IGCSE Biology · Cambridge CIE 0610 · Topic 7
The seven nutrient groups, what each does, what happens when they're missing, and how protein-energy malnutrition shows up. Mark scheme rewards precise sources, functions, and deficiency-disease names.
The five food groups in approximate proportions for a balanced diet.
A diet containing all seven nutrient groups — carbohydrates, fats, proteins, vitamins, mineral salts, fibre and water — in the correct proportions for the body's needs.
examinerNaming all seven groups and "correct proportions" wins both marks.
Energy and nutrient needs differ with age, gender, and activity level, and are increased during pregnancy and breast-feeding.
e.g. A pregnant woman needs extra iron, calcium, folate and protein for fetal growth.
Composed of carbon, hydrogen and oxygen. The body's immediate energy source; also used to build up other nutrients.
sourcesBread, pasta, rice, potatoes, fruit. Deficiency: low energy, reduced muscle mass, low BMI.
Composed of carbon, hydrogen and oxygen. Provide extra energy (more per gram than carbs) and insulate the body against excessive heat loss.
deficiencyDry skin, dandruff, brittle nails, dry eyes, poor wound healing, cracked skin on heels.
Composed of carbon, hydrogen, oxygen and nitrogen (with traces of sulphur). Used for growth and repair of cells, and to make enzymes, hormones and antibodies.
deficiencyOedema, weight loss, brittle hair, weakness, slow wound healing.
Promotes a healthy immune system, helps wounds heal, maintains blood vessels and connective tissue, and aids absorption of iron. Water-soluble; RDA ~60 mg.
deficiencyScurvy — weakness, anaemia, bruising, bleeding gums, loose teeth. Citrus fruits, peppers, leafy greens.
Promotes the body's absorption of calcium, essential for healthy bones and teeth. Fat-soluble; DRI ~5 µg. Made by the body when skin is exposed to sun.
deficiencyRickets — weakening and softening of the bones due to extreme calcium loss.
Essential for the formation and maintenance of bones and teeth, blood clotting, normal heart beat and hormone secretion.
deficiencyOsteoporosis. Sources: dairy, leafy greens, salmon, sardines.
Needed for the formation of haemoglobin and certain enzymes. Vitamin C aids its absorption.
deficiencyAnaemia — tiredness, pallor, breathlessness. Lean red meat, eggs, cereals.
Mainly cellulose from plant cell walls. Not digested, but gives bulk and stimulates peristalsis, keeping the alimentary canal working properly.
sourcesWholegrains, bran, leafy vegetables, fruit skins.
Solvent for chemical reactions; transports substances in blood and lymph; supports digestion, waste removal, kidney function and temperature regulation.
rememberWater is one of the seven nutrient groups — don't leave it off the list.
Caused by diets deficient in protein (but adequate calories). Age 6 months to 3 years. Oedema present; subcutaneous fat preserved; enlarged fatty liver; lethargic.
treatmentAdequate amounts of protein.
Caused by deficiency of both protein and calories. Common in infants under 1 year. Oedema absent; subcutaneous fat not preserved; ribs very prominent; severe muscle wasting; alert and irritable.
treatmentAdequate protein, fats and carbohydrates.
Focus
Examiners love this contrast. The two facts that flip: oedema (present in K, absent in M) and subcutaneous fat (preserved in K, lost in M). Age is also a tell — K is 6 months–3 years; M is infants <1 year.
trapDon't say "Kwashiorkor is just protein deficiency, Marasmus is just calorie deficiency" — that loses marks. Marasmus is both.
Substances added to food to improve appearance, prolong shelf-life, enhance taste, or stabilise mixtures. Each group has a distinctive E-number range from the International Numbering System.
A worldwide system assigning a number (with or without an "E" prefix in the EU) to each approved additive. The number tells you the category.
rememberKnowing the ranges matters more than memorising specific numbers.
Improve the appearance of food. Includes natural and synthetic dyes.
e.g. Tartrazine (102) — yellow/orange in sweets and drinks; linked to hyperactivity in some children. Caramel — brown, made by heating sugar.
Prolong shelf-life by killing bacteria or stopping their growth.
e.g. Vinegar, salt, sugar (traditional). Sulfur dioxide in fruit juice and dried fruit. Sodium nitrite in cured meats — increases cancer risk.
Prevent oxidation — especially browning of cut fruit and rancidity of fats.
e.g. Ascorbic acid (300) — vitamin C as a flour improver and to stop browning.
Enhance or alter the taste of food.
e.g. MSG (621) in soups, stock cubes, Chinese food — enhances savoury flavours. Saccharin — artificial sweetener. Vanillin — vanilla flavour.
Allow fats and oils to mix with water in a stable mixture.
e.g. Lecithin (471, 475) in chocolate milk and ice cream — stops the fat separating out.
How yoghurt and mycoprotein are produced industrially, and the two main fermentation strategies. Watch the precise role of each microorganism and what's added to the fermenter.
Global food supply is limited by: political unrest, high food prices, weather and climate, population pressure, unequal distribution, and lack of modern technology in some regions.
remember"Not enough food" is the wrong answer — world production is actually sufficient; distribution is the issue.
Milk (cattle, sheep, goat or buffalo) is pasteurised — heated to ~70 °C to kill other microorganisms. It's then cooled, the starter bacterium is added, and the mixture is kept warm.
why pasteuriseTo prevent other microbes spoiling the yoghurt and to give an unpleasant taste-free starting point.
The bacterium added to warm milk. It converts lactose (milk sugar) into lactic acid plus energy. The acid gives the sour taste and causes the milk proteins to curdle, producing a solid curd and a liquid whey.
equationlactose → lactic acid + energy
A meat substitute (sold as Quorn) providing protein for vegetarians. Made from a fungus, not an animal — suitable for low-fat diets.
key idea"Myco" = fungus. The protein comes from the fungal hyphae.
The fungus Fusarium is grown in a fermenter at 30 °C. It is supplied with glucose as an energy source and ammonia (NH3) as a nitrogen source. It grows long thread-like hyphae made of protein.
finishingThe protein is harvested, heat-treated, dried, then flavoured and shaped into burgers or sausages.
Industrial fermenters control: temperature (water-cooled jacket), pH (probes), oxygen (sterile air supply), nutrient input, and waste-gas removal. A stirrer keeps contents mixed.
e.g. The Quorn fermenter takes in NH3, glucose and air, removes waste gases, and harvests mycoprotein at the bottom.
Fermentation in a closed fermenter — nothing added or removed during the run. The microorganisms and medium are sealed in, the product is separated at the end, and the vessel is cleaned for the next batch.
use forPenicillin and other secondary metabolites — products made under stress conditions that batch culture can create.
Fermentation in an open fermenter — nutrients are added and product removed at a steady rate. Microorganisms are held at the exponential phase of growth indefinitely.
advantageHigher productivity in smaller vessels. disadvantageFoaming and cell clumping can block inlets; can't create stress conditions for secondary metabolites.
Focus
Pick by product. If the product is a primary metabolite (made during growth — mycoprotein, ethanol, yoghurt): continuous. If a secondary metabolite (made under stress — penicillin, other antibiotics): batch.
trap"Batch is safer because contamination only affects one batch" — true, but examiners want the primary/secondary distinction.
The continuous, hollow, muscular tube that food travels through — mouth to anus — and the five processes that happen along the way. Definitions here are verbatim mark-scheme: learn them precisely.
The alimentary canal (teal) is open at both ends. Accessory organs (orange dashed) secrete substances into it but food doesn't pass through them.
The taking of substances — e.g. food and drink — into the body through the mouth.
mark scheme"Taking substances into the body through the mouth" — use this exact phrasing.
The breakdown of food into smaller pieces without chemical change to the food molecules. Done by teeth (chewing), stomach (churning), and bile (emulsification).
whyIncreases the surface area for enzymes to work on.
The breakdown of large, insoluble molecules into small, soluble molecules — done by enzymes.
e.g. Amylase: starch → maltose. Pepsin: protein → polypeptide. Lipase: fat → fatty acids + glycerol.
The movement of small food molecules and ions through the wall of the intestine into the blood.
whereMostly the small intestine (via villi). Water also in the large intestine.
Focus
These two are the most-confused pair in this topic. Absorption = molecules pass from the gut into the blood. Assimilation = the digested molecules then move from the blood into body cells, where they become part of the cells.
trap"Food gets absorbed into the cells" loses marks — absorption is into the blood, not into cells.
The passing out of food that has not been digested or absorbed, as faeces, through the anus.
not the same asExcretion — that's removal of metabolic waste (urea, CO2). Egestion is undigested food, not waste from the body's own reactions.
Canal organs (food passes through): mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus. Accessory organs (secrete into the canal): tongue, teeth, salivary glands, liver, gall bladder, pancreas.
testCould food pass through it? If yes, canal. If no, accessory.
Diarrhoea: the loss of watery faeces, often caused by infection (e.g. cholera) or food poisoning. Dangerous because of dehydration and loss of salts.
treatmentOral rehydration therapy — a solution of clean water, salts and glucose to replace what's lost.
Teeth types, tooth structure, dental decay, and the comparative anatomy of herbivore vs carnivore dentition. Diastema and carnassial are the two technical terms most often dropped.
Site of ingestion, mechanical digestion by the teeth, and partial chemical digestion of carbohydrate by salivary amylase.
rememberOnly carbs start digesting here — protein and fat start further down.
A muscular sense organ with taste receptors (bitter, sour, salty, sweet). Encourages ingestion, mixes food with saliva, rolls it into a bolus, and aids swallowing.
bolusA ball-like mass of partially digested food shaped by the tongue.
Incisors — chisel-shaped, for biting. Canines — pointed, for tearing flesh (well developed in carnivores). Premolars and molars — broad and ridged, for crushing and grinding (well developed in herbivores).
human formulaPer side, per jaw: 2 incisors, 1 canine, 2 premolars, 3 molars = 32 in total.
Enamel — hardest substance in the body, harder than ivory. Dentine — living tissue, softer than enamel but harder than bone, shock-resistant. Pulp — living core with blood vessels and nerves. Cement anchors the root.
partsCrown = visible above gum. Root = hidden below gum.
20 baby teeth (temporary) — start falling out between ages 6–14. Replaced by 32 adult teeth (permanent), including the four wisdom teeth that emerge in late adolescence.
whyA child's jaw isn't big enough for 32 adult-sized teeth.
A multifactorial disease caused by plaque (a biofilm of bacteria) + carbohydrate. Bacteria respire the sugar and produce acid, which dissolves enamel and demineralises the tooth.
stages(1) chalky white spot, (2) brown lesion, (3) cavity into dentine, (4) cavity reaches pulp — pain begins.
Focus
Mark-scheme phrasing: plaque bacteria + dietary sugar → respiration producing acid → demineralisation of enamel. All four steps needed for full marks.
preventionBrush teeth (removes plaque), limit sugary food, regular dentist visits, calcium & vitamin C in diet.
Saliva contains water (softens food), mucus (lubrication for swallowing), an alkaline buffer (neutralises mouth acid), and the enzyme amylase — which catalyses the hydrolysis of starch → maltose.
rememberSalivary amylase only starts the job — pancreatic amylase finishes it in the small intestine.
Incisors only on the lower jaw; the upper jaw has a horny pad. Canines reduced or absent — leaves a diastema, a gap that lets chewed grass be moved by the tongue. Premolars and molars have alternating ridges of hard enamel and softer dentine that wear at different rates, keeping the grinding surface rough.
e.g. Cow, sheep, horse, deer.
Incisors on both jaws (no horny pad). Canines long, sharp, well developed for stabbing prey. Carnassial teeth — modified premolars and molars that slide past each other like scissors, slicing flesh and crushing bone.
e.g. Dog, cat, lion, tiger.
How food moves down the oesophagus, what the stomach actually does, and the enzymes that break large molecules into small ones. The peristalsis mechanism comes up almost every paper.
Peristalsis: rhythmical contraction of the smooth muscle behind the bolus pushes it along. Circular muscle contracts behind, relaxes in front; longitudinal muscle does the reverse.
A 22–25 cm muscular tube transporting food from the mouth to the stomach. Food doesn't fall down — it's pushed by peristalsis, which works even if you swallow upside down.
structureTwo muscle layers: circular (rings round the tube) and longitudinal (running along its length).
A rhythmical contraction of involuntary smooth muscle. Behind the bolus: circular muscles contract, longitudinal relax (tube narrows, lengthens). Ahead of the bolus: circular relax, longitudinal contract (tube widens, shortens).
rememberThe wave of contraction pushes the bolus along — it doesn't pull it.
A C- or J-shaped muscular bag. Temporary food store and site of partial protein digestion. Walls churn and squeeze (mechanical); glands secrete gastric juice (chemical).
outputThe watery, partially digested mixture leaving the stomach is called chyme.
Four key ingredients: HCl (acid), mucus, water, and the enzyme pepsin. Together they create a low-pH protein-digesting soup.
don't forgetMucus is what stops the stomach digesting itself.
Focus
(1) Kills bacteria in the food. (2) Provides the optimum low pH for pepsin to work. (3) Helps break down connective tissue in meat and denatures harmful enzymes in food.
trap"HCl digests protein" loses marks — HCl provides the conditions, pepsin does the digestion.
A protease — an enzyme that catalyses the hydrolysis of protein → polypeptide. Works at the low pH of the stomach (~pH 1.5–2).
paired withTrypsin — the protease in the small intestine, working at higher pH.
The semi-liquid mixture of partially digested food and gastric juice that leaves the stomach. It is acidic — needs neutralising before enzymes can work in the small intestine.
next stepBile and pancreatic juice (both alkaline) neutralise chyme in the duodenum.
Amylase: starch → maltose (then maltase: maltose → glucose). Protease (pepsin, trypsin): protein → amino acids. Lipase: fats → fatty acids + glycerol.
whereAmylase — mouth & small intestine. Pepsin — stomach only. Trypsin & lipase — small intestine.
Where digestion finishes, how the small intestine is adapted for absorption, what bile actually does, and the precise mark-scheme chain for cholera.
A villus: one-cell-thick epithelium, vast surface area from microvilli, capillaries for glucose & amino acids, and a central lacteal for fatty acids & glycerol.
Three regions: duodenum (first ~25 cm, where chyme is neutralised and enzymes act), jejunum, and ileum (main absorption site). Convoluted to slow food and maximise absorption.
lengthAbout 6 m long in an adult — a huge digestion-and-absorption surface.
Produced by the liver, stored in the gall bladder. Yellow-green alkaline fluid that does two things: (1) emulsifies fats — breaks fat globules into droplets, increasing surface area for lipase; (2) neutralises the acidic chyme from the stomach, giving enzymes a suitable pH.
keyBile is mechanical digestion of fat, not chemical.
Focus
Bile emulsifies fat — physically breaks large fat globules into smaller droplets. The actual chemical breakdown into fatty acids and glycerol is done by lipase, an enzyme from the pancreas.
trap"Bile breaks down fats into fatty acids and glycerol" — common error, no marks. Bile makes the surface area bigger so lipase can do that job faster.
Secretes pancreatic juice into the duodenum. Contains bicarbonate (neutralises chyme), water, and three enzymes: amylase (starch → maltose), trypsin (protein → amino acids), lipase (fat → fatty acids + glycerol).
alsoEndocrine role: produces insulin and glucagon (different topic).
Maltose —maltase→ glucose. Polypeptide —trypsin→ amino acids. Fat droplets —lipase→ fatty acids + glycerol. Maltase works on the membranes of the epithelial cells — right at the absorption surface.
why therePutting maltase on the epithelium means glucose is released exactly where it can be absorbed.
(1) Finger-like projections increase surface area. (2) Covered in microvilli — massive further surface area increase. (3) One-cell-thick epithelium — short diffusion distance. (4) Rich blood supply + central lacteal maintain a steep concentration gradient.
what goes whereGlucose & amino acids → capillaries (blood). Fatty acids & glycerol → lacteal (lymph).
Most water is absorbed in the small intestine, alongside the digested nutrients. The remaining water is reclaimed in the large intestine (colon).
trap"Water is absorbed in the large intestine" is incomplete — most happens earlier.
Comprises the caecum, appendix, colon, rectum, and anus. Job: absorb the remaining water from the indigestible residue, leaving solid faeces for temporary storage in the rectum until egestion.
compositionFaeces are mostly water, fibre (cellulose), bacteria, and dead cells from the gut lining.
A water-borne disease caused by a bacterium (Vibrio cholerae) ingested in contaminated food or water. The bacterium produces a toxin.
treatmentAntibiotics + oral rehydration therapy (water, salts, glucose).
Focus
Examiners want all four links: (1) toxin causes chloride ions (Cl−) to be secreted into the small intestine → (2) water moves out of cells by osmosis into the gut → (3) diarrhoea → (4) dehydration and loss of salts from the blood.
trap"Bacteria cause diarrhoea" gets zero. You need Cl− → osmosis → water loss.
Feeding types and how gut anatomy is adapted to diet. The ruminant four-chamber stomach and the rabbit's caecum are favourite exam targets — learn names and order.
Food flow: rumen → reticulum → omasum → abomasum. The first two chambers ferment cellulose with bacteria; the last is the true acid stomach.
Energy is lost at each trophic level. Roughly: producers 1000 → herbivores 100 → carnivores 10 → top carnivores 1. So large carnivores need huge ranges.
why lostRespiration, heat, movement — only ~10% of energy passes up the chain.
Grazers (zebra, cow) feed on herbage on the ground. Foragers (giraffe, panda) feed on leaves and shoots higher up. Both eat large quantities frequently because cellulose is hard to digest and plants are nutritionally dilute.
teethBroad ridged molars + diastema + horny pad (no upper incisors).
Meals are long and irregular — meat is concentrated nutrition. Large amounts of energy used for stalking, catching, killing. May not eat for days between kills.
teethSharp canines for stabbing, carnassial teeth for slicing.
Eats both plants and animals. Often highly intelligent and adaptable, with a varied diet reflecting flexible behaviour.
e.g. Humans, pigs, dogs, chimpanzees, bears.
Rumen (~270 L) — largest; rich in bacteria that digest cellulose and produce vitamins B, E, K + methane. Reticulum — holds water; with rumen forms the "fermentation vat". Omasum — food kneaded; water absorbed. Abomasum — the "true" acid stomach.
e.g. Cow, sheep, goat, deer. Camels are unusual — well-developed reticulum but no omasum.
Focus
Memorise this order: Rumen → Reticulum → Omasum → Abomasum. Note the rumen and reticulum can pass food back to the mouth for re-chewing (chewing the cud / regurgitation).
trick"Rabbits Rarely Order Apples" — or any mnemonic with R, R, O, A.
Rabbits and other rodents have a large caecum packed with cellulose-digesting bacteria. It's less efficient than a rumen, so they eat their own soft night-time faeces and produce a second, harder pellet — getting two passes through the gut.
nameThis behaviour is called coprophagy (or caecotrophy).
Birds have a beak (lighter than teeth, helps flight). Crop — a bag-like structure for temporary food storage before the stomach. Gizzard — a hollow, muscular grinder; seed-eaters swallow pebbles that sit in the gizzard to crush food.
special casePigeons and doves secrete crop milk — a regurgitated nutritive substance to feed their young.
Question types mix: reveal, multiple choice, fill-in-the-blank, and order-the-steps. The round adapts — questions you mark "review again" come back sooner, and Focus topics are weighted higher.