PRELIMS

Human Digestive System and Enzymes

Nutrition is a complex process consisting of five stages: Ingestion, Digestion, Absorption, Assimilation, and Defecation.

0. Sequence of Digestive Organs

Mouth → Pharynx → Oesophagus → Stomach → Duodenum → Small Intestine (Jejunum, Ileum) → Large Intestine (Caecum, Colon, Rectum) → Anus.

• Oesophagus: Moves food by peristalsis (rhythmic muscular contractions); no digestion occurs here.
• Total length of alimentary canal: ~8–9 metres (in an adult).

I. Digestion from Mouth to Stomach

1. Mouth: Digestion starts here. Saliva (pH 6.8, ~1.5L/day) contains Ptyalin and Maltase to convert starch into sugar.
2. Stomach: Food stays for ~4 hours. Gastric Juice (light yellow, acidic) is secreted.
   • HCl: Secreted by Oxynic cells; kills bacteria; activates enzymes.
   • Pepsin: Breaks down Proteins into Peptones.
   • Renin: Breaks down Caseinogen into Casein (milk protein).

II. Digestion in Duodenum and Intestine

1. Duodenum: Receives Bile (alkaline, from liver) and Pancreatic Juice (alkaline).
   • Trypsin: Proteins/Peptones Polypeptides/Amino acids.
   • Amylase: Starch Soluble sugar.
   • Lipase: Emulsified fats Glycerol/Fatty acids.
2. Small Intestine: Digestion completes here. Intestinal Juice (alkaline, ~2L/day) is secreted.
   • Erepsin: Remaining proteins/peptones Amino acids.
   • Maltase/Sucrase/Lactase: Convert corresponding sugars into Glucose/Fructose/Galactose.

III. Summary of Digestive Enzymes

• Saliva: Enzyme: Ptyalin/Maltase. Substrate: Starch. Product: Maltose/Glucose
• Gastric Juice: Enzyme: Pepsin/Renin. Substrate: Proteins/Casein. Product: Peptones/Calcium Paracasein
• Pancreatic: Enzyme: Trypsin/Amylase/Lipase. Substrate: Proteins/Starch/Fat. Product: Polypeptides/Sugar/Glycerol
• Intestinal: Enzyme: Erepsin/Sucrase/Lactase. Substrate: Proteins/Sugars. Product: Amino acids/Glucose

IV. Absorption and Assimilation

• Absorption: Digested food enters the blood via Villi in the small intestine.
• Assimilation: The use of absorbed food in the body.
• Defecation: Undigested food reaches the Large Intestine, where bacteria turn it into faeces.

UPSC Relevance

The biology of digestion explains metabolic health and nutritional science.

• Enzymatic Specificity: Knowing which enzyme acts on what (e.g., Trypsin on proteins) is a recurring MCQ point.
• Secretion Sites: Differentiating between the roles of Liver (Bile), Pancreas (Trypsin), and the Stomach (Pepsin).
• Absorption Mechanism: The role of Villi in increasing surface area and the significance of the small intestine as the primary absorption site.
• Confused Pair — Pepsin vs Trypsin: Pepsin acts in an acidic medium (stomach, HCl-activated) on proteins → peptones; Trypsin acts in an alkaline medium (duodenum, pancreatic) on proteins/peptones → polypeptides/amino acids.
• Confused Pair — Ptyalin vs Amylase: Ptyalin (salivary amylase, mouth) and Pancreatic Amylase (duodenum) both break starch into sugar, but act in different regions and pH environments.