8.1 Diet
A balanced diet contains all the nutrients in the correct amounts for energy and health: carbohydrates, lipids, proteins, vitamins, minerals, fibre and water. How much is needed depends on age, sex, activity level and whether a woman is pregnant or breast-feeding.
| Nutrient | Main sources | Importance |
|---|---|---|
| carbohydrates | bread, rice, potatoes, pasta, sugar | the main source of energy |
| lipids (fats and oils) | butter, oil, nuts, meat, fish | a store of energy, insulation, and part of cell membranes |
| proteins | meat, fish, eggs, beans, milk | growth and repair of tissues; make enzymes and hormones |
| vitamin C | citrus fruits, tomatoes, green vegetables | keeps skin and gums healthy; helps wounds heal |
| vitamin D | oily fish, eggs, milk; made in the skin in sunlight | helps the body absorb calcium, for strong bones and teeth |
| calcium | milk, cheese, green vegetables | strong bones and teeth |
| iron | red meat, liver, spinach | makes haemoglobin in red blood cells |
| fibre (roughage) | vegetables, fruit, whole grains | gives bulk to the food so that peristalsis can push it along, preventing constipation |
| water | drinks and food | a solvent; makes up blood plasma, cytoplasm and urine; needed for all chemical reactions |
Deficiency diseases
| Missing nutrient | Disease | Symptoms |
|---|---|---|
| vitamin C | scurvy | bleeding gums, slow healing of wounds, loose teeth |
| vitamin D | rickets | soft, weak bones that bend (bow legs) in children |
| calcium | rickets | soft, weak bones, and weak teeth |
| iron | anaemia | not enough haemoglobin, so tiredness, weakness and pale skin |
8.2 Human digestive system
Why food must be digested
Most food molecules are large and insoluble, so they cannot pass through the wall of the gut into the blood. They must be digested into small, soluble molecules that can be absorbed.
- Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules (chewing and the churning of the stomach). It increases the surface area of the food for enzymes to act on.
- Chemical digestion is the breakdown of large molecules into small molecules by enzymes.
Teeth
| Type | Function in physical digestion |
|---|---|
| incisors | cutting and biting food |
| canines | tearing and piercing food |
| premolars | crushing and grinding food |
| molars | crushing and grinding food |
Parts of a tooth: enamel (the hard outer covering of the crown), dentine (a bone-like layer beneath it), pulp (the soft centre with nerves and blood vessels), and cement (which fixes the root into the jaw). Teeth are embedded in the gum.
The regions of the digestive system
| Region | Function |
|---|---|
| mouth | ingestion; physical digestion by teeth; chemical digestion of starch by amylase in saliva |
| salivary glands | secrete saliva containing amylase |
| oesophagus | carries food to the stomach by peristalsis |
| stomach | physical digestion (churning); chemical digestion of protein by protease (pepsin); secretes hydrochloric acid |
| small intestine (duodenum and ileum) | chemical digestion of starch by amylase, maltose by maltase, protein by protease and lipids by lipase; absorption of nutrients in the ileum |
| liver | makes bile; stores glycogen |
| gall bladder | stores bile |
| pancreas | secretes an alkaline juice containing amylase, protease and lipase into the duodenum |
| large intestine (colon) | absorbs water (and some ions) |
| rectum and anus | the rectum stores faeces and the anus egests them |
The digestive enzymes
| Enzyme | Substrate | Products | Where |
|---|---|---|---|
| amylase | starch | maltose | mouth (saliva) and small intestine (from the pancreas) |
| maltase | maltose | glucose | small intestine |
| protease (pepsin and trypsin) | protein | amino acids | pepsin in the stomach; trypsin in the small intestine |
| lipase | lipids (fats and oils) | fatty acids and glycerol | small intestine (from the pancreas) |
- Hydrochloric acid in the stomach kills bacteria that are eaten with the food, and provides the acid conditions (about pH 2) in which pepsin works best.
- The proteases in the stomach and the duodenum work best at different pH values: pepsin in acid and trypsin in the alkaline conditions of the small intestine (the pancreatic juice and bile neutralise the acid from the stomach).
Bile
Bile does not contain enzymes, and it does not chemically digest fat.
Peristalsis
The circular muscles contract behind the food, narrowing the gut, while the longitudinal muscles contract to shorten that section; the muscles ahead of the food relax, so the food moves forwards.
Amylase breaks starch down to maltose. Maltase, in the small intestine, breaks maltose down to glucose.
8.3 Absorption and assimilation
The small intestine (mainly the ileum) is where most nutrients are absorbed. Absorption happens by diffusion, osmosis and active transport. Water is absorbed in the small intestine and the colon, but most of the water is absorbed in the small intestine.
Villi
The inner wall of the ileum is covered in many tiny finger-like projections called villi. The cells on their surface have even smaller projections called microvilli.
| Feature of a villus | Function |
|---|---|
| villi and microvilli | a very large internal surface area for absorption |
| a wall one cell thick | a short diffusion distance |
| a network of capillaries | a good blood supply that takes away glucose and amino acids, keeping a steep concentration gradient |
| a lacteal (a lymph vessel) | absorbs fatty acids and glycerol |
The hepatic portal vein
The liver then regulates what reaches the rest of the body: it stores glucose as glycogen, turns excess amino acids into urea, and removes toxins.
Assimilation
The nutrients that are carried in the blood are used by the cells: glucose in respiration, amino acids to build proteins, and fatty acids and glycerol to make cell membranes and for energy storage.
Practise this chapter
Every past-paper question on these topics, with the mark scheme.