Digestion and AbsorptionNEET Zoology · Class 11 · NCERT Chapter 16

Introduction

Your body cannot use the food you eat in its original form. Large molecules like starch, proteins and fats must first be broken down into smaller, soluble molecules that can cross the gut wall and enter the blood. This process is called digestion.

Food has six components: carbohydrates, proteins, fats, vitamins, minerals and water. Of these, carbohydrates, proteins and fats are macromolecules that need digestion. Vitamins, minerals and water are already small enough to be absorbed directly.

Expect 1 to 2 NEET questions from this chapter every year. The most reliable scoring areas are: the dental formula, digestive enzymes at each site, the role of bile, how fats are absorbed via lacteals, and the two protein energy malnutrition disorders.

Human Alimentary Canal

The alimentary canal is a long muscular tube that runs from the mouth to the anus. It is about 8 to 10 metres long in an adult. Food moves through it by a process called peristalsis (rhythmic muscular contractions). Different regions are specialised for different functions.

Mouth and Buccal Cavity

The mouth leads into the buccal cavity. Digestion begins here. The buccal cavity contains the teeth, tongue and openings of three pairs of salivary glands.

Teeth: Human teeth are described by three terms.

• Thecodont: teeth are set in sockets (alveoli) in the jawbone.
• Diphyodont: two sets of teeth appear during a lifetime. The first set (milk or deciduous teeth, 20 in total) is replaced by permanent teeth (32 in total).
• Heterodont: four different kinds of teeth serve different purposes: incisors (cutting), canines (tearing), premolars (grinding) and molars (grinding).

The dental formula is written as the number of each type of tooth in one half of one jaw (upper/lower). Human dental formula: 2123/2123 (incisors/canine/premolars/molars).

Tongue: a muscular organ covered with taste buds. It helps in mixing food with saliva, swallowing (deglutition) and taste. The tongue has small projections called papillae.

Pharynx: the common passage for food and air. The epiglottis (a flap of cartilage) closes over the larynx during swallowing, preventing food from entering the airway.

Oesophagus and Stomach

Oesophagus: a muscular tube about 25 cm long connecting the pharynx to the stomach. It passes through the diaphragm. Its only job is to carry food to the stomach by peristalsis. No digestion happens here.

Stomach: a J-shaped muscular organ on the left side of the abdomen. It has four regions:

• Cardiac region: near the entry of the oesophagus (cardiac sphincter controls entry).
• Fundic region: dome-shaped upper part, often contains swallowed air.
• Body (corpus): the main central region. Contains most of the gastric glands.
• Pyloric region: narrows into the pyloric canal. The pyloric sphincter controls the release of chyme into the duodenum.

Gastric glands in the body and fundus secrete gastric juice, which contains pepsinogen, HCl, rennin (in infants) and mucus.

Small Intestine

The small intestine is about 6 to 7 metres long and has three parts:

• Duodenum: the first short C-shaped part (about 25 cm). Receives pancreatic juice from the pancreas and bile from the liver via the common bile duct. Most chemical digestion happens here.
• Jejunum: the next 2 to 2.5 metres. Digestion continues and absorption begins.
• Ileum: the last and longest part (about 3.5 metres). The main site of absorption. Has villi, microvilli (brush border) and circular folds (plicae circulares) that hugely increase absorptive surface area.

Large Intestine

The large intestine is about 1.5 metres long and has three parts:

• Caecum: a small pouch at the junction with the small intestine. The vermiform appendix hangs from the caecum. It is a vestigial organ in humans.
• Colon: the main tube of the large intestine. Absorbs water, salts and some vitamins (like vitamin K produced by gut bacteria). Converts liquid waste into semi-solid faeces.
• Rectum: stores faeces until defecation. Opens to the outside via the anus.

Digestive Glands

Digestive glands produce secretions that carry enzymes and other chemicals into the alimentary canal.

• Salivary glands (3 pairs): parotid (near the ear, largest), submandibular (below the jaw) and sublingual (under the tongue). They secrete saliva containing salivary amylase (ptyalin) and mucus. Saliva keeps the mouth moist, binds food into a bolus and starts starch digestion.
• Liver: the largest gland in the body. It produces bile(about 600 to 1000 mL per day). Bile does not contain enzymes. It contains bile salts that emulsify fats (break large fat droplets into tiny ones), which increases the surface area for lipase to act. Bile also contains bile pigments (bilirubin, biliverdin) from broken red blood cells, cholesterol and water.
• Gall bladder: a small sac under the liver. Stores and concentrates bile between meals. Releases bile into the duodenum (via the bile duct) when fatty food arrives.
• Pancreas (mixed gland): lies behind the stomach. Its exocrine part (acinar cells) secretes pancreatic juice via the pancreatic duct into the duodenum. Its endocrine part (islets of Langerhans) secretes insulin (B cells) and glucagon (A cells) directly into the blood. Because it has both exocrine and endocrine functions, it is called a mixed gland.

Digestion of Food

Digestion is a stepwise process. Each section of the gut adds its own enzymes and chemicals, breaking macromolecules down to absorbable units.

Digestion in the Mouth

Chewing (mastication) breaks food into smaller pieces, increasing surface area. Saliva moistens the food and binds it into a soft mass called a bolus.

• Salivary amylase (ptyalin) acts at slightly alkaline pH and breaks starch (amylose) into maltose and dextrins. About 30% of starch is digested in the mouth. Salivary amylase is inactivated once food reaches the acidic stomach.
• Mucus in saliva lubricates the bolus and protects the mucosa.
• Proteins and fats undergo no chemical digestion in the mouth.

Digestion in the Stomach

In the stomach, the bolus is churned by muscular contractions with gastric juice, producing a semi-liquid mixture called chyme. The stomach wall secretes:

• HCl (from parietal/oxyntic cells): creates a strongly acidic environment (pH about 1.8 to 2). Kills bacteria in food. Converts inactive pepsinogen to active pepsin. HCl also inactivates salivary amylase.
• Pepsin (from chief/peptic cells as pepsinogen): breaks proteins into shorter polypeptides (proteoses and peptones). It is a protease that works at low pH.
• Rennin (from gastric glands in infants): coagulates milk protein casein so that it stays longer in the stomach for pepsin to act on it.
• Mucus (from goblet cells): coats the stomach lining and protects it from being digested by the pepsin and HCl.

The stomach does not digest fats or carbohydrates significantly. It mainly starts protein digestion.

Digestion in the Small Intestine

Most digestion is completed in the duodenum, using secretions from the pancreas, liver and the intestinal wall itself.

Pancreatic juice (alkaline, pH about 8) contains:

• Trypsinogen and chymotrypsinogen: inactive proteases activated by enterokinase (and by trypsin itself) in the duodenum. Active trypsin and chymotrypsin break proteins and polypeptides into shorter peptides.
• Carboxypeptidase: breaks peptides from the carboxyl (C-terminal) end, releasing amino acids.
• Pancreatic amylase: breaks any remaining starch into maltose.
• Pancreatic lipase: breaks emulsified triglycerides into fatty acids and glycerol. This enzyme needs bile salts to work well.
• Nucleases (DNase, RNase): break DNA and RNA into nucleotides.

Bile from the gall bladder emulsifies fats: it breaks fat globules into tiny droplets, dramatically increasing the surface area for lipase.

Succus entericus (intestinal juice) is secreted by the lining of the small intestine and contains:

• Maltase, lactase, sucrase: break disaccharides into monosaccharides. Maltase: maltose to glucose + glucose. Lactase: lactose to glucose + galactose. Sucrase: sucrose to glucose + fructose.
• Peptidases (dipeptidases, aminopeptidase): break dipeptides and tripeptides into individual amino acids.
• Intestinal lipase: further breaks fats.
• Nucleotidases and nucleosidases: break nucleotides to bases, sugars and phosphates.

At the end of digestion in the small intestine: carbohydrates are broken down to monosaccharides (glucose, fructose, galactose); proteins to amino acids; fats to fatty acids and glycerol; nucleic acids to nucleotides and their components.

Absorption of Digested Products

Absorption means moving the digested molecules from inside the gut lumen into the blood or lymph. Almost all absorption happens in the small intestine.

The inner surface of the small intestine has three levels of folds that increase surface area enormously:

• Plicae circulares (circular folds): semi-permanent folds of the mucosa.
• Villi: finger-like projections of the mucosa (0.5 to 1 mm tall). Each villus contains a blood capillary network and a central lymph capillary called a lacteal.
• Microvilli (brush border): tiny projections on each villus epithelial cell. Enormously increase surface area; also contain many of the final digestive enzymes.

Absorption occurs by three mechanisms:

• Active transport: against concentration gradient, needs energy (ATP) and carrier proteins. Used for glucose, amino acids, Na+.
• Facilitated diffusion: down concentration gradient, needs carrier proteins but no energy. Used for fructose.
• Passive diffusion: down concentration gradient, no carrier proteins or energy needed. Used for water, small lipid-soluble molecules.

After absorption, glucose, amino acids and water-soluble vitamins travel directly into blood capillaries inside the villi. They then move to the liver via the hepatic portal vein.

Fat Absorption and Chylomicrons

Fatty acids and glycerol are lipid-soluble and can diffuse into epithelial cells. Inside the cells, they are reassembled into triglycerides. These triglycerides are then packaged with cholesterol, phospholipids and proteins into spherical lipoprotein particles called chylomicrons.

Chylomicrons are large particles and cannot enter the narrow blood capillaries. Instead, they enter the lacteals (the lymph capillaries inside each villus). The lacteals drain into larger lymph vessels, then into the thoracic duct, which empties into the bloodstream at the left subclavian vein. In this way, absorbed fats reach the blood without first passing through the liver.

Fat-soluble vitamins (A, D, E, K) are also packaged in chylomicrons and absorbed via this lymphatic route.

Assimilation

After absorption, nutrients are distributed to cells throughout the body via the blood and lymph. Assimilation is the process by which cells take up these nutrients and use them for energy (cellular respiration), growth and repair, or storage:

• Glucose is oxidised in cells for energy (cellular respiration). Excess glucose is stored as glycogen in the liver and muscles, or converted to fat.
• Amino acids are used to build proteins. Excess amino acids cannot be stored; the amino group is removed (deamination in the liver) and the carbon skeleton is used for energy or fat synthesis.
• Fatty acids are used for energy, cell membrane construction and hormone synthesis. Excess is stored as adipose (fat) tissue.

Digestive Disorders

Common disorders affecting the digestive system:

• Jaundice: yellow discolouration of skin and eyes caused by excess bilirubin in blood. The liver is not clearing bilirubin properly (due to hepatitis, cirrhosis, bile duct obstruction or haemolysis).
• Vomiting: forcible expulsion of stomach contents through the mouth, a reflex coordinated by the vomiting centre in the medulla. Caused by irritants, toxins, motion sickness or many other triggers.
• Diarrhoea: abnormally frequent, loose or watery stools. Caused by infections (viral, bacterial, parasitic), food intolerance or inflammation. Results in loss of water and electrolytes; severe diarrhoea can cause dehydration.
• Constipation: infrequent, hard stools that are difficult to pass. Usually caused by a low-fibre diet, inadequate water intake or reduced physical activity. Too much water is absorbed in the colon, making the stool hard.
• Indigestion (dyspepsia): feeling of discomfort or fullness in the upper abdomen after eating. Common causes: eating too fast, fatty or spicy food, stress or excess acid.

Protein Energy Malnutrition

Protein energy malnutrition (PEM) is a serious nutritional condition, most common in children in developing countries.

• Marasmus: caused by severe deficiency of both protein and calories. The body uses up all its fat stores and then breaks down muscle. Key signs: extreme wasting, loose wrinkled skin, very low body weight (below 60% of expected weight), muscle wasting and a wizened "old person" appearance. No oedema.
• Kwashiorkor: caused by severe protein deficiency even when calorie intake is adequate (often a diet of carbohydrate-rich but protein-poor food). Key signs: oedema (swelling of limbs and abdomen, pot belly), depigmented or flaky skin, thin reddish or discoloured hair, anaemia and poor growth. The oedema occurs because low plasma proteins reduce oncotic pressure, causing fluid to leak into tissues.

Worked NEET Problems

Summary Cheat Sheet

• Six components of food: carbohydrates, proteins, fats, vitamins, minerals, water. First three need digestion.
• Dental formula: 2123/2123. 32 permanent teeth. Thecodont (in sockets), diphyodont (two sets), heterodont (different types).
• Salivary glands: 3 pairs (parotid, submandibular, sublingual). Salivary amylase breaks starch to maltose.
• Stomach regions: cardiac, fundic, body, pyloric. Gastric juice: HCl + pepsinogen (to pepsin) + rennin (infants) + mucus.
• Small intestine: duodenum (25 cm, main digestion site), jejunum, ileum (main absorption site).
• Large intestine: caecum (vermiform appendix), colon (absorbs water), rectum.
• Liver: produces bile (emulsifies fats). Bile stored in gall bladder. Bile has NO enzymes.
• Pancreas (mixed gland): exocrine = pancreatic juice (trypsin, chymotrypsin, carboxypeptidase, amylase, lipase, nucleases); endocrine = insulin, glucagon.
• Enterokinase: activates trypsinogen to trypsin. Secreted by intestinal mucosa.
• Succus entericus: intestinal juice. Contains maltase, lactase, sucrase, peptidases, lipase, nucleotidases.
• End products: carbohydrates to monosaccharides; proteins to amino acids; fats to fatty acids + glycerol; nucleic acids to nucleotides.
• Absorption site: mainly ileum. Glucose/amino acids to blood capillaries (then to hepatic portal vein). Fats to lacteals as chylomicrons (then to thoracic duct).
• Absorption mechanisms: active transport (glucose, amino acids), facilitated diffusion (fructose), passive diffusion (water, lipid-soluble molecules).
• Chylomicrons: lipoprotein particles carrying fats from intestinal cells into lacteals.
• Jaundice: excess bilirubin, yellow skin and eyes.
• Diarrhoea: watery stools, loss of water and electrolytes.
• Constipation: hard infrequent stools; too much water absorbed in colon.
• Marasmus: deficiency of both protein AND calories; extreme wasting, no oedema.
• Kwashiorkor: protein deficiency with adequate calories; oedema, pot belly, discoloured hair.

Next: use the interactive learning widgets to explore the alimentary canal, trace digestion of any food type step by step, and explore digestive disorders, or work through the 14+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test. You can also compare this with how the excretory system works in Excretory Products and Their Elimination or study the chemistry behind digestion in Biomolecules.