NEET-BIOLOGY-CHAPTER 16-DIGESTION & ABSORPTION-QUICK REVISION

• Name of the Italian scientist who began his scientific career studying the cardiovascular systems of reptiles. Later , he turned his attention to the mammalian auditory systems- It was ALFONSO CORTI. • LIST OF Important Vitamins and Scientific names- Vitamin A : RETINOL, Vitamin B1 : Thiamine, Vitamin B2: Riboflavine, Vitamin B5 : Pantothenic Acid, Vitamin B6: Pyridoxine, Vitamin B3: Niacin or Nicotinic Acid, Vitamin B7: Biotin, Vitamin B9: Folic acid, Vitamin B12: Cobalamin, Vitamin C : Ascorbic acid, Vitamin D : Calciferol, Vitamin E : Tocopherol, Vitamin K : Phytomenadione. • Each tooth is embedded in a socket of jaw bone . This type of attachment is called thecodont. • The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum. • The upper surface of the tongue has small projections called papillae, some of which bear taste buds. • pharynx which serves as a common passage for food and air. • Structure of stomach is J shaped. • A muscular sphincter (gastro-oesophageal) regulates the opening of oesophagus into the stomach. • The stomach, located in the upper left portion of the abdominal cavity, has three major parts – a cardiac portion into which the oesophagus opens, a fundic region and a pyloric portion which opens into the first part of small intestine. • Small intestine is distinguishable into three regions, a ‘U’ shaped duodenum, a long coiled middle portion jejunum and a highly coiled ileum. • The opening of the stomach into the duodenum is guarded by the pyloric sphincter. • Ileum opens into the large intestine. It consists of caecum, colon and rectum. • Caecum is a small blind sac which hosts some symbiotic micro-organisms. • A narrow finger-like tubular projection, the vermiform appendix which is a vestigial organ, arises from the caecum. • The caecum opens into the colon. The colon is divided into three parts – an ascending, a transverse and a descending part. The descending part opens into the rectum which opens out through the anus. • The wall of alimentary canal from oesophagus to rectum possesses four layers namely serosa, muscularis, sub-mucosa and mucosa. • Serosa is the outermost layer and is made up of a thin mesothelium which is epithelium of visceral organs). • Question can be asked like : please name the epithelium of visceral organs and the answer would be mesothelium. • Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal. • Mucosal epithelium has goblet cells which secrete mucus that help in lubrication. • Mucosa also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn). • Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek), the sub-maxillary/sub-mandibular (lower jaw) and the sublinguals (below the tongue). • Liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human. • The hepatic lobules are the structural and functional units of liver containing hepatic cells arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule. • The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder. • The duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct. • The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of Oddi. • The pancreas is a compound (both exocrine and endocrine) elongated organ situated between the limbs of the ‘U’ shaped duodenum. The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon. • Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus. • The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition. • The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis. • The gastro-oesophageal sphincter controls the passage of food into the stomach. • The saliva secreted into the oral cavity contains electrolytes (Na+ , K+ , Cl– , HCO– ) and enzymes, salivary amylase and lysozyme. • The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase. • About 30 per cent of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose. • Lysozyme present in saliva acts as an antibacterial agent that prevents infections. • The mucosa of stomach has gastric glands. Gastric glands have three major types of cells namely - (i) mucus neck cells which secrete mucus; (ii) peptic or chief cells which secrete the proenzyme pepsinogen; and (iii) parietal or oxyntic cells which secrete HCl and intrinsic factor (factor essential for absorption of vitamin B12). • The stomach stores the food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme. • The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach. • Pepsin converts proteins into proteoses and peptones (peptides). • The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid. • HCl provides the acidic pH (pH 1.8) optimal for pepsins. • Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins. Please be careful regarding Rennin whose spelling is r e n n i n whereas other Renin which is secreted at kidneys has spelling of R E N I N. • Small amounts of lipases are also secreted by gastric glands. • The bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine. • Pancreatic juice and bile are released through the hepato-pancreatic duct. • The pancreatic juice contains inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. • Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice. • The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids but no enzymes. • Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. • Bile also activates lipases. • The intestinal mucosal epithelium has goblet cells which secrete mucus. • The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus. • Intestinal juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases. • The mucus alongwith the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. Sub-mucosal glands (Brunner’s glands) also help in this. • Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice which trypsin ,chymotrypsin and Carboxypeptidase converting them into Dipeptides. • Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides. • Fats are broken down by lipases with the help of bile into di-and monoglycerides. • Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides. • The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter. It is temporarily stored in the rectum till defaecation. • Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. It is carried out by passive, active or facilitated transport mechanisms. • Small amounts of monosacharides like glucose, amino acids and some of electrolytes like chloride ions are generally absorbed by simple diffusion. • However, some of the substances like fructose and some amino acids are absorbed with the help of the carrier ions like Na+. This mechanism is called the facilitated transport. • Transport of water depends upon the osmotic gradient. • Active transport occurs against the concentration gradient and hence requires energy. Various nutrients like amino acids, monosacharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism. • Fatty acids and glycerol being insoluble, cannot be absorbed into the blood. They are first incorporated into small droplets called micelles which move into the intestinal mucosa. They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the blood stream. • Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. However, maximum absorption occurs in the small intestine. • The absorbed substances finally reach the tissues which utilise them for their activities. This process is called assimilation. • The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal. The egestion of faeces to the outside through the anal opening (defaecation) is a voluntary process and is carried out by a mass peristaltic movement.