Life Processes | Class 7 Science Notes

Digestion in human beings

Digestion in humans is a complex process involving both mechanical and chemical breakdown of food as it travels through the alimentary canal. The journey begins in the mouth where teeth crush and chew food into smaller pieces, a mechanical digestion process. Saliva, secreted by salivary glands, moistens food and contains an enzyme that starts chemical digestion by breaking down starch (a carbohydrate) into sugar. This is why starchy foods like chapati taste sweet when chewed for a long time. The tongue helps mix food with saliva and pushes it into the oesophagus or food pipe. The oesophagus moves food to the stomach by rhythmic contractions called peristalsis. In the stomach, food is churned and mixed with gastric juice containing digestive enzymes, acid, and mucus. The enzymes break down proteins into simpler forms, acid kills harmful bacteria and aids digestion, and mucus protects the stomach lining from acid damage. The partially digested food then moves into the small intestine, the longest part of the alimentary canal, which receives secretions from the liver (bile) and pancreas (pancreatic juice). Bile neutralizes stomach acid and emulsifies fats, while pancreatic juice digests carbohydrates, proteins, and fats further. The small intestine's inner lining has finger-like projections called villi that increase surface area for absorption of nutrients into the blood. The remaining undigested food passes into the large intestine, which absorbs water and salts, forming semi-solid waste called stool, stored in the rectum and expelled through the anus. This detailed process ensures that nutrients from food are efficiently digested and absorbed to sustain life.

📊 Diagram: Figure 9.2 depicts the movement of food down the oesophagus by peristalsis. Figure 9.3 shows the stomach where food is churned and mixed with gastric juice. Figure 9.4 illustrates the stretched alimentary canal highlighting the small intestine's length and associated organs like liver and pancreas. Figure 9.5 shows the inner lining of the small intestine with villi.

🧪 Activity: Activity 9.1: Investigate the action of saliva on starch by comparing boiled rice and chewed rice in test tubes with iodine solution to observe colour change indicating starch presence or breakdown.

🔗 Connection: This section leads to understanding variations in digestion among different animals and introduces the next topic of respiration in animals.