Answer of Question of Nutrition & Digestion
Q.13a. Describe digestion in a bivalve mollusc.
Ans. Digestion in Bivalves Digestion in bivalve molluscs is a coordination of three cycles: (1) Feeding,
(2) extracellular digestion, and (3) intracellular digestion. A resting stage is preparative for extracellular digestion. Feeding: Bivalve molluscs are mostly suspension and filter feeders and ingest small food particles. The gills trap food particles brought into the mantle cavity along with the incurrent water. The food trapping is
unclear, but once food particles are trapped, cilia move them to the gill’s ventral margin. Cilia along the ventral margin of the gills then move food toward the mouth. Cilia covering leaf like labial palps on either side of the mouth also sort filtered food particles. Cilia carry small particles into mouth. Digestion and Absorption: The digestive
tract has a short esophagus opening into a stomach. As food enters the stomach, the
rotating crystalline style and the enzymes released by the gastric shield (both present in stomach) mechanically and enzymatically break it down. the small food particles move into the
digestive diverticulae for intracellular digestion. These diverticulae in the stomach increase the surface area for absorption and intracellular digestion. Intracellular digestion releases the nutrients into the blood and produces the fragmentation spherules that both excrete wastes and lower the pH for optimal extracellular digestion. Sorting of food separates fine particles from indigestible coarse materials which are sent on to the intestine. Partially digested food from the stomach enters a digestive gland for intracellular digestion. Cilia carry indigested wastes in the digestive gland back to the stomach and then to the intestine. There is no marked division of gut into midgut and hind gut. The intestine empties through the anus near the excurrent siphon.
Q.14. How do heterotrophic protozoa feed?
Ans. Ciliated protozoa utilize heterotrophic nutrition. Ciliary Buccal cavity action directs the food from the Cytostome environment into the buccal Food vacuole cavity and cytostome. The cytostome opens into the cytopharynx cytopharynx, which enlarges as leaves food food enters and pinches off a food containing vacuole. The detached food vacuole moves through the cytoplasm. During this movement excess water moves out of the vacuole by exosmosis, the contents are acidified and then made alkaline and lysosomes add digestive enzymes. The food particles are digested within the vacuole (intracellular digestion) in cytoplasm. The residual vacuole then excretes its wastes via the cytopyge/ cytoproct. Fig. 5.4.
Q.14a. Give an aceount of digestion in cnidarians I Hydra, and a planarian.
Ans. Cnidarians have a sac like digestive gut i.e., the gut is a closed sac called a gastrovascular cavity, Fig. 5.5(a). It has only one opening that is both entrance for food, water, and exit for wastes. Some specialized cells in the body wall lining the cavity secrete digestive enzymes that begin the extracellular digestion. Other phagocytic cells that line the cavity engulf food materials and continue intracellular digestion inside the food vacuoles. Flatworms, such as planarians have similar patterns, however the digestive gut is branched providing more surface area.
In planaria there is a gastrovascular cavity which is extensively branched. It is also an incomplete digestive tract with only one opening. When a planarian feeds, its sticks its muscular pharynx out of its mouth and sucks in food. The gastrovascular cavity is branched so as to increase the absorptive surface area. The cavity is saclike Pharyngeal glands secrete enzymes. The food is partly digested extracellularly and digested food is absorbed in cells lining the cavity. In the digestive cavity, phagocytic cells engulf small food particles, and digestion is completed in intracellular vesicles..
.15. Describe process of digestion In an insect.
Ans. Insects, such as a grasshopper have a complete digestive tract, and the digestion is extracellular. Mandibles and maxillae cut and masticate the food (leaves) mixed with saliva from salivary glands, which is taken into the mouth and passed to the crop via esophagus. Saliva lubricates the food, and enzyme amylase in it begins the digestion of carbohydrates. In the crop food is stored temporarily where digestion continues. Enzymes, carbohydrases, lipases, proteases secreted from midget enter-the crop. From the crop food passes to the stomach, where it is mechanically grinded and nutrients are stored. Large particles are returned to crop for reprocessing, small particles enter the gastric cecae, where extracellular digestion is completed. Absorption then occurs in the intestine. Undigested food then passes to rectum, where water and ions are absorbed. Solid fecal pellets then pass out of the body via the anus. Fig. 5.5
Q.16. How do vertebrate teeth reflect feeding habits of various animals?
Ans. In vertebrates teeth are specialized according to the food and feeding habits of animals. The teeth of sharks and snakes, for example slope backwards to aid in the retention of prey while swallowing. Carnivores, such as members of the dog and cat families, generally have pointed incisors and canines that can be used to kill prey and rip away pieces of flesh. The jagged premolars and molars are modified for crushing and shredding, In contrast, herbivorous mammals, such as horses deer and cows, usually have teeth with broad, ridged surfaces that work like millstones for grinding tough plant material. The incisors and canines are generally modified for biting off pieces of vegetation. Humans, being omnivores adapted for eating both vegetation and meat, have a relatively unspecialized dentition. The permanent (adult) set of teeth is 32 in number. Beginning at the midline of the upper and lower jaw are two blade like incisors for biting, a pointed canine for tearing, two premolars for grinding, and three molars. 1 2 3 for crushing. The dental formula of man is
Beavers have front teeth chisel-like to cut branches and stems of trees. Elephants have two upper front teeth (tusks) specialized as weapons and for moving objects. Carnivores, such as cats have camassial teeth.for shearing the flesh. Fig.5.6.
Q.17. How does stomach of a ruminant function?
Ans. Ruminant mammals, such as cows, sheep, and deer show some of the most unusual modifications of the stomach for storing large amounts of food which they chew later, and providing an opportunity for large number of microorganisms which digest cellulose thus compensating disability of animals to digest it. The upper portion of the stomach expands to form a large pouch, the rumen, and a smaller reticulum. The lower portion of the stomach consists of a small antechamber, the omasum, followed by abomasum which is the true stomach (contains cardia, pylorus and fundus mucosa). Food first enters the rumen which secretes copious fluid into it and churns the food.
Microorganisms, symbiotic bacteria breakdown cellulose and release fatty acids as by-products of their metabolism. This food enters the reticulum; the animal periodically regurgitates and rechews the cud. which further breaks down the fibres, making them more accessible to further bacterial action. Later the pulpy mass moves into the reticulum and passed to omasum where water is removed. The cud containing great numbers of bacteria finally passes to the abomasum where digestive enzymes of the animal continue digestive process. Fig. 5.7.
Q.18. Name the component parts of the mammalian gastrointestinal tract. Name the accessory structures concerned with digestion.
Ans. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts. Following is a summary of primary and accessory organs in a human body. Fig.5.8.
Primary Organs of Digestion in Man
- Mouth; an opening for ingress into alimentary canal, guarded by muscular lips to prevent escape of food, leads into buccal cavity.
- In buccal cavity, the teeth grind the food. Food is tasted, moistened, and lubricated by mixing with saliva. Posteriorly buccal cavity leads into pharynx. The tongue contain taste buds and it also help push food back into pharynx.
- Pharynx is the intersection that leads to both the esophagus and the windpipe (trachea). A flap-like structure, the epiglottis closes the entrance to trachea when food is swallowed.
- Esophagus conducts food from pharynx to the stomach.
- Stomach is located on the left side of the abdominal cavity, just below the diaphragm. Digestion of proteins and churning of food occurs here.
- Small intestine, a long tube, the first 25 cm of it is called duodenum where bile from gall bladder and pancreatic juice from pancrease are added to acidic food, the remaining part is called jejunum and ileum. Digestion and much of the absorption of food occurs in small intestine.
- Large intestine or colon, connected to the small intestine at a T-oshaped junction, is a wider tube where some D absorption of salts and water takes place. It opens into rectum.
- opens into rectum. Rectum, here feces are stored until they can be eliminated.
- Accessory Organs/Glands The accessory glands of the mammalian digestive system are three pairs of salivary glands, the pancreas and the liver with its storage organ, the gall bladder.
- 1. Salivary glands; submaxillary,sublingual, and parotid are paired glands that secrete saliva that contain bicarbonates, salivary amylase, mucin, water, and thiocyanate ions.2 Pancreas releases assorted enzymes, and bicarbonate into duodenum
through pancreatic ducts.
- Liver , a large brownish organ synthesizes bile salts and bicarbonates. which are Stored in gall bladder.
4.Gall bladder stores and concentrates bile received from liver.
Q.19. How peristalsis and segmentation differ?
Ans. Two types of rhythmic or coordinated muscular
movements i.e., peristalsis and segmentation mix the food material with various
secretions and move the food from oral cavity to anus. In peristalsis, rings of circular muscles contract behind a mass of food material, and the mechanical pressure propels the material forward. As it moves, the mass expands the tube wall; the expansion stimulates peristalsis, and moves down the tract in a wave-like manner. Segmentation is another type of rhythmic muscular contractions which are oscillating back-and-forth movements in the same place in small and large intestine. This movement mixes the food with digestive secretions and increases the efficiency of absorption.
Q 20. How is gastrointestinal motility controlled?
Ans. The gastrointestinal tract is innervated with a network of nerves in sub mucosa, and longitudinal and circular muscle layers. The nerves receive information from; chemoreceptors, which respond to materials in gut i.e., carbohydrates, lipids, proteins; and mechanoreceptors, which respond to distension of the walls. Sympathetic and parasympathetic nerves in gut walls work antagonistically in controlling peristalsis and segmentation. Signals from parasympathetic nerves usually increase activity in the tract. Signals from sympathetic nerves cause contraction of some sphincters and thus control rate at which materials move forward. Many different hormones help regulate secretion of enzymes, digestion, and absorption. The best known hormones are gastrin, secretin, cholecystokinin, and gastric inhibitory peptide (GIP).
.21. What is the role of saliva?
Ans. In human beings, more than a litre of saliva is secreted into the oral cavity each
- day from three pairs of salivary glands-sub-maxillary (submandibular), sublingual, and parotids. Following are the functions of saliva: (1) Dissolved in saliva is a slippery glycoprotein called mucin, which protects the soft lining of the mouth from abrasion and lubricates the food for easy swallowing. (2) saliva contains
- Q.22. What are the functions of stomach?
Ans. The stomach is a muscular, distensible sac due to very elastic walls and accordion-like folds. It performs three important functions:
It stores and mixes the food bolus received from esophagus.
The epithelium that lines the lumen of the stomach secretes gastric juice that contains enzymes and hydrochloric acid which start digesting proteins. Mucus is also secreted which add water, and also coats the inner surface of the stomach and protects it from HCI and digestive enzymes.
It helps regulate the passage of chyme (pulpy food) into intestine with the help of pyloric sphincter.
Q.23. Describe process of digestion in stomach.
Ans. The inner epithelium that lines the lumen of the stomach contains thousands of gastric glands. Three types of secretary cells are preset in these glands; parietal cells secrete HCI; chief cells secrete pepsinogen which the HCI coverts into pepsin; and mucous cells that secrete mucus. Parietal cells and chief cells are in the pits of gastric glands, while mucous cells are at the surface epithelium surrounding the openings of the glands. Gastric secretion is controlled by a combination of nervous impulses and hormones. When we see smell, or taste food, impulses from the brain to the stomach initiate the secretion of the gastric juice. Then certain food substances (proteins) in the food stimulate the glands in stomach walls to release the hormone gastrin into the blood which when reaches back to stomach wall, the hormone stimulates further secretion of gastric juice. Fig. 5.10. Gastric pits
buffers (bicarbonate ions) that help prevent tooth decay by neutralizing acid in the mouth, (3) saliva contains antibacterial agents such as thiocyanate Ions that kill many of the bacteria that enter the mouth with food, (4) salivary amylase, a digestive enzyme that hydrolyses the glucose polymers, starch (from plants), and glycogen (from animals), is also present in saliva.
- Hydrochloric acid in the gastric juice converts pesinogen to active pepsin by removing a short segment of the proteins polypeptide chain, an alteration that exposes the active site of pepsin. Pepsin hydrolyzes proteins into smaller polypeptides. Hydrolysis is incomplete because pepsin can only break peptide bonds adjacent to specific amino acids. As a result of mixing and enzyme action the meal becomes a nutrient broth called acid chyme.Q.24. Why does not gastric juice digest walls of the stomach?Ans. A coating of mucus secreted by the epithelial cells helps protect the stomach lining from being digested by the pepsin and acid in gastric juice. Still, the epithelium is constantly eroded, and mitosis generates enough cells to completely replace the stomach lining every three days. Lesions in the stomach lining, called gastric ulcers, are caused mainly by bacterial (Helicobacter pylon), but they may worsen if pepsin and acid destroy the lining faster than it can regenerate.Q.25. Describe in detail digestion in the small intestine.Ans. Although some digestion of starch in oral cavity, and partial digestion of proteins in the stomach has already started, however most of the digestion of macromolecules in food occurs in the small intestine. It is about 7-8 metres in length and has 4cm diameter.The first 25cm or so of the small intestine is called the duodenum. It is here that acid chyme seeping from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and gland cells of the intestinal wall itself.The digestion of the carbohydrates, starch and glycogen begun by salivary amylase in the oral cavity continues in the small intestine. Pancreatic amylases hydrolyze starch, glycogen, and smaller polysaccharides into disaccharides, including maltose. The enzyme maltase completes the digestion of maltose, splitting it into two molecules of the simple sugar glucose. Maltase is one of a family of disaccharidases, each one specific for the hydrolysis of a different disaccharide. Sucrase, for instance, hydrolyzes table sugar (sucrose), and lactase digests milk sugar (lactose), (in general adults have much less lactase than children). The disaccharidases are built into the membranes and extracellular matrix covering the intestinal epithelium. Thus the terminal steps in carbohydrate digestion occur at the site of sugar absorption.Protein digestion in the small intestine involves completion of the work begun by pepsin in the stomach. Enzymes in the duodenum dismantle polypeptides into their component amino acids or into small peptides (fragments only two or three amino acids long). Trypsin and chymotrypsin are specific for peptide bonds adjacent to certain amino acids, and thus, like pepsin, break large polypeptides into shorter chains. Carboxypeptidase splits off one amino acid at a time, beginning at the end of the polypeptide that has a free carboxyl group. Aminopeptldase works in the opposite direction. Either aminopeptidase or carboxypeptidase alone could completely digest a protein. But teamwork among these enzymes and the trypsin and chymotrypsin that attack the interior of the proteins speeds up hydrolysis tremendously. Other enzymes called dipeptidases, attached to the intestinal lining, further hasten digestion by splitting small peptides.
The protein digesting enzymes, including trypsin, chymotrypsin, and carboxypeptidase, are secreted as inactive zymogens by the pancreas. An intestinal enzyme called enteropeptidase triggers activation of these enzymes within the lumen of the small intestine.
The digestion of nucleic acids involves a hydrolytic assault similar to that mounted on proteins. A team of enzymes called nucleases hydrolytizes DNA and RNA in food into their component nucleotides. Other hydrolytic enzymes then break nucleotides down further into nucleosides, nitrogenous bases, sugars, and phosphates.
Nearly all the fat in a meal reaches the small intestine completely undigested. Hydrolysis of fats is a special problem, because fat molecules are insoluble in water. Bile salts secreted into the duodenum coat tiny fat droplets and keep them from coalescing, a process called emulsification. Because the droplets are small, there is a large surface area of fat exposed to lipase, an enzyme that hydrolyzes the fat molecules.
Thus, the macromolecules from food are completely hydrolyzed to their component monomers as peristalsis moves the mixture of chyme and digestive juices along the small intestine. Most digestion is completed early in this journey, while the chyme is still in the duodenum. The remaining regions of the small intestine, the jejunum and ileum, function mainly in the absorption of nutrients.
Q.26. Give an account of hormonal control of digestion in humans.
Ans. At least four regulatory hormones help ensure that digestive secretions are present only when needed. We have already seen that gastrin is released from the stomach lining in response to the presence of food. The acidic pH of the chyme that enters the duodenum stimulates the intestinal wall to release a second hormone, secretin. This hormone signals the pancreas to release bicarbonate, which neutralizes the acid chyme. A third hormone, cholecystokinin (CCK), produced by cells in the lining of the duodenum, causes the gallbladder to contract and release bile into the small intestine. CCK also triggers the release of pancreatic enzymes. The chyme, particularly if rich in fats, also causes the duodenum to release a fourth hormone, enterogastrone, which inhibits peristalsis in the stomach, thereby slowing down the entry of food into the small intestine. Fig. 5.11. Let’s now follow the action of enzymes from the pancreas and intestinal wall in digesting macromolecules.
Q.27. Give an account of absorption of products of digestion in small intestine.
Ans. To enter the body, nutrients that accumulate in the lumen when food is digested must cross the lining of the digestive tract. A limited number of nutrients are absorbed in the stomach and large intestine, but most absorption occurs in the small intestine. Fig. 5.12.
The lining of the small intestine has a huge surface area of about 300 m2. Large circular folds in the lining bear fingerlike projections called villi, and each of the epithelial cells of a villus as many microscopic appendages called microvilli, which are exposed to the lumen of the intestine. Commonly called a brush border for its bristlelike appearance, the huge microvillar surface is an adaptation well suited to the task of absorbing nutrients.
Only two single layers of epithelial cells separate nutrients in the lumen of the intestine from the blood stream. Penetrating the core of each villus is a net of microscopic blood vessels, (capillaries) and a small vessel of the lymphatic system called a lacteal, (in addition to their circulatory system that carries blood, vertebrates have an auxiliary system of vessels — the lymphatic system — which carries a clean fluid called lymph. Nutrients are absorbed across the epithelium and then across the unicellular wall of the capillaries or lacteals. In some cases, the transport is passive. The simple sugar fructose, for example, is apparently absorbed by deffusion down its concentration gradient from the lumen of the intestine into the epithelial cells, then out of the epithelial cells into capillaries. Other nutrients, including amino acids, small peptides, vitamins, glucose, and several other simple sugars, are pumped against gradients by the epithelial membranes. The absorption of some nutrients seems to be coupled to the active transport of sodium across the membranes of the epithelial cells. The membrane pumps sodium out of the cell and into the lumen, and the passive reentry of the
Amino acids and sugars pass through the epithelium, enter capillaries, and are carried away from the intestine by the bloodstream. After glycerol and fatty acids are absorbed by epithelial cells, they are recombined within those cells to form fats again. The fats are then mixed with cholesterol and coated with special proteins, forming small globules called chylomicrons, which arb transported by exocytosis out of the epithelial cell and into a lacteal.
The capillaries and veins that drain nutrients away from the villi all converge into a single circulatory channel, the hepatic portal vessel, which leads directly to the liver. The rate of flow in this large vessel, about 1L per minute, ensures that the liver, which has the metabolic versatility to interconvert various organic molecules, has first access to nutrients absorbed after a meal is digested. The blood that leaves the liver may have a very different balance of nutrients from the blood that entered via the hepatic portal vessel. From the liver, blood travels to the heart, which pumps the blood and the nutrients it contains to all parts of the body.
Q.28. What role does pancreas play in the body of humans?
Ans. Pancreas is an important gland in the body.. It has both endocrine and exocrine functions.
Endocrien Functions Islets of langerhans release two important hormones i.e., insulin from p cells which lowers blood sugar level, and glucagon from a cells which increases blood sugar level by breaking down glucagon into glucose.
Exocrine cells (pancreatic acini) secrete a number of digestive enzymes into pancreatic duct which merges with the hepatic duct from the liver to form a
common bile duct that enters duodenum. The enzymes in active form (zymogens) released from pancreas are procarboxypeptidase, chymotripsinogen, and trypsinogen. An enzyme called enteropeptidase which is bound to the intestinal epithelium converts trypsinogen to trypsin, which then activates procarboxypeptidase to carboxypeptidase, and chymotrypsinogen to active chymotrypsin. These enzymes i.e. trypsin, carboxypeptidase and chymotrypsin digest proteins into small peptides and individual amino acids. Pancreatic lipases split triglycerides to glycerol and free fatty acids. Amylases convert polysaccharides to disaccharides and monosacharides. Pancreas also secretes bicarbonate that help neutralize acid food coming from stomach i.e. raise pH from 2 to 7.
Q.29. What is the function of large intestine?
Ans. Large intestine has no circular folds, villi, or microvilli. Small intestine opens into large intestine near a blind sac, the secum, with an extension called appendix both are storage sites. Appendix has lymphoid tissue and functions as part of the immune system. The major functions of large intestine include the absorption of water, and minerals, and formation and storage of feces. When water reabsorption is insufficient, diarrhoea results ,and when reabsorption is too Much, constipation results. Bacterial (escherichia cob) and fungi exist as symbionts. They secrete amino acids and vitamin K, which the host’s gut absorbs. Feces are expelled out through anus.
Q.30. Describe role of liver and gall gladder.
Ans. The liver is the largest organ in the _mammalian body. In the liver, millions of
specialized cells called hepatocytes take up nutrients absorbed from intestines and release them into the bloodstream. Some major functions of the liver include:
- The liver removes excess glucose’from the blood and stores it as glycogen, and reconverts glycogen into glucose to maintain the blood sugar level when the incoming supply is ineOficient.
- It resynthesizes glycogen from some of the lactic acid produced by muscles during glycolysis.
- It plays a major role in the interconversion of various nutrients, such as the conversion of carbohydrates into fats, of incoming fats into fats more typical of the organism’s own body, of amino acids into carbohydrates or fats.
- It deaminates amino acids, converts the ammonia thus obtained into urea, uric acid, or some other compound, and releases the nitrogenous wastes into the blood.
- It detoxifies a great variety of injurious chemical compounds and is thus one of the body’s most important defenses against poisons such as alcohol and barbiturates.
- It manufactures many of the plasma proteins, including fibrinogen, prothrombin, albumin, and some globulin.