Human nutrition

• One meal, five verbs, in order: ingest → digest → absorb → assimilate, with egest for whatever never crossed the gut wall.
• Excretion is what your cells throw out. Egestion is what your gut never let in.
• A balanced diet supplies all required nutrients, plus fibre and water, in suitable proportions — suitable for that person.
• For every nutrient, learn two sources, one role. That is exactly the shape statement 7.1.2 asks for.
• Mouth → oesophagus → stomach → duodenum → ileum → colon → rectum → anus. Learn it as one unbroken sentence; the four associated organs hang off it.
• Physical changes how many pieces. Chemical changes what the molecules are.
• Enzyme names end in -ase, and most are named after the substrate, not the product: amylase acts on amylose (starch), protease on protein, lipase on lipid — fats and oils.
• Nutrients are absorbed in the small intestine. Most water is absorbed there too; the colon takes up some of the rest.

• I can describe what is meant by a balanced diet (7.1.1) — and I can explain why requirements differ with age, biological sex, activity level, pregnancy, state of health and body size, which is supporting context rather than a numbered statement.
• I can state the principal dietary sources and describe the importance of carbohydrates, fats and oils, proteins, vitamins C and D, calcium, iron, fibre (roughage) and water. (7.1.2)
• I can state the causes of scurvy and of rickets. (7.1.3)
• I can identify, on a diagram or image, the mouth, oesophagus, stomach, duodenum, ileum, colon, rectum and anus, together with the salivary glands, pancreas, liver and gall bladder. (7.2.1)
• I can describe the functions of those organs in relation to ingestion, digestion, absorption, assimilation and egestion, using the syllabus definition of each. (7.2.2)
• I can describe physical digestion as the breakdown of food into smaller pieces without chemical change, and state that it increases the surface area of food for enzyme action. (7.3.1, 7.3.2)
• I can identify incisors, canines, premolars and molars, describe their functions in physical digestion, and label enamel, dentine, pulp, nerves, blood vessels and cement on a tooth embedded in bone and gums. (7.3.3, 7.3.4, 7.3.5)
• I can describe the function of the stomach in physical digestion. (7.3.6)
• I can describe chemical digestion as the breakdown of large insoluble molecules into small soluble molecules, and state why that matters for absorption. (7.4.1, 7.4.2)
• I can describe the functions of amylase, proteases and lipase, and state where each is secreted and where each acts. (7.4.3, 7.4.4)
• I can describe the two functions of hydrochloric acid in gastric juice. (7.4.5)
• I can state that the small intestine is where nutrients are absorbed, and that most water is absorbed there with some also absorbed from the colon. (7.5.1, 7.5.2)
• I can outline the role of bile in emulsifying fats and oils to increase the surface area for chemical digestion. (7.3.7)
• I can describe the digestion of starch as amylase breaking starch down to maltose, and maltase breaking maltose down to glucose on the membranes of the epithelium lining the small intestine. (7.4.6)
• I can describe pepsin breaking down protein in the acidic conditions of the stomach and trypsin breaking down protein in the alkaline conditions of the small intestine. (7.4.7)
• I can explain that bile is an alkaline mixture that neutralises the acidic mixture of food and gastric juices entering the duodenum, to provide a suitable pH for enzyme action. (7.4.8)
• I can explain the significance of villi and microvilli in increasing the internal surface area of the small intestine. (7.5.3)
• I can describe the structure of a villus. (7.5.4)
• I can describe the roles of capillaries and lacteals in villi. (7.5.5)

Why this chapter matters. Topic 4 told you what carbohydrates, proteins and fats are; Topic 5 told you how enzymes work; Topic 3 told you how substances cross membranes. Topic 7 is where those three ideas meet a real organ system. Every enzyme you meet here is an ordinary enzyme obeying the ordinary rules — a specific substrate, an optimum pH, denaturation outside it. Nothing new is being asked of you biochemically. What is new is the anatomy, the sequence, and the discipline of saying exactly where a substance is secreted and exactly where it acts.

• “A balanced diet is the same for everyone.” Why it is wrong Requirements change with age, biological sex, activity level, pregnancy, state of health and body size. A balanced diet for a six-year-old is not a balanced diet for an adult doing heavy manual work. Write instead "A balanced diet supplies all the required nutrients, plus fibre and water, in suitable proportions for that person."
• “Vitamins give you energy.” Why it is wrong Vitamins are required in tiny amounts and are not respiratory substrates. No energy is released from them in respiration. Write instead "Carbohydrate is the main respiratory substrate; vitamin C is needed for healthy connective tissue and wound healing."
• “Fibre is digested into glucose.” Why it is wrong Humans produce no enzyme that digests cellulose, so fibre passes through the gut undigested and is never absorbed. If it were digested it would not be fibre. Write instead "Fibre is not digested. It adds bulk, which helps the muscles of the gut move material along and prevents constipation."
• “Digestion and absorption are the same thing.” Why it is wrong Digestion is a chemical and physical breakdown that happens in the cavity of the gut. Absorption is a movement across the gut wall. Different events, different places. Write instead "Digestion breaks large molecules into small soluble ones; absorption then moves those molecules through the intestinal wall into the blood."
• “Physical digestion changes the food molecules.” Why it is wrong Chewing a piece of bread leaves the starch molecules in it entirely unchanged. Only the size of the pieces has changed. Write instead "Physical digestion breaks food into smaller pieces without changing the food molecules, increasing the surface area for enzymes."
• “Bile is an enzyme.” Supplement 7.3.7 Why it is wrong Bile contains no enzyme, catalyses no reaction and breaks no chemical bond. Write instead "Bile is an alkaline liquid made by the liver. It contains no enzymes."
• “Bile chemically digests fat.” Supplement 7.3.7 Why it is wrong Bile changes the size of the droplets, not the molecules. That is physical digestion by definition. Write instead "Bile emulsifies lipid, increasing its surface area so that lipase digests it faster. Lipase does the chemical digestion."
• “Amylase turns starch into glucose.” Why it is wrong Glucose is two enzymes away, not one. The destination is right but a whole step has been skipped, and the answer required differs by route. Write instead · Core “Amylase breaks down starch to simple reducing sugars.” Write instead · Supplement “Amylase breaks down starch to maltose; maltase then breaks maltose down to glucose, on the membranes of the epithelium lining the small intestine.”
• “Proteases digest protein into fatty acids.” Why it is wrong Fatty acids come from lipid, not protein. Protein is built from amino acids, so amino acids are what it breaks down into. Write instead “Proteases break down protein to amino acids. Lipase breaks down fats and oils to fatty acids and glycerol.”
• “Hydrochloric acid digests the protein in the stomach.” Why it is wrong Acid is not a catalyst and is not an enzyme. Statement 7.4.5 limits its functions to two, and digesting protein is neither of them. Write instead “Hydrochloric acid kills harmful microorganisms in food and provides an acidic pH for optimum enzyme activity. The protease does the digesting.”
• “Enzymes are secreted where they act.” Why it is wrong Pancreatic amylase is secreted by the pancreas and acts in the small intestine. Statement 7.4.4 asks for both precisely because they differ. Write instead “Amylase is secreted by the salivary glands and by the pancreas, and acts in the mouth and in the small intestine.”
• “Food moves down the gut because of gravity.” Why it is wrong You can swallow lying flat. Movement is produced by muscle, not by falling. Write instead “Waves of contraction and relaxation of the muscles in the wall of the canal push the food along.” The mechanism is supporting context in this syllabus, but “gravity” is still wrong.
• “Absorption and assimilation are two words for the same thing.” Why it is wrong Absorption is entry into the blood across the gut wall. Assimilation is what a cell then does with the molecule. A glucose molecule in the plasma has been absorbed and not yet assimilated. Write instead "Absorption is movement into the blood; assimilation is uptake and use by cells."
• “All nutrients enter the blood in the same way.” Why it is wrong Water enters by osmosis; glucose and ions enter by diffusion when a gradient allows and by active transport when it does not. On the Extended route there is a further difference: the products of fat digestion enter the lacteal, not a blood capillary. Write instead Name the process that fits the substance in the question, and say why that process is the one required.
• “A villus is a cell.” Supplement 7.5.4 Why it is wrong A villus is a projection of the intestinal wall made of many cells, containing a capillary network and a lacteal. A single cell contains neither. Write instead "A villus is a finger-like projection of the wall of the small intestine, covered by an epithelium one cell thick."
• “All the water is absorbed in the colon.” Why it is wrong Most water is absorbed in the small intestine, where the villus surface is enormous. The colon absorbs the remainder, which is why the visible change to firm faeces happens there. Write instead "Most water is absorbed in the small intestine; further water is absorbed in the colon."
• “Egestion is a kind of excretion.” Why it is wrong Faeces are mostly undigested material that was never absorbed and never entered a cell. Excretion removes substances the body itself made, or absorbed in excess. Write instead "Egestion is the removal of undigested food as faeces through the anus; excretion is the removal of toxic materials, substances in excess and the waste products of metabolism."

• Where the tiering bites in Topic 7 — and it bites hard. Unlike most topics, the Supplement here is not a scattering of extra sentences. It is three blocks of genuinely separate content: everything bile does (7.3.7 and 7.4.8), the second step of starch and protein digestion (7.4.6 maltase, 7.4.7 pepsin and trypsin), and the entire villus (7.5.3, 7.5.4, 7.5.5). Seven statements, three ideas.

• Topic 7 is assessed through a small number of recurring question shapes. Six of them are set out below, each with the answer structure the wording of the syllabus statement calls for. One of the six rests on Supplement statements and is marked as such.
• A Core candidate sits Paper 1 and Paper 3; an Extended candidate sits Paper 2 and Paper 4. Both routes sit either Paper 5 or Paper 6. There is no paper on which an Extended candidate can leave Supplement content unlearned.
• A digestive system outline with four or five leader lines. Marks are for the exact organ, spelled correctly. Small intestine is not enough when the line points at the duodenum. Core, both routes.
• Substrate, product, where secreted, where it acts — one distinct fact per cell. The Core table has three rows — amylase, protease, lipase. Extended papers can add a maltase row and ask for pepsin and trypsin by name.
• One organ, one to three marks. Give one distinct function per mark, and do not repeat the same idea in different words. Core, both routes.
• In this topic that structure is the villus, which is Supplement 7.5.3–7.5.5. Use feature → location → effect → advantage. A feature named without its consequence answers only half the question. Papers 2 and 4 only.