Chapter-16 Digestion and Absorption(Not in Syllabus)
Food is one of the basic requirements of all living organisms. The major components of our food are carbohydrates, proteins and fats. Vitamins and minerals are also required in small quantities. Food provides energy and organic materials for growth and repair of tissues. The water we take in, plays an important role in metabolic processes and also prevents dehydration of the body. Biomacromolecules in food cannot be utilised by our body in their original form. They have to be broken down and converted into simple substances in the digestive system. This process of conversion of complex food substances to simple absorbable forms is called digestion and is carried out by our digestive system by mechanical and biochemical methods. General organisation of the human digestive system is shown in Figure 16.1.
16.1 DIGESTIVE SYSTEM
The human digestive system consists of the alimentary canal and the associated glands.
16.1.1 Alimentary Canal
The alimentary canal begins with an anterior opening - the mouth, and it opens out posteriorly through the anus. The mouth leads to the buccal cavity or oral cavity. The oral cavity has a number of teeth and a muscular tongue. Each tooth is embedded in a socket of jaw bone (Figure16.2). This type of attachment is called thecodont. Majority of mammals including human being forms two sets of teeth during their life, a set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth. This type of dentition is called diphyodont. An adult human has 32 permanent teeth which are of four different types (Heterodont dentition), namely, incisors (I), canine (C), premolars (PM) and molars (M). 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 . The hard chewing surface of the teeth, made up of enamel, helps in the mastication of food. 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.
The oral cavity leads into a short pharynx which serves as a common passage for food and air. The oesophagus and the trachea (wind pipe) open into the pharynx. A cartilaginous flap called epiglottis prevents the entry of food into the glottis - opening of the wind pipe - during swallowing. The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach. 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 four major parts - a cardiac portion into which the oesophagus opens, a fundic region, body (main central region) and a pyloric portion which opens into the first part of small intestine (Figure 16.3). 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. 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 four parts - an ascending, a transverse, descending part and a sigmoid colon. 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 (Figure 16.4) namely serosa, muscularis, sub-mucosa and mucosa. Serosa is the outermost layer and is made up of a thin mesothelium (epithelium of visceral organs) with some connective tissues. Muscularis is formed by smooth muscles usually arranged into an inner circular and an outer longitudinal layer. An oblique muscle layer may be present in some regions. The submucosal layer is formed of loose connective tissues containing nerves, blood and lymph vessels. In duodenum, glands are also present in sub-mucosa. The innermost layer lining the lumen of the alimentary canal is the mucosa. This layer forms irregular folds (rugae) in the stomach and small finger-like foldings called villi in the small intestine (Figure 16.5). The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance. These modifications increase the surface area enormously. 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). All the four layers show modifications in different parts of the alimentary canal.
16.1.2 Digestive Glands
The digestive glands associated with the alimentary canal include the salivary glands, the liver and the pancreas.
Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek), the sub maxillary/sub-mandibular (lower jaw) and the sub- linguals (below the tongue). These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.
Liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human. It is situated in the abdominal cavity, just below the diaphragm and has two lobes. 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 (Figure 16.6). 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 ‘C’ shaped duodenum. The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin and glucagon. The process of digestion is accomplished by mechanical and chemical processes.
The buccal cavity performs two major functions, mastication of food and facilitation of swallowing. The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly. 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 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. Salivary Amylase
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. Small amounts of lipases are also secreted by gastric glands.
Various types of movements are generated by the muscularis layer of the small intestine. These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion. 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. This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. 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 as given below:
Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.
Amylase Polysaccharides (starch ) → Disaccharides
Fats are broken down by lipases with the help of bile into di-and monoglycerides. Lipases
Fats → Diglycerides → Monoglycerides
Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides
Nucleases → Nucleotides → Nucleosides Nucleic acids
The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms. These final steps in digestion occur very close to the mucosal epithelial cells of the intestine.
Dipeptidases
Dipeptides → Amino acids
Maltase Maltose → Glucose + Glucose
Lactase Lactose → Glucose + Galactose Sucrase Sucrose → Glucose + Fructose Nucleotidases Nucleosidases Nucleotides → Nucleosides → Sugars + Bases
Lipases Di and Monoglycerides → Fatty acids + Glycerol
The breakdown of biomacromolecules mentioned above occurs in the duodenum region of the small intestine. The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine. The undigested and unabsorbed substances are passed on to the large intestine.
No significant digestive activity occurs in the large intestine. The functions of large intestine are: (i) absorption of some water, minerals and certain drugs; (ii) secretion of mucus which helps in adhering the waste (undigested)
particles together and lubricating it for an easy passage. 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.
The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts. The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva. Gastric and intestinal secretions are also, similarly, stimulated by neural signals. The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS. Hormonal control of the secretion of digestive juices is carried out by local hormones produced by the gastric and intestinal mucosa.
CALORIFIC VALUE OF PROTEIN, CARBOHYDRATE AND FAT (Boxed item - Not for evaluation)
The energy requirements of animals, and the energy content of food, are expressed in terms of measure of heat energy because heat is the ultimate form of all energies. This is often measured to as calorie (cal) or joule (J), which is the amount of heat energy required to raise the temperature of 1 g of water by 1 °C. Since this value is tiny amount of energy, physiologists commonly use kilocalorie (kcal) or kilo joule (kJ). One kilo calorie is the amount of energy required to raise the temperature of 1 kg of water by 1 °C. Nutritionists, traditionally refer to kcal as the Calorie or Joule (always capitalised). The amount of heat liberated from complete combustion of 1 g food in a bomb calorimeter (a closed metal chamber filled with O2) is its gross calorific or gross energy value. The actual amount of energy combustion of 1 g of food is the physiologic value of food. Gross calorific values of carbohydrates, proteins and fats are 4.1 kcal/g, 5.65 kcal/g and 9.45 kcal/g, respectively, whereas their physiologic values are 4.0 kcal/g, 4.0 kcal/g and 9.0 kcal/g, respectively.
16.3 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. It is carried out by passive, active or facilitated transport mechanisms. Small amounts of monosaccharides like glucose, amino acids and some electrolytes like chloride ions are generally absorbed by simple diffusion. The passage of these substances into the blood depends upon the concentration gradients. However, some substances like glucose and amino acids are absorbed with the help of carrier proteins. 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, monosaccharides 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. A summary of absorption (sites of absorption and substances absorbed) is given in Table 16.1.
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.
16.4 DISORDERS OF DIGESTIVE SYSTEM
The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections. The infections are also caused by the parasites of the intestine like tapeworm, roundworm, threadworm, hookworm, pin worm, etc.
Jaundice: The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.
Vomiting: It is the ejection of stomach contents through the mouth. This reflex action is controlled by the vomit centre in the medulla. A feeling of nausea precedes vomiting.
Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the absorption of food.
Constipation: In constipation, the faeces are retained within the colon as the bowel movements occur irregularly.
Indigestion: In this condition, the food is not properly digested leading to a feeling of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, over eating, and spicy food.
PEM
Dietary deficiencies of proteins and total food calories are widespread in many underdeveloped countries of South and South-east Asia, South America, and West and Central Africa. Protein-energy malnutrition (PEM) may affect large sections of the population during drought, famine and political turmoil. This happened in Bangladesh during the liberation war and in Ethiopia during the severe drought in mid-eighties. PEM affects infants and children to produce Marasmus and Kwashiorkar.
Marasmus is produced by a simultaneous deficiency of proteins and calories. It is found in infants less than a year in age, if mother’s milk is replaced too early by other foods which are poor in both proteins and caloric value. This often happens if the mother has second pregnancy or childbirth when the older infant is still too young. In Marasmus, protein deficiency impairs growth and replacement of tissue proteins; extreme emaciation of the body and thinning of limbs results, the skin becomes dry, thin and wrinkled. Growth rate and body weight decline considerably. Even growth and development of brain and mental faculties are impaired.
Kwashiorkar is produced by protein deficiency unaccompanied by calorie deficiency. It results from the replacement of mother’s milk by a high calorielow protein diet in a child more than one year in age. Like marasmus, kwashiorkor shows wasting of muscles, thinning of limbs, failure of growth and brain development. But unlike marasmus, some fat is still left under the skin; moreover, extensive oedema and swelling of body parts are seen.
Summary
The digestive system of humans consists of an alimentary canal and associated digestive glands. The alimentary canal consists of the mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and the anus. The accessory digestive glands include the salivary glands, the liver (with gall bladder) and the pancreas. Inside the mouth the teeth masticates the food, the tongue tastes the food and manipulates it for proper mastication by mixing with the saliva. Saliva contains a starch digestive enzyme, salivary amylase that digests the starch and converts it into maltose (disaccharide). The food then passes into the pharynx and enters the oesophagus in the form of bolus, which is further carried down through the oesophagus by peristalsis into the stomach. In stomach mainly protein digestion takes place. Absorption of simple sugars, alcohol and medicines also takes place in the stomach.
The chyme (food) enters into the duodenum portion of the small intestine and is acted on by the pancreatic juice, bile and finally by the enzymes in the succus entericus, so that the digestion of carbohydrates, proteins and fats is completed. The food then enters into the jejunum and ileum portions of the small intestine. Carbohydrates are digested and converted into monosaccharides like glucose. Proteins are finally broken down into amino acids. The fats are converted to fatty acids and glycerol.
The digested end products are absorbed into the body through the epithelial lining of the intestinal villi. The undigested food (faeces) enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter. Most of the water is absorbed in the large intestine. The undigested food becomes semi-solid in nature and then enters into the rectum, anal canal and is finally egested out through the anus.
Exercises
1. Choose the correct answer among the following :
(a) Gastric juice contains
(i) pepsin, lipase and rennin
(ii) trypsin, lipase and rennin
(iii) trypsin, pepsin and lipase
(iv) trypsin, pepsin and renin
(b) Succus entericus is the name given to
(i) a junction between ileum and large intestine
(ii) intestinal juice
(iii) swelling in the gut
(iv) appendix
Answer (a): (i) Pepsin, lipase, and rennin Gastric juice contains pepsin, lipase, and rennin. Pepsin is secreted in an inactive form as pepsinogen, which is activated by $HCl$. Pepsin digests proteins into peptones. Lipase breaks down fats into fatty acids. Rennin is a photolytic enzyme present in the gastric juice. It helps in the coagulation of milk. (b): (ii) Intestinal juice Succus entericus is another name for intestinal juice. It is secreted by the intestinal gland. Intestinal juice contains a variety of enzymes such as maltase, lipases, nucleosidases, dipeptidases, etc.Show Answer
| Column I | Column II |
|---|---|
| (a) Bilirubin and biliverdin | (i) Parotid |
| (b) Hydrolysis of starch | (ii) Bile |
| (c) Digestion of fat | (iii) Lipases |
| (d) Salivary gland | (iv) Amylases |
AnswerShow Answer
Column I
Column II
(a) Bilirubin and biliverdin
(ii) Bile
(b) Hydrolysis of starch
(iv) Amylases
(c) Digestion of fat
(iii) Lipases
(d) Salivary gland
(i) Parotid
(a) Why are villi present in the intestine and not in the stomach?
(b) How does pepsinogen change into its active form?
(c) What are the basic layers of the wall of alimentary canal?
(d) How does bile help in the digestion of fats?
Answer (a) The mucosal wall of the small intestine forms millions of tiny finger-like projections known as villi. These villi increase the surface area for more efficient food absorption. Within these villi, there are numerous blood vessels that absorb the digested products of proteins and carbohydrates, carrying them to the blood stream. The villi also contain lymph vessels for absorbing the products of fat-digestion. From the blood stream, the absorbed food is finally delivered to each and every cell of the body. The mucosal walls of the stomach form irregular folds known as rugae. These help increase the surface area to volume ratio of the expanding stomach. (b) Pepsinogen is a precursor of pepsin stored in the stomach walls. It is converted into pepsin by hydrochloric acid. Pepsin is the activated in the form of pepsinogen. Pepsinogen $\xrightarrow{HCl}$ Pepsin + Inactive peptide (Inactive) (Active) (c) The walls of the alimentary canal are made up of four layers. These are as follows: (i) Serosa is the outermost layer of the human alimentary canal. It is made up of a thin layer of secretory epithelial cells, with some connective tissues underneath. (ii) Muscularis is a thin layer of smooth muscles arranged into an outer longitudinal layer and an inner circular layer. (iii) Sub-mucosa is a layer of loose connective tissues, containing nerves, blood, and lymph vessels. It supports the mucosa. iv. Mucosa is the innermost lining of the lumen of the alimentary canal. It is mainly involved in absorption and secretion. (d) Bile is a digestive juice secreted by the liver and stored in the gall bladder. Bile juice has bile salts such as bilirubin and biliverdin. These break down large fat globules into smaller globules so that the pancreatic enzymes can easily act on them. This process is known as emulsification of fats. Bile juice also makes the medium alkaline and activates lipase.Show Answer
Answer Pancreatic juice contains a variety of inactive enzymes such as trypsinogen, chymotrypsinogen, and carboxypeptidases. These enzymes play an important role in the digestion of proteins. Physiology of protein-digestion The enzyme enterokinase is secreted by the intestinal mucosa. It activates trypsinogen into trypsin. Trypsinogen $\xrightarrow{\text{ Enterokinase }}$ Trypsin + Inactive peptide Trypsin then activates the other enzymes of pancreatic juice such as chymotrypsinogen and carboxypeptidase. Chymotrypsinogen is a milk-coagulating enzyme that converts proteins into peptides. $ \underset{\text{(Inactive)}}{\text{Chymotrypsinogen}} \xrightarrow{\text{ Trypsin }} \underset{\text{(Active)}}{\text{Chymotrypsin}}$ Proteins $\xrightarrow{\text{ Chymotrypsin }}$ Peptides Carboxypeptidase acts on the carboxyl end of the peptide chain and helps release the last amino acids. Hence, it helps in the digestion of proteins. Peptides $\xrightarrow{\text{ Carboxypeptidase }}$ Smaller peptide chain + Amino acids Thus, in short, we can say that the partially-hydrolysed proteins present in the chyme are acted upon by various proteolytic enzymes of the pancreatic juice for their complete digestion. Proteins, peptones $\xrightarrow[\text{ Carboxypeptidase }]{\text{ Trypin/Chymotrypsin }} \text{Dipeptide and proteases}$Show Answer
Answer The digestion of proteins begins in the stomach and is completed in the small intestine. The digestive juice secreted in the gastric glands present on the stomach walls is called gastric juice. The food that enters the stomach becomes acidic on mixing with this gastric juice. The main components of gastric juice are hydrochloric acid, pepsinogen, mucus, and rennin. Hydrochloric acid dissolves the bits of food and creates an acidic medium so that pepsinogen is converted into pepsin. Pepsin is a protein- digesting enzyme. It is secreted in its inactive form called pepsinogen, which then gets activated by hydrochloric acid. The activated pepsin then converts proteins into proteases and peptides. Proteins $\xrightarrow{\text{ Pepsin }}$ Proteases + Peptides Rennin is a proteolytic enzyme, released in an inactive form called prorennin. Rennin plays an important role in the coagulation of milk.Show Answer
Answer Thedental formula expresses the arrangement of teeth in each half of the upper jaw and the lower jaw. The entire formula is multiplied by two to express the total number of teeth. The dental formula for milk teeth in humans is: $\frac{2102}{2102} \times 2=20$ Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, and 2 molars. Premolars are absent in milk teeth. The dental formula for permanent teeth in humans is: $\frac{2123}{2123} \times 2=20$ Each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2 premolars, and 3 molars. An adult human has 32 permanent teeth.Show Answer
Answer Bile is a digestive juice secreted by the liver. Although it does not contain any digestive enzymes, it plays an important role in the digestion of fats. Bile juice contains bile salts, bile pigments like bilirubin, biliverdin and phospholipids. Bile salts break down large fat globules into smaller globules so that the pancreatic enzymes can easily act on them. This process is known as emulsification of fats. Bile juice also makes the medium alkaline and activates lipase.Show Answer
Answer The enzyme trypsin (present in the pancreatic juice) activates the inactive enzyme chymotrypsinogen into chymotrypsin. $\underset{\text{(Inactive)}}{\text{Chymotrypsinogen}} \xrightarrow{\text{ Trypsin }} \underset{\text{(Active)}}{\text{Chymotrypsin}}$ The activated chymotrypsin plays an important role in the further breakdown of the partially-hydrolysed proteins. Proteins $\xrightarrow{\text{ Chymotrypsin }}$ Peptides The other digestive enzymes of the same category are trypsinogen and carboxypeptidase. These are secreted by the same source-gland, pancreas. Trypsinogen is present in an inactive form in the pancreatic juice. The enzyme enterokinase is secreted by the intestinal mucosa is activates trypsinogen into trypsin. Trypsinogen $\xrightarrow{\text{ Enterokinase }}$ Trypsin + Inactive peptide The activated trypsin then further hydrolyses the remaining trypsinogen and activates other pancreatic enzymes such as chymotrypsinogen and carboxypeptidase. Trypsin also helps in breaking down proteins into peptides. Proteins $\xrightarrow{\text{ Trypsin }}$ Peptides Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing the last amino acids. Peptides $\xrightarrow{\text{ Carboxypeptidase }}$ Small peptide chain + Amino acidsShow Answer
Answer The digestion of carbohydrates takes place in the mouth and the small intestine region of the alimentary canal. The enzymes that act on carbohydrates are collectively known as carbohydrases. Digestion in the mouth: As food enters the mouth, it gets mixed with saliva. Saliva is secreted by the salivary glands is contains a digestive enzyme called salivary amylase. This enzyme breaks down starch into sugar at $pH 6.8$. Starch $\xrightarrow[\text{ pH } 6.8]{\text{ Salivary amylase }}$ Maltose + Isomaltose + Limit dextrins Salivary amylase continues to act in the oesophagus, but its action stops in the stomach as the contents become acidic. Hence, carbohydrate-digestion stops in the stomach. Digestion in the small intestine: Carbohydrate-digestion is resumed in the small intestine. Here, the food gets mixed with the pancreatic juice and the intestinal juice. Pancreatic juice contains the pancreatic amylase that hydrolyses the polysaccharides into disaccharides. Starch $\xrightarrow{\text{ Amylase }}$ Disaccharides (Polysaccharides) Similarly, the intestinal juice contains a variety of enzymes (disaccharidases such as maltase, lactase, sucrase, etc.). These disaccharidases help in the digestion of disaccharides. The digestion of carbohydrates is completed in the small intestine. $\text{Maltose} \xrightarrow{\text{Maltose}} \text{2~Glucose}$ $\text{Lactose} \xrightarrow{\text{Lactose}} \text{Glucose + Galactose}$ $\text{Sucrose} \xrightarrow{\text{Sucrose}} \text{Glucose + Fructose}$Show Answer
Answer Hydrochloric acid is secreted by the glands present on the stomach walls. It dissolves bits of food and creates an acidic medium. The acidic medium allows pepsinogen to be converted into pepsin. Pepsin plays an important role in the digestion of proteins. Therefore, if $HCl$ were not secreted in the stomach, then pepsin would not be activated. This would affect protein digestion. A pH of about 1.8 is necessary for proteins to be digested. This $pH$ is achieved by $HCl$.Show Answer
Answer Digestion of fats: Butter is a fat product and gets digested in the small intestine. The bile juice secreted by the liver contains bile salts that break down large fat globules into smaller globules, so as to increase their surface area for the action of lipase. This process is referred to as emulsification of fats. After this, the pancreatic lipase present in the pancreatic juice and the intestinal lipase present in the intestinal juice hydrolyse the fat molecules into triglycerides, diglycerides, monoglycerides, and ultimately into glycerol. Fats $\xrightarrow[\text{ lipase }]{\text{ Pancreatic }}$ Triglycerides + Diglycerides Diglycerides and monoglycerides $\xrightarrow{\text{ Lipases }}$ Fatty acids + Glycerol Absorption of fats: Fat absorption is an active process. During fat digestion, fats are hydrolysed into fatty acids and glycerol. However, since these are water insoluble, they cannot be directly absorbed by the blood. Hence, they are first incorporated into small droplets called micelles and then transported into the villi of the intestinal mucosa. They are then reformed into small microscopic particles called chylomicrons, which are small, protein-coated fat globules. These chylomicrons are transported to the lymph vessels in the villi. From the lymph vessels, the absorbed food is finally released into the blood stream and from the blood stream, to each and every cell of the body.Show Answer
Answer The digestion of proteins begins in the stomach and is completed in the small intestine. The enzymes that act on proteins are known as proteases. Digestion in the stomach: The digestive juice secreted in the gastric glands present on the stomach walls is called gastric juice. The main components of gastric juice are $HCl$, pepsinogen, and rennin. The food that enters the stomach becomes acidic on mixing with this gastric juice. The acidic medium converts inactive pepsinogen into active pepsin. The active pepsin then converts proteins into proteases and peptides. Proteins $\xrightarrow{\text{ Pepsin }}$ Proteases + Peptides The enzyme rennin plays an important role in the coagulation of milk. Digestion in the small intestine: The food from the stomach is acted upon by three enzymes present in the small intestine is pancreatic juice, intestinal juice (known as succus entericus), and bile juice. Action of pancreatic juice Pancreatic juice contains a variety of inactive enzymes such as trypsinogen, chymotrypsinogen, and carboxypeptidases. The enzymes are present in an inactivated state. The enzyme enterokinase secreted by the intestinal mucosa activates trypsinogen into trypsin. Trypsinogen $\xrightarrow{\text{ Enterokinase }}$ Trypsin + Inactive peptide Theactivated trypsin then activates the other enzymes of pancreatic juice. Chymotrypsinogenis a proteolytic enzyme that breaks down proteins into peptides. Chymotrypsinogen $\xrightarrow{\text{ Trypsin }}$ Chymotrypsin Proteins $\xrightarrow{\text{ Chymotrypsin }}$ Peptides Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing the last amino acids. Peptides $\xrightarrow{\text{ Carboxypeptidase }}$ Smaller peptide chain + Amino acids Action of bile juice Bile juice has bile salts such as bilirubin and biliverdin which break down large, fat globules into smaller globules so that pancreatic enzymes can easily act on them. This process is known as emulsification of fats. Bile juice also makes the medium alkaline and activates lipase. Lipase then breaks down fats into diglycerides and monoglycerides. Action of intestinal juice Intestinal juice contains a variety of enzymes. Pancreatic amylase digests polysaccharides into disaccharides. Disaccharidases such as maltase, lactase, sucrase, etc., further digest the disaccharides. The proteases hydrolyse peptides into dipeptides and finally into amino acids. Dipeptides $\xrightarrow{\text{ Dipeptidases }}$ Amino acids Pancreatic lipase breaks down fats into diglycerides and monoglycerides. The nucleases break down nucleic acids into nucleotides and nucleosides.Show Answer
Answer Thecodont is a type of dentition in which the teeth are embedded in the deep sockets of the jaw bone. Ankylosis is absent and the roots are cylindrical. Examples include living crocodilians and mammals. Diphyodont is a type of dentition in which two successive sets of teeth are developed during the lifetime of the organism. The first set of teeth is deciduous and the other set is permanent. The deciduous set of teeth is replaced by the permanent adult teeth. This type of dentition can be seen in humans.Show Answer
Answer There are four different types of teeth in an adult human. They are as follows: (i) Incisors The eight teeth in the front are incisors. There are four incisors each in the upper jaw and the lower jaw. They are meant for cutting. (ii) Canines The pointy teeth on either side of the incisors are canines. They are four in number, two each placed in the upper jaw and the lower jaw. They are meant for tearing. (iii) Premolars They are present next to the canines. They are eightin number, four each placed in the upper jaw and the lower jaw. They are meant for grinding. (iv) Molars They are present at the end of the jaw, next to the premolars. There are twelve molars, six each placed in the upper jaw and the lower jaw. Hence, the dental formula in humans is $
\frac{2123}{2123} \times 2=32
$ This means each half of the upper jaw and the lower jaw has 2 incisors, 1 canine, 2 premolars, and 3 molars. Hence, an adult human has 32 permanent teeth.Show Answer
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Liver is the largest and heaviest internal organ of the body. It is not directly involved in digestion, but secretes digestive juices. It secretes bile which plays a major role in the emulsification of fats.
