Diet and digestion connects what you eat to how your body actually uses it. It is one of the most commonly examined areas of the Edexcel IGCSE Human Biology specification, and a thorough understanding here pays dividends across both papers.
Key facts
- A balanced diet provides carbohydrates, proteins, lipids (fats and oils), vitamins, mineral ions, fibre and water in the correct proportions
- Digestion breaks large, insoluble food molecules into small, soluble ones that can be absorbed into the blood
- Enzymes are biological catalysts that speed up digestion without being used up
- Absorption of digested food occurs mainly in the small intestine through villi
- BMI (body mass index) is used to assess whether a person's weight is healthy for their height
What is a balanced diet?
A balanced diet supplies all the nutrients the body needs in the right amounts. The Edexcel IGCSE specification requires you to know the sources, functions and recommended dietary intakes of specific nutrients. Here they are:
| Nutrient | Source | Function |
|---|---|---|
| Carbohydrates | Bread, rice, pasta, potatoes | Primary source of energy for cellular respiration |
| Proteins | Meat, fish, eggs, legumes | Growth and repair of tissues; making enzymes and antibodies |
| Lipids (fats and oils) | Butter, oil, nuts, fatty fish | Energy store; insulation; protection of organs; component of cell membranes |
| Vitamin A | Carrots, liver, dairy products | Needed for healthy vision (especially night vision) and healthy skin |
| Vitamin C | Citrus fruits, peppers, broccoli | Needed for healthy connective tissue (collagen); aids wound healing |
| Vitamin D | Oily fish, eggs, sunlight on skin | Needed for calcium absorption and healthy bones |
| Calcium | Dairy products, leafy greens | Needed for strong bones and teeth; blood clotting; muscle contraction |
| Iron | Red meat, spinach, beans | Needed to make haemoglobin in red blood cells for oxygen transport |
| Fibre (dietary) | Wholegrain cereals, fruit, vegetables | Adds bulk to food; stimulates peristalsis; prevents constipation |
| Water | Drinks, food | Solvent for chemical reactions; transport medium; temperature regulation |
Deficiency diseases
The specification limits deficiency diseases to four:
- Scurvy (lack of vitamin C) - symptoms include bleeding gums, slow wound healing, weakness
- Anaemia (lack of iron) - symptoms include tiredness, pale skin, shortness of breath (not enough haemoglobin to carry oxygen)
- Blindness (lack of vitamin A) - affects night vision first; prolonged deficiency can cause permanent blindness
- Kwashiorkor (lack of protein) - symptoms include swollen abdomen (fluid retention), wasting of muscles, stunted growth
Variations in diet
Diet requirements vary with age (children need more protein for growth; elderly need more calcium for bone maintenance), pregnancy (additional iron, calcium, folic acid and energy), climate (more energy-rich foods in cold climates to maintain body temperature) and occupation (manual workers need more carbohydrates for energy than office workers).
The alimentary canal
The alimentary canal is the continuous tube from mouth to anus through which food passes. Each section has a specific role in digestion and absorption:
- Mouth - mechanical digestion (teeth break food into smaller pieces, increasing surface area); chemical digestion begins (salivary amylase starts breaking starch into maltose); tongue mixes food with saliva and forms a bolus
- Oesophagus - moves the bolus from the mouth to the stomach by peristalsis (wave-like muscular contractions)
- Stomach - churns food and mixes it with gastric juice containing hydrochloric acid (kills bacteria, provides acidic pH for pepsin) and pepsin (a protease that begins protein digestion)
- Small intestine (duodenum and ileum) - the main site of chemical digestion and absorption. Pancreatic juice and bile are released into the duodenum. Digestion is completed here, and the products are absorbed through the villi in the ileum
- Large intestine - absorbs water from the remaining indigestible food, forming faeces
- Pancreas - produces pancreatic juice containing amylase, protease (trypsin) and lipase, which are released into the duodenum
Peristalsis and fibre
Peristalsis is the rhythmic contraction and relaxation of muscles in the gut wall that pushes food along the alimentary canal. Dietary fibre adds bulk to the food, giving the muscles something to push against, which keeps food moving efficiently. A diet low in fibre can slow peristalsis and lead to constipation.
Digestive enzymes
Three groups of enzymes are responsible for chemical digestion. The exam tests their names, where they are produced, where they act, and what they produce:
| Enzyme | Substrate | Product | Produced in | Acts in |
|---|---|---|---|---|
| Amylase (salivary) | Starch | Maltose | Salivary glands | Mouth |
| Amylase (pancreatic) | Starch | Maltose | Pancreas | Small intestine |
| Maltase | Maltose | Glucose | Small intestine lining | Small intestine |
| Pepsin (protease) | Proteins | Shorter polypeptides | Stomach wall | Stomach (acidic pH) |
| Trypsin (protease) | Proteins | Amino acids | Pancreas | Small intestine (alkaline pH) |
| Lipase | Lipids | Fatty acids + glycerol | Pancreas | Small intestine |
The role of bile
Bile is produced by the liver and stored in the gall bladder. It is released into the duodenum (the first part of the small intestine) and performs two functions:
- Emulsification - bile breaks large fat droplets into smaller droplets, increasing the surface area for lipase to act on. This speeds up lipid digestion.
- Neutralisation - bile is alkaline. It neutralises the acidic chyme arriving from the stomach, creating the slightly alkaline conditions needed for intestinal enzymes (such as trypsin and lipase) to work at their optimum pH.
Bile is not an enzyme. It does not chemically break down food molecules. It assists digestion physically (emulsification) and chemically (pH adjustment).
Absorption in the small intestine
The small intestine is adapted for efficient absorption through its villi. Each villus has features that maximise the rate of absorption:
- Large surface area (millions of villi, each covered in microvilli)
- Thin epithelial wall (one cell thick, reducing the diffusion distance)
- Dense network of blood capillaries (maintains a concentration gradient by carrying absorbed nutrients away quickly)
- Lacteal (a lymph vessel that absorbs fatty acids and glycerol)
Glucose and amino acids are absorbed into the blood capillaries. Fatty acids and glycerol are absorbed into the lacteal. The blood from the villi travels to the liver via the hepatic portal vein, where excess nutrients are processed and stored.
Teeth
The specification requires knowledge of the types, structure and functions of teeth:
- Incisors - chisel-shaped; cutting and biting food
- Canines - pointed; tearing food
- Premolars and molars - flat with ridges; grinding and crushing food
Tooth structure includes enamel (hard outer layer), dentine (beneath enamel, forms the bulk of the tooth), pulp cavity (contains blood vessels and nerves) and cement (holds the tooth in the jawbone). Tooth decay is caused by bacteria in the mouth that convert sugars into acid, which dissolves the enamel. Prevention includes regular brushing, reducing sugar intake, using fluoride toothpaste and regular dental check-ups.
BMI and obesity
Body mass index (BMI) is calculated as:
BMI = body mass (kg) / height2 (m2)
A BMI over 30 is classified as obese. Obesity is a risk factor for type 2 diabetes, cardiovascular disease and joint problems. High cholesterol levels, often associated with a diet rich in saturated fats, contribute to atherosclerosis: the build-up of fatty deposits (plaques) inside artery walls, narrowing the lumen and restricting blood flow. This increases the risk of heart attack and stroke.
Worked example: BMI
Question: A person weighs 85 kg and is 1.7 m tall. Calculate their BMI and state whether they are classified as overweight. (3 marks)
Model answer:
BMI = 85 / (1.7)2 = 85 / 2.89 = 29.4 kg/m2 (2 marks)
A BMI of 29.4 is above 25 but below 30, so this person is classified as overweight (1 mark).
Food hygiene
The specification requires understanding of hygienic methods of food preparation, cooking, storage and preservation:
- Preparation - wash hands before handling food; use separate chopping boards for raw meat and vegetables; keep work surfaces clean
- Cooking - cook food thoroughly to kill bacteria (especially meat); heat food to a high enough internal temperature
- Storage - refrigerate perishable foods to slow bacterial growth; keep raw and cooked foods separate
- Preservation - methods include canning, freezing, drying, salting and pasteurisation. All work by either killing bacteria or creating conditions in which bacteria cannot reproduce
How diet and digestion appears in the exam
This topic is commonly examined on both Edexcel IGCSE papers. Questions may ask you to label a diagram of the alimentary canal, explain the role of a specific enzyme, calculate BMI, describe how the villus is adapted for absorption, or discuss the effects of a nutrient deficiency. The edexcel igcse human biology explained approach is always the same: name the structure, state the function, give the specific details the mark scheme requires. Knowing what is diet and digestion igcse content on the 4HB1 specification versus what goes beyond it will save you time and keep your answers focused.
Self-check questions
- State two functions of protein in the body.
- Name the enzyme that digests maltose and state its product.
- Explain the two functions of bile in digestion.
- Describe three features of a villus that make it well adapted for absorption.
- A pregnant woman is advised to increase her intake of iron. Explain why.
- Calculate the BMI of a person with a mass of 60 kg and a height of 1.65 m. State whether this is in the healthy range.
- Explain how a diet high in saturated fat can lead to cardiovascular disease.
- Distinguish between mechanical digestion and chemical digestion. Give one example of each.
For more edexcel igcse diet and digestion resources and diet and digestion explained content, use the Green Bridge CBT platform to practise topic-specific questions with instant feedback. As an edexcel igcse human biology definition resource, the platform covers every nutrient, enzyme and process the specification demands, keeping your edexcel igcse human biology notes organised and exam-ready.
Edexcel IGCSE diet and digestion explained: nutrients, enzymes, the alimentary canal, absorption, BMI, and food hygiene.
Comentario(s)