Topic No. & Title: Topic 8 Digestion & Absorption of Dietary Components

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1 [Academic Script] Digestion & Absorption of Dietary Components Subject: Zoology Course: B.Sc. 2 nd Year Paper No. & Title: Z-202B Physiology Topic No. & Title: Topic 8 Digestion & Absorption of Dietary Components Lecture Title: Digestion & Absorption of Dietary Components

2 Academic Script 1.Introduction: The major dietary components on which the body lives can be classified as carbohydrates, fats, and proteins. They generally cannot be absorbed in their natural forms through the gastrointestinal mucosa and useless as nutrients without preliminary digestion. Therefore, this chapter firstly discusses the processes by which carbohydrates, fats, and proteins are digested into small compounds for absorption, secondly the hormonal and neural regulation of digestion and lastly the mechanisms by which the digestive end products as well as water, electrolytes, and other substances are absorbed. 2.Digestion of Dietary Components: The term digestion may be defined as the process by which the complex food particles are broken down into simple unitary fragments suitable for absorption and assimilation. There are four digestive juices-saliva, gastric juice, pancreatic juice and succus entericus. Bile may be taken as the fifth one. Digestion is carried out by enzymes by process of hydrolysis. Digestion of Carbohydrates: - The different forms in which carbohydrates are taken in diet are (a) (b) Polysaccharides- starch and cellulose. Disaccharides- lactose, maltose, cane sugar

3 (c) Monosaccharides- glucose and fructose. Cellulose cannot be digested in the human alimentary canal. Monosaccharides need no further digestion, because all carbohydrates are absorbed in the form of monosaccharides. Hence, digestion of carbohydrates includes only the digestion of starch and disaccharides. When food is chewed, it is mixed with saliva, which contains digestive enzyme, a salivary amylase or ptyalin. This enzyme hydrolyzes starch and produce maltose and other small polymers of glucose. Gastric HCl can carry on some hydrolysis of cane sugar. Pancreatic juice contains two enzymes- pancreatic amylase acting on starch and dextrin and maltase acting on maltose. After pancreatic digestion starch is completely digested into maltose. So, when carbohydrates reach small intestine i.e. succus entericus, they come in form of disaccharides mainly maltose, lactose and cane sugar. The enzymes, the substrates upon which they act and respective end products are 1. Sucrase or invertase acts on cane sugar producing one molecule of fructose and one molecule of glucose. 2. Lactase acts on lactose giving one molecule of glucose and one molecule of galactose. 3. Maltase acts on maltose giving two glucose molecules. 4. Amylase acts on little quantity of starch and dextrin which might have escaped pancreatic digestion. The disaccharides-splitting enzymes are also found in intestinal mucosa. Some amount of disaccharides may enter the epithelial cells as such, and undergo final digestion into monsaccharides inside these cells.

4 Digestion of Proteins: - The different forms in which proteins are taken in diet are (a) (b) (c) (d) (e) (f) Various types of albumins and globulins. Nucleo-protein. Caseinogen of milk. Collagen and gelatin. Mucin. Elastin. Of these proteins, elastin cannot be digested. Proteins are formed from multiple amino acids that are bound together by peptide linkages. Pepsin is the proteolytic enzyme of gastric juice and converts all digestible proteins upto the peptone stage. Trypsin, chymotrypsin, carboxypolypeptidase and proelastase are the proteolytic enzyme of pancreatic juice that acts on proteose, peptones, polypeptides and convert them into lower peptides. Both trypsin and chymotrypsin split protein molecules into lower polypeptides; carboxypolypeptidase then cleaves individual amino acids from the carboxyl ends of the polypeptides. In prolonged tryptic digestion even some amino acids may be formed. Proelastase, in turn, is converted into elastase, which then digest elastin fibers. Peptidases acts upon lower peptides and converts them completely into amino acids, in which form they are absorbed.

5 Digestion of Lipids and Fats: - Forms in which fats are taken in diet are (a) Neutral fats. (b) Phospholipids. (c) cholesterol, fatty acids and glycerine. The last group does not require any digestion, because they are absorbed as such. Neutral fats are digested into fatty acids and glycerol. A small amount of triglycerides is digested in the stomach by enzyme lipase. Instead, all fat digestion occurs in the small intestine. Fat digestion begins when chyme, an acidic fluid mixture of gastric juice and partly digested food, passes from the stomach into the initial section of the small intestine, the duodenum. Digestion requires many specialized enzymes, as well as other secretions. The liver synthesizes bile which aids in digesting lipids. When the chyme enters the duodenum, a hormonal signal causes the walls of the gall bladder to contract rhythmically and squeezing the bile into the common bile duct and down to the duodenum. The pancreas produces lipases, essential in fat digestion. Fats are hydrophobic molecules, that is, they are not soluble in water.

6 However, most enzymes, lipase included, are water soluble and require an aqeous medium to function. Fat molecules would aggregate to form large globules that would present only on a surface area where the water soluble lipases could act. The bile molecules have a hydrophobic end, which is absorbed into the fat droplet, and a hydrophilic end, which sticks out. This hydrophilic coating prevents the fat droplets from aggregating. These stabilized tiny droplets of fat are called micelles. Once stabilized in micelles, lipase breaks down the fats into fatty acids and monoglycerides. 3.Hormonal & Neural Regulation of Digestion: Regulation of digestion by hormones occurs in 3 phases: 1. Cephalic phase 2. Gastric phase 3. Intestinal phase 1. Cephalic Phase During the cephalic phase of digestion, the smell, sight, thought, or initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem. The brain stem then activates the facial, glossopharyngeal, and vagus nerves. The facial and glossopharyngeal nerves stimulate the salivary glands to secrete saliva, while the vagus nerves stimulate the gastric glands to secrete gastric juice. The purpose of the cephalic phase of digestion is to prepare the mouth and stomach for food that is about to be eaten. 2. Gastric Phase

7 Once food reaches the stomach, the gastric phase of digestion begins. Neural and hormonal mechanisms regulate the gastric phase of digestion to promote gastric secretion and gastric motility. Neural regulation: Food of any kind distends the stomach and stimulates stretch receptors in its walls. Chemoreceptors in the stomach monitor the ph of the stomach chyme. When the stomach walls are distended or ph increases because proteins have entered the stomach and buffered some of the stomach acid, the stretch receptors and chemoreceptors are activated, and a neural negative feedback loop is set in motion from the stretch receptors and chemoreceptors. Nerve impulses propagate to the submucosal plexus, where they activate parasympathetic and enteric neurons. The resulting nerve impulses cause waves of peristalsis and continue to stimulate the flow of gastric juice from gastric glands. The peristaltic waves mix the food with gastric juice; when the waves become strong enough, a small quantity of chyme undergoes gastric emptying into the duodenum. The ph of the stomach chyme decreases, so it becomes more acidic and the distension of the stomach walls lessens because chyme has passed into the small intestine, suppressing secretion of gastric juice. Hormonal regulation: Gastric secretion during the gastric phase is also regulated by the hormone gastrin. Gastrin is released from the G cells of the gastric glands in response to several stimuli: distension of the stomach by chyme, partially digested proteins in chyme, the high ph of chyme

8 due to the presence of food in the stomach, caffeine in gastric chyme, and acetycholine released from parasympathetic neurons. Once it is released, gastrin enters the bloodstream, makes a roundtrip through the body, and finally reaches its target organs in the digestive system. Gastrin stimulates gastric glands to secrete large amounts of gastric juice. It also strengthens the contraction of the lower esophageal sphincter to prevent reflux of acid chyme into the esophagus, increases motility of the stomach, and relaxes the pyloric sphincter, which promotes gastric emptying. Gastrin secretion is inhibited when the ph of gastric juice drops below 2.0 and is stimulated when the ph rises. This negative feedback mechanism helps provide an optimal low ph for the functioning of pepsin, the killing of microbes, and the denaturing of proteins in the stomach. 3. Intestinal Phase The intestinal phase of digestion begins once food enters the small intestine. In contrast to reflexes initiated during the cephalic and gastric phases, which stimulate secretory activity and motility of stomach, those occurring during the intestinal phase have inhibitory effects that slow the exit of chyme from the stomach. This prevents the duodenum from being overloaded with more chyme than it can handle. In addition, responses occurring during the intestinal phase promote the continued digestion of foods that have reached the small intestine. These activities of the intestinal phase of digestion are regulated by neural and hormonal mechanisms. Neural regulation:

9 Distension of the duodenum by the presence of chyme causes the enterogastric reflex. Stretch receptors in the duodenal wall send nerve impulses to the medulla oblongata, where they inhibit parasympathetic stimulation and stimulate the sympathetic nerves to the stomach. As a result, gastric motility is inhibited and there is an increase in the contraction of the pyloric sphincter, which decreases gastric emptying. Hormonal regulation: The intestinal phase of digestion is mediated by two major hormones secreted by the small intestine: cholecystokinin and secretin. Cholecystokinin i.e. CCK is secreted by the CCK cells of the small intestinal crypts in response to chyme containing amino acids from partially digested proteins and fatty acids from partially digested triglycerides. CCK stimulates secretion of pancreatic juice that is rich in digestive enzymes. It also causes contraction of the wall of the gallbladder, which squeezes stored bile out of the gallbladder into the cystic duct and through the common bile duct. In addition, CCK causes relaxation of the sphincter of the hepatopancreatic ampulla, which allows pancreatic juice and bile to flow into the duodenum. CCK also slows gastric emptying by promoting contraction of the pyloric sphincter, produces satiety i.e. a feeling of fullness by acting on the hypothalamus in the brain, promotes normal growth and maintenance of the pancreas, and enhances the effects of secretin. Acidic chyme entering the duodenum stimulates the release of secretin from the S cells of the small intestinal crypts. In turn, secretin stimulates the flow of pancreatic juice that is rich in bicarbonate i.e. HCO - 3 ions to buffer the acidic chyme that enters the duodenum from the small intestine.

10 Besides this major effect, secretin inhibits secretion of gastric juice, promotes normal growth and maintenance of the pancreas, and enhances the effects of CCK. Overall, secretin causes buffering of acid in chyme that reaches the duodenum and slows the production of acid in the stomach. MAJOR HORMONES THAT CONTROL DIGESTION 1. Hormone : Gastrin Stimulus & Site of Secretion : Stomach chyme stimulate gastrin secretion by enteroendocrine G cells, located in the mucosa of pyloric antrum of stomach. Actions : Major effects: Promotes secretion of gastric juice, gastric mucosa and increases gastric motility. Minor effects: Constricts lower esophageal sphincter, relaxes pyloric sphincter. 2. Hormone : Secretin Stimulus & Site of Secretion : Acidic chyme stimulates secretion of secretin by enteroendocrine S cells in the mucosa of the duodenum. Actions : Major effects: Stimulates secretion of pancreatic juice and bile that are rich in HCO 3 -

11 Minor effects: Inhibits secretion of gastric juice and enhances effects of CCK. 3. Hormone : Cholecysto-kinin (CCK) Stimulus & Site of Secretion : Partially digested proteins, triglycerides, and fatty acids stimulate secretion of CCK by enteroendocrine CCK cells in the mucosa of the small intestine; it is also released in the brain. Actions : Major effects: Stimulates secretion of pancreatic juice rich in digestive enzymes. Minor effects: Inhibits gastric emptying, promotes normal growth and maintenance of the pancreas, and enhances effects of secretin. 4. Absorption of Dietary Components: Passage of digested nutrients from the gastrointestinal tract into the blood or lymph is called absorption. Absorption is an essential factor of complete digestion. Because, if not removed by absorption, the end products of digestion will accumulate and set up a reversible enzyme action. Absorption of materials occurs via passive diffusion, facilitated diffusion, osmosis, and active transport. About 90% of all absorption of nutrients occurs in the small intestine; the remaining 10% occurs in the stomach and large intestine. Any undigested or unabsorbed material left in the small intestine passes on to the large intestine.

12 Some substances, such as fructose molecules, need a little assistance to enter and exit cells. Proteins embedded in the cell membrane lightly bind to these substances and then change shape to allow the substance to move across the cell membrane. This process is called facilitated diffusion. Here substances are moved from an area of high concentration to an area of lower concentration. Amino acids need a lot of assistance to enter and exit cells. In addition to a protein in the membrane to serve as a carrier, energy in the form of ATP is also needed. This type of absorption called active transport. Here substances move against a concentration gradient. Water, fats, fat-soluble vitamins, and other small molecules such as oxygen, carbon dioxide and alcohol move easily across cell membranes. In a process called passive diffusion, these substances move from an area of higher concentration to an area of low concentration either by diffusing diretly across the cell membrane, or through a protein channel embedded in the membrane. Absorption of Water: Isosmotic Absorption: - Water is transported through the intestinal membrane by diffusion. Therefore, when the chyme is dilute enough, water is absorbed through the intestinal mucosa into the blood of the villi almost entirely by osmosis. Absorption of Ions: That is active transport of Sodium: -

13 20 to 30 grams of sodium is secreted by the intestinal secretions each day. In addition, the average person eats 5 to 8 grams of sodium each day. Therefore, to prevent net loss of sodium into the faeces, the intestine must absorb 25 to 35 grams of sodium each day. It is rapidly absorbed through the intestinal mucosa. Absorption of Chloride Ions : - In the upper part of the small intestine, chloride ion absorption is rapid and occurs mainly by diffusion that is, absorption of sodium ions through the epithelium creates electronegativity in the chyme and electropositivity in the paracellular spaces between the epithelial cells. Then chloride ions move along this electrical gradient to follow the sodium ions. Absorption of Bicarbonate Ions : - Large quantities of bicarbonate ions must be reabsorbed from the upper small intestine. The bicarbonate ion is absorbed in an indirect way, when sodium ions are absorbed; moderate amounts of hydrogen ions are secreted into the lumen of the gut in exchange for some of the sodium. These hydrogen ions in turn combine with the bicarbonate ions to form carbonic acid i.e. H 2 CO 3, which then dissociates to form water and carbon dioxide. The water remains as part of the chyme in the intestines, but the carbon dioxide is readily absorbed into the blood and subsequently expelled through the lungs. Thus, this is so-called active absorption of bicarbonate ions. Absorption of Carbohydrates: -

14 All carbohydrates are absorbed as monosaccharides. The capacity of the small intestine to absorb monosaccharides is estimated to be 120 grams per hour. As a result, all dietary carbohydrates that are digested normally are absorbed, leaving only indigestible cellulose and fibers in the faeces. Monosaccharides pass from the lumen through the apical membrane via facilitated diffusion or active transport. Fructose is transported via facilitated diffusion whereas glucose and galactose are transported into absorptive cells of the villi via secondary active transport. The transporter has binding sites for one glucose molecule and two sodium ions. Neither of these substance is transported unless all three sites are filled. Galactose competes with glucose to ride the same transporter. Because Na - and glucose or galactose moves in the same direction, they are symporter. Monosaccharides then move out of the absorptive cells via facilitated diffusion and enter the capillaries of the villi. Absorption of Proteins: - Most proteins are absorbed as amino acids via active transport. About half of the absorbed amino acids are present in food and the other half come from the body itself as proteins in digestive juices and dead cells that slough off the mucosal surface % of the protein present in the small intestine is digested and absorbed. Different transporters carry different types of amino acids. Some amino acids enter absorptive cells of the villi via Na - dependent secondary active transport processes that are similar to the glucose transporter. Other amino acids are actively transported by themselves. At least one symporter brings in dipeptides and tripeptides together with H -. The peptides are then hydrolyzed to

15 single amino acids inside the absorptive cells. Amino acids move out of the absorptive cells via diffusion and enter capillaries of the villus. Both monosaccharides and amino acids are transported in the blood to the liver by way of the hepatic portal system. If not removed by hepatocytes, they enter the general circulation. Absorption of Lipids and Fats: - Fats are digested to form monoglycerides and free fatty acids. Because these products are lipid-soluble, they can easily pass through the plasma membrane in the intestinal mucosa and enters the epithelial cells. Once inside the cells, the fatty acids and monoglycerides enter the endoplasmic reticulum, where they are synthesized into triglycerides, combined with cholesterol and phospholipids, and coated with a protein to form chylomicrons. The protein coat serves to make the chylomicrons water soluble, and to facilitate exocytosis. Chylomicrons serve as a shuttle to transport lipids through the body. The chylomicrons leave the mucosal cells via exocytosis and enter lymphatic vessels in the submucosa. From here, they are transported to the thoracic duct where they enter the bloodstream. 5.Summary: This chapter has been studied in three parts. The first part of the chapter has focussed on the different forms in which carbohydrates, proteins, lipids and fats are taken in diet and are digested to its lower forms under the influence of several enzymes. The second part has discussed hormonal and neural regulation of digestion under assistance of certain major hormones and neural reflexes. Lastly the

16 absorption of dietary components that mainly occurs via passive diffusion, facilitated diffusion and active transport has been elaborated. In this manner digestion and absorption of all dietary components has been studied in detail.

Topic No. & Title: Topic 8 Digestion & Absorption of Dietary Components

[Summary] Digestion & Absorption of Dietary Components Subject: Zoology Course: B.Sc. 2 nd Year Paper No. & Title: Z-202B Physiology Topic No. & Title: Topic 8 Digestion & Absorption of Dietary Components