CUET UG Biology: Animal Physiology - Digestion and Absorption, Enzymes, Absorption Sites

Must-Know

• Salivary amylase (ptyalin) hydrolyzes starch into maltose and dextrins at pH 6.8; it is inactivated in the stomach by HCl.
• Gastric juice contains pepsinogen, which is activated to pepsin by HCl; pepsin digests proteins into proteoses and peptones.
• Chief cells in the gastric mucosa secrete pepsinogen and gastric lipase; parietal cells secrete HCl and intrinsic factor.
• HCl in stomach kills microbes, activates pepsinogen, and provides acidic pH for pepsin activity (optimum pH 1.8–2.0).
• Rennin is present in infants and curdles milk by converting caseinogen to casein; absent in adults.
• Pancreatic juice contains inactive enzymes: trypsinogen, chymotrypsinogen, procarboxypeptidase, and proelastase.
• Enterokinase (enteropeptidase), secreted by intestinal mucosa, activates trypsinogen to trypsin; trypsin then activates other enzymes.
• Trypsin and chymotrypsin digest proteins into peptides; carboxypeptidase removes amino acids from C-terminal end.
• Pancreatic amylase digests starch into maltose, isomaltose, and ?-dextrins; acts at neutral pH (optimum pH 7.0).
• Pancreatic lipase hydrolyzes triglycerides into monoglycerides and free fatty acids; requires bile salts for emulsification.
• Bile juice, produced by hepatocytes, contains bile salts (e.g., taurocholate), bilirubin, cholesterol, and phospholipids; no digestive enzymes.
• Bile salts emulsify fats, form micelles with fatty acids and monoglycerides, and activate pancreatic lipase.
• Intestinal juice (succus entericus) contains maltase, lactase, sucrase, dipeptidases, nucleosidases, and enterokinase.
• Maltase breaks maltose into two glucose molecules; lactase hydrolyzes lactose into glucose and galactose.
• Sucrase splits sucrose into glucose and fructose; deficiency causes sucrose intolerance.
• Dipeptidases hydrolyze dipeptides into amino acids; located on brush border of intestinal epithelial cells.
• Final digestion occurs at brush border membrane of small intestine; absorption occurs mainly in jejunum.
• Glucose and galactose are absorbed via secondary active transport (Na?-dependent co-transport); amino acids use similar mechanism.
• Fructose is absorbed by facilitated diffusion using GLUT5 transporter; di- and tripeptides by H?-coupled transport.
• Fatty acids and monoglycerides are absorbed into enterocytes, re-synthesized into triglycerides, and packaged into chylomicrons for lymphatic transport via lacteals.

Difficulty Level

Intermediate — requires understanding of enzyme activation cascades and site-specific absorption mechanisms, but facts are directly from NCERT.

Common CUET Traps

• Trap: Thinking bile contains digestive enzymes. Avoid: Bile has no enzymes; it emulsifies fats and aids lipase action.
• Trap: Believing pepsin works in alkaline medium. Avoid: Pepsin functions only in acidic pH (1.8–2.0), provided by HCl.
• Trap: Assuming all carbohydrates are absorbed as glucose. Avoid: Disaccharides are digested to monosaccharides (glucose, fructose, galactose), each with different absorption mechanisms.

Practice MCQs

1. Which enzyme is responsible for activating trypsinogen to trypsin in the small intestine?
   A. Carboxypeptidase
   B. Chymotrypsin
   C. Enterokinase
   D. Elastase
   Answer: C
   Explanation: Enterokinase, secreted by intestinal mucosa, activates trypsinogen.
   Why others fail: Carboxypeptidase is a proteolytic enzyme, not an activator.

2. Where does the final digestion of proteins occur before absorption?
   A. Stomach
   B. Duodenum
   C. Brush border of small intestine
   D. Ileum
   Answer: C
   Explanation: Dipeptidases on the brush border complete protein digestion into amino acids.
   Why others fail: Duodenum initiates digestion, but final breakdown occurs at brush border.

3. Which of the following is NOT a function of bile?
   A. Emulsification of fats
   B. Activation of lipase
   C. Digestion of proteins
   D. Excretion of bilirubin
   Answer: C
   Explanation: Bile does not contain proteolytic enzymes; it aids fat digestion only.
   Why others fail: Students confuse bile’s role in fat digestion with protein digestion.

4. Fructose is absorbed into the blood from the small intestine via:
   A. Active transport with Na?
   B. Facilitated diffusion using GLUT5
   C. Simple diffusion
   D. Co-transport with H?
   Answer: B
   Explanation: Fructose uses facilitated diffusion through GLUT5 transporters.
   Why others fail: Glucose uses Na?-co-transport, so students misapply it to fructose.

5. Which cells in the gastric glands secrete intrinsic factor essential for vitamin B absorption?
   A. Chief cells
   B. Goblet cells
   C. Parietal cells
   D. Paneth cells
   Answer: C
   Explanation: Parietal cells secrete intrinsic factor required for B absorption in ileum.
   Why others fail: Chief cells secrete pepsinogen, leading to confusion with parietal cell function.

Last?Minute Revision

• Salivary amylase acts in mouth and stops in stomach due to low pH.
• Pepsinogen-pepsin by HCl; trypsinogen-trypsin by enterokinase.
• Rennin present only in infants; curdles milk protein.
• Pancreatic juice pH: ~8.0 due to bicarbonate ions.
• Bile: no enzymes, contains bile salts and bilirubin.
• Emulsification = physical breakdown of fats by bile salts.
• Micelles transport fatty acids and monoglycerides to brush border.
• Chylomicrons formed in enterocytes; enter lymph, not blood directly.
• Glucose and galactose: Na?-dependent co-transport.
• Fructose: facilitated diffusion via GLUT5.
• Amino acids: active transport with Na?.
• Dipeptides: H?-coupled co-transport into enterocytes.
• Lactose intolerance due to lactase deficiency in adults.
• Sucrase deficiency causes sucrose malabsorption.
• Brush border enzymes: maltase, sucrase, lactase, dipeptidases.
• Maximum absorption occurs in jejunum.
• Vitamin B absorbed in ileum with intrinsic factor.
• Water absorbed in large intestine by osmosis.
• No significant digestion occurs in large intestine.
• Mnemonic: "Parietal cells = HCl + Intrinsic Factor" — PIN.