Dyspepsia means imperfect or difficult digestion. It causes symptoms like upper abdominal discomfort, bloating, nausea, and heartburn. It can result from overeating, stress, enzyme deficiency, or gastroparesis.

The ileocaecal (ileocolic) sphincter/valve is located at the junction between the ileum (end of small intestine) and caecum (beginning of large intestine). It prevents backflow of faecal material into the small intestine.

The cardiac sphincter (gastro-oesophageal sphincter/lower oesophageal sphincter) is at the junction between the oesophagus and stomach. It prevents reflux of stomach acid into the oesophagus. Pyloric sphincter is at stomach-duodenum junction.

Constipation results when too much water is absorbed from the colon, making faeces hard, dry, and difficult to pass. Causes include low fibre diet, insufficient water intake, lack of exercise, and certain medications.

Amylases (salivary and pancreatic) digest starch and glycogen into maltose and dextrins by hydrolysing alpha-1,4 glycosidic bonds. Maltose is then further hydrolysed to glucose by maltase.

Trypsin (secreted as trypsinogen by the pancreas and activated by enterokinase) splits proteins into smaller peptides (peptones and polypeptides). It is an endopeptidase that cuts within the protein chain.

Egestion (defecation) is the elimination of undigested and indigestible material from the body as faeces through the anus. It is the final step of holozoic nutrition, different from excretion (removal of metabolic wastes).

Nematocysts are specialised stinging organelles within cnidocytes (stinging cells) on Hydra's tentacles. They can discharge a venomous thread coil to paralyse or capture prey organisms.

The gastrovascular cavity of Hydra is also called the coelenteron — a blind-ended sac with one opening (mouth/anus). Extracellular digestion begins here and intracellular digestion is completed by gastrodermal cells.

Ingestion is the taking in of food into the body through the mouth. It is the first step of holozoic nutrition, followed by digestion, absorption, assimilation, and egestion.

The cockroach digestive system is divided into three parts: foregut (stomodaeum — includes mouth, pharynx, oesophagus, crop, gizzard), midgut (mesenteron — stomach), and hindgut (proctodaeum — intestine, rectum, anus).

Amoeba uses pseudopodia (false feet — temporary cytoplasmic projections) to surround and engulf food particles by phagocytosis. The food is enclosed in a food vacuole where intracellular digestion occurs.

The human large intestine consists of: caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. Together these sections form the approximately 1.5 m long large intestine.

Entamoeba histolytica lives in the large intestine (colon) of humans, causing amoebic dysentery. It can invade the intestinal wall and spread to the liver (causing amoebic liver abscess) in severe cases.

The foregut of a cockroach consists of: mouth, pharynx, oesophagus, crop (food storage), and gizzard (grinding). The midgut (stomach) and hindgut (intestine, rectum, anus) are separate regions.

Ticks and mites (Arachnida) are common ectoparasites of non-human mammals, feeding on blood. Aphids parasitise plants. Fleas and lice can parasitise both humans and non-human mammals.

Animals that feed on detritus (dead and decomposing organic matter) are called detritivores. Examples include earthworms, woodlice, millipedes, some beetles, and many crabs. They play a key role in decomposition.

Mosquitoes are fluid feeders — they pierce skin and suck blood using their proboscis. Other fluid feeders include aphids (plant sap), butterflies (nectar), leeches (blood), and hummingbirds (nectar).

Cytochromes contain iron (Fe) in their porphyrin haem groups. Iron alternates between Fe²⁺ and Fe³⁺ (reduced and oxidised states) as it accepts and donates electrons in the electron transport chain.

Mycorrhizae is the mutualistic association between fungi and plant roots. The fungus (mycorrhiza) increases plant nutrient and water absorption (especially phosphorus) while the plant provides carbohydrates to the fungus.

Earthworms are the classic example of detritivores — they ingest soil containing decomposing organic matter, extract nutrients, and excrete casts that enrich soil. They are vital for soil health and nutrient recycling.

All three macronutrients provide energy: carbohydrates (4 kcal/g) are the primary fuel, fats (9 kcal/g) are the most energy-dense, and amino acids (4 kcal/g) can be used when carbohydrates and fats are insufficient.

Nutrition refers to the intake and utilisation of food substances that supply energy and raw materials for metabolism. It encompasses all processes by which organisms obtain and use nutrients.

Dyspepsia (indigestion) is discomfort in the upper abdomen caused by excessive acidity, poor digestion, or gastric dysfunction. Symptoms include bloating, nausea, heartburn, and upper abdominal pain after eating.

A peptic ulcer is a sore (erosion) in the lining of the stomach or duodenum, caused by Helicobacter pylori infection or NSAIDs. Without mucus protection, HCl and pepsin digest the gut wall itself.

Botulism is a severe, potentially fatal food poisoning caused by the neurotoxin of Clostridium botulinum bacteria in improperly canned/preserved foods. The toxin blocks neuromuscular transmission causing paralysis.

Diarrhoea results when water is not properly absorbed in the large intestine. Infection (e.g., cholera, Salmonella), drugs, or stress can accelerate gut motility so that water cannot be absorbed, producing watery stools.

The large intestine (colon) consists of: caecum (with appendix), ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. Its main functions are water absorption and faeces formation.

Maltase is a brush-border enzyme that hydrolyses maltose (a disaccharide) into two glucose molecules. Glucose and galactose come from lactase acting on lactose; sucrose is hydrolysed by sucrase.

The jejunum is the middle section (second portion) of the small intestine, between the duodenum and the ileum. It is about 2.5 metres long and is the primary site of carbohydrate and protein absorption.

The liver secretes bile — a digestive fluid containing bile salts, bilirubin, cholesterol, and bicarbonate. Pancreatic juice comes from the pancreas, insulin from the pancreatic islets, secretin from the duodenum.

Villi are most numerous and well-developed in the ileum — the last and longest section of the small intestine, the primary site of nutrient absorption. The ileum has the most extensive villous surface.

The ileum is the primary site of nutrient absorption in the small intestine, having the longest length, most villi, and microvilli. It also specifically absorbs vitamin B12, bile salts, and fat-soluble vitamins.

Erypsin (dipeptidase/peptidase) is a brush-border enzyme that acts on dipeptides, hydrolysing the remaining peptide bonds to release individual amino acids — the final products of protein digestion.

Both hepatitis (viral liver inflammation) and chronic alcoholism damage the liver, causing inflammation, fatty liver, cirrhosis (scarring), and ultimately liver failure if untreated. The liver has limited ability to regenerate.

Gallstones form when bile components (mainly cholesterol, also bile pigments and calcium salts) crystallise in the gallbladder. They can obstruct the bile duct, causing severe pain, jaundice, and infection.

Bile is produced continuously by the liver and stored and concentrated in the gallbladder between meals. When fatty food enters the duodenum, CCK stimulates gallbladder contraction to release bile.

Chyme is the semi-liquid, acidic mixture of partially digested food and gastric juice produced by stomach churning. A bolus is the swallowed ball of food before it enters the stomach.

Oxyntic cells (parietal cells) in the gastric glands secrete hydrochloric acid (HCl) and intrinsic factor. Zymogen cells (chief cells) secrete pepsinogen. Mucous cells secrete protective mucus.

Peristalsis involves coordinated contraction of both circular muscles (constrict the lumen ahead of food) and longitudinal muscles (shorten the gut behind food). This produces the wave-like movement propelling food forward.

Pancreatic amylase is also called amylopsin. It hydrolyses starch into maltose and dextrins in the duodenum, completing the starch digestion begun by salivary amylase.

The pancreas is a mixed exocrine-endocrine gland. Its exocrine portion (acini) secretes pancreatic juice (digestive enzymes + bicarbonate) through the pancreatic duct into the duodenum. Its endocrine portion (islets) secretes hormones.

Gastrin is the hormone that stimulates gastric gland secretion. It is produced by G-cells in the stomach lining in response to the presence of food. Gastrin stimulates parietal cells to produce HCl and chief cells to produce pepsinogen.

The oral cavity in man is bounded by: palate (roof), cheeks (sides), tongue (floor), and lips (anterior opening). Together these structures contain and manipulate food during mechanical digestion.

The epiglottis is the leaf-shaped cartilage flap at the top of the trachea (windpipe). During swallowing, it folds down to cover the glottis (entrance to trachea), preventing food from entering the airways.

Swallowing has two phases: the initial phase (pushing food to the back of the mouth with the tongue) is voluntary. Once the food reaches the pharynx, swallowing becomes involuntary/reflex (controlled by the swallowing centre in the medulla).

Food selection uses all senses: sight (visual assessment), smell (olfaction detects food quality and safety), and taste (taste receptors on tongue detect sweet, sour, salty, bitter, umami). All three senses guide food selection.

Digestion in humans occurs at multiple sites: oral cavity (starch digestion by salivary amylase, mechanical breakdown), stomach (protein digestion by pepsin), and small intestine (all macromolecules digested completely).

Both lice (Pediculus) and fleas (Siphonaptera) are common ectoparasites of non-human mammals. Lice are host-specific and stay on the animal; fleas can jump between different mammalian hosts.

In the cockroach, the midgut is the stomach — a short narrow tube where most digestion and absorption occurs. The foregut includes the crop and gizzard; the hindgut includes ileum and rectum.

Planaria performs both extracellular digestion (enzymes secreted into the gut cavity by gland cells) and intracellular digestion (cells engulf partially digested particles for final digestion in food vacuoles).

The tubular (tube-within-a-tube) digestive system of higher animals is most efficient because it has a one-way flow, allows specialisation of different regions, and processes food continuously.

Fleas are ectoparasites (external parasites) that live in the fur of mammals and feathers of birds, feeding on their blood. Leeches are also ectoparasites but live on skin, not in fur/feathers.

Earthworms are classic detritivores — they consume dead and decaying organic matter in soil. Rodents are omnivores, crows are omnivores/scavengers, and baleen whales are filter feeders.

Detritus is the collective term for dead particulate organic matter from decomposing plants and animals. Organisms that feed on detritus are called detritivores. Detritus is a key component of many food webs.

Macrophagous feeders consume food in large pieces relative to their body size. Examples include lions, snakes, sharks, and eagles. Microphagous feeders consume many small food particles (e.g., filter feeders).

Cats, dogs, lions, and tigers are carnivorous macrophagous feeders — they consume large chunks of prey. The term 'filter feeders' applies to animals like baleen whales and flamingos that filter small particles from water.

Herbivores (cows, sheep, rabbits) lack canine teeth or have reduced canines because they do not tear flesh. They have well-developed incisors (for cropping plants) and molars/premolars (for grinding plant material).

Animals are classified by diet as herbivores (plants), carnivores (animals), omnivores (both), detritivores (dead organic matter), and filter feeders. All these groups represent different nutritional strategies.

Dodder (Cuscuta) is a parasitic plant that penetrates the host plant's vascular tissue with haustoria and extracts water, minerals, and organic nutrients. It is an obligate parasite with no chlorophyll.

Venus flytrap (Dionaea muscipula) has bilobed leaves with a midrib hinge between them. When trigger hairs are touched, the lobes snap shut trapping insects. The trap reopens after digestion.

Insectivorous (carnivorous) plants are autotrophic — they perform photosynthesis to make their own food. They supplement their nutrition with insects only for nitrogen and minerals, not for energy.

Cuscuta (dodder) is a leafless, chlorophyll-lacking parasitic plant that twines around host plants and penetrates their vascular tissue with haustoria to steal water and organic nutrients.

Autotrophic organisms (plants, algae, cyanobacteria, chemolithotrophs) can live in purely inorganic environments because they synthesise their own organic food from inorganic raw materials (CO₂, water, minerals, light or chemical energy).

Potassium deficiency can cause premature death of plants. Potassium is essential for stomatal regulation, enzyme activation, protein synthesis, and phloem transport. Its deficiency causes wilting, scorched leaf margins, and death.

Pakistan commonly uses urea (provides nitrogen), superphosphate (provides phosphorus), and ammonium nitrate (provides nitrogen) as chemical fertilisers to boost crop production, especially wheat and rice.

Chlorophyll contains a porphyrin ring with magnesium (Mg²⁺) at its centre, along with carbon, hydrogen, oxygen, and nitrogen. Magnesium is the key metallic element in chlorophyll, essential for light absorption.

Plants require carbon (from CO₂), hydrogen (from H₂O), and oxygen (from both CO₂ and H₂O) as the primary elements for synthesising organic molecules like carbohydrates, proteins, and lipids through photosynthesis.

The stomach lining is protected from HCl primarily by mucin (mucus) secreted by goblet cells. Gastrin stimulates acid production rather than protects against it. Mucin forms a physical barrier over the epithelium.

Protein-rich and fatty foods (like roast chicken) stay longest in the stomach because they require more time for pepsin and gastric acid to work. Fat slows gastric emptying. Carbohydrates pass through quickest.

The primary function of villi is to absorb the products of digestion — monosaccharides and amino acids into blood capillaries, and fatty acids/glycerol into lacteals. Villi greatly increase absorptive surface area.

Enzymes are the most important agents in food digestion. They catalyse the hydrolysis of all food macromolecules (amylases for starch, proteases for proteins, lipases for fats) at body temperature.

Magnesium deficiency causes chlorosis — yellowing of leaves — because magnesium is the central atom of the chlorophyll molecule. Without Mg, chlorophyll cannot be synthesised and leaves turn yellow.

The stomach has the lowest pH in the GI tract — about 1.5-2.5 — due to HCl secretion by parietal cells. This acidic environment activates pepsin, kills microbes, and begins protein denaturation.

Potatoes are primarily a starch (carbohydrate) source — they contain only about 2% protein. Eggs, meat, and milk are all rich protein sources containing all essential amino acids.

Among these organisms, Hydra has a relatively specialised digestive system with a gastrovascular cavity, nematocysts for prey capture, and specialised digestive cells. Earthworm also has a complex tubular gut.

Earthworms are detritivores that feed primarily on decaying organic matter in soil — decomposing plant material, bacteria, fungi, and other microorganisms. They play a vital role in soil nutrient cycling.

Bile is produced by the liver and stored in the gallbladder. It contains bile salts (emulsify fats), bile pigments (bilirubin from haemoglobin breakdown), cholesterol, and bicarbonate.

In frogs, the bile duct and pancreatic duct join to form a common hepatopancreatic duct before opening into the duodenum. In humans, they usually also join but the anatomy varies.

The spleen is a lymphoid organ involved in immune function and red blood cell destruction — it is NOT a digestive gland. The liver and pancreas are the major digestive glands; salivary glands and gastric glands are also digestive glands.

Lacteals are lymphatic capillaries in each intestinal villus that absorb chylomicrons (fat droplets containing fatty acids and glycerol). These enter the lymphatic system and eventually reach the blood via the thoracic duct.

Peristalsis (rhythmic muscular contractions) occurs throughout the alimentary canal. It is most essential in the oesophagus to propel food to the stomach against gravity — this is why you can swallow upside down.

The Islets of Langerhans are clusters of endocrine cells in the pancreas. Alpha cells produce glucagon (raises blood glucose), beta cells produce insulin (lowers blood glucose), and delta cells produce somatostatin.

Bile (from liver/gallbladder) and pancreatic juice (from pancreas) enter the duodenum — the first part of the small intestine — through the hepatopancreatic duct at the ampulla of Vater.

Mammalian teeth are thecodont (teeth set in sockets/alveoli in the jaw bone) and heterodont (different tooth types: incisors, canines, premolars, molars). They are also diphyodont (two sets: milk and permanent).

The liver stores many substances including glycogen, fat-soluble vitamins (A, D, E, K), and minerals like iron and copper. It acts as a metabolic storehouse regulating the supply of nutrients to the blood.

Amoeba ingests food particles by phagocytosis — it extends pseudopodia around the food, enclosing it in a food vacuole. This is the classic example of phagocytic ingestion in unicellular organisms.

Digestion is primarily carried out by enzymes — biological catalysts that accelerate the hydrolysis of food macromolecules. Acid (HCl) activates pepsin and kills bacteria but does not itself digest food.

Mucin (mucus glycoprotein) in saliva lubricates food, forming a slippery bolus that slides easily down the oesophagus. Ptyalin (salivary amylase) digests starch; water moistens food.

The liver is the main detoxification organ. It converts toxic substances (drugs, alcohol, ammonia, bilirubin) into less toxic or water-soluble forms that can be excreted by the kidneys.

Heterodont dentition (different types of teeth — incisors, canines, premolars, molars) is found in most mammals including rabbits, bats, cats, and humans. Homodont animals have teeth all of the same type.

Oxyntic cells (parietal cells) are located in the gastric glands of the stomach. They secrete hydrochloric acid (HCl) and intrinsic factor. HCl creates the acidic environment needed for pepsin activity.

The pylorus is the narrow distal end of the stomach that connects to the duodenum. It contains the pyloric sphincter that controls the rate of chyme release into the small intestine.

Humans are diphyodont — teeth grow twice. The 20 deciduous (milk/baby) teeth are replaced by 20 of the 32 permanent teeth. The remaining 12 permanent teeth (including wisdom teeth) have no deciduous predecessors.

In herbivores (rabbits, horses), the large caecum contains symbiotic bacteria that produce cellulase to digest cellulose (plant cell walls). In humans, the caecum is small and vestigial with no significant digestive function.

The dental formula 2/2, 1/1, 2/2, 3/3 (representing incisors, canines, premolars, molars per half jaw) totalling 32 teeth is found in humans, cats, and many other placental mammals.

Giraffes have the longest oesophagus of any land animal — about 2 metres long — needed to transport food from their mouth (when grazing) or from elevated acacia branches down to their stomach.

Omnivores (like humans, bears, rats, pigs) eat both plant-based and animal-based foods. This dietary flexibility is an evolutionary advantage as it allows survival when specific food types are scarce.

The stomach wall is protected from its own HCl and pepsin by a thick layer of alkaline mucus secreted by goblet cells and mucous neck cells. Breakdown of this mucus layer leads to peptic ulcers.

The liver processes fatty acids through beta-oxidation and may produce ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone) especially during fasting or low-carbohydrate states. Ketone bodies serve as alternative fuel.

Insectivorous (carnivorous) plants grow in nitrogen-poor soils like bogs and fens. They supplement their nitrogen supply by trapping and digesting insects, as nitrogen is essential for protein and nucleic acid synthesis.

Food serves two main purposes: it provides raw materials for building and repairing tissues (growth) and provides energy for all metabolic activities through cellular respiration.

Insectivorous plants like Venus flytrap, sundew, and pitcher plant trap and digest insects. The insect proteins are digested by enzymes and the amino acids are absorbed to fulfil nitrogen requirements.

Inorganic solutes (mineral ions) move through the cytoplasm of root cells via the symplast pathway, and also through cell walls via the apoplast pathway, before entering the xylem for upward transport.

Microelements (trace elements/micronutrients) are mineral elements required by plants in very small amounts — examples include iron, manganese, zinc, copper, boron, and molybdenum.

Proteins, carbohydrates, fats, and vitamins are all organic compounds — they contain carbon and are produced by living organisms. They are the main organic nutrients in food.

Mineral nutrition refers to the requirement of various inorganic mineral elements for normal plant growth and metabolism. These include macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn).

Saprophytes (saprotrophs) are organisms that obtain nutrition from dead and decaying organic matter by secreting digestive enzymes externally and absorbing the soluble products. Many fungi and bacteria are saprophytes.

Passive absorption of water and minerals occurs through physical processes (diffusion, osmosis) without expenditure of metabolic energy. Ions move along their concentration gradient without ATP.

Food nutrients include both organic compounds (carbohydrates, proteins, fats, vitamins) and inorganic substances (water, mineral salts). Together they provide energy, building materials, and regulatory functions.

Salts and water are inorganic substances — they do not contain carbon-hydrogen bonds. They are essential nutrients for all living organisms, required for various metabolic and structural functions.

All major biomolecules are digested: carbohydrates (by amylases), lipids (by lipases), proteins (by proteases), and nucleic acids (by nucleases). All require specific enzymes for hydrolysis.

Absorption across intestinal epithelium uses all mechanisms: diffusion (small molecules), osmosis (water), active transport (against concentration gradient with ATP), and endocytosis (large molecules).

The colon is the main part of the large intestine in mammals. It consists of ascending, transverse, descending, and sigmoid sections. Its main functions are water absorption and formation of faeces.

Salivary amylase (ptyalin) is the main enzyme in saliva. It begins the digestion of starch in the mouth, converting it into maltose and dextrins. Bile is not an enzyme; pepsin and lipase are gastric/pancreatic enzymes.

Omnivores eat both plants and animals. Humans are classic omnivores — our digestive system and dentition are adapted to process both plant-based and animal-based foods.

Malnutrition arises from either undernutrition (deficiency of nutrients) or overnutrition (excess). It includes protein-energy malnutrition (kwashiorkor, marasmus), vitamin deficiencies, and obesity.

Chemical digestion is mainly hydrolysis — the addition of water to break down large food molecules into smaller absorbable units. Each type of macromolecule (protein, carbohydrate, fat) is hydrolysed by specific enzymes.

Villi are finger-like projections of the small intestinal mucosa that greatly increase the absorptive surface area. Each villus has microvilli on its epithelial cells (brush border), further multiplying the surface area.

Digestion is the process of converting large, insoluble food molecules into small, soluble molecules that can be absorbed. It involves both mechanical breakdown (chewing, churning) and chemical hydrolysis by enzymes.

Water-soluble vitamins (B group, C) cannot be stored in large amounts in the body and excess is excreted in urine. They must be consumed regularly. Fat-soluble vitamins (A, D, E, K) can be stored in liver and fat tissue.

Pancreatic juice contains multiple enzymes: amylase (digests starch), lipase (digests fats), and proteases (trypsin, chymotrypsin — digest proteins). Amylase is indeed a component but all three are present.

Amoeba uses pseudopodia (false feet) — temporary cytoplasmic extensions — to surround and engulf food particles by phagocytosis. This is the simplest form of ingestion in animals.

Intracellular digestion occurs inside the cell, within food vacuoles (phagosomes). Lysosomes fuse with food vacuoles and digest the contents with their hydrolytic enzymes. This occurs in Amoeba, sponges, and some cells.

Amylase breaks down starch into maltose (a disaccharide of two glucose units) by hydrolysing alpha-1,4 glycosidic bonds. Maltose is then further hydrolysed to glucose by maltase in the brush border.

Paramecium ingests food through its oral groove (cytostome) using cilia to sweep bacteria and food particles into it. Food vacuoles form and merge with lysosomes for intracellular digestion.

Extracellular digestion occurs outside cells — in the internal body cavity (coelenteron of Hydra), stomach, gut, or alimentary canal. Enzymes are secreted into these spaces to digest food.

Heterotrophic organisms cannot synthesise their own organic food and must obtain it from other organisms. They include animals, fungi, most bacteria, and non-photosynthetic protists.

Cellular respiration involves the oxidation of absorbed food molecules (primarily glucose) — they lose electrons (hydrogen) to oxygen. This controlled oxidation releases energy captured as ATP.

Hydra is a simple heterotrophic animal that performs extracellular digestion. It secretes enzymes into its gastrovascular cavity (coelenteron) to partly digest prey before cells complete digestion intracellularly.

Hydra first digests food extracellularly in its gastrovascular cavity (coelenteron/gastrocoel) using enzymes secreted by gastrodermal cells, then cells engulf partially digested particles for intracellular completion.

Planaria's highly branched (ramified) intestine extends throughout the body, allowing nutrients absorbed from digested food to reach all cells directly without a separate circulatory system.

As organisms evolved from intracellular to extracellular digestion, they developed complex feeding apparatus — specialised structures like mouths, teeth, pharynx, and digestive glands for capturing and processing food.

Higher heterotrophic organisms (animals) have a complete tubular digestive system with mouth at one end and anus at the other. This one-way flow allows specialisation of different regions for different digestive functions.

Planaria feeds through a muscular pharynx that can be extended through the mouth to suck up food. The pharynx secretes enzymes that begin extracellular digestion before food is drawn into the branched intestine.

Earthworms are detritivores that feed on all of the above: soil containing organic matter, decaying vegetation, dead plant material, and bacteria. They swallow soil and extract nutrients from organic matter in it.

The earthworm alimentary canal has: mouth, buccal cavity, pharynx, oesophagus, crop (food storage), gizzard (grinding), intestine (digestion and absorption), and anus — a complete digestive system.

The liver and pancreas are the two major digestive glands. The liver produces bile (emulsifies fats) and the pancreas produces pancreatic juice containing amylase, lipase, and proteases.

Proteolytic enzymes (proteases) break down proteins by hydrolysing peptide bonds. They include pepsin (stomach), trypsin, chymotrypsin, and peptidases (small intestine) working in sequence.

The pancreas is the most important digestive gland, secreting pancreatic juice containing multiple enzymes (amylase, lipase, trypsin, chymotrypsin) and bicarbonate into the duodenum through the pancreatic duct.

Saliva performs all three functions: it moistens food for easier swallowing, lubricates food with mucus for smooth passage, and contains salivary amylase (ptyalin) to begin starch digestion.

HCl in gastric juice kills most ingested bacteria (sterilising food), activates pepsinogen to pepsin, provides optimal acid pH for pepsin activity, and denatures proteins for easier enzymatic digestion.

Pepsin is secreted by chief cells (zymogen cells) in the gastric glands of the stomach wall as inactive pepsinogen. HCl from parietal cells converts pepsinogen to active pepsin in the stomach lumen.

Monosaccharides (sugars) and amino acids are absorbed into blood capillaries in the villi and transported via the hepatic portal vein to the liver. Fatty acids and glycerol enter lacteals as chylomicrons.

The colon absorbs large amounts of water (about 1-1.5 L/day) from the indigestible residue, converting liquid chyme into semi-solid faeces. It also absorbs electrolytes (Na⁺, Cl⁻).

Faeces (undigested matter, bacteria, dead cells, water) are stored in the rectum before being expelled through the anus during defecation. The rectum is the final segment of the large intestine.

Excess glucose is converted to glycogen (glycogenesis) in the liver and muscles for storage. When blood glucose falls, glycogen is hydrolysed back to glucose (glycogenolysis) to restore blood sugar levels.

The liver performs deamination — removing amino groups from excess amino acids. The amino group is converted to urea (excreted by kidneys) and the carbon skeleton is used for energy or glucose synthesis (gluconeogenesis).