The human digestive system
The substances a person needs are of two groups: organic (proteins, fats, carbohydrates, vitamins) and inorganic (water, minerals). Carbohydrates are the main source of energy; proteins are the main building material of cells; fats are an energy source and store; vitamins support metabolism, growth and development; minerals are needed for strong bones and growth; water makes up about two thirds of body mass. The organic substances in food are large molecules and must be broken down before they can enter the blood; this is digestion. The digestive tract: mouth cavity → pharynx → oesophagus → stomach → small intestine → large intestine. Its glands: salivary glands, stomach and intestinal glands, pancreas, liver. In the mouth teeth crush food, the tongue mixes it and saliva moistens it. In the stomach food is broken down for several hours by digestive juice. The small intestine is the longest part – nutrients are completely broken down and absorbed into the blood there. Undigested remains pass to the large intestine and leave the body.
Experiment “Saliva at work”: put a piece of clean bread or an unsalted cracker in your mouth and chew slowly for 1–2 minutes without swallowing. It tastes slightly sweeter because saliva begins to break down starch. Compare results with the class. Then a “stomach model”: put a piece of bread and a little water in a zip bag and squeeze it by hand for 2–3 minutes. Safety: pupils with food allergies do not take part, hands must be clean, and do not eat the mixture from the bag.