Digestive System — ICSE Class 9 Biology Important Questions

Answer: (b) Ptyalin (salivary amylase).

Salivary amylase (ptyalin) in the mouth begins the digestion of cooked starch, converting it into maltose in a slightly alkaline medium.

Answer: (c) Liver.

Bile is secreted by the liver and stored temporarily in the gall bladder before being released into the duodenum through the bile duct.

Answer: (c) Amino acids.

Proteins are digested by pepsin, trypsin and erepsin in stages to peptones, polypeptides and finally amino acids, which are absorbed.

Answer: (c) Fats.

Bile emulsifies fats so that lipase can act efficiently. If the bile duct is blocked, fats are not emulsified, so their digestion and absorption are greatly reduced.

Answer: (a) The villi and their microvilli greatly increase the internal surface area, and this larger area is exactly why absorption is efficient, so R correctly explains A.

(i) Digestion: The process by which large, insoluble, complex food molecules are broken down (mechanically and chemically by enzymes) into small, soluble, absorbable molecules.

(ii) Assimilation: The process by which the absorbed, digested food is utilised by the body cells to release energy, build new protoplasm, repair tissues and store reserves.

Hydrochloric acid secreted by the gastric glands: (i) provides the acidic medium (about $\text{pH } 2$pH 2) needed for the enzyme pepsin to act on proteins; (ii) kills germs and bacteria entering with food, and stops the action of salivary amylase. It also softens the food and converts inactive pepsinogen to active pepsin.

A villus is a tiny finger-like projection of the inner wall of the small intestine.

Structure: It has a single-cell-thick epithelial lining bearing microvilli, a central lacteal (lymph vessel) and a dense network of blood capillaries.

Adaptations: (i) Large numbers of villi with microvilli give a huge surface area for absorption; (ii) the thin, one-cell-thick wall allows quick diffusion; (iii) rich blood supply carries absorbed glucose and amino acids away rapidly, maintaining a concentration gradient; (iv) the lacteal absorbs digested fats (fatty acids and glycerol).

The human digestive system consists of the alimentary canal (mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus) and glands (salivary glands, liver, pancreas).

Ptyalin acts in the alkaline saliva of the mouth; pepsin needs the acidic stomach medium; pancreatic and intestinal lipase act on emulsified fats in the small intestine.

Mouth: The bread is chewed by the teeth (mechanical digestion) and mixed with saliva. Salivary amylase (ptyalin) begins converting starch into maltose. The tongue rolls it into a bolus which is swallowed.

Oesophagus: The bolus is pushed down to the stomach by peristalsis (wave-like muscular contractions); no digestion occurs here.

Stomach: Gastric juice provides HCl and pepsin. Pepsin acts on the proteins of the bread, converting them into peptones. The food is churned into a semi-liquid chyme. Salivary amylase stops in the acidic medium.

Small intestine (duodenum): Bile emulsifies any fat; pancreatic juice (amylase, trypsin, lipase) and intestinal juice complete digestion. Remaining starch $\rightarrow$→ glucose, peptones $\rightarrow$→ amino acids.

Small intestine (ileum): The soluble products (glucose, amino acids) are absorbed through the villi into the blood; fats enter the lacteals.

Large intestine: Water and minerals are absorbed; undigested residue forms faeces, egested through the anus.

Digestion of proteins: Begins in the stomach where pepsin (in acidic medium) converts proteins to peptones. In the small intestine, trypsin (pancreatic) and erepsin (intestinal) convert peptones and polypeptides to amino acids, which are absorbed into the blood. No prior physical treatment such as emulsification is required.

Digestion of fats: No chemical digestion of fat occurs in the mouth or stomach. Fats are insoluble in water and form large globules, so enzymes cannot act on them easily.

Why fat digestion needs both liver and pancreas: (i) The liver produces bile whose bile salts emulsify the large fat globules into tiny droplets, greatly increasing the surface area exposed to enzymes. (ii) The pancreas produces the enzyme lipase, which then chemically breaks the emulsified fats into fatty acids and glycerol. Without bile the fat stays as large globules and lipase acts too slowly; without lipase no chemical breakdown happens at all. Hence both are essential, whereas protein digestion needs only the enzymes.

(i) Starch digestion first begins in the mouth, by the enzyme salivary amylase (ptyalin), which acts best near neutral/slightly alkaline pH.

(ii) The stomach is strongly acidic ($\text{pH } 2.0$pH 2.0) due to HCl. Salivary amylase is denatured/inactivated in this acidic medium, so it stops working.

(iii) Bile (from the liver) and pancreatic juice (from the pancreas), together with intestinal secretions, neutralise the acid and make the duodenum alkaline ($\text{pH } 8.0$pH 8.0).

(iv) The alkaline pH provides the correct medium for the pancreatic and intestinal enzymes (trypsin, amylase, lipase) to act, and neutralises the harmful acidic chyme entering from the stomach.