Nutrition and Human Digestive System, revision notes

Overview

Food must be broken down into small, soluble molecules the body can absorb. This chapter follows food through the digestive system, covering digestion, absorption, assimilation, a balanced diet, and related health issues.

Knowing which enzyme acts where, and the products it forms, is heavily examined.

Undigested material, mainly fibre, together with dead cells and bacteria, passes into the large intestine, where water and some mineral ions are reabsorbed before the remaining waste is expelled through the anus as faeces, a process called defaecation. A diet low in fibre slows this process and is linked to constipation, while poor eating habits more broadly are linked to health issues such as obesity, diabetes, gastric ulcers and colon problems, all of which examiners connect back to a specific part of the digestive system.

Digestion is tested practically as well as theoretically. The Paper 3 food test experiment uses iodine solution to detect starch, Benedict's solution to detect reducing sugar, and biuret solution to detect protein, so candidates must know the colour change each test gives and be able to link a positive result to a specific nutrient.

The same logic extends to testing saliva, gastric juice or pancreatic juice on starch, egg white or fat to show which enzyme has acted.

This chapter is assessed through Paper 1 items on enzymes and organs, and through Paper 2 structured questions that give a described diet, a set of food-test results or a health scenario and ask candidates to explain the underlying digestion, absorption or health concept. Naming the correct organ, enzyme and product together, rather than any one alone, is what separates a full-mark answer from a partial one.

9.1 Digestive System: Alimentary Canal and Glands

The human digestive system is built around one continuous tube, the alimentary canal, running from the mouth to the anus: mouth, oesophagus, stomach, small intestine and large intestine, ending at the anus. Food is pushed along this tube mainly by peristalsis, the wave-like contraction of muscle in the gut wall, rather than by gravity alone.

The salivary glands, liver, pancreas and gall bladder all add secretions into the canal without forming part of it themselves. The salivary glands release saliva containing salivary amylase into the mouth.

The liver produces bile, which is stored and concentrated in the gall bladder before release into the duodenum. The pancreas releases pancreatic juice, containing enzymes such as pancreatic amylase, trypsin and pancreatic lipase, into the same region.

Each gland's secretion is timed to reach the gut section where its enzyme or fluid is needed.

• Mouth
• Oesophagus
• Stomach
• Small intestine (duodenum, jejunum, ileum)
• Large intestine (caecum, colon, rectum)
• Anus

9.2 Digestion: Enzymes Across the Gut

Digestion has two parts that work together: mechanical digestion, such as chewing in the mouth and churning in the stomach, breaks food into smaller pieces and increases its surface area; chemical digestion, carried out by enzymes, breaks large molecules into small soluble ones. Table 1 lists the main digestive enzymes in the order they act, from the mouth to the small intestine.

Each enzyme in the table is specific to one substrate and works best at a particular pH, which is why the acidic stomach and the alkaline small intestine each favour a different set of enzymes.

9.3 Absorption at the Villus

Absorption takes place mainly in the small intestine, whose inner wall is folded into millions of finger-like villi that increase the surface area available for the digested food to cross into the blood. Glucose and amino acids diffuse and are actively transported across the villus wall into the capillary network, then travel in the hepatic portal vein to the liver before entering general circulation.

Fatty acids and glycerol take a different route: they are absorbed into the lacteal at the centre of each villus, enter the lymphatic system as a milky fluid, and are only added to the bloodstream later, near the heart.

• Wall one cell thick, giving a short diffusion distance
• Millions of villi, each covered in microvilli, giving a large surface area
• Dense network of capillaries maintaining a steep concentration gradient
• A lacteal to absorb fatty acids and glycerol into the lymphatic system

9.4 Assimilation

Assimilation is what happens to nutrients after they reach body cells. Glucose is respired to release energy, or converted to glycogen and stored in the liver and muscles when supply exceeds immediate need.

Amino acids are built into new proteins for growth, repair, enzymes and antibodies; the liver deaminates any amino acids in excess of what the body can use, producing urea for excretion.

Fatty acids and glycerol are used to build cell membranes, to make hormones, or are stored as fat for insulation and long-term energy reserve. Vitamins and mineral ions absorbed alongside these nutrients are assimilated directly into structures such as bone, or act as cofactors for enzymes, without themselves being broken down for energy.

9.5 Defaecation and the Large Intestine

Material that reaches the large intestine has already had almost all its digestible nutrients removed; what remains is mainly fibre, together with dead intestinal cells and bacteria. As this material passes through the colon, water and some mineral ions are reabsorbed back into the blood, gradually turning a liquid residue into semi-solid faeces.

The compacted faeces are stored in the rectum and expelled through the anus, a process called defaecation. Because the material removed by defaecation was never absorbed into the blood, it is not the same as excretion, which removes waste substances that body cells actually produced during metabolism, such as urea and carbon dioxide.

9.6 Balanced Diet

A balanced diet supplies carbohydrates and lipids mainly for energy, proteins mainly for growth and repair, and fibre to keep the digestive system working efficiently, together with the vitamins, minerals and water the body cannot make for itself. 'Balanced' describes a proportion suited to the individual, not a single fixed menu.

The right proportion changes with age, gender, body size, activity level and physiological state: a growing teenager and a pregnant woman both need proportionally more protein and certain minerals than a sedentary older adult, while an athlete in training needs more energy from carbohydrates than someone with a desk-based routine.

9.7 Health Issues Related to the Digestive System and Eating Habits

Common health issues such as obesity, diabetes, gastric ulcer and constipation can each be traced to a specific, describable eating pattern rather than to 'unhealthy food' in general. Obesity develops when energy intake from fatty and sugary food consistently exceeds the energy the body uses, so the surplus is stored as fat; this same imbalance is linked to a higher risk of diabetes.

A gastric ulcer can form when irregular eating, excess stomach acid or a bacterial infection damages the lining of the stomach or duodenum.

Constipation is linked to a diet low in fibre and fluids, which reduces the bulk of the colon's contents and slows the muscular movement that pushes waste toward the rectum. In each case, examiners expect the specific habit or mechanism to be named, not just a general warning about diet.

Key concepts to master

• Digestive system, The alimentary canal from mouth to anus, plus glands such as the salivary glands, pancreas and liver.
• Digestion, Large molecules are broken down by enzymes: carbohydrases to glucose, proteases to amino acids, lipases to fatty acids and glycerol.
• Absorption, Digested food is absorbed into the blood mainly through the villi of the small intestine.
• Villus adaptations, Thin walls, a large surface area, many capillaries and a lacteal make absorption efficient.
• Assimilation, Absorbed nutrients are used by cells to build tissues and release energy.
• Balanced diet, The right proportions of carbohydrates, proteins, lipids, vitamins, minerals, fibre and water for the person's needs.
• Defaecation, Undigested material such as fibre, dead cells and bacteria moves from the small intestine into the large intestine. There, water and some mineral ions are reabsorbed back into the blood, and the remaining waste is compacted and expelled through the anus as faeces in a process called defaecation, distinct from excretion of metabolic waste.
• Health issues from poor eating habits, Eating too much fatty or sugary food with too little exercise is linked to obesity and diabetes; eating irregularly or too quickly can lead to a gastric ulcer when stomach acid damages the stomach lining; and a diet low in fibre and fluids is linked to constipation.
• Enzyme specificity in digestion, Each digestive enzyme acts on only one type of substrate: amylase breaks down starch, protease breaks down protein, and lipase breaks down fat, and each enzyme works best at a particular pH and temperature matched to the part of the gut where it is released.
• Food tests, Iodine solution turns blue-black in the presence of starch, Benedict's solution forms a brick-red precipitate with a reducing sugar on heating, and biuret solution turns purple in the presence of protein, so a food test result identifies which nutrient, and therefore which enzyme's product, is present.
• Peristalsis and the alimentary canal, Peristalsis, the wave-like contraction and relaxation of muscle in the wall of the alimentary canal, pushes food from the oesophagus through to the rectum; it is a separate mechanical process from the chemical digestion carried out by enzymes.
• Vitamin and mineral deficiency, A diet lacking specific vitamins or minerals causes a named deficiency disease: too little vitamin C causes scurvy with bleeding gums, too little iron causes anaemia and fatigue, and too little vitamin D or calcium weakens bones, causing rickets in children.
• Determining vitamin C content, In the Paper 3 practical, the vitamin C content of a fruit juice is estimated by adding the juice drop by drop to a fixed volume of blue DCPIP dye until the dye just turns colourless; a juice that needs more drops to decolourise the same volume of dye contains less vitamin C, while one that needs fewer drops contains more.

Quick recall checklist

1. Can you define and explain Digestive system?
2. Can you define and explain Digestion?
3. Can you define and explain Absorption?
4. Can you define and explain Villus adaptations?
5. Can you define and explain Assimilation?
6. Can you define and explain Balanced diet?
7. Can you define and explain Defaecation?
8. Can you define and explain Health issues from poor eating habits?
9. Can you define and explain Enzyme specificity in digestion?
10. Can you define and explain Food tests?
11. Can you define and explain Peristalsis and the alimentary canal?
12. Can you define and explain Vitamin and mineral deficiency?
13. Can you define and explain Determining vitamin C content?

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