Human Digestive System: Digestion in the Mouth-PDF

CHAPTER 3- [1.3.] Digestion: What is on the Plate? Scope:  The human digestive system comprises the alimentary canal and digestive glands.  The alimentary canal is structurally adapted to perform various functions during the digestive process.  The secretions of chemicals from different digesti...

CHAPTER 3- [1.3.] Digestion: What is on the Plate? Scope:  The human digestive system comprises the alimentary canal and digestive glands.  The alimentary canal is structurally adapted to perform various functions during the digestive process.  The secretions of chemicals from different digestive glands aid in chemical digestion  The human digestive system consists primarily of the digestive tract, or the series of structures and organs through which food and liquids pass during their processing into forms that can be absorbed into the bloodstream.  The system also consists of the structures through which wastes pass in the process of elimination and of organs that contribute juices necessary for the digestive process.  In order to function properly, the human body requires nutrients. Some such nutrients serve as raw materials for the synthesis of cellular materials, while others help regulate chemical reactions or, upon oxidation, yield energy. Many nutrients, however, are in a form that is unsuitable for immediate use by the body; to be useful, they must undergo physical and chemical changes, which are facilitated by digestion.  The digestive system uses mechanical and chemical activities to break food down into absorbable substances during its journey through the digestive system. Digestive Processes The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation. Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. In chemical digestion, starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids). These secretions vary in composition, but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine. Organs of the Digestive System It consists of two parts-i. Alimentary canal and digestive glands. Alimentary canal consists of- 1. MOUTH The mouth is a transverse slit like opening bounded by two soft movable lips. The process of taking food into the body is called ingestion. Ingestion happens through mouth. The mouth leads to the buccal cavity or oral cavity. CHAPTER 3. DIGESTION/SC /2024/12 1  2. BUCCAL CAVITY/ ORAL CAVITY Buccal cavity is bound by- 1. Above- Palate roof of the buccal cavity has two parts –  hard palate with rugae and posterior part the soft palate that makes swallowing easier.  small flap the uvula or velum palati closes the internal nares during swallowing of food bolus. 2. Below- throat 3. Sides- jaws Inside the buccal cavity is present a muscular tongue and teeth arranged on the jaws.  TONGUE Is a skeletal muscle, helps to moves food in mouth, and to taste. Attached at the floor of the oral cavity by a ligamentous fold FRENULUM LINGUAE. The dorsal side of the tongue has a V shaped furrow-SULCUS TERMINALIS. Taste buds- have papillae  Fungiform - mushroom shaped  Filliform - thread like  Circumvalate - inverted V at the posterior Taste regions --Sweet-tip of the tongue/Salt- on the sides/Bitter- on the posterior part Functions-  Helps in ingestion and tasting food  For swallowing  Helps in speech  TEETH CHAPTER 3. DIGESTION/SC /2024/12 2 FOUR types of teeth- incisors, canines, premolars, molars The human teeth is  Heterodont: Humans have four types present. Incisors, canines, premolars and molars  Diphyodont: There are two sets of teeth. The milk teeth are replaced by permanent teeth.  Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human adults is 2123/2123 and in infants 2102/2102. 3.PHARYNX It is a common passageway for air and food. It participates in swallowing  Gullet- the opening of the esophagus  Glottis opening of the larynx or trachea Opening of buccal cavity into the pharynx. It has three parts-  Nasopharynx – nasal chamber  Oropharynx – behind buccal cavity  Laryngopharynx- lowest part with two openings- glottis and gullet. 4. OESOPHAGUS  It connects pharynx to stomach  Structure: mix of skeletal and smooth muscle  Mucus secreting cells: assist passage of food  Food motility: gravity and peristalsis  Peristalsis- contraction and relaxation of muscles which propagates in a wave down the muscular tube. 5. STOMACH Large, muscular, somewhat J shaped sac, occupying the upper left part of the abdominal cavity. Four regions- Cardiac, fundic, corpus and Pyloric. Shows the presence of longitudinal folds called rugae. Oesophagus opens into the stomach via the cardiac sphincter. Stomach opens into the duodenum by the pyloric sphincter. Functions:  Food storage, digestion, regulation of delivery.  Produces gastric juice for digestion. CHAPTER 3. DIGESTION/SC /2024/12 3   Hydrochloric acid: produces a pH of about 2, breaks down large bits of food.  Intrinsic factor; needed to absorb Vitamin B12  Mucus: protects stomach lining from acid.  Pepsinogen: with acid, begins protein breakdown.  Gastrin: hormone secreted for stimulating digestion. What is heartburn or acidity? When the cardiac sphincter fails to close fully, the acidic chyme squeezes into the oesophagus. This irritates the lining of oesophagus and causes heartburn. CARDIAC SPHINCTER [LOWER OESOPHAGUS SPHINCTER]-It is less effective in infants hence regurgitation is more common among infants. Cardiac sphincter lies between the esophagus and the stomach, opening at the approach of food that can then be swept into the stomach by rhythmic peristaltic waves. PYLORIC SPHINCTER- The pyloric sphincter is a band of smooth muscle at the junction between the pylorus of the stomach and the duodenum of the small intestine. It plays an important role in digestion, where it acts as a valve to controls the flow of partially digested food from the stomach to the small intestine 6. SMALL INTESTINE  Small intestine is distinguishable into three regions, a ‘C’ 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.  The small intestine is highly coiled and is about 5 meters long. It receives secretions from the liver and the pancreas. Besides, its wall also secretes juices.  The digested food passes into the blood vessels in the wall of the intestine. This process is called absorption.  The inner walls of the small intestine have thousands of finger-like outgrowths. These are called villi (singular villus). The villi increase the surface area for absorption of the digested food.  Villi are supplied with a network of capillaries and a large lymph (a colourless fluid containing white blood cells) vessel called the lacteal.  The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by the body. This is called assimilation.  In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water, and energy is released.  The food that remains undigested and unabsorbed then enters into the large intestine. 7. LARGE INTESTINE  The large intestine is wider and shorter than small intestine.  It consists of caecum, colon and rectum. Caecum is a small blind sac which hosts some symbiotic micro-organisms. 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. CHAPTER 3. DIGESTION/SC /2024/12 4  A narrow finger -like tubular projection, the vermiform appendix which is a vestigial organ [small remnant of something that was once more noticeable], arises from the caecum. Appendix was helpful in digesting roughage (fibrous indigestible material in vegetable foodstuffs which aids the passage of food and waste products through the gut). Thousands of years ago, when man used to eat roots, leaves, etc., it was essential. But now it has lost its significance.  No significant digestive activity occurs in the large intestine. The functions of large intestine are: absorption of some water, minerals and certain drugs; secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.  Micro-organisms in the large intestine [Gut bacteria include Lactobacillus, the bacteria commonly used in probiotic foods such as yogurt, and E. coli bacteria]. Escherichia coli - is a type of bacteria that normally lives inside our intestines, where it helps the body break down and digest the food we eat [cellulose]. Gut bacteria have several roles in the body. For example, intestinal bacteria:  Produce vitamin B12 and vitamin K.  Control the growth of harmful bacteria.  Break down poisons in the large intestine.  Break down some substances in food that cannot be digested, such as fiber and some starches and sugars. Bacteria produce enzymes that digest carbohydrates in plant cell walls. Most of the nutritional value of plant material would be wasted without these bacteria. These help us digest plant foods like spinach. 1.3.1 B. DIGESTIVE GLANDS The digestive glands associated with the alimentary canal include the salivary glands, the liver, gastric glands, pancreas and the intestinal glands. 1. Salivary glands Saliva is mainly produced by three pairs of salivary glands, i. Parotid glands- (cheek), which opens into the vestibule through the Stensen’s duct. ii. Sub-maxillary/Sub mandibular glands (lower jaw) opens by the Wharton’s duct and iii. Sub-lingual glands- (below the tongue) opens by the duct of Rivinus. [The parotid glands are enclosed in sheaths that limit the extent of their swelling when inflamed, as in mumps.] These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity. The saliva breaks down the starch into sugars Human saliva is 99.5% water, but also contains many important substances, including electrolytes, mucus, antibacterial compounds and various enzymes. /Water: 99.5%/Electrolytes: sodium, potassium, calcium, etc./Mucus (mucus in saliva mainly consists of mucopolysaccharides and glycoproteins)/Antibacterial compounds (thiocyanate ions)/Various enzymes; there are three major enzymes found in saliva: /Salivary amylase or ptyalin, and Lingual lipase Antimicrobial enzymes that kill bacteria -Lysozyme CHAPTER 3. DIGESTION/SC /2024/12 5  2. Gastric Gland: These are simple or branched tubular glands are present in the mucosa of the stomach. The gastric glands have the following cells- i. Chief cells/peptic cells/zymogen cells —Located primarily in the basal regions of gastric glands are chief cells, which secrete pepsinogen, the inactive proenzyme form of pepsin. ii. Parietal cells/oxyntic cells—Located primarily in the middle region of the gastric glands are parietal cells. These relatively large cells produce hydrochloric acid (HCl), bicarbonates and Castle’s intrinsic factor. Castle’s Intrinsic factor is a glycoprotein necessary for the absorption of vitamin B12 in the stomach. iii. Goblet cells/neck cells—Gastric glands in the upper part of the stomach contain goblet cells that secrete thin, acidic mucus that is responsible for maintaining the inner walls of the stomach. iv. Argentaffin cells-secretes serotonin. [vasoconstrictor] v. Endocrine cells—secrete various hormones into the interstitial fluid. These include gastrin, which is released mainly by endocrine G cells. Function of Hydrochloric Acid: i. It kills the germs present in the food ii. It inactives the ptyalin enzyme and stops the digestion of carbohydrates. iii. It makes the inactive pepsinogen and prorennin active by converting it into pepsin and rennin. CHAPTER 3. DIGESTION/SC /2024/12 6  iv. It makes the medium acidic where pepsin could easily facilitate protein digestion. v. It softens the food and dissolves the cementing materials vi. It controls the opening and closing of pyloric opening of stomach Pepsin and rennin help in the digestion of protein. It converts proteins into pepto 3. Intestinal Glands: - It is present on the walls of small intestine. Microscopic glands present in the mucosa of small intestine. i. Crypts of Lieberkuhn- multicellular, straight, tubular glands present throughout the mucosa of the small intestine. These glands have the following cells- a. Paneth cells-secretes lysozymes b. Argentaffin cells-secretes serotonin and secretin. c. Goblet cells- secretes mucus. d. Enzyme secreting cells- secretes enzymes. ii. Brunner’s glands- compound tubular glands, found in the duodenum. Secrete alkaline enzyme-free watery juice. The secretions from the two intestinal glands is called succus entericus or intestinal juice Intestinal enzymes include ----  Carbohydrate digesting enzymes [Intestinal amylase, Maltase Sucrase, Isomaltase, Lactase, Limit dextrase]  Protein digesting enzymes [Enterokinase, Aminopeptidase, Dipeptidase, Tripeptidase]  Fat digesting enzymes[Intestinal lipase] iii. Liver  The liver is a reddish-brown gland situated in the upper part of the abdomen on the right side. It is the largest gland in the body. It has two lobes. The hepatic lobules are the structural and functional units of liver containing hepatic cells [hepatocytes]. Hepatocytes contain large amounts of rough endoplasmic reticulum and free ribosomes. Hepatocytes are involved in:  Protein synthesis.  Protein storage.  The transformation of carbohydrates.  The synthesis of cholesterol, bile salts, and phospholipids.  The detoxification, modification, and excretion of exogenous and endogenous substances.  Hepatocytes also initiate the formation and secretion of bile.  Bile, or gall, is a bitter-tasting, dark-green to yellowish-brown fluid that aids the process of digestion of lipids in the small intestine.  Bile is stored in the gallbladder, and upon eating is discharged into the duodenum through the bile duct. Bile is a composition of the following materials: water (85%), bile salts (10%), mucus and pigments (3%), fats (1%), inorganic salts (0.7%), and cholesterol (0.3%).  Bile Pigments- bilirubin and biliverdin and bile Salts- [Na glycocholate, Na taurocholate and Na bicarbonate]. Bile acts as a surfactant, helping to emulsify the fats in the food, in the CHAPTER 3. DIGESTION/SC /2024/12 7  same way that soap emulsifies fat. The bile salts are ionically charged, with a hydrophobic end and a hydrophillic end. Bile salts also act as bactericides, destroying many of the microbes that may be present in the food. The bile plays an important role in the digestion of fats [Emulsification] 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 common bile duct joins the pancreatic duct to form hepato- pancreatic duct to open into the duodenum. The hepato-pancreatic duct is guarded by a sphincter called the sphincter of Oddi. The primary functions of the liver are:  Bile production and excretion  Synthesis of urea  Deamination of excess amino acids in ammonia and pyruvic acid.  Excretion of bilirubin, cholesterol, hormones, and drugs  Metabolism of fats, proteins, and carbohydrates Glycogenesis-Excess glucose in the blood is converted into glycogen and stored. Glycogenoglysis-When required glucose is obtained by hydrolysis of glycogen. Glyconeogenesis-Glucose is synthesized from amino acids or fatty acids and glycerol.  Lipogenesis- excess of glucose is converted to fats  Enzyme activation  Storage of glycogen, vitamins, and minerals  Synthesis of plasma proteins, such as albumin, and clotting factors  Blood detoxification and purification  phagocytosis/formation of RBC iv. Pancreas Is a heterocrine gland with both endocrine and exocrine parts.  The endocrine part is represented by patches of cells, islets of langerhans formed of four types of cells- i. Alpha cells that produce glucagon and make up 15–20% of total islet cells. Glucagon is a hormone that raises blood glucose levels by stimulating the liver to convert its glycogen into glucose. ii. Beta cells that produce insulin and makes up 65–80% of the total islet cells. Insulin lowers blood glucose levels by stimulating cells to take up glucose out of the blood stream. iii. Delta cells that produce somatostatin and make up 3–10% of the total islet cells. Somatostatin is a hormone that suppresses the release of the other hormones made in the pancreas. iv. Gamma cells or pancreatic polypeptide cells [PP Cells] that produce pancreatic polypeptide and make up 3–5% of the total islet cells. Pancreatic polypeptide regulates both the endocrine and exocrine pancreatic secretions. CHAPTER 3. DIGESTION/SC /2024/12 8  The exocrine portion secretes an alkaline pancreatic juice containing enzymes, which are- i. For proteins –Trypsinogen, Chymotrypsinogen and Procarboxypeptidase. ii. For carbohydrates-Pancreatic amylase (amylopsin) iii. For fats-Pancreatic lipase (steapsin) The pancreatic juice acts on carbohydrates, fats and proteins and changes them into simpler forms.  Pancreatic juice is alkaline in nature due to its high concentration of bicarbonate ions that neutralize the gastric acid and allow effective enzymic action.  Pancreatic juice secretion is regulated by the hormones secretin and cholecystokinin. It is produced by the walls of the duodenum upon detection of acid food, proteins, fats, and vitamins. Pancreatic secretion consists of an aqueous bicarbonate component from the duct cells and an enzymatic component from the acinar cells.  Because the pancreas is a sort of storage depot for digestive enzymes, injury to the pancreas is potentially fatal. A puncture of the pancreas generally requires prompt and experienced medical intervention.  A variety of factors cause a high pressure within pancreatic ducts. Pancreatic duct rupture and pancreatic juice leakage cause pancreatic self-digestion. 1.3.2. Physiology of Digestion Scope: Digestion occurs by physical and chemical breakdown of food. During physical digestion, the larger food chunks are broken down into smaller particles due to chewing and movements. Chemical digestion involves enzymatic hydrolysis of food molecules to soluble forms in different parts of the alimentary canal. Objectives  Develop a model that explains the chemical breakdown of food materials in the alimentary canal by enzymes (limited to carbohydrates, fats, and proteins). 1. Ingestion: - It is also called in taking of food. It taken through mouth. 2. Deglutition - The passage of food bolus from oral cavity to stomach. This is also called swallowing. 3. Digestion: - The breakdown of food into its simpler components so that it could be easily absorbed is called digestion. It starts from mouth and ends in small intestine. Digestion is of two kinds- Mechanical and Chemical 4. Absorption- The process by which the digested nutrient molecules are taken into the cells of a living organism. 5. Assimilation- The process by which the absorbed nutrients are incorporated into the cells/tissues. 6. Egestion- The elimination of the undigested food. The process is called defaecation.  Food can be digested by a combination of two methods – mechanical digestion and chemical digestion. CHAPTER 3. DIGESTION/SC /2024/12 9 Mechanical Digestion Chewing (Mouth)  Food is initially broken down in the mouth by the grinding action of teeth (chewing or mastication)  The tongue pushes the food towards the back of the throat, where it travels down the esophagus as a bolus  The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the bolus from entering the nasal cavity Churning (Stomach)  The stomach lining contains muscles which physically squeeze and mix the food with strong digestive juices ('churning’)  Food is digested within the stomach for several hours and is turned into a creamy paste called chyme. Eventually the chyme enters the small intestine (duodenum) where absorption will occur. Movement of Food Peristalsis  Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut  Continuous segments of longitudinal smooth muscle rhythmically contract and relax  Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus) Segmentation  Segmentation involves the contraction and relaxation of non-adjacent segments of circular smooth muscle in the intestines  Segmentation contractions move chyme in both directions, allowing for a greater mixing of food with digestive juices  While segmentation helps to physically digest food particles, its bidirectional propulsion of chyme can slow overall movement Chemical digestion  In chemical digestion, food is broken down by the action of chemical agents (such as enzymes, acids and bile) CHAPTER 3. DIGESTION/SC /2024/12 10 DIGESTION OF FOOD 1. Digestion in the oral cavity [mouth]  Enzyme present in saliva- Salivary amylase (ptyalin) Hydrolyses starch to maltose, isomaltose and small dextrins (limit dextrins)  Food remains in the buccal cavity for a very short time, hence only 30% of digestion takes place here.  Mastication breaks the food into small pieces hence, facilitates the action of salivary amylase.  Saliva does not contain enzymes for protein and fat digestion. Thus, proteins and fats are not digested here.  The partially digested food leaving the oral cavity is now the BOLUS. 2. Digestion – Enzyme Action in Stomach 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. Gastric juice contains i. Two proteolytic enzymes in inactive form-Pepsinogen and Prorennin.  Pepsinogen- is activated by HCl and the active form pepsin acts on proteins.  Proteins in the presence of pepsin is converted to proteases and peptones. (Pepsin works at acidic pH) Acidic pH is maintained by HCl.  Pepsin also helps in coagulation of milk. It hydrolyses casein [milk protein] to paracasein and whey proteins. Paracasein is precipitated as calcium paracaseinate to form solid curd.  Prorennin is also activated by HCl to active rennin. Rennin also helps in coagulation of milk.  Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins. It is absent in adults. Other functions of HCl -  Disinfects food by killing bacteria.  Stops the action of salivary enzymes as well as gastric amylase.  Softens the food. ii. The gastric juice in the stomach does not contain any carbohydrate digesting enzymes. The HCl present here also destroys ptyalin. Hence there is no carbohydrate digestion in the stomach. iii. Gastric lipase-are not able to digest fats.  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 pepsin. 3. Digestion – Enzyme Action in Small Intestine Small intestine receives the following- i. Bile ii. Pancreatic juice iii. Intestinal juice  Bile and pancreatic juice is poured into the duodenum by the hepato-pancreatic duct.  Bile salts emulsify fats i.e., large fat globules are broken down into fine droplets called micelles. Innumerable number provide larger surface area for enzyme action. Bile also activates LIPASES. CHAPTER 3. DIGESTION/SC /2024/12 11  Pancreatic juice contains-pancreatic amylase (amylopsin), pancreatic lipase(steapsin) and inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases (for protein digestion) a. Pancreatic amylase hydrolysis remaining 70% starch to maltose, isomaltose and dextrins. b. Pancreatic lipase hydrolyses fats into diglycerides and monoglycerides. c. Pro-enzymes-Trypsinogen is activated to trypsin by enterokinase. Trypsin further activates chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase. Trypsin converts basic proteins to peptones and peptides. Chymotrypsin converts proteins to peptides. Carboxypeptidase hydrolyze the terminal peptide bonds in a peptide chain acting on the carboxyl group.  Intestinal juice  The intestinal mucosal epithelium has goblet cells which secrete mucus. The secretions of the mucosa along with the secretions of the goblet cells constitute the intestinal juice.  This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities.  The intestinal juice contains six carbohydrases- Maltase--------------maltose---------------------glucose+glucose Isomaltase---------isomaltose----------------- glucose+Dextrin Lactase--------------Lactose----------------------glucose + galactose Sucrase------------- Sucrose---------------------glucose+ fructose Limit dextrase---------------limit dextrins-------------------- glucose.  Intestinal juice contains proteases like- Enterokinase that activates trypsinogen. Aminopeptidase hydrolyses the terminal peptide bond acting on amino group. Dipeptidase hydrolyse dipeptides to amino acids.  Intestinal lipase hydrolyses diglycerides and monoglycerides into fatty acids and glycerol. Role of large intestine- Escherichia coli can be commonly found in larger intestines of human and mammals. When E. coli locates in human large intestines, it can help digestion processes, food breakdown [cellulose] and absorption, and vitamin K production. Digestion for Carbohydrates  Carbohydrate digestion begins in the mouth. The salivary glands in the mouth secrete saliva, which helps to moisten the food.  The food is then chewed while the salivary glands also release the enzyme salivary amylase, which begins the process of breaking down the polysaccharides in the carbohydrate food. CHAPTER 3. DIGESTION/SC /2024/12 12  [30% of starch is broken down to maltose, isomaltose and α-dextrins].  Lysozyme present in saliva helps in killing bacteria found along with food. Stomach[need not mention-just for understanding] After the carbohydrate food is chewed into smaller pieces and mixed with salivary amylase and other salivary juices, it is swallowed and passed through the esophagus. The mixture enters the stomach where it is known as chyme. There is no further digestion of chyme, as the stomach produces acid which destroys bacteria in the food and stops the action of the salivary amylase.  After being in the stomach, the chyme enters the beginning portion of the small intestine, or the duodenum. In response to chyme being in the duodenum, the pancreas releases the enzyme pancreatic amylase(amylopsin), which hydrolyses the remaining 70% of starch into its respective disaccharides along with bicarbonates. Bicarbonate is a base and critical to neutralizing the acid coming into the small intestine from the stomach.  The small intestine then produces enzymes called lactase, sucrase and maltase, isomaltase and dextrinase which break down the disaccharides into monosaccharides.  Large Intestine (Colon) Carbohydrates that were not digested and absorbed by the small intestine reach the colon where they are partly broken down by intestinal bacteria. Fiber, which cannot be digested like other carbohydrates, is excreted with feaces or partly digested by the intestinal bacteria. Protein digestion  Protein digestion begins in the stomach. The food in the stomach comes in contact with the hydrochloric acid and proenzymes- pepsinogen and prorennin along with gastric lipase and gastric amylase. Due to its low pH, the gastric juice has an antiseptic action, killing most bacteria and other foreign cells, HCL of the gastric juice activates pepsinogen, to pepsin, and prorennin to rennin and inactivates gastric lipase and amylase.  The proenzyme [inactive precursor of an enzyme] pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme PEPSIN, the proteolytic (breakdown of proteins or peptides into amino acids) enzyme of the stomach.  Pepsin converts proteins into proteoses and peptones (peptides).  Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins.  Casein (milk protein) is converted by Rennin and pepsin as Paracasein. Paracasein precipates as calcium paracaseinate.  In the small intestine-which receives o pancreatic juice containing inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases.  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. o Intestinal juice-This juice contains enzymes like dipeptidases and aminopeptidases which breaks down dipeptides into amino acids. Digestion of fats Fat digestion occurs in the small intestine which receives bile from the gall bladder. Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates LIPASES. Small amounts of lipases are secreted by gastric glands. CHAPTER 3. DIGESTION/SC /2024/12 13 Pancreatic juice as well as intestinal juice contains enzymes for the digestion of fats, which as pancreatic lipase and intestinal lipase. Fats are hydrolysed into fatty acids and glycerol. 1.3.3 -ABSORPTION & ASSIMILATION OF DIGESTED FOOD Scope: The digested food particles are absorbed into the bloodstream through blood capillaries and lacteals. Blood transports the absorbed food to the liver for detoxification and is ultimately assimilated in the cells. OBJECTIVE  Construct scientific explanation on how the end products of digestion are absorbed and assimilated in the cells. Absorption of Digested Products Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. The small intestine is the region where digested food is absorbed. Most absorption happens in the ileum. This is the longest part of the small intestine and is between 2-4 meters long. The small intestine has a large internal surface area for absorption to happen quickly and efficiently. The villi The villi (one is called a villus) are tiny, finger-shaped structures that increase the surface area. They have several important features:  wall just one cell thick - ensures that there is only a short distance for absorption to happen by diffusion and active transport  network of blood capillaries - transports glucose and amino acids away from the small intestine in the blood  internal structure called a lacteal - transports fatty acids and glycerol away from the small intestine in the lymph Absorption is achieved by either of the following mechanisms:  Simple diffusion: Simple diffusion is defined as the movement of solute from the higher concentration to the lower concentration through the membrane. After digestion, a few monosaccharides diffuse into the blood based on the concentration gradient. Example: Glucose, amino acids, and ions like chloride. CHAPTER 3. DIGESTION/SC /2024/12 14  Active transport: Active transport may be defined as the process of solute movement from the lower concentration of the higher concentration by the expense of energy. Electrolytes like sodium ions are absorbed by active transport into the blood.  Facilitated transport: Facilitated transport is defined as the process of movement of solutes across the biological membrane with the help of specific carrier proteins. Some digested amino acids and glucose are absorbed into the blood by this method.  Passive transport: Passive transport is defined as the process of solute movement across a cell membrane without a requirement of energy. After digestion, simpler food substances are absorbed into the blood by passive transport. Absorption of carbohydrates 1. Glucose and Galactose  They are transported from the intestinal lumen into the cells by a Na+-dependent co- transport in the luminal membrane.  The sugar is transported “uphill” and Na+ is transported “downhill.”  They are then transported from cell to blood by facilitated diffusion. 2. Fructose  Fructose is transported exclusively by facilitated diffusion; therefore, it cannot be absorbed against a concentration gradient. Absorption of Proteins 1. Free amino acids  Na+-dependent amino acid cotransport occurs in the luminal membrane. It is analogous to the cotransporter for glucose and galactose.  The amino acids are then transported from cell to blood by facilitated diffusion. 2. Dipeptides and tripeptides  They are absorbed faster than free amino acids.  H+-dependent cotransport of dipeptides and tripeptides also occurs in the luminal membrane.  After the dipeptides and tripeptides are transported into the intestinal cells, cytoplasmic peptidases hydrolyze them to amino acids.  The amino acids are then transported from cell to blood by facilitated diffusion. Absorption of Fats  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. Micelles bring the products of lipid digestion into contact with the absorptive surface of the intestinal cells.  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. Chylomicrons are transported out of the intestinal cells by exocytosis.  Because chylomicrons are too large to enter the capillaries, they are transferred to lymph vessels and are added to the bloodstream via the thoracic duct.  These lymph vessels ultimately release the absorbed substances into the blood stream Summary of Absorption in Different Parts of Digestive System Mouth Stomach Small Intestine Large Intestine Certain drugs coming Absorption of Principal organ for Absorption of water, in contact with the water, simple absorption of nutrients. The some minerals and mucosa of mouth and sugars, and digestion is completed here drugs take place lower side of the alcohol etc. and the final products of tongue are absorbed takes place. digestion such as glucose, CHAPTER 3. DIGESTION/SC /2024/12 15  into the blood fructose, fatty acids, glycerol capillaries lining them and amino acids are absorbed through the mucosa into the blood stream and lymph ASSIMILATION. The absorbed substances finally reach the tissues which utilize them for their activities. Digested and undigested foods have different outcomes once they have passed through the alimentary canal (gut). Assimilation is the movement of digested food molecules into the cells of the body where they are used. Assimilation of Proteins, Carbohydrates and Fats: (i) Protein  Amino acids are not stored but are taken up by the cells in connection with the synthesis of proteins. Proteins are used for growth, repair, etc.  Excess amino acids can be converted into glucose and then to fat and are thus stored. This is an irreversible reaction.  Amino acids can also be converted to glucose and used as fuel for the cell. During their conversion to glucose the amino acids are deaminated (removal of amino groups NH 2).  The liver is chief site for deamination, i.e., a process by which the amino group is removed from the amino acids resulting in the production of ammonia. The ammonia is soon converted into urea, which is filtered from the blood in the kidney. (ii) Carbohydrates:  The excess of the monosaccharide’s; the glucose, fructose and galactose are usually stored in the liver and muscle cells in the form of glycogen (glycogenesis). Whenever, there is a deficiency of glucose in the blood the glycogen is converted into glucose (glycogenolysis).  Muscle glycogen is utilized during muscle contraction. Glucose is utilized in the production of energy for various body activities. A considerable amount of glucose is converted into fat and stored as such. (iii) Fats:  The fat is stored in the fat deposits of the body, such as subcutaneous layers, mesenteries, etc. The fat stored is a readily available source of fuel for the cells.  Fat has important insulating properties in connection with the conservation of heat and maintenance of body temperature.  Fat also plays a protective role as filling or around packing material and between organs. In the liver phospholipids are formed which are returned to the blood to be used by all the cells.  In the liver cells the fats are converted into amino acids and carbohydrates. Vitamins, salts and water are also useful for various metabolic processes. Egestion The small intestine absorbs most of the water in the contents of the gut. By the time the contents reach the end of the small intestine, most of the digested food has also been absorbed. The remaining material consists of:  water  bacteria (living and dead)  cells from the lining of the gut  indigestible substances - such as cellulose from plant cell walls CHAPTER 3. DIGESTION/SC /2024/12 16 The colon is the first part of the large intestine. It absorbs most of the remaining water. This leaves semi-solid waste material called faeces. The faeces are stored in the rectum, the last part of the large intestine. Egestion happens when these faeces pass out of the body through the anus. 1.3.4. BALANCED DIET & COMMON DIGESTIVE DISORDERS SCOPE-The growth, development, and health of an organism depend on its food. The human lifestyle is undergoing a rapid change due to which people depend more on processed food (fast foods) that barely meets the dietary requirements. Healthy dietary practices contribute to good immunity and help in preventing diseases (e.g., malnourishment, heart diseases, stroke depressed immune functions, etc.) Balanced Diet: The World Health Organization’s (WHO) definition of a healthy diet emphasizes the importance of starting healthy eating habits in early life (notably through breastfeeding) and limiting the intake of free sugars and salt. It advises people to eat plenty of fruits and vegetables, wholegrains, fibre, nuts and seeds, while limiting free sugars, sugary snacks and beverages, processed meats and salt, and replacing saturated and industrial trans fats with unsaturated fats.  A diet is said to be balanced when various nutritional materials i.e., proteins, carbohydrates, fats, minerals, vitamins, roughage and water are present in sufficient amount and proper proportion.  Various constituents of the balanced diet provide energy, growth, repair, replacement of cells, and physiological regulation. Our food should contain the various nutrients in such proportions as can satisfy all the needs of our body.  It has been discovered that of our energy requirement, we obtain about 50% from carbohydrates, 35% from fats and 15% from proteins. Thus, we daily require about 400 to 500 grams of carbohydrates, 60 to 70 grams of fats and 65 to 75 grams of proteins. Balanced diet of each individual can be determined according to his or her needs. Nutritional Requirements of Humans: (i) Energy yielding nutrients: Carbohydrates and lipids (fats) are chief energy giving nutrients. Proteins can also give energy. (ii) Body building nutrients: Proteins are chief body building nutrients. (iii) Metabolic regulators: E.g., vitamins, water and mineral salts. (iv) Hereditary substances: E.g., Nucleic acids (DNA and RNA). Besides carbohydrates, proteins, fats, vitamins, minerals and water, roughage is also essential in diet. Calorific Value of Carbohydrate, Protein and Fat:  Carbohydrates, proteins and fats serve as the chief sources of energy in humans. These are oxidized and transformed into ATP, the chemical energy form used by cells for their various activities.  Because heat is the ultimate form of all energy, the energy value of food (or any fuel) is expressed in terms of a measure of heat energy it produces on combustion. The heat energy released by combustion of one gram of food is usually known as its gross calorific value. A balanced diet includes:  Plenty of fruit and vegetables. According to NHS guidelines, getting your five-a-day means eating at least five 80g portions of fruit and vegetables – which is easier than you might think. Fresh, canned, dried, and frozen vegetables all count towards this aspect of a healthy balanced diet, and it’s simple to up your intake by stirring vegetables into stews, curries, soups, and CHAPTER 3. DIGESTION/SC /2024/12 17  casseroles, or topping your breakfast, snack, or dessert with a portion of fruit. The main exception to the five-a-day rule is potatoes – they are considered starchy foods (see below).  Starchy foods, including bread, pasta, rice, grains, and potatoes. In the balanced diet chart, starchy foods cover about a third of overall food consumption. Providing energy and essential nutrients, starchy foods can also be a good source of fibre, particularly the wholegrain or brown varieties and potatoes eaten with their skins. Studies on oats suggest that a daily intake of 3g beta-glucan (a kind of fibre found in oats) can lower cholesterol as part of a healthy diet and lifestyle*.  Non-dairy protein, including fish, pulses, nuts, eggs, and meat. Protein contributes to the growth and maintenance of muscles. When it comes to maintaining a desirable cholesterol level, the type of fat contained in the protein we eat is important. Nuts and oily fish are good sources of unsaturated fat, which can help reduce cholesterol levels when used as a replacement for saturated fats as part of a healthy balanced diet. Foods within this group can also be a great source of fibre: pulses (such as beans, lentils, and peas) and nuts are all good options for increasing the amount of fibre in your diet.  Milk and other dairy foods. Milk and other dairy products like cheese, cream, and butter are also good sources of protein or minerals like calcium, but are often high in saturated fat – especially full fat variants. For a healthy balanced diet, opt for reduced fat or reduced saturated fat versions, such as skimmed milk or vegetable oil based soft spreads.  Fat and sugar. As you can see from the healthy balanced diet pie chart above, visible fat (fat you can see, such as oils, butter, or the fat you can cut off meat) and sugar should account for a quite small portion of our overall diet. Fats supply us with energy and essential fatty acids the body needs, but cannot produce. However – as with the “invisible fat” found in other kinds of food – the visible types of fat we eat need to be as good as possible. Total fat and total saturated fat intake through our diet needs to be within the recommended limits. According to the NHS:  The average man should eat under 30g of saturated fat a day, 95g of fat in total.  The average woman should eat under 20g of saturated fat a day, 70g of fat in total. Sugar is also a source of energy, but foods high in sugar often contain few other nutrients – it is important to keep sugar consumption within the recommended limits.  High cholesterol is a risk factor in the development of coronary heart disease. There are many risk factors for coronary heart disease and it is important to take care of all of them to reduce overall risk of it. CHAPTER 3. DIGESTION/SC /2024/12 18 How does processed food affect human life? Heavily processed foods are often high in sugar, fat and empty calories. Consuming lots of these foods has long been linked to an increased risk of a wide variety of health problems that can lead to heart disease or an early grave, such as obesity, high blood pressure, elevated cholesterol, cancer and depression MALNUTRITION Malnutrition has been clearly described by the World Health Organization, among other entities dealing with the professional aspects of the subject matter. The nutritional imbalance can be classified in various ways, especially for purposes of proper and adequate management. Currently, the categories of malnutrition include undernutrition, overweight, and obesity. Globally, inadequate food intake is the most common cause of malnutrition: In developing countries, inadequate food intake may be due to poverty, insufficient, or inappropriate food supplies or early weaning and premature stopping of breastfeeding. Ignorance about the need to have a balanced diet (and lack of adequate knowledge about the appropriate food and the right quantities needed by each family member) and ignorance about the importance of breastfeeding may be important contributing factor(s) in some places, especially if those endowed with the responsibility of disseminating the right knowledge fail to do it appropriately. Psychosocial issues, such as premature death of a breastfeeding mother, deliberate maternal deprivation from whatever reason may also contribute to childhood malnutrition. Single mothers (due to death of a spouse, separation, or divorce) may also face the challenge of premature stoppage of breastfeeding, owing to the fact that such mothers have to constantly provide food and other needs for the family, without the help of a spouse. HEART DISEASE Heart disease results from the narrowing of the arteries that supply the heart with blood through a process known as atherosclerosis. Fatty deposits (or plaque) gradually build up on the inside of the artery walls, narrowing the space in which blood can flow to heart. Atherosclerosis can start when you are young, so by the time you reach middle age, it can be quite advanced. Plaque build-up can be considered as stable or unstable. If there is too much build-up of stable plaque, it narrows the arteries, causing pain and discomfort due to not enough blood reaching the heart – this is called angina and it needs to be treated. A diet high in salt is linked to hypertension (high blood pressure), which can increase your risk of heart disease and stroke. Most of us consume more than 10 times the amount of salt we need to meet our sodium requirements (salt contains sodium and chloride). Nutrition and Immunity During the flu season or times of illness, people often seek special foods or vitamin supplements that are believed to boost immunity. Vitamin C and foods like citrus fruits, chicken soup, and tea with honey are popular examples. Yet the design of our immune system is complex and influenced by an ideal balance of many factors, not just diet, and especially not by any one specific food or nutrient. However, a balanced diet consisting of a range of vitamins and minerals, combined with healthy lifestyle factors like adequate sleep and exercise and low stress, most effectively primes the body to fight infection and disease. CHAPTER 3. DIGESTION/SC /2024/12 19 What role does nutrition play in boosting the immune system? These nutrients help the immune system in several ways: working as an antioxidant to protect healthy cells, supporting growth and activity of immune cells, and producing antibodies. Epidemiological studies find that those who are poorly nourished are at greater risk of bacterial, viral, and other infections. Factors contributing to good nutrition  Good agricultural practices  Good and vibrant economy  Healthy enabling environment  Good education  Healthy social and family life  Good antenatal and perinatal care  Early screening and control of preventable diseases What are digestion problems? The digestive system is an intricate and extensive part of the body. It ranges all the way from the mouth to the rectum. The digestive system helps your body absorb essential nutrients and is responsible for getting rid of waste. Digestion problems can mean more than unwanted symptoms. Minor problems that are left untreated can lead to more serious, chronic illnesses. Digestion problems, such as constipation, diarrhea, heartburn, and bloating are very common conditions related to digestion and are treatable at home. The simplest form of treating conditions associated with digestion is by using plants. Chronic constipation Chronic constipation indicates a problem with getting rid of waste. This most often occurs when the colon can’t pass or move stools through the rest of the digestive tract. You may experience abdominal pain and bloating as well as fewer bowel movements (which are more painful than usual). Food intolerance Food intolerance occurs when your digestive system can’t tolerate certain foods. Unlike food allergies, which can cause hives and respiratory problems, an intolerance only affects digestion. Symptoms of food intolerance include:  bloating and/or cramps, diarrhea, headache, heartburn, irritability, gas and vomiting Celiac disease, an autoimmune disorder, is one type of food intolerance. It causes digestive problems when you eat gluten (a protein in wheat, barley, and rye). People with celiac disease must follow a gluten-free diet to minimize symptoms and damage to the small intestine. GERD Gastroesophageal reflux disease (GERD) is a common condition in which the stomach contents move up into the esophagus. Heartburn is an occasional occurrence for many adults and a milder form of GERD. This happens when stomach acids go back up into the esophagus, causing chest pain and the trademark burning sensation. Antacids help to treat heartburn (indigestion). They work by neutralizing the stomach acid that causes heartburn. If you have more frequent heartburn, you might have gastroesophageal reflux disease (GERD). Such frequent episodes can interfere with your daily life and damage your esophagus. CHAPTER 3. DIGESTION/SC /2024/12 20 How are plants used in treating conditions associated with digestion? Healing starts with the gut. A healthy digestive system supports our mental and emotional well-being, while also processing nutrients and delivering them through the entire body. And it’s not just about what you eat, but rather, what you assimilate. Through the release of the liver’s bile and digestive enzymes and an orchestra of other processes, our bodies are capable of creating fuel from the foods we eat and absorbing essential vitamins and minerals. Herbs for the Digestive System Peppermint (Mentha x piperita): cool and uplifting, an herbal carminative that alleviates digestive discomfort. Dandelion Leaf and Root (Taraxacum officinale): while the whole plant can be enjoyed as a medicinal tea for its bitter liver supporting properties, its leaves can be enjoyed in salad, pesto, and more. Fennel (Foeniculum vulgare): this sweet, soothing tea might not taste like herbal medicine, but each seed contains essential oils rich with anethole and fenchone, known for their capacity to ease bloating and gas. Ginger (Zingiber officinale): a warming carminative traditionally used for motion sickness, stomach upset, and cramping. Lemon Balm (Melissa officinalis): used for thousands of years as an aromatic nervine to support digestion and calm frazzled nerves. Chamomile (Matricaria recutita): this common flower isn’t just a sleep aid but also a calming flower that eases digestion and relaxes twitchy tummies. Slippery Elm (Ulmus rubra): while we famously infuse this (inner) bark into our Throat Coat tea, this demulcent herb soothes gastrointestinal tissues, too. Calendula (Calendula officinalis): herbalists prize this golden flower for lending its bright notes to herbal butters and salads. Traditionally, they use it as a gentle demulcent to moisten and soothe digestive tissue. Lavender (Lavandula angustifolia): this common flower isn’t just a lovely scent, but also a treasure trove of essential oils that work to calm nerves and upset tummies. BOTANOCHEMICALS AND ITS USE IN DIGESTIVE DISORDERS Botanochemicals include substances which can be extracted. directly from plants, such as Plant species Common name Medicinal uses 1. Artemisia common mugwort Treatment of diarrhea, constipation, nausea and vulgaris L. (KHEMPA) vomitting 2. Zingiber officinale Ginger [Contains treat chronic indigestion gingerol] help lower cholesterol levels, help fight infections, highly effective against nausea 3. Allium sativum Garlic reduce blood sugar levels, reduce total cholesterol and low-density lipoprotein, improve liver health 4. Bistorta red knotweed Antidiarrheal, antidysentery and alleviates stomach macrophylla [spang-ram] pain. 5. Aloe vera Improves digestive health CHAPTER 3. DIGESTION/SC /2024/12 21  [irritable bowel syndrome] (IBS). improving oral hygiene and reducing plaque. 6. Ocimum sanctum Holy basil help to cure indigestion and loss of appetite 7. Cinnamomum cinnamon Used for treatment of gastric ulcers, diarrhea verum 8. Mentha piperita, Peppermint Gastric pain, dyspepsia 9. Psidium guajaba Guava, Ulcers, diarrhea, dyspepsia 10. Matricaria recuita Chamomile Gastrointestinal colic, dyspepsias, diarrhea Answer the following Questions 1. How would the absence of enzymes impact digestion? 2. Design an experiment to demonstrate the effect of amylase on starch. 3. Do digestive juices help in maintaining the internal environment of the alimentary canal? Give reasons for your answer. 4. Drinking water right after eating is thought to dilute enzymes and hinder digestion. Comment your views on the statement giving reasons 5. How are end products of digestion of carbohydrate and fat absorbed in the small intestine? 6. What happens to the various components of food that are absorbed into the blood or lacteals? 7. Why is it important to take a diet that is rich in protein? 8. Identify some plants and their products that are used in traditional practices for treating digestive problems. 9. Mention a fruit that you would advise a person to take if he is suffering from diarrhea due consumption of milk. 10. Investigate the dietary habits of Bhutanese to determine if the nutritional requirements for a healthy lifestyle are met. 11. Why is eating a balanced diet important? 12. Would you prefer home remedy or pharmaceutical medicines to treat problems related to digestion? Justify with ONE reason. 13. What are the daily nutritional requirements for an average normal person? 14. What are the possible consequences of unhealthy dietary habits? 15. Insulin is also synthetically produced to treat diabetes mellitus and is usually administered in the form of injection rather than in tablets. Are there some physiological benefits associated with the practise? 16. “A healthy diet is less about choosing the right foods and more about choosing the right ingredients.” Do you agree with the statement? Justify giving reasons. 17. How would the absence of physical digestion affect human nutrition? 18. Explain how different parts of the alimentary canal are structurally adapted to carry out various functions of digestion. 19. Design an experiment to test the presence of different nutrients (carbohydrate, protein, and fats) in food. CHAPTER 3. DIGESTION/SC /2024/12 22