Digestion of Food — Revision Notes

⚡ 30-Second Revision

Mouth: — Salivary Amylase (Starch $ightarrow$ Maltose), pH 6.7-7.0.
Stomach: — HCl (activates Pepsinogen $ightarrow$ Pepsin), Pepsin (Proteins $ightarrow$ Proteoses/Peptones), pH 1.5-3.5. Rennin (milk protein coagulation in infants).
Small Intestine:

- Bile: Emulsifies fats (no enzymes). - Pancreatic Juice: Pancreatic Amylase (Starch $ightarrow$ Disaccharides), Trypsinogen $xrightarrow{\text{Enterokinase}}$ Trypsin (Proteins $ightarrow$ Dipeptides), Lipase (Fats $ightarrow$ Fatty Acids/Glycerol), Nucleases (Nucleic Acids $ightarrow$ Nucleotides).

- Intestinal Juice (Succus Entericus): Disaccharidases (Maltase, Lactase, Sucrase $ightarrow$ Monosaccharides), Dipeptidases (Dipeptides $ightarrow$ Amino Acids), Nucleotidases/Nucleosidases (Nucleotides $ightarrow$ Sugars/Bases), Intestinal Lipase.

Final Products: — Monosaccharides, Amino Acids, Fatty Acids, Glycerol, Nitrogenous Bases, Pentose Sugars.

2-Minute Revision

Digestion is the breakdown of complex food into simple, absorbable forms. It starts in the mouth with mechanical chewing and chemical action of salivary amylase on starch. The food then moves to the stomach, where HCl denatures proteins and activates pepsinogen to pepsin, initiating protein digestion.

In the small intestine, the major events occur. Bile from the liver emulsifies fats, increasing their surface area. Pancreatic juice, rich in enzymes like pancreatic amylase, trypsinogen (activated to trypsin by enterokinase), and lipase, acts on carbohydrates, proteins, and fats respectively.

Intestinal juice (succus entericus) contains disaccharidases, dipeptidases, and nucleosidases that complete the digestion, yielding monosaccharides, amino acids, fatty acids, glycerol, and nitrogenous bases.

These final products are then absorbed, primarily in the small intestine. The large intestine absorbs water and forms feces, with no digestive enzyme activity.

5-Minute Revision

Digestion is a crucial process involving both mechanical and chemical breakdown of food. Mechanical digestion begins in the mouth with mastication and continues with churning in the stomach and segmentation in the small intestine. Chemical digestion relies on specific enzymes.

In the mouth, salivary amylase (ptyalin) starts carbohydrate digestion, breaking starch into maltose. The bolus then travels to the stomach, where the highly acidic environment (pH 1.5-3.5) due to HCl activates pepsinogen into pepsin, initiating protein digestion into proteoses and peptones. HCl also denatures proteins and kills microbes. In infants, rennin coagulates milk proteins.

The partially digested food, now called chyme, enters the small intestine. Here, it mixes with three key secretions:

1
Bile juice — (from liver/gallbladder): Contains bile salts that emulsify fats, increasing their surface area for enzyme action. It contains no enzymes.
2
Pancreatic juice — (from pancreas): Contains bicarbonate to neutralize acidic chyme and a powerful array of enzymes:

* Pancreatic amylase: Continues starch digestion to disaccharides. * Trypsinogen, Chymotrypsinogen, Procarboxypeptidases: Inactive proteases activated by enterokinase (from intestinal wall) and trypsin, breaking proteins into dipeptides and amino acids. * Pancreatic lipase: Digests emulsified fats into fatty acids and glycerol. * Nucleases: Digest nucleic acids into nucleotides.

1
Intestinal juice (succus entericus) — (from intestinal wall): Contains enzymes that complete digestion:

* Disaccharidases (Maltase, Lactase, Sucrase): Break disaccharides into monosaccharides (glucose, fructose, galactose). * Dipeptidases: Break dipeptides into amino acids. * Nucleotidases and Nucleosidases: Break nucleotides into nucleosides, sugars, and nitrogenous bases.

The final products – monosaccharides, amino acids, fatty acids, glycerol, and nitrogenous bases – are then absorbed into the bloodstream or lymphatic system. The large intestine primarily absorbs water and electrolytes, forming feces, with no enzymatic digestion.

Prelims Revision Notes

Mouth:

- Mechanical: Mastication (chewing). - Chemical: Salivary amylase (ptyalin) on starch $ightarrow$ maltose, dextrins. pH 6.7-7.0. Lingual lipase (inactive).

Esophagus: — Peristalsis, no digestion.
Stomach:

- Mechanical: Churning. - Chemical: HCl (parietal cells) activates pepsinogen to pepsin, denatures proteins, kills microbes. Pepsin (chief cells) on proteins $ightarrow$ proteoses, peptones. pH 1.5-3.5. Rennin (infants) coagulates milk casein. Gastric lipase (limited action).

Small Intestine (Duodenum, Jejunum, Ileum): — Major site of digestion and absorption.

- Bile (Liver/Gallbladder): Emulsifies fats. No enzymes. - Pancreatic Juice (Pancreas): Alkaline (bicarbonate). - Pancreatic amylase: Starch $ightarrow$ disaccharides. - Trypsinogen $xrightarrow{\text{Enterokinase}}$ Trypsin.

Trypsin activates chymotrypsinogen $ightarrow$ chymotrypsin, procarboxypeptidases $ightarrow$ carboxypeptidases. Proteases act on proteins $ightarrow$ dipeptides. - Pancreatic lipase: Emulsified fats $ightarrow$ fatty acids + glycerol/monoglycerides.

- Nucleases (DNase, RNase): Nucleic acids $ightarrow$ nucleotides. - Intestinal Juice (Succus Entericus): - Disaccharidases: Maltase (maltose $ightarrow$ glucose+glucose), Lactase (lactose $ightarrow$ glucose+galactose), Sucrase (sucrose $ightarrow$ glucose+fructose).

- Dipeptidases: Dipeptides $ightarrow$ amino acids. - Intestinal lipase: Remaining fats $ightarrow$ fatty acids + glycerol. - Nucleotidases: Nucleotides $ightarrow$ nucleosides + phosphate. - Nucleosidases: Nucleosides $ightarrow$ sugars + nitrogenous bases.

Final Products: — Monosaccharides (glucose, fructose, galactose), Amino acids, Fatty acids, Glycerol, Nitrogenous bases, Pentose sugars.
Large Intestine: — Water and mineral absorption. No digestion. Feces formation. Symbiotic bacteria produce Vitamin K, B vitamins.
Hormonal Regulation: — Gastrin (stomach motility, HCl), Secretin (pancreatic bicarbonate), CCK (pancreatic enzymes, bile release), GIP (inhibits gastric secretion/motility).

Vyyuha Quick Recall

To remember the main enzymes and their substrates in the small intestine: Please Try Lots of New Carbs, Proteins, Lipids, and Nucleic Acids!

Pancreatic Trypsin (for Proteins)
Pancreatic Lipase (for Lipids)
Pancreatic Nucleases (for Nucleic Acids)
Pancreatic Amylase (for Carbohydrates)

And for intestinal enzymes: My Little Sugar Dipper Needs New Lipstick!

Maltase, Lactase, Sucrase (for Disaccharides)
Dipeptidases (for Dipeptides)
Nucleotidases, Nucleosidases (for Nucleotides/Nucleosides)
Lipase (for Lipids)