Digestion is the breakdown of food into molecules small enough to cross into blood. The chapter covers anatomy of the gut, the enzymes at each stop, and the hormones that coordinate secretion. CBSE Class 11 and NEET test it heavily — expect two questions a year on enzymes alone.
The human digestive system is essentially a 9-metre tube that turns a complex meal into glucose, amino acids and fatty acids. Each section of the tube has a specific job, specific secretions, and specific pH. If you can trace a bite of food from mouth to absorption and name the enzyme acting at each stage, you own this chapter.
Core Concepts
Alimentary canal layout
Mouth → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (caecum, colon, rectum) → anus. Total length about 8 to 9 metres. The small intestine does most digestion and almost all absorption.
Mouth and oesophagus
Mechanical breakdown by teeth — incisors cut, canines tear, premolars and molars grind. The tongue mixes food with saliva and forms a bolus.
Saliva from three pairs of salivary glands (parotid, submandibular, sublingual):
- Salivary amylase (ptyalin) — starts carbohydrate digestion: starch → maltose. Works at pH 6.8. Inactivated in the stomach acid.
- Lysozyme — antibacterial enzyme
- Mucin — lubricates the bolus for swallowing
The bolus is swallowed and pushed down the oesophagus by peristalsis — waves of muscular contraction. The cardiac sphincter at the junction with the stomach prevents reflux.
Stomach
A J-shaped muscular bag with four regions (cardia, fundus, body, pylorus). Secretes about 2-3 litres of gastric juice per day.
| Secretion | Source | Function |
|---|---|---|
| HCl | Parietal (oxyntic) cells | Kills microbes, activates pepsinogen, provides acidic pH (1.5-2.5) |
| Pepsinogen | Chief (peptic) cells | Activated by HCl to pepsin, digests proteins to peptides |
| Mucus | Mucous neck cells | Protects stomach lining from acid and pepsin |
| Intrinsic factor | Parietal cells | Required for vitamin B12 absorption in ileum |
The stomach churns food into a semi-liquid called chyme, which is released into the duodenum through the pyloric sphincter in controlled portions.
For NEET, remember that the stomach digests proteins (pepsin) but NOT carbohydrates or fats. Salivary amylase is inactivated by stomach acid. Fat digestion begins only in the duodenum with bile and pancreatic lipase.
Small intestine — the main site
Three regions: duodenum (25 cm, receives bile and pancreatic juice), jejunum (middle, most absorption), ileum (longest, absorbs B12 and bile salts).
Pancreatic juice (from pancreas, enters duodenum via pancreatic duct):
- Pancreatic amylase — starch → maltose
- Trypsinogen → activated to trypsin by enterokinase → proteins → peptides
- Chymotrypsin — proteins → peptides
- Pancreatic lipase — fats → fatty acids + glycerol (after bile emulsification)
- Nucleases — DNA/RNA → nucleotides
- Bicarbonate — neutralises stomach acid, raises pH to ~7.5-8
Bile (from liver, stored in gallbladder):
- No digestive enzymes
- Bile salts emulsify fats — break large fat droplets into tiny micelles, increasing surface area for lipase
- Bilirubin — waste pigment from haemoglobin breakdown, gives faeces brown colour
Brush border enzymes (from the intestinal mucosa):
- Maltase — maltose → glucose + glucose
- Sucrase — sucrose → glucose + fructose
- Lactase — lactose → glucose + galactose
- Peptidases — dipeptides → amino acids
- Nucleotidases — nucleotides → nucleosides → bases + sugars + phosphates
Absorption
Occurs primarily in the jejunum and ileum through villi and microvilli (brush border), which increase surface area to about 200 m.
| Nutrient | Absorption mechanism | Absorbed as |
|---|---|---|
| Monosaccharides | Active transport (glucose, galactose via SGLT1) or facilitated diffusion (fructose via GLUT5) | Glucose, fructose, galactose |
| Amino acids | Active transport (Na-dependent co-transport) | Individual amino acids |
| Fatty acids | Diffusion into enterocytes → re-esterified to triglycerides → packaged in chylomicrons → enter lacteals (lymph) | Chylomicrons |
| Water | Osmosis | HO |
| Vitamins | Fat-soluble (A, D, E, K) with fats; water-soluble (B, C) by specific transporters | Various |
| Iron | Active transport in duodenum | Fe |
| B12 | Binds intrinsic factor, absorbed in ileum by receptor-mediated endocytosis | B12-IF complex |
Large intestine
Absorbs water and electrolytes. Hosts gut flora (~100 trillion bacteria) that produce vitamin K and some B vitamins, ferment undigested fibre to short-chain fatty acids, and prevent pathogen colonisation.
Caecum is a small pouch at the junction with the small intestine; the appendix is a vestigial structure attached to it. Colon (ascending, transverse, descending, sigmoid) absorbs water and compacts faeces. Rectum stores faeces until defecation.
Hormonal control of digestion
| Hormone | Source | Stimulus | Action |
|---|---|---|---|
| Gastrin | G cells (stomach) | Food in stomach | Stimulates HCl and pepsinogen secretion |
| Secretin | S cells (duodenum) | Acidic chyme in duodenum | Stimulates bicarbonate from pancreas |
| CCK (Cholecystokinin) | I cells (duodenum) | Fats and peptides | Stimulates bile release from gallbladder, enzyme release from pancreas |
| GIP (Gastric Inhibitory Peptide) | K cells (duodenum) | Glucose and fats | Inhibits gastric acid, stimulates insulin |
Worked Examples
Pepsinogen is an inactive zymogen. HCl in the stomach converts it to active pepsin by cleaving a small peptide fragment. Pepsin itself works best at pH 1.5-2.5 — its active site is optimally shaped at this pH. At neutral pH (small intestine), pepsin is irreversibly denatured. The acid environment is not for ‘burning food’ but for enzyme activation and killing ingested bacteria.
Adults with low lactase expression cannot break down lactose (milk sugar) in the small intestine. Undigested lactose passes to the colon, where bacteria ferment it, producing gas (CO, H, CH) and drawing water by osmosis — causing bloating, cramps and diarrhoea. Many Indian adults tolerate curd (where lactose has already been fermented by bacteria to lactic acid) better than fresh milk.
Fats enter the duodenum as large globules. Bile salts emulsify them into tiny micelles (increasing surface area). Pancreatic lipase then cleaves triglycerides into fatty acids and monoglycerides at the water-lipid interface. These products diffuse into enterocytes, are re-assembled into triglycerides, packaged with proteins into chylomicrons, and released into lacteals (lymph vessels) rather than blood capillaries. Chylomicrons eventually enter the bloodstream via the thoracic duct.
The exocrine part (acinar cells) secretes digestive enzymes into the duodenum via the pancreatic duct. The endocrine part (Islets of Langerhans) secretes insulin and glucagon directly into the blood. Same organ, two completely different functions, two different delivery systems.
Dietary iron comes in two forms: haem iron (from meat, well-absorbed) and non-haem iron (from plants, poorly absorbed). Non-haem iron must be reduced from Fe to Fe by duodenal cytochrome b (DcytB) and vitamin C before it can be absorbed via the DMT1 transporter. This is why eating citrus fruits with iron-rich foods improves absorption.
Common Mistakes
Saying bile contains digestive enzymes. It does not — bile salts emulsify fats so lipases can act, but bile itself has no enzymes. It is a detergent, not an enzyme.
Writing that the stomach digests most of the food. The small intestine does most digestion (all three macronutrients) and almost all absorption. The stomach only starts protein digestion.
Confusing trypsin and pepsin. Pepsin works in the stomach (acidic pH). Trypsin works in the small intestine (alkaline pH, from pancreas). Both digest proteins but in different environments.
Calling the pancreas only an endocrine gland. It is both endocrine (islets → insulin, glucagon) and exocrine (acini → digestive enzymes). This distinction is tested almost every year.
Forgetting that fat absorption goes via lymph (lacteals), not directly into blood capillaries. Amino acids and monosaccharides enter blood capillaries in villi, but fats take the lymphatic route as chylomicrons.
Exam Weightage and Strategy
Digestion and Absorption carries 5-7 marks in CBSE Class 11 boards. NEET asks 2-3 questions per year — the most common being on enzyme-substrate matching, hormonal control, and absorption mechanisms. This is one of the most table-friendly chapters in biology.
Build a master table — organ, secretion, enzyme, substrate, product — covering mouth, stomach, duodenum (pancreatic juice, bile, brush border enzymes), and large intestine. That single table is your entire revision for this chapter and answers most PYQs directly.
Practice Questions
Q1. Trace the digestion of a protein molecule from ingestion to absorption.
Mouth: no protein digestion. Stomach: HCl denatures protein, pepsinogen → pepsin cleaves proteins into large peptides. Duodenum: trypsin and chymotrypsin (from pancreas) break peptides into smaller peptides. Brush border peptidases convert dipeptides and tripeptides into individual amino acids. Amino acids are absorbed by Na-dependent active transport into blood capillaries in villi, then travel via the hepatic portal vein to the liver.
Q2. What is the role of enterokinase?
Enterokinase (enteropeptidase) is a brush border enzyme in the duodenum that converts inactive trypsinogen (from pancreas) into active trypsin. Trypsin then activates other zymogens: chymotrypsinogen → chymotrypsin, procarboxypeptidase → carboxypeptidase. This cascade ensures that pancreatic enzymes are only activated inside the duodenum, not inside the pancreas (where they would cause self-digestion — pancreatitis).
Q3. Why does the stomach not digest itself?
Three protective mechanisms: (1) Mucus — mucous neck cells secrete a thick alkaline mucus layer that coats the stomach lining, creating a pH gradient (pH 1-2 in lumen, pH 7 at epithelial surface). (2) Bicarbonate — trapped in the mucus layer, neutralises acid near the surface. (3) Rapid cell turnover — stomach epithelial cells are replaced every 3-6 days. When these defences fail (H. pylori infection, NSAIDs), peptic ulcers result.
Q4. Distinguish between peristalsis and segmentation.
Peristalsis: wave-like contraction behind the food bolus and relaxation ahead of it, pushing food forward in one direction. Occurs throughout the GI tract. Segmentation: rhythmic contraction and relaxation of non-adjacent segments of the small intestine, mixing chyme with digestive juices and increasing contact with the absorptive surface. Segmentation does not propel food forward — it mixes and churns.
FAQs
Why is the small intestine so long?
To maximise absorption. Digestion products need time and surface area to be absorbed. The 6-metre length, combined with circular folds, villi, and microvilli, creates a surface area of about 200 m — roughly the size of a tennis court. This enormous surface ensures that virtually all nutrients are absorbed before food reaches the large intestine.
What happens if the gallbladder is removed?
Bile is still produced by the liver and flows directly into the duodenum through the common bile duct. However, without the gallbladder to concentrate and store bile, the person may have difficulty digesting large fatty meals. Small, frequent meals and low-fat diet help compensate.
What is the role of gut bacteria?
The gut microbiome (100 trillion bacteria) ferments dietary fibre into short-chain fatty acids (energy source for colon cells), synthesises vitamins K and B12, trains the immune system, prevents colonisation by pathogens, and even influences mood via the gut-brain axis. A healthy microbiome is increasingly recognised as essential for overall health.
Digestion is a relay race — each stop does a specific job and hands off to the next. Know the specialty of each stop and you know the chapter.