Chapter 16: DIGESTION AND ABSORPTION
All the living organisms require the presence of energy to do various functions of life. They get this energy from the food they eat. Food is also needed for growth and development of the body. Nutrition is defined as, the substance in total from which an organism derives its energy to do work and other materials for its growth, development and maintenance of life.
“The breaking down of complex and insoluble organic substances such as carbohydrates, proteins and fats into simpler and soluble substances like glucose, amino acids and fatty acids respectively so that they can be easily absorbed into the body is known as digestion. This is a hydrolytic process and is carried out by various enzymes.”
Alimentary or Digestive system:
Alimentary canal is a tube present in all higher animals starting from mouth and reaching up to anus. Various glands located on its wall produce digestive juices that help in the process of digestion. Two glands namely liver and pancreas are also associated with it. They also produce the digestive juices. The digested food is also absorbed into the alimentary canal and undigested and indigestible food is passed out of the body through anus.
Mammalian Alimentary System:
In man the total length of alimentary canal is about 21 feet and consists of the following parts:
Mouth leads into a buccal cavity. The opening of the mouth is provided with lips. At the floor of the buccal cavity a muscular tongue is present. It helps in the ingestion, mastication and swallowing of food. It has got taste buds on its surface. Most of the mammals possess teeth on both the jaws. They are present in the cavity or socket of gums (thecodont dentition). The number and types of teeth vary in mammals. In man, there are 32 teeth of four different types namely incisors, canines, premolars and molars. This type of dentition is known as heterodont dentition. Their number can be represented by the dental formula: In each half of jaw;
Upper jaw I(2); C(1); PM(2); M(3)
----------------------------------------------- = 32
Lower jaw I (2); C (1); PM (2); M (3)
The incisor teeth are chisel-shaped and have sharp cutting edges. Canines are dagger-shaped and pierce the food. They are very large and well developed in predatory animals. Premolars and molars are broad and strong crushing teeth. Thus the incisors are used for biting; the canines for tearing the food; and premolars and molars for grinding the food. With the help of the teeth, tongue and jaw movements, food is chewed and mixed with saliva in the mouth.
There are three pairs of salivary glands namely parotids, submaxillary (submandibular) and sublingual glands. Their secretion is collectively known as saliva that is poured into the buccal cavity. Saliva usually contains enzymes and mucin. The enzyme present in saliva is known as ptyalin that helps in the digestion of carbohydrates; while mucin helps to lubricate the food for swallowing.
The mouth leads to a funnel-shaped pharynx, which communicates with a long muscular tube called oesophagus. The oesophagus opens into a muscular sac like structure called as stomach. In man, it is somewhat J-shaped and occupies the left side of the abdomen. The stomach opens into the small intestine. The stomach has many glands on its wall. Stomach wall produces gastric juice, which chiefly contains HCl, mucin and two protein digesting enzymes – rennin and pepsin. The muscles of the stomach wall churn and mix the food with gastric juice. Stomach through its pyloric region opens into small intestine. It is differentiated into three regions viz., duodenum, jejunum and ileum. Duodenum is U-shaped and gets the common bile duct and pancreatic duct from the gall bladder and pancreas. Jejunum is longer and more coiled. Ileum is the last part of small intestine and opens into the large intestine. Its wall has numerous long, finger-like projections called villi, which enhance absorption. Small intestine is the main region where digestion and absorption of food occurs. It has large number of tubular glands that produce the intestinal juice containing a number of enzymes, which digest various types of food. Digestion of different nutrients is completed in the small intestine by the action of pancreatic juice, intestinal juice and bile juice. The end products of digestion are then absorbed from the small intestine.
The small intestine opens into the large intestine. It is comparatively much shorter and wider than the small intestine. It does not have villi. It is also differentiated into three regions: caecum, colon and rectum. Caecum is a small pouch-like structure and its main part is vermiform appendix. However, caecum is very well developed in herbivorous animals like horse and ass. The colon is longest and has four parts; ascending colon, transverse colon, descending colon and pelvic colon. The pelvic colon opens into the rectum. Rectum is the last part of large intestine. Both in colon and rectum most of the water is reabsorbed back while the undigested food is removed from the body as faecal matter through anus. This is known as Egestion.
Glands associated with alimentary canal:
Pancreas: It is located in between the loops of duodenum. It is the second largest gland of the body. It secretes pancreatic juice that contains large number of digestive enzymes for digesting starch, lipids, proteins and nucleic acids. The pancreatic juice is released into the pancreatic duct, which joins with the common bile duct.
Liver: It is the largest gland of the body lying immediately below the diaphragm in the right upper part of abdomen. The cells of the liver (hepatic cells) produce bile juice that contains bile pigments and bile salts. These bile salts help in the digestion and absorption of fats. Bile juice does not contain any enzyme. Bile juice flows out of the liver through hepatic ducts forming the common bile duct that opens into the duodenum (when the food is present in the duodenum). When there is no food in the duodenum, then bile juice is stored in the gall bladder. The gall bladder is a small elongated, muscular sac below the liver. When the food comes into duodenum, it contracts to release the bile juice.
Digestion of Carbohydrates:
Carbohydrates are of three types: polysaccharides, disaccharides and monosaccharides. During the process of digestion both poly-and disaccharides are broken down to monosaccharides and in this form they can be absorbed into the body. Some of these complex carbohydrates are starch and cellulose, present in cereal grains, potato, fruits and tubers; sucrose present in cane sugar; lactose present in milk etc. Enzymes that act on carbohydrates are collectively known as carbohydrases.
In the mouth cavity, the food is mixed with saliva. It contains an enzyme called salivary amylase or ptyalin. Salivary amylase acts on starch and convert it into maltose, isomaltose and small dextrins or `limit dextrin’(disaccharides). Chewing and mastication of food increases the action of salivary amylase on starch by increasing the surface area of food on which the enzyme acts. About 30 percent of starch present in food is hydrolysed in the mouth. The action of salivary amylase continues for sometime even in the stomach but soon HCl present in the gastric juice destroys the entire enzyme.
Starch --------------> Maltose + Isomaltose + Dextrin
Pancreatic juice and intestinal juice also contain carbohydrates digesting enzymes. Pancreatic juice contains pancreatic amylase that acts on starch to digest it into maltose, isomaltose and dextrin. Intestinal juice contains number of carbohydrates like maltase, isomaltase and sucrase and lactase. Maltase and isomaltase act on maltose, isomaltose and dextrins and convert into glucose; sucrase acts on sucrose to convert it into glucose and fructose; and lactase acts on lactose to convert it into glucose and the galactose.
Starch ------------> Maltose + Isomaltose + Dextrin
Maltose + Isomaltose + Dextrin ------------> Glucose
Sucrose ------------> Glucose + Fructose
Lactose -----------> Glucose + Galactose
Only human being can digest lactose present in the milk. But with advancing age, they also cannot digest milk. This is because less of lactase is produced. In them, lactose remains undigested and gets fermented in the intestine producing gases and acids. This results in intestinal disorder and diarrhoea. So these persons must consume curd or yoghurt (sweetened curd) as lactase is fermented to lactic acid in them. This will not pose any digestive problem to them.
Many of the herbivorous animals can digest cellulose by the microorganisms (bacteria and protozoa) present in their alimentary canal. These microbes ferment cellulose into short chain fatty acids such as acetic acid and propionic acid. These acids are then absorbed and utilized by the animal. This is, thus, an example of symbiotic digestion. Microbes may be present in the rumen and reticulum part of stomach (cow and buffaloes); or in the large intestine (horse and donkeys).
Digestion of proteins:
Proteins are complex organic compounds made up of single units called amino acids. In the process of digestion, proteins are broken down to amino acids. Enzymes that hydrolyze protein are collectively known as proteases or peptides. Many of these enzymes are secreted in their inactive form or proenzymes. These inactive enzymes are converted to their active form only at the site of action.
Protein digestion starts in the stomach. The gastric glands of stomach produce a light coloured, thin and transparent gastric juice. It contains hydrochloric acid and pepsinogen. The H+ ions present in HCl converts pepsinogen into pepsin. The presence of HCl makes the medium highly acidic so that pepsin can act on proteins to convert them into peptones. HCl also helps to kill bacteria and other harmful organisms that may be present along with the food. Calf gastric juice contains another milk coagulating protease, called rennin. It is secreted as inactive pro-rennin. In the presence of HCl, the inactive prorennin is converted into their active form, i.e., rennin. Rennin acts on the casein protein of milk and converts it into paracasein, which in the presence of calcium ions forms calcium paracaseinate (curdling of milk). The function of rennin is then taken over by pepsin and other milk-coagulating enzymes. Adult cows or human infants do not produce rennin.
Both pancreatic juice and intestinal juice are poured into small intestine. Pancreatic juice contains trypsinogen, chymotrypsinogen, carboxypeptidases, lipases, amylases, DNAases and RNAases. All these enzymes of pancreatic juice can act only in the alkaline medium. This change in the medium of food, from acidic to alkaline, is done by the bile juice. Therefore, bile juice acts on the food before the action of pancreatic juice. In the intestinal lumen, pancreatic and intestinal juices mix together. Then a protease of intestinal juice, called Enteropeptidase or Enterokinase acts in coordination with pancreatic proteases. This enterokinase converts inactive trypsinogen into active trypsin. In predatory animals, trypsins can hydrolyse fibrinogen of blood into fibrin leading to blood coagulation. But it is unable to bring about coagulation of milk. The inactive Chymotrypsinogen is activated to chymotrypsin by trypsin. Chymotrypsins can hydrolyse casein into paracasein, which then coagulates to form calcium paracaseinate. But it acts in the alkaline medium. Chymotrypsin acts on other proteins and converts them into peptides. Carboxypeptidase hydrolyses the terminal carboxyl groups from peptide bonds to release the last amino acids from the peptides thus making the peptide shorter.
The intestinal juice contains aminopeptidases and dipeptidases; and enterokinase or enteropeptidase. Out of these enterokinase activates the trypsinogen. Aminopeptidase hydrolyses the terminal amino group from peptide bonds to release the last amino acid from the peptides thus making the peptide shorter. Dipeptidase acts on dipeptides to release the individual amino acids.
Digestion of fats:
Fat digestion starts only when the food reaches the small intestine. It starts with the action of bile juice from liver. Bile juice contains bile salts, which are secreted by the liver in the bile. Bile salts break down the bigger molecules of fat globules into smaller droplets by reducing the surface tension of fat droplets. This process is known as emulsification of fats.
Lipase is the enzyme that acts on emulsified fats. It is present both in the pancreatic juice and intestinal juice. Lipase converts emulsified fats into diglycerides and monoglycerides releasing fatty acids at each step. At the end of digestion, all fats are converted into fatty acids, glycerol and monoglycerides.
During the process of digestion proteins are changed to amino acids, carbohydrates to glucose, fructose and galactose, fats to fatty acids, glycerol and monoglycerides. These end products of digestion are finally absorbed in small intestine. So absorption can be defined as a process by which nutrient molecules are taken into the cells of the body. For this purpose, intestine has vast surface area of absorption by the presence of numerous villi. Further, this area is increased by microvilli present on the free surface of epithelial cells.
Passive absorption: When the nutrients are absorbed by simple diffusion, then it is known as passive absorption. Various amino acids and monosaccharides diffuse into the blood capillaries of villi. This is dependent on the fact that these nutrients are more in concentration in the intestine than in the cells. Further, these molecules are small and water soluble. All the amino acids and monosaccharides are not absorbed in this way.
Water is absorbed from the intestine to the intestinal cells and finally to the blood by the process of osmosis. This occurs when the solute concentration in the blood is higher (hypertonic). Thus, whenever any solute is absorbed from the intestine, it also results in the absorption of water.
Active absorption: This process occurs against the concentration gradient, i.e., nutrients may be more in intestinal cells than in the lumen of intestine. It requires the expenditure of energy i.e., ATP. Various nutrients like amino acids, glucose, galactose, Na+ ions can be absorbed by active transport. After their passive absorption, they are completely absorbed by active transport. For the active absorption of Na+ ions, a mechanism of sodium pump operates in the cell membranes.
Micelles in fat absorption/Role of bile juice in the absorption of fats:
As the fatty acids and glycerol are insoluble in water, the intestine cannot directly absorb them. So they cannot reach the blood stream directly. Instead, they are passed into lymph capillaries of the villi called lacteals. Digested fats are first incorporated into small, spherical droplets called micelles with the help of bile salts and phospholipids in the intestinal lumen. In the lacteals, fats are resynthesised into very small fat molecules called chylomicrons. An obstruction in the bile duct may prevent the entry of bile juice into the small intestine (obstructive jaundice) as a result unabsorbed fats are removed from the body along with the faecal matter. Thus bile plays an important role in the absorption of fats.
Balanced diet: To maintain normal functioning of our body, we need varieties of food so that all the systems are well maintained. A diet, which contains adequate amount of all the essential nutrients, is known as balanced diet. It varies according to age and occupation. A balanced diet should have the following three qualities:
· It must be rich in various essential nutrients like vitamins, minerals and some amino acids.
· It should provide enough raw materials needed for the growth and development, repair and replacement of cells, tissues and organs of the body.
· It should provide the necessary energy required by the body.
Disorders of Digestive System:
1. Jaundice: The liver is affected; skin and eyes turn yellow due to the deposit of bile pigment.
2. Vomiting: It is the ejection of stomach contents through the mouth and controlled by the centre in the medulla oblongata.
3. Diarrhoea: Abnormal bowel movement and the faecal discharge with more liquidity, which leads to dehydration.
4. Constipation: the feces are retained within the rectum due to irregular bowel movement.
5. Indigestion: food is not properly digested leading to a feeling of fullness due to inadequate enzyme secretion, anxiety, food poisoning, over eating nd spicy food.