breakdown of large insoluble molecules into small soluble molecules.
Its role: to produce small soluble molecules that can be absorbed.
Those two adjectives are the whole reason the process exists. A starch molecule is far too big to cross a cell membrane, and it will not dissolve, so it cannot be transported in blood plasma even if it did get through. Glucose is small and it dissolves. Digestion is not about making food “usable” in some vague sense — it is about producing molecules that will fit through a membrane and dissolve in blood.
An answer that says food is “broken down” without naming the molecules is describing what everybody can already see. Name the substrate and name the products, every time.
The Three Enzymes
For each enzyme Cambridge wants four things, and questions ask for them in any combination: the substrate, the products, where it is secreted and where it acts. Those last two are a separate syllabus statement because for pancreatic enzymes they are different places — nothing is digested inside the pancreas.
Hydrochloric Acid: Two Jobs, Neither of Them Digestion
Gastric juice contains pepsin and hydrochloric acid. The acid has exactly two functions in the syllabus:
1. It kills harmful microorganisms taken in with the food.
2. It provides the acidic pH at which pepsin has its optimum activity — about pH 2.
Note what is not on that list: the acid does not digest anything. Writing “the acid breaks down the protein” hands the enzyme’s job to the environment, and it leaves you unable to explain why the stomach bothers to make pepsin at all. The acid creates the conditions; the enzyme does the work.
Supplement: The Detail Behind Each Enzyme
Starch is digested in two steps, in two places
Amylase breaks starch down to maltose. That is all it does. Maltose is still a disaccharide and is still not what a cell respires.
Maltase breaks maltose down to glucose, and it does so on the membranes of the epithelium lining the small intestine — not floating free in the fluid of the gut. The final step therefore happens at exactly the surface the glucose is about to be absorbed through, which is elegant and is also a favourite exam point.
A separate detail worth carrying: salivary amylase carried into the stomach is denatured by the acid. That is why starch digestion has to be restarted in the duodenum by pancreatic amylase.
Protein is digested by two proteases, in two very different environments
Pepsin breaks protein down in the acidic conditions of the stomach, optimum about pH 2.
Trypsin breaks protein down in the alkaline conditions of the small intestine, optimum about pH 8.
Put either enzyme in the other’s organ and it is denatured within minutes. This is why a graph question showing two protease curves peaking at pH 2 and pH 8 can be answered without knowing anything else: match each optimum to the region whose pH it fits.
Bile’s second job: neutralising the acid
The mixture leaving the stomach is at about pH 2. Every pancreatic enzyme — amylase, trypsin, lipase — has an optimum around pH 8. If nothing were done, all three would be denatured on arrival.
Bile is an alkaline mixture. It neutralises the acidic mixture of food and gastric juices entering the duodenum from the stomach, providing a suitable pH for enzyme action.
So bile does two separate things, one physical and one chemical: it emulsifies fats, and it changes the pH. Both are examinable and candidates who remember only one lose half the marks in any question about the duodenum.
Iodine detects starch and nothing else, so the loss of blue-black tells you starch is disappearing. Benedict’s detects reducing sugars in general — and both maltose and glucose are reducing sugars, so the test cannot tell them apart.
Maltose, not glucose. So the results are entirely consistent with a tube full of maltose and no glucose at all.
From maltase, acting on the membranes of the epithelium lining the small intestine. Three marks: Benedict’s cannot distinguish maltose from glucose; amylase produces maltose; maltase completes the job elsewhere.
Where It Happens
Two statements to learn exactly as Cambridge words them:
The small intestine is the region where nutrients are absorbed.
Most water is absorbed from the small intestine, but some is also absorbed from the colon.
The wording of the second one matters. “Water is absorbed in the colon” is a true sentence that scores nothing, because it leaves out where most of the job is done. And it is a great deal of water: you swallow about two litres a day and secrete roughly seven litres of digestive juices on top of that, nearly all of which has to be recovered before the material reaches the rectum.
Supplement: Why the Small Intestine Is Shaped the Way It Is
Absorption is diffusion and active transport across a surface, and everything that speeds up movement across a surface comes down to three things: more surface area, a shorter distance, and a steeper concentration gradient. You met exactly this argument in Topic 3 with root hair cells. The small intestine solves all three problems at once.
The structure of a villus
A villus is a finger-like projection of the lining of the small intestine, about one millimetre long. There are millions of them, standing on a lining that is already thrown into folds.
- Its wall is a single layer of epithelial cells — one cell thick, so the diffusion distance into the blood is tiny.
- The surface of each epithelial cell carries thousands of microvilli, microscopic projections that multiply the surface area again.
- Inside is a dense network of blood capillaries.
- Running up the centre is a lacteal, a vessel of the lymphatic system.
Significance of villi and microvilli: together with the folding of the lining they increase the internal surface area of the small intestine many times over compared with a smooth tube. More surface means more molecules of digested food are absorbed per second.
The roles of the capillaries and the lacteal
Blood capillaries absorb and carry away glucose and amino acids, together with water-soluble vitamins and mineral ions. Blood flowing through them constantly removes what has just been absorbed, so the concentration inside stays low and the concentration gradient is maintained. Without that flow, absorption would slow to a stop as the blood filled up.
The lacteal absorbs and carries away fatty acids and glycerol, the products of fat digestion, into the lymphatic system, which eventually empties into the blood. This is why lymph looks milky after a fatty meal — a detail worth remembering, because it makes the fact impossible to forget.
Questions frequently exclude one adaptation to check that you know the others: “Apart from the large surface area, describe two features…”. If you answer with surface area again you score nothing. The other two are always available: the wall one cell thick (short diffusion distance) and the rich blood supply (maintains the concentration gradient).
Diffusion, and When It Is Not Enough
Most of the time the concentration of glucose in the gut after a meal is higher than in the blood, so glucose moves into the blood by diffusion — down a concentration gradient, no energy required. But towards the end of a meal the gut runs low and the gradient reverses, and yet absorption continues. That can only be active transport: movement against the concentration gradient, using protein carriers in the cell membrane and energy from respiration. It is why the epithelial cells of a villus are packed with mitochondria.
Osmosis is the movement of water. Glucose, amino acids and mineral ions move by diffusion or active transport, never by osmosis. This single word appears in a large minority of answers about villi and it scores nothing every time. The clue for which process is wanted is always the direction: uphill movement cannot be diffusion.
Flattened villi means a much smaller internal surface area, and fewer epithelial cells carrying carrier proteins. The wall is still thin, but there is far less of it.
Less absorption means more glucose left in the contents, not less. This is the direction candidates most often get backwards — damaged absorption leaves nutrients in the gut, it does not make them disappear.
Nutrients that are not absorbed pass out in the faeces by egestion, so the patient loses body mass, becomes tired and may become anaemic if iron absorption is also reduced. Four marks: smaller surface area; less absorbed; more glucose remaining; weight loss or anaemia from nutrients lost in the faeces.
The Six Errors That Cost the Most Marks
Every one of these is a sentence that sounds sensible, gets written by thousands of candidates every year, and earns nothing. If you eliminate these six you will be several marks up before you have learned anything new.
| Never write | Write instead | Why |
|---|---|---|
| “Bile digests fat” | “Bile emulsifies fat, increasing the surface area for lipase” | Bile contains no enzymes. It changes droplet size, not molecules |
| “The nutrients are assimilated into the blood” | “Nutrients are absorbed into the blood, then assimilated by cells” | Absorption reaches the blood; assimilation reaches a cell. Two events, in that order |
| “The food is broken down” | “Starch is broken down to maltose by amylase” | Name the substrate, the product and the enzyme. Vague breakdown earns nothing |
| “Faeces are excreted” | “Undigested food is egested” | Excretion removes waste made by the body’s reactions. Faeces were never absorbed |
| “The enzyme was killed by the acid” | “The enzyme was denatured; the shape of the active site changed” | An enzyme is a protein molecule and was never alive |
| “Glucose enters the villus by osmosis” | “Glucose enters by diffusion, or by active transport against the gradient” | Osmosis is the movement of water only |
Reading the Command Word
State / Name — one word or one short phrase. No explanation needed, and adding one wastes time.
Describe — say what happens or what something looks like. For a villus this means listing the parts, not the advantages.
Explain — say why. Every mark needs a because. This is where the mechanism words live.
Suggest — apply what you know to something unfamiliar. There is often more than one acceptable answer, and the mark is for the reasoning.
Evaluate — say what the evidence does support, what it does not, and reach a judgement. An answer that only agrees or only disagrees is half an answer.
Calculate — show the working. Working earns a mark even when the final number is wrong.
How to Attack a Data Question
Topic 7 papers are full of tables: nutrient contents, enzyme activities, pH along the canal, mass remaining at each region. Work through them in the same order every time.
1. Read the headings and the units before looking at a single number. A percentage of each enzyme’s own maximum is not the same as a rate, and a dry mass is not the same as a mass.
2. Say which way each column points — rising, falling, peaking. Do this before choosing an answer.
3. Describe with figures, then explain. Quoting two numbers from the table is very often a mark on its own.
4. Check the direction of your prediction. Damaged absorption leaves more in the gut. A shorter time means a faster reaction.
5. Do not claim more than the data allow. If there is no reading at time zero you cannot say what the starting value was.
Three Scenarios to Test Yourself On
Bile is produced by the liver, not by the gall bladder. Removing the store does not remove the supply, so bile still trickles into the duodenum and fat is still emulsified and digested.
The gall bladder’s job is to store and concentrate bile and release a large quantity at once when a meal arrives. Without it there is no surge, so a large fat load is emulsified more slowly, the surface area available to lipase is smaller, and some fat passes on undigested.
Starch digestion genuinely does begin in the mouth, because saliva contains amylase. So something chemical really is happening while the food is being chewed.
Chewing itself is physical digestion — smaller pieces, unchanged molecules. And food spends only a short time in the mouth, so most starch is digested later by pancreatic amylase in the small intestine. A person with no teeth still digests starch; it is simply slower, because the reduced surface area slows the enzymes down.
Plenty of vitamin D, so absorption works perfectly — but there is almost no calcium in the diet to absorb. Bones cannot be hardened with material that was never eaten.
Plenty of calcium, but almost no vitamin D, so the calcium is not absorbed and passes out in the faeces. It is present in the gut and absent from the blood.
Bone needs the mineral and the means of absorbing it. A table designed like this is testing whether you can hold two variables in mind at once instead of picking the first column you recognise.
The Night-Before Checklist
The definition of a balanced diet, in two clauses. A source and a job for all seven nutrients. Scurvy and rickets, with mechanisms. The alimentary canal in order, and the four associated organs. The five processes in order, with definitions of absorption, assimilation and egestion. Physical digestion defined, and why surface area matters. The four tooth types with a function each. Enamel, dentine, pulp, cement. Emulsification — the word and the sentence. Chemical digestion defined. Amylase, protease and lipase: substrate, products, secreted where, acting where. The two functions of hydrochloric acid. Amylase → maltose, maltase → glucose on the epithelial membranes. Pepsin at pH 2, trypsin at pH 8. Bile neutralises the acid. Where nutrients and water are absorbed. The structure of a villus, and the roles of the capillaries and the lacteal.
That list is the entire sub-topic. If you can produce it out loud in four minutes, you are ready.