Digestion and Absorption — Revision Notes

⚡ 30-Second Revision

Mouth: — Salivary amylase (starch $\rightarrow$ maltose), pH 6.8.
Stomach: — Pepsin (protein $\rightarrow$ proteoses/peptones), HCl, pH 1.5-3.5. Intrinsic factor for B12.
Pancreas: — Pancreatic amylase, Trypsinogen (activated by Enterokinase), Chymotrypsinogen, Lipase, Nucleases. Bicarbonate for alkaline pH.
Liver: — Produces bile (emulsifies fats), stored in gallbladder. No enzymes.
Small Intestine: — Succus Entericus (Maltase, Sucrase, Lactase, Dipeptidases, Intestinal Lipase).
Absorption: — Monosaccharides/Amino acids $\rightarrow$ Blood capillaries. Fatty acids/Glycerol $\rightarrow$ Micelles $\rightarrow$ Chylomicrons $\rightarrow$ Lacteals.
Hormones: — Gastrin (gastric HCl), Secretin (pancreatic bicarbonate), CCK (pancreatic enzymes, gallbladder contraction), GIP (inhibits gastric activity).
Villi/Microvilli: — Increase surface area for absorption.

2-Minute Revision

Digestion is the breakdown of complex food into simpler, absorbable forms. It starts in the mouth with salivary amylase breaking down starch. The stomach, with its acidic pH and pepsin, primarily digests proteins.

The small intestine is the main site for both digestion and absorption. Here, pancreatic juice (containing amylase, proteases like trypsin, and lipase) and bile (for fat emulsification) complete the chemical breakdown.

Intestinal enzymes (disaccharidases, dipeptidases, intestinal lipase) finalize the process. Monosaccharides and amino acids are absorbed into blood capillaries, while fatty acids and glycerol are absorbed into lacteals via chylomicrons.

The large intestine absorbs water and forms feces. The entire process is regulated by neural and hormonal signals, ensuring efficient nutrient extraction and waste elimination.

5-Minute Revision

The human digestive system is a sophisticated assembly of the alimentary canal and associated glands, orchestrating the breakdown and uptake of nutrients. Mechanical digestion begins with mastication in the mouth, followed by chemical digestion of starch by salivary amylase.

The bolus then travels via the esophagus to the stomach, where strong muscular churning and gastric juice (HCl, pepsin) initiate protein digestion in a highly acidic environment. The resulting chyme moves into the small intestine, the powerhouse of digestion and absorption.

In the duodenum, chyme mixes with bile from the liver (emulsifies fats, no enzymes) and pancreatic juice. Pancreatic juice is rich in enzymes: pancreatic amylase for carbohydrates, trypsinogen and chymotrypsinogen (activated to trypsin and chymotrypsin) for proteins, and pancreatic lipase for fats.

The intestinal wall itself secretes succus entericus, containing disaccharidases (maltase, sucrase, lactase), dipeptidases, and intestinal lipase, which complete the digestion of all major macromolecules into their monomeric units: monosaccharides, amino acids, fatty acids, and glycerol.

Absorption primarily occurs in the jejunum and ileum of the small intestine, facilitated by its vast surface area created by villi and microvilli. Monosaccharides (glucose, galactose by active transport; fructose by facilitated diffusion) and amino acids (active transport) enter the blood capillaries directly.

Fatty acids and monoglycerides, being hydrophobic, form micelles with bile salts, enter intestinal cells, are re-esterified into triglycerides, and then packaged into chylomicrons. These chylomicrons are absorbed into the lacteals (lymphatic vessels), eventually reaching the bloodstream.

Water and electrolytes are absorbed throughout the small and large intestines.

Regulation is both neural and hormonal. Hormones like Gastrin (stimulates gastric acid), Secretin (pancreatic bicarbonate), and Cholecystokinin (CCK, pancreatic enzymes and gallbladder contraction) ensure coordinated activity. Undigested food and excess water pass into the large intestine, where water is reabsorbed, and feces are formed and stored before egestion. Key disorders include jaundice, vomiting, diarrhea, and constipation.

Prelims Revision Notes

Digestion and Absorption: NEET Quick Recall

I. Alimentary Canal Pathway:

Mouth $\rightarrow$ Pharynx $\rightarrow$ Esophagus $\rightarrow$ Stomach $\rightarrow$ Small Intestine (Duodenum, Jejunum, Ileum) $\rightarrow$ Large Intestine (Caecum, Colon, Rectum) $\rightarrow$ Anus.

II. Digestive Glands & Secretions:

Salivary Glands: — Saliva (Salivary Amylase/Ptyalin, Lysozyme).
Gastric Glands (Stomach): — Gastric Juice (HCl, Pepsinogen, Pro-rennin, Intrinsic Factor).
Liver: — Produces Bile (Bile salts, Bile pigments, Cholesterol, Phospholipids). Stored in Gallbladder. No enzymes.
Pancreas: — Pancreatic Juice (Pancreatic Amylase, Trypsinogen, Chymotrypsinogen, Procarboxypeptidases, Lipases, Nucleases). Bicarbonate.
Intestinal Glands (Small Intestine): — Succus Entericus (Maltase, Sucrase, Lactase, Dipeptidases, Intestinal Lipase, Nucleosidases, Enterokinase).

III. Enzyme Actions & Products:

Enzyme	Source	Substrate	Product	Optimal pH
Salivary Amylase	Saliva	Starch	Maltose	~6.8
Pepsin	Gastric Juice	Proteins	Proteoses, Peptones	1.5-3.5
Pancreatic Amylase	Pancreatic Juice	Starch	Maltose	~7-8
Trypsin/Chymotrypsin	Pancreatic Juice	Proteins, Proteoses, Peptones	Dipeptides	~7-8
Carboxypeptidases	Pancreatic Juice	Polypeptides	Dipeptides, Amino acids	~7-8
Lipase	Pancreatic/Intestinal	Fats (Triglycerides)	Fatty acids, Monoglycerides	~7-8
Maltase	Succus Entericus	Maltose	Glucose + Glucose	~7-8
Sucrase	Succus Entericus	Sucrose	Glucose + Fructose	~7-8
Lactase	Succus Entericus	Lactose	Glucose + Galactose	~7-8
Dipeptidases	Succus Entericus	Dipeptides	Amino acids	~7-8

IV. Absorption Mechanisms:

Monosaccharides (Glucose, Galactose): — Active transport (Na+ co-transport) into blood capillaries.
Monosaccharides (Fructose): — Facilitated diffusion into blood capillaries.
Amino Acids: — Active transport into blood capillaries.
Fatty Acids & Glycerol: — Form micelles $\rightarrow$ enter intestinal cells $\rightarrow$ re-esterified to triglycerides $\rightarrow$ packaged into chylomicrons $\rightarrow$ absorbed into lacteals (lymphatic system).
Water: — Osmosis.
Vitamins: — Fat-soluble (A, D, E, K) with fats; Water-soluble (B, C) by diffusion/active transport. B12 needs Intrinsic Factor.

V. Hormonal Control:

Gastrin: — Stimulates gastric HCl and pepsinogen secretion.
Secretin: — Stimulates pancreatic bicarbonate and inhibits gastric secretion.
Cholecystokinin (CCK): — Stimulates pancreatic enzyme secretion and gallbladder contraction (bile release).
Gastric Inhibitory Peptide (GIP): — Inhibits gastric motility and secretion.

VI. Key Structures:

Villi & Microvilli: — Increase surface area for absorption in small intestine.
Lacteals: — Lymphatic capillaries in villi, absorb fats.
Hepatic Portal System: — Transports absorbed carbohydrates and proteins from intestine to liver.

VII. Disorders:

Jaundice: — Liver affected, bile pigments accumulate (yellow skin/eyes).
Vomiting: — Ejection of stomach contents via mouth.
Diarrhea: — Abnormal frequency of bowel movement, increased liquidity of fecal discharge.
Constipation: — Infrequent bowel movements, hard feces.
Indigestion: — Food not properly digested, feeling of fullness.

Vyyuha Quick Recall

To remember the sequence of major digestive enzymes and their primary substrates:

All People Try Lots of Meat, Sugar, Lactose, Dipeptides.

All (Amylase) $\rightarrow$ Starch
People (Pepsin) $\rightarrow$ Protein
Try (Trypsin) $\rightarrow$ Protein
Lots of (Lipase) $\rightarrow$ Lipids (Fats)
Meat (Maltase) $\rightarrow$ Maltose
Sugar (Sucrase) $\rightarrow$ Sucrose
Lactose (Lactase) $\rightarrow$ Lactose
Dipeptides (Dipeptidases) $\rightarrow$ Dipeptides