Human Nutrition

• One meal, five verbs, in order: ingest → digest → absorb → assimilate, with egest for whatever never made it through the 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 of the mark scheme.
• 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), maltase on maltose, protease on protein, lipase on lipid.
• Absorption happens mainly in the ileum, because that is where the villi are.

• I can give the principal dietary sources and the biological importance of carbohydrates, lipids, proteins, vitamin C, vitamin D, calcium, iron, dietary fibre and water.
• I can link vitamin C deficiency to scurvy, vitamin D or calcium deficiency to rickets, and iron deficiency to anaemia, and give a symptom of each.
• I can define a balanced diet and explain why requirements differ with age, biological sex, activity level, pregnancy, health and body size.
• I can define ingestion, digestion, absorption, assimilation and egestion, and distinguish egestion from excretion.
• I can identify the mouth, oesophagus, stomach, duodenum, ileum, colon, rectum and anus on a diagram, together with the salivary glands, pancreas, liver and gall bladder.
• I can state the function of each of those regions and associated organs.
• I can distinguish physical from chemical digestion and explain why physical digestion speeds chemical digestion up.
• I can name the four types of tooth, describe their shapes and functions, and label a tooth in cross-section.
• I can explain peristalsis in terms of circular and longitudinal muscle.
• I can state the substrate, the product, the site of production and the site of action of amylase, maltase, protease and lipase.
• I can explain the two functions of hydrochloric acid in the stomach and why different proteases have different optimum pH values.
• I can explain what bile does, where it is made and stored, and why emulsification is physical rather than chemical digestion.
• I can distinguish absorption from assimilation and describe how villus structure suits absorption, feature by feature.
• I can separate the blood-capillary route from the lacteal route, and state that most water is absorbed in the small intestine.
• I can state what the hepatic portal vein carries and where it carries it.

Why this chapter matters. Chapter 4 told you what carbohydrates, proteins and lipids are; chapter 5 told you how enzymes work; chapter 3 told you how substances cross membranes. Chapter 8 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 thing is made and exactly where it acts.

Deficiency Disease

A disease caused by the long-term shortage of a particular nutrient in the diet, in which the body process that depends on that nutrient begins to fail. Shortage of vitamin C causes scurvy, shortage of vitamin D or calcium contributes to rickets in children, and shortage of iron causes anaemia.

Balanced Diet

A diet that supplies all the nutrients the body requires, together with dietary fibre and water, in the proportions and quantities needed to maintain health. The required quantities are not fixed: they vary with age, biological sex, activity level, pregnancy, state of health and body size, so two balanced diets can look entirely different.

Nutrient

A substance in food that the body requires for energy, for growth and repair, or for the correct working of its processes. Carbohydrates, lipids and proteins are required in large amounts; vitamins and mineral ions are required in small amounts. Dietary fibre and water are required too, although neither is a nutrient in the strict sense because neither is absorbed and used in the same way.

Alimentary Canal

The continuous muscular tube running from the mouth to the anus through which food passes: mouth, oesophagus, stomach, duodenum, ileum, colon, rectum and anus. Associated organs such as the salivary glands, liver, gall bladder and pancreas add secretions to the canal through ducts, but food never passes through them.

Liver

A large organ associated with the digestive system that produces bile, which is stored in the gall bladder and released into the duodenum to emulsify lipids. The liver also receives most of the absorbed nutrients directly from the ileum through the hepatic portal vein, and stores excess glucose as glycogen.

Absorption

The movement of digested food molecules through the wall of the intestine into the blood or the lymphatic system. It happens mainly in the ileum, and different substances cross by different processes: glucose and amino acids by diffusion and by active transport, water by osmosis, and the products of lipid digestion into the lacteal.

Physical Digestion

The breakdown of food into smaller pieces without any chemical change to the food molecules themselves. It is carried out by chewing in the mouth, by the churning of the stomach and by the emulsification of lipid droplets by bile, and its purpose is to increase the surface area of the food that is available to digestive enzymes.

Digestive Enzyme

A biological catalyst secreted into the alimentary canal that breaks a large food molecule into smaller soluble molecules. Amylase breaks starch into maltose, maltase breaks maltose into glucose, protease breaks protein into amino acids, and lipase breaks lipids into fatty acids and glycerol. Each is specific to one substrate and works fastest at a particular pH.

Protease

A digestive enzyme that breaks down protein into amino acids. Protease produced in the stomach works best in the acidic conditions created by hydrochloric acid, while protease produced by the pancreas works best in the alkaline conditions of the small intestine.

Assimilation

The uptake and use of absorbed nutrients by the cells of the body. Glucose is taken into cells and respired to release energy, or stored as glycogen in the liver and muscles; amino acids are used to build proteins including enzymes; and the products of lipid digestion are used to build cell membranes or stored as fat.

Pulp Cavity

The soft living centre of a tooth, enclosed by dentine, containing nerves and blood vessels. The blood vessels supply the living cells of the tooth with oxygen and nutrients, and the nerves make the tooth sensitive to pressure, temperature and damage.

Enamel

The hard, non-living outer layer that covers the crown of a tooth. It is the hardest substance produced by the human body and resists the wear of biting and grinding, but because it contains no living cells it cannot repair itself once it is worn away or damaged.

Villus

A small finger-like projection of the wall of the small intestine, made of many cells, which increases the surface area available for absorption. Its epithelium is one cell thick, its exposed surface carries microvilli, and inside it are a dense network of blood capillaries and a central lacteal.

Small Intestine

The long, narrow region of the alimentary canal between the stomach and the colon, made up of the duodenum and the ileum. Chemical digestion of carbohydrate, protein and lipid is completed here using enzymes from the pancreas and the intestinal wall, and it is the main site of absorption of nutrients and of most of the water.

Lacteal

A blind-ended lymph vessel running up the centre of a villus. It receives the products of lipid digestion — fatty acids and glycerol — after they have passed through the epithelial cells, and carries them into the lymphatic system, which is a different initial route from the blood capillaries that take up glucose and amino acids.

Peristalsis

Waves of contraction and relaxation of the circular and longitudinal muscles in the wall of the alimentary canal, which move food along it. Circular muscle contracts behind the bolus, narrowing the canal and pushing the bolus forward, while circular muscle ahead relaxes so that the canal widens to receive it.

Chemical Digestion

The breakdown of large food molecules into small soluble molecules by enzyme-controlled reactions. Amylase, maltase, protease and lipase each act on one kind of substrate, breaking the bonds that hold the large molecule together so that the products are small enough and soluble enough to be absorbed through the wall of the intestine.

Hepatic Portal Vein

The blood vessel that carries most of the molecules and ions absorbed in the ileum directly from the capillaries of the small intestine to the liver. It is unusual among veins because it begins in one capillary network and ends in another rather than returning blood to the heart.

Bile

An alkaline liquid produced by the liver and stored in the gall bladder before being released into the duodenum. It contains no enzymes. Bile emulsifies lipids, breaking large fat droplets into many small ones, which greatly increases the total lipid surface area available to lipase and so increases the rate of lipid digestion.

Lipase

A digestive enzyme that breaks down lipids into fatty acids and glycerol. It is produced by the pancreas and acts in the small intestine. Its rate of action depends strongly on how much lipid surface is available, which is why bile emulsifies large fat droplets into small ones before lipase works on them.

Amylase

A digestive enzyme that breaks down starch into maltose. It is produced by the salivary glands, where it acts in the mouth, and by the pancreas, where it acts in the small intestine. It does not produce glucose: a second enzyme, maltase, is needed to break maltose into glucose.

Hydrochloric Acid

An acid secreted by the wall of the stomach. It has two functions: it provides the acidic conditions in which stomach protease works best, and it kills many of the microorganisms that are swallowed with food. It is not an enzyme and does not itself digest protein.

Dentition

The set of teeth in the mouth, described by their types, shapes and arrangement. Human dentition contains four types: chisel-shaped incisors at the front for biting and cutting, pointed canines for gripping and tearing, and premolars and molars at the back with broad ridged surfaces for crushing and grinding. Each shape suits a different part of physical digestion.

• “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.” 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.” 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 Amylase produces maltose. A second enzyme, maltase, is needed to break maltose into glucose. The destination is right but a whole enzyme has been skipped. Write instead "Amylase breaks starch into maltose; maltase then breaks maltose into glucose."
• “Protease digests 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 "Protease breaks protein into amino acids. Lipase breaks lipid into 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. It creates the conditions in which the stomach's protease works, and it kills many ingested microorganisms. Write instead "Hydrochloric acid provides the acidic conditions the stomach protease needs, and kills many ingested microorganisms. The protease does the digesting."
• “Enzymes are made where they act.” Why it is wrong Pancreatic amylase is made in the pancreas and acts in the small intestine; bile is made in the liver and acts in the duodenum. Questions ask for both columns precisely because they differ. Write instead "Amylase is produced by the salivary glands and the pancreas, and acts in the mouth and 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 "Peristalsis moves food along: circular muscle contracts behind the bolus and relaxes in front of it, and the wave travels along the canal."
• “Absorption and assimilation are two words for the same thing.” Why it is wrong Absorption is entry into the blood or lymph 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; the products of lipid 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.” 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."
• “The hepatic portal vein carries food from the stomach to the liver.” Why it is wrong Two errors. It carries blood, containing absorbed molecules and ions, not food; and it starts at the capillaries of the ileum, not the stomach. Write instead "The hepatic portal vein carries most of the molecules and ions absorbed in the ileum to the liver."
• “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."

• Human nutrition is unusually predictable. The same six question shapes come round, and each has a fixed answer structure worth learning as a habit.
• 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.
• Substrate, product, site of production, site of action, sometimes pH. One mark per cell. Learn the table as a table; do not try to reconstruct it from a story.
• One organ, one to three marks. Give one distinct function per mark, and do not repeat the same idea in different words.
• Almost always the villus. Use feature → location → effect → advantage. A feature alone scores nothing.
• Physical from chemical; absorption from assimilation; egestion from excretion. Write matched statements: same property, both sides.