Digestion is the mechanical (chewing, mixing) and chemical (enzyme) breakdown of food into simple substances — monomers — that can be absorbed into the blood and lymph. Proteins, fats and carbohydrates in food are large molecules that cannot enter cells directly; digestion turns them into amino acids, glycerol and fatty acids, and glucose, from which the body builds its own substances and obtains energy. Exams often ask about the enzymes and conditions in each part of the gut, absorption, the roles of the liver and pancreas, vitamins and I. P. Pavlov’s experiments.
§1. Nutrients and the digestive organs
The main substances in food are proteins, fats and carbohydrates (energy and building material), plus water, mineral salts and vitamins (absorbed without being broken down).
The alimentary canal: mouth → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (caecum with the appendix, colon, rectum) → anus. Food is moved along by waves of contraction of the smooth muscle in the wall — peristalsis.
Digestive glands:
salivary glands — 3 pairs of large glands (parotid, submandibular, sublingual) and many small ones;
the liver — the largest gland: it makes bile (stored in the gall bladder), detoxifies harmful substances in the blood (barrier function), stores excess glucose as glycogen and makes urea from ammonia;
the pancreas — a mixed gland: it releases digestive juice into the gut and the hormones insulin and glucagon into the blood;
small glands in the walls of the stomach and intestine.
Enzymes are biological catalysts made of protein. Each enzyme acts only on its own substrate, works best at body temperature (about 37 °C) and at its own pH, and is denatured and inactivated by boiling.
§2. Digestion in the mouth
Teeth break food up. An adult has 32 teeth: 8 incisors, 4 canines, 8 premolars and 12 molars. Children have 20 milk teeth. A tooth has a crown, neck and root; most of it is dentine, the crown is covered with enamel (the hardest tissue in the body), and inside is the pulp with nerves and vessels.
The tongue mixes food and senses taste.
Saliva is slightly alkaline to neutral (pH about 7); it contains amylase (breaks starch down to maltose), lysozyme (kills bacteria) and mucin (makes the bolus slippery for swallowing). About 1–1.5 l of saliva is produced a day.
Food stays in the mouth only briefly, so starch is not fully digested there; good chewing makes later digestion easier.
§3. Digestion in the stomach
The stomach is a muscular bag; its wall has three layers of smooth muscle, and its lining contains millions of glands. Gastric juice is strongly acidic (pH about 1.5–2):
hydrochloric acid (HCl) kills microbes, makes proteins swell and activates pepsin;
pepsin breaks proteins into polypeptides (not into amino acids);
lipase breaks down only small amounts of emulsified milk fat;
mucus protects the stomach wall from being digested by its own juice.
Food stays in the stomach for about 3–6 hours (fatty food longer) and then passes in small portions into the duodenum. Water, alcohol, some drugs and a little glucose can be absorbed in the stomach.
§4. Digestion and absorption in the small intestine
The duodenum is the main site of digestion; conditions are slightly alkaline. Two juices enter it:
pancreatic juice breaks down all the main nutrients: trypsin (proteins and polypeptides to amino acids), lipase (fats to glycerol and fatty acids), amylase (starch to glucose);
bile (from the liver) breaks fat into tiny droplets (emulsifies it), activates lipase and stimulates peristalsis. Bile is not an enzyme.
Enzymes of the intestinal juice complete digestion; part of it happens on the surface of the gut wall — membrane (contact) digestion.
Absorption takes place mainly in the small intestine. Its lining has folds and millions of villi, which multiply the absorbing surface many times. Each villus contains blood capillaries and a lymph capillary (lacteal).
amino acids and glucose are absorbed into the blood and travel through the hepatic portal vein first to the liver (which removes harmful substances and turns excess glucose into glycogen);
glycerol and fatty acids are rebuilt into human-type fat in the epithelial cells and enter the lymph.
Protein → amino acids (pepsin, trypsin); fat → glycerol + fatty acids (lipase, helped by bile); starch → glucose (amylase).
In the large intestine mainly water is absorbed and faeces are formed. Helpful bacteria partly break down cellulose and make vitamin K and B vitamins. Inflammation of the appendix is appendicitis.
§5. Control of digestion
I. P. Pavlov won the Nobel Prize in 1904 for his work on the physiology of digestion. He used the fistula method in dogs: saliva or gastric juice was led outside the body and measured. In the “sham feeding” experiment the oesophagus was cut and brought out: food never reached the stomach, yet as soon as it entered the mouth, gastric juice was secreted. Conclusions:
secretion of juices is controlled by unconditioned reflexes (food stimulating receptors in the mouth) and conditioned reflexes (the smell or sight of food, even talking about it);
then hormonal control joins in: hormones released when food enters the stomach and intestine increase secretion.
That is why food eaten with appetite and attractively served is digested better.
§6. Vitamins and healthy eating
Vitamins are organic substances needed in small amounts; most form part of enzymes. Their existence was shown by N. I. Lunin (1880), and the word “vitamin” was introduced by C. Funk (1912).
A — vision, skin (night blindness); B₁ — nervous system (beriberi); C — blood-vessel walls and immunity (scurvy, bleeding gums); D — bones (rickets; made in the skin under UV light); K — blood clotting.
Water-soluble vitamins: the B group and C; fat-soluble: A, D, E, K.
Avitaminosis — complete lack of a vitamin, hypovitaminosis — shortage, hypervitaminosis — excess (especially from overdosing on A and D supplements).
Vitamin C is quickly destroyed by cooking — eating fresh fruit and vegetables raw is useful.
Healthy eating: 3–4 meals a day at regular times, thorough chewing, washing hands and produce (preventing gut infections and worms), limiting sweets (tooth decay), avoiding too much fatty and salty food.
Worked example
Starch solution was put into test tubes with: 1) saliva at 37 °C; 2) boiled saliva at 37 °C; 3) saliva at 0 °C. After 20 minutes iodine was added. In which tube will no blue colour appear?
Iodine turns blue where starch is present.
In tube 1 amylase is active — the starch is broken down, so no blue.
In tube 2 the enzyme was denatured by boiling; in tube 3 it does not work at low temperature — starch remains, so blue.
Answer: only in tube 1.
Worked example
A patient’s bile duct is blocked by a stone. Digestion of which substances is impaired, and why?
Bile cannot reach the intestine.
Bile emulsifies fats and activates lipase.
Without bile, fat stays in large drops and lipase digests it poorly.
Answer: mainly fat digestion is impaired; absorption of fat-soluble vitamins also falls.
Worked example
An adult has 32 teeth. What percentage are premolars and molars?
Premolars 8, molars 12: 8 + 12 = 20.
Share: 20 / 32 × 100% = 62.5%.
Answer: 62.5%.
Check yourself
In what conditions does pepsin work?
What does bile do?
Where are the products of fat digestion absorbed?
Which vitamins do bacteria in the large intestine make?
Lack of which vitamin causes rickets?
How many teeth does an adult have?
Which organ do amino acids and glucose reach first after absorption into the blood?
Who carried out the “sham feeding” experiment?
Common mistakes
Calling bile an enzyme — it only emulsifies fats.
Writing that most absorption happens in the stomach — it happens in the small intestine.
Saying salivary amylase digests proteins — it digests starch.
Thinking glucose is absorbed into the lymph — it goes into the blood.
Writing that pepsin breaks proteins down to amino acids — only to polypeptides.
Treating the pancreas as only an exocrine gland — it also makes insulin.