Digestion in Small Intestine — Enzymes and Absorption
Question
Describe the digestion process in the small intestine in detail. What secretions enter the small intestine, which enzymes are involved, and how are the final products absorbed? What adaptations of the small intestine improve absorption efficiency?
Solution — Step by Step
Step 1: What Arrives at the Small Intestine
When chyme (partially digested semi-liquid food) exits the stomach through the pyloric sphincter and enters the duodenum, it contains:
- Partially digested proteins (as polypeptides)
- Partially digested carbohydrates (some as starch, some as maltose)
- Undigested fats (large globules)
- Nucleic acids (DNA, RNA)
- Strong acid (HCl from stomach, pH ~2)
The small intestine must: (1) neutralise the acid, (2) complete digestion of all macromolecules, (3) absorb all nutrients. It receives help from three secretions: bile, pancreatic juice, and intestinal juice.
Step 2: Secretion 1 — Bile (from Liver, stored in Gall Bladder)
Bile is secreted by the liver, stored in the gall bladder, and released into the duodenum via the bile duct (which merges with the pancreatic duct at the hepatopancreatic ampulla).
Composition of bile:
- Bile salts (sodium glycocholate, sodium taurocholate)
- Bile pigments (bilirubin, biliverdin — from broken-down haemoglobin)
- Cholesterol, phospholipids
- Water and electrolytes
Functions:
- Emulsification of fats: Bile salts are amphipathic (have both hydrophilic and hydrophobic regions). They surround large fat globules and break them into tiny fat droplets — this is emulsification. It dramatically increases the surface area available for pancreatic lipase to act on. Without emulsification, lipase can only work on the outer surface of large fat globules.
- Neutralisation: Bile is slightly alkaline — helps neutralise acidic chyme
- Activation: Creates the optimal pH for intestinal enzymes
🎯 Exam Insider
A very common NEET question: "Does bile contain any digestive enzymes?" Answer: NO. Bile contains no digestive enzymes — only bile salts (for emulsification) and bile pigments (waste products). The actual enzymes that digest fats come from the pancreas (pancreatic lipase) and intestinal glands. Getting this wrong is one of the most common mistakes in this chapter.
Step 3: Secretion 2 — Pancreatic Juice (from Pancreas)
The pancreas secretes a mixture of digestive enzymes and sodium bicarbonate into the duodenum via the pancreatic duct.
Sodium bicarbonate (NaHCO₃): Neutralises the acidic chyme from the stomach → raises pH to 7–8 (alkaline) → optimal for intestinal enzymes
Pancreatic enzymes:
For carbohydrates:
- Pancreatic amylase: Starch and remaining glycogen → maltose and dextrins
For proteins (secreted as inactive zymogens):
- Trypsinogen → activated by enterokinase (from intestinal lining) → trypsin
- Chymotrypsinogen → activated by trypsin → chymotrypsin
- Procarboxypeptidase → activated by trypsin → carboxypeptidase
- Trypsin cleaves at arginine/lysine residues; chymotrypsin cleaves at phenylalanine/tyrosine/tryptophan residues; carboxypeptidase cleaves from the C-terminus
For fats:
- Pancreatic lipase: Emulsified fat droplets → fatty acids + monoglycerides + glycerol
For nucleic acids:
- DNase: DNA → deoxyribonucleotides
- RNase: RNA → ribonucleotides
Step 4: Secretion 3 — Intestinal Juice / Succus Entericus (from Intestinal Glands)
Secreted by the intestinal glands (crypts of Lieberkühn) in the mucosal lining.
Intestinal enzymes (brush border enzymes — on the surface of enterocytes):
- Enterokinase: Activates trypsinogen → trypsin (key initiating step for all protease cascade)
- Maltase: Maltose → glucose + glucose
- Sucrase (invertase): Sucrose → glucose + fructose
- Lactase: Lactose → glucose + galactose
- Aminopeptidase: Cleaves amino acids from N-terminus of peptides
- Dipeptidase: Dipeptides → amino acids
- Intestinal lipase: Additional fat digestion
Step 5: End Products of Digestion
| Macromolecule | Final products |
|---|---|
| Carbohydrates (starch, glycogen) | Monosaccharides: glucose, fructose, galactose |
| Proteins | Amino acids |
| Fats (triglycerides) | Fatty acids, monoglycerides, glycerol |
| Nucleic acids | Nucleotides (then nucleosides, nitrogenous bases, sugars) |
Step 6: Absorption — How Nutrients Enter the Blood
The small intestine has three structural adaptations that maximise absorption surface area:
- Long tube (6–7 metres total in adult)
- Villi (finger-like projections of mucosa, ~1 mm tall) — increase surface area ~10×
- Microvilli (on each villus epithelial cell — brush border) — increase surface area ~600×
Combined effect: surface area of the small intestine ≈ 200–300 m² (size of a tennis court).
Absorption Routes
Glucose and amino acids:
- Absorbed by active transport (sodium-linked cotransport) across the intestinal epithelial cells
- Enter the capillary blood in each villus
- Blood goes: capillaries → venules → mesenteric veins → portal vein → liver
- Liver processes: stores some as glycogen, regulates blood glucose, metabolises amino acids
Fatty acids, monoglycerides, and glycerol:
- Enter intestinal epithelial cells by diffusion
- Inside cells: re-synthesised into triglycerides
- Packaged with cholesterol and proteins into chylomicrons (lipoprotein particles)
- Chylomicrons enter the lacteal (a lymph capillary in each villus) — NOT the blood capillaries
- Lymph travels: lacteals → lymphatic vessels → thoracic duct → subclavian vein → blood
- Fats bypass the liver initially (enter blood circulation directly)
Fat-soluble vitamins (A, D, E, K): Absorbed with fat via lacteals Water-soluble vitamins (C, B group): Absorbed with water into blood capillaries Water and electrolytes: Absorbed throughout the small intestine; more in large intestine
💡 Expert Tip
Remember: fats go via lymph (lacteals → thoracic duct → subclavian vein). Glucose and amino acids go directly into blood (portal vein → liver). This is why people with liver disease may have normal fat digestion but abnormal blood glucose levels — glucose goes through the liver first, fat doesn't.
Why This Works — The Logical Order
The digestion cascade in the small intestine is highly organised:
- Acid neutralised first (bicarbonate) — otherwise no intestinal enzyme can work at pH 2
- Fats emulsified (bile salts) — increases lipase efficiency enormously
- Zymogens activated in the right place (enterokinase triggers the protease cascade only in the duodenum, not in the pancreas) — prevents self-digestion
- Final digestion at the brush border (maltase, lactase, etc.) — ensures products are ready for immediate absorption
Each step is sequentially triggered — a beautifully coordinated biochemical production line.
Common Mistake
⚠️ Common Mistake
Mistake: Writing that "bile digests fats."
Correct: Bile does NOT digest fats. Bile emulsifies fats (breaks large fat globules into tiny droplets — a physical/mechanical process). The actual chemical digestion of fats is done by lipase (pancreatic lipase and intestinal lipase). Emulsification only prepares fats for lipase to act more efficiently.
Think of it this way: bile is like a detergent (breaks up oil into drops); lipase is the actual chemical that breaks the fat molecule's bonds. Both are needed, but they do different jobs.
Second common mistake: Saying fat is absorbed into blood capillaries directly. Fats are absorbed into lacteals (lymph capillaries), not blood capillaries. They travel via lymph to enter blood at the subclavian vein, completely bypassing the liver in the first pass.