Contents: 7 sections
Cambridge IGCSE Biology 0610 · Core and Extended
Syllabus points
- Describe what makes a balanced diet and the sources and functions of the main nutrients.
- Describe the effects of malnutrition.
- Identify the organs of the alimentary canal and describe their functions.
- Describe the types of human teeth and the causes and prevention of dental decay.
- Describe physical and chemical digestion and the roles of the digestive enzymes.
- Describe absorption and explain how the small intestine is adapted for it.
A balanced diet
A balanced diet contains all the nutrients in the right proportions and the right amount for the person's age, sex and activity.
| Nutrient | Function | Source |
|---|---|---|
| Carbohydrate | main energy source | bread, rice, potatoes |
| Protein | growth and repair | meat, fish, eggs, beans |
| Fat | energy store, insulation, membranes | butter, oils, nuts |
| Vitamin C | forms connective tissue | citrus fruit |
| Vitamin D | helps the body absorb calcium | oily fish, made in skin in sunlight |
| Calcium | strong teeth and bones | milk, cheese |
| Iron | makes haemoglobin | red meat, liver |
| Fibre | provides bulk so muscles can grip during peristalsis | vegetables, wholegrains |
| Water | solvent for reactions and transport | drinks, most foods |
Deficiency diseases follow directly from the functions:
- Scurvy — too little vitamin C: bleeding gums, poor wound healing.
- Rickets — too little vitamin D or calcium: soft, deformed bones.
- Anaemia — too little iron: not enough haemoglobin, so tiredness and breathlessness.
- Kwashiorkor — too little protein: poor growth and a swollen abdomen.
- Constipation — too little fibre.
Malnutrition means an unbalanced diet, not only too little. Too much energy-rich food leads to obesity, raising the risk of heart disease and type 2 diabetes.
Teeth
Four types, each shaped for a job:
- Incisors — chisel-shaped, at the front, for biting and cutting.
- Canines — pointed, for tearing.
- Premolars and molars — broad with cusps, at the back, for chewing and grinding.
Dental decay happens in a fixed sequence, and questions usually ask for it in order:
- Bacteria and food form a layer of plaque on the teeth.
- Bacteria respire the sugar in the food and produce acid.
- The acid dissolves the enamel.
- Decay reaches the dentine and then the pulp, causing pain.
Prevention follows from each step: brush to remove plaque, eat less sugar so the bacteria produce less acid, and use fluoride toothpaste to strengthen the enamel.
The alimentary canal
The route, in order: mouth → oesophagus → stomach → small intestine (duodenum then ileum) → large intestine (colon then rectum) → anus.
Two organs are not part of the tube but empty into it:
- Liver — makes bile.
- Pancreas — makes amylase, protease and lipase, and releases them into the duodenum.
The gall bladder stores bile until it is needed.
The five processes
Getting these apart is worth easy marks.
- Ingestion — taking food into the body through the mouth.
- Digestion — breaking large insoluble molecules into small soluble ones.
- Absorption — moving those small molecules into the blood.
- Assimilation — cells taking them up and using them.
- Egestion — passing out undigested food as faeces.
Egestion is not excretion, since the material never entered a cell or took part in metabolism.
Physical and chemical digestion
Physical digestion breaks food into smaller pieces without changing it chemically. Teeth chew it, the stomach churns it, and bile emulsifies fat, breaking large droplets into many small ones. In every case the point is the same: a larger surface area for enzymes to work on, so chemical digestion is faster.
Peristalsis moves food along. Circular muscles contract behind the food and relax in front of it, squeezing it forward in waves.
Chemical digestion uses enzymes to break the bonds in large molecules.
| Enzyme | Made in | Substrate | Products |
|---|---|---|---|
| Amylase | salivary glands, pancreas | starch | maltose |
| Protease (pepsin, trypsin) | stomach, pancreas | protein | amino acids |
| Lipase | pancreas | fats | fatty acids and glycerol |
Bile is not an enzyme. It does two things: it emulsifies fat, and it is alkaline, so it neutralises the acid arriving from the stomach and gives the intestinal enzymes the pH 8 they need. Questions often offer "bile digests fat" as a wrong option.
The stomach produces hydrochloric acid, which gives pepsin its pH 2 optimum and kills most bacteria in food.
Absorption
Absorption happens mainly in the small intestine, which is adapted for it in ways that all come down to surface area and distance:
- Very long, several metres.
- Folded, and covered in millions of villi, each covered in microvilli. Together these give a huge surface area.
- The wall of a villus is one cell thick, so the diffusion distance is short.
- Each villus has a dense capillary network, which carries absorbed molecules away and keeps the concentration gradient steep.
- Each villus has a lacteal, which absorbs fatty acids and glycerol.
Glucose and amino acids pass into the capillaries; fatty acids and glycerol pass into the lacteal. Most absorption is by diffusion, but when the concentration in the gut has fallen below that in the blood, active transport finishes the job, which is why the cells lining a villus are full of mitochondria.
The colon absorbs water, and it is the failure of this step that causes diarrhoea.