Zoology MCQs for NEET — Practice Questions with Answers

(C) Secretin and cholecystokinin (CCK) are two main gastrointestinal (GI) hormones secreted in duodenum of alimentary canal. CCK stimulates gall bladder contraction and thus increases the flow of bile salts into the intestine. Secretin stimulates the release of an alkaline pancreatic fluid that neutralises stomach acid as it enters the intestine.

The saliva secreted into the oral cavity contains electrolytes (Na⁺, K⁺, Cl^–, HCO^–) and enzymes, salivary amylase and lysozyme and has an optimum pH of action- 6.8.

 $\mathrm{Starch}\underset{\mathrm{pH} 6.8}{\overset{\mathrm{Salivary} \mathrm{Amylase}}{\to }}\mathrm{Maltose}$

(page-262)

Fatty acids and glycerol are first incorporated into small droplets called micelles which move into the intestinal mucosa. They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the blood stream. (page- 265)

Amylase is secreted in pancreatic juice. Gastric juice contains hydrochloric acid and enzymes like rennin, pepsin, etc. (page-262)

Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice (page-262).

The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of Oddi. (page- 261).

Parietal or oxyntic cells present in mucosa of stomach secrete HCl and intrinsic factor (factor essential for absorption of vitamin B12). If these cells become partially non-functional, then HCl would not be secreted in adequate amounts, thus it would not allow the conversion of pepsinogen to pepsin which converts proteins into proteoses and peptones. (page-262).

Mucosa forms small finger-like foldings called villi in the small intestine. The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance. These modifications increase the surface area enormously. Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal. (page- 260).

Absorption of fats: Fatty acids and glycerol being insoluble, cannot be absorbed into the blood after digestion and thus needs to be modified. They are first incorporated into small droplets called micelles which move into the intestinal mucosa. They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the blood stream.

Carbohydrates and proteins: Amino acids and glucose do not need any modification because they can be directly absorbed by active, passive and facilitated diffusion. (page-265).