Q.5 Describe the functions of hormones secreted by ovary in human beings. (2007)
Related Question -
Q. Describe functions of Ovarian harmones. (2016)
Ans. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are called gonadotropins because stimulate the gonads - in males, the testes, and in females, the ovaries. They are not necessary for life, but are essential for reproduction. These two hormones are secreted from cells in the anterior pituitary called gonadotrophs. Most gonadotrophs secrete only LH or FSH, but some appear to secrete both hormones.
As describe for thyroid-simulating hormone, LH and FSH are large glycoproteins composed of alpha and beta subunits. The alpha subunit is identical in all three of these anterior pituitary hormones, while the beta subunit is unique and endows each hormone with the ability to bind its own receptor.
Physiologic Effects of Gonadotropins: -
Physiologic effects of the gonadotrophins are known only in the ovaries and testes. Together, then regulate many aspects of gonadal function in both males and females.
Luteinizing Hormone: -
In both sexes, LH stimulates secretion of sex steroids from the gonads. In the testes, LH binds to receptors on Leydig cells, stimulating synthesis and secretion of testosterone. Theca cells in the ovary respond to LH stimulation by secretion of testosterone, which is converted into estrogen by adjacent granulosa cells.
In females, ovulation of mature follicles on the ovary is induced by a large burst of LH secretion known as the preovulatory LH surge. Residual cells within ovulated follicles proliferate to form corpora lutea, which secrete the steroid hormones progesterone and estradiol. Progesterone is necessary for maintenance of pregnancy, and, in most mammals, LH is required for continued development and function of corpora lutea. The name luteinizing hormone derives from this effect of inducing luteinization of ovarian follicles.
Follicle-Stimulating Hormone: -
As its name implies, FSH stimulates the maturation of ovarian follicles. Administration of FSH to humans and animals induces “superovulation”, or development of more than the usual number of mature follicles and hence, an increased number of mature gametes.
FSH is also critical for sperm production. It supports the function of Sertoli cells, which in turn support many aspects of sperm cell maturation.
Control of Gonadotropin Secretion: -
The principle regulator of LH and FSH secretion is gonadotropin-releasing hormone (GnRH, also known as LH-releasing hormone). GnRH is a ten amino acid peptide that is synthesized and secreted from hypothalamic neurons and binds to receptors on gonadotrophs.
In a classical negative feedback loop, sex steroids inhibit secretion of GnRH and also appear to have direct negative effects on gonadotrophs.
This regulatory loop leads to pulsatile secretion of LH and, to a much lesser extent, FSH. The number of pulses of GnRH and LH varies from a few per day to one or more per hour. In females, pulse frequency is clearly related to stage of the cycle.
Numerous hormones influence GnRH secretion, and positive and negative control over GnRH and gonadotropin secretion is actually considerably more complex than depicted in the figure. For example, the gonads secrete at least two additional hormones - inhibin and activin - which selectively inhibit and activate FSH secretion from the pituitary.
Disease States: -
Diminished secretion of LH or FSH can result in failure of gonadal function (hypogonadism). This condition is typically manifest in males as failure in production of normal numbers of sperm. In females, cessation of reproductive cycles is commonly observed.
Elevated blood levels of gonadotropins usually reflect lack of steroid negative feedback. Removal of the gonads from either males or females, as is commonly done to animals, leads to persistent elevation in LH and FSH. In humans, excessive secretion of FSH and/or LH most commonly the result of gonadal failure or pituitary tumors. In general, elevated levels of gonadotropins per se have no biological effect.
Q.6 Describe how carbon dioxide is transported from tissues to lungs by blood. (2006)
Ans. Transport of carbon dioxide is not nearly so great a problem as transport of oxygen because even in the most abnormal conditions carbon dioxide can usually be transported in far greater quantity than oxygen.
Under normal resting conditions an average of 4 mililitres of carbon dioxide is transported from the tissues to the lungs in each deciliter of the blood.
To begin the process of carbon dioxide transport carbon dioxide diffuses out of the tissue cells in the dissolved molecular form. On entering the capillary the carbon dioxide initiates a host of almost instantaneous physical and chemical reactions that are essential for carbon dioxide transport.
Transport of Carbon Dioxide in the Dissolved State- :
A small portion of carbon dioxide is transported in the dissolved state to the lungs. It will be recalled that the Pco2 of venous blood is 45 mmhg and that of arterial blood is 40 mmhg.The amount of carbon dioxide dissolved in the fluid of the blood at 45 mmhg is about 2.7 ml/dl.The amount dissolved at 40 mmhg is about 2.4 ml or a difference of 0.3 ml. Therefore only about 0.3 ml of carbon dioxide is transported in the form of dissolved carbon dioxide be each deciliter of blood. This is about 7%of all the carbon dioxide transported.
Transport of Carbon Dioxide in the Form of Bicarbonateions: -
Effect of Carbonic Anhydrase: -
The dissolved carbon dioxide in the blood reacts with water to form carbonic acid. This reaction would occur much too slowly to be of importance were it not for the fact that inside the red blood cells is an enzyme called carbonic anhydrase which catalyzes the reaction between carbon dioxide and water accelerating its rate about 500-fold.Instead of requiring many seconds or minutes to occur as is true in the plasma, the reaction occurs so rapidly in the red blood cells that it reaches almost complete equilibrium within a fraction of second. This allows tremendous amounts of carbon dioxide to react with the red cell water even before the blood leaves the tissue capillaries.
Dissociation of the Carbonic Acid into Bicarbonate and Hydrogenions: -
In another fraction of a second ,the carbonic acid formed in the red blood cells dissociates into hydrogen and bicarbonate ions. Most of the hydrogen ions then combine with the hemoglobin in the red blood cells because the hemoglobin protein is a powerful acid-base buffer. In turn many of the bicarbonate ions diffuse from the red cells into the plasma while chloride ions diffuse into the red cells to take their place. This is made possible by the presence of a special bicarbonate-chloride carrier protein in the red cell membrane that shuttles these two ions in opposite directions at rapid velocities. Thus the chloride content of venous red blood cells is greater than that of arterial red cells a phenomena called the chloride shift.
The reversible combination of carbon dioxide with water in the red blood cells under the influence of carbonic anhydrase accounts for about 70% of the carbon dioxide transported from the tissues to the lungs. Thus this means of transporting carbon dioxide is by far the most important of all the methods for transport.
Transport of Carbon Dioxide in Combination with Hemoglobin and Plasma Proteins-Carbaminohemoglown: -
In addition to reacting with water carbon dioxide reacts with amine radicals of the hemoglobin molecules to form the compound carbaminohemoglobin.This combination of carbon dioxide with the hemoglobin is a reversible reaction that occurs with a loose bond so that the carbon dioxide is easily released into the alveoli where the P co2 is lower than in the tissue capillaries.A small amount of carbon dioxide also reacts in the same way with the plasma proteins ,but this is much less significant because the quantity of these proteins is only one fourth as the quantity of hemoglobin.
The theoretical quantity of carbon dioxide that can be carried from the tissues to the lungs in the carbamino combination with hemoglobin and plasma proteins is about 30% of the total quantity transported.
Q.7 Explain the process of digestion and assimilation of fat. (2005, 2008)
Ans. Fats of the diet: -
By far the most abundant fat of the diet are the neutral fats also known as triglycerides each molecule of which is composed of a glycerol nucleus and three fatty acids .In the usual diet are also small quantities of phospholipids, chlolesterol and cholesterol esters.the phospholipids and cholesterol esters contain fatty acid and therefore can be considered fat themselves.
Digestion of Fats in the Intestine: -
A small amount of triglycerides is digested in the stomach by lingual lipase that is secreted by lingual glands in the mouth and swallowed with the saliva. The amount of digestion is less than 10%and generally unimportant. Instead essentially all fat digestion occurs in the small intestine.
Emulsification of Fat by Bile Acids and Lecithin: -
The first step in fat digestion is to break the fat globules into small sizes so that the water-soluble digestive enzymes can act on the globule surfaces .This process is called emulsification of fat and it is achieved partly by agitation in the stomach along with the products of stomach digestion but mainly under the influence of bile, the secretion of the liver that does not contain any digestive enzymes. However bile does contain a large quantity of bile salts as well as the phospholipids lecithin, both of which but especially the lecithin are extremely important for the emulsification of fat. The polar parts of the bile salts and lecithin molecules are highly soluble in water, whereas most of the remaining portions of their molecules are highly soluble in fat.Therefore the fat-soluble portions dissolve in the surface layer of the fat globule with the polar portions projecting outward and soluble in the surrounding fluids, this effect greatly decreases the interfacial tension of the fat.
When the interfacial tension of a globule of nonmiscible fluid is low this nonmiscible fluid, on agitation can be broken up into many minute particles far more easily than it can when interfacial tension is great. Consequently a major function of the bile salts and lecithin especially the lecithin, in the bile is to make the fat globules readily fragmentable by agitation in the small bowel. This action is the same as that of many detergents that are widely used in household cleaners for removing grease.
Each time the diameters of the fat globules are decreased by a factor of 2 as a result of agitation in the small intestine, the total surface area of the fat increases two times. In other words the total surface area of the fat particles in the intestinal contents is inversely proportional to the diameters of the particles. Because the average size of the emulsified fat particle in the intestine is less than I micrometer, this represents an increase of as much as 1000 fold in the total surface area of the fats caused by the emulsification process.
The lipases are water–soluble compounds and can attack the fat globules only on their surfaces. Consequently, it can be readily understood how important this detergent function of bile salts is for digestion of fats.
Digestion of Triglycerides by Pancreatic Lipase: -
By far the most important enzyme for the digestion of triglycerides is pancreatic lipase in the pancreatic juice. This is present in enormous quantities in pancreatic juice, enough to digest all triglycerides that it can reach within a few minutes. In addition the enterocytes of the small intestine contain a minute quantity of lipase known as enteric lipase, but this is usually unimportant.
End Product of Fat Digestion: -
Most of the triglycerides of the diet are split by pancreatic lipase into free fatty acids and 2 –monoglycerides .
Role of Bile Salts in Accelerating Fat digestion-Formation of Micelles: -
The hydrolysis of triglycerides is a highly reversible process therefore accumulation of monoglycerides and free fatty acids in the vicinity of digesting fat quickly blocks further digestion. The bile salts play an important role in removing the monoglycerides and free fatty acids from the vicinity of the digesting fat globules almost as rapidly as these end products of digestion are formed. This occurs in the following way.
Bile salts when in high enough concentration, have the propensity to form micelles, which are small spherical, cylidrical globules of 3 to 6 nanometers diameter composed of 20 to 40 molecules of bile salt. They develop because each bile salt molecule is composed of a sterol nucleus most of which is highly fat soluble, and a polar group that is highly water-soluble. The sterol nuclei of the 20 to 40 bile salt molecules of the micelle aggregate together with the fat digestates, to form a small fat globule in the middle of the micelle with the polar groups of the bile salts projecting outward to cover the surface of the micelle. Because these polar groups are negatively charged, They allow the entire fat micelle to become dissolved in the water of the digestive fluids and to remain in the stable solution despite the large size of the micelle.
During triglyceride digestion, as rapidly as the monoglycerides and free fatty acids are formed, the fatty acid portions of them become dissolved in the central fatty acid portion of the micelles, which immediately reduces the concentrations of these products of digestion in the vicinity of the digesting fat globules. Consequently the digestive process can proceed unabated.
The bile salt micelles also act as a transport medium to carry monoglyceride s and the free fatty acids, both of which would otherwise be relatively insoluble, to the brush border of the intestinal epithelial cells. There the monoglycerides and free fatty acids are absorbed. On delivery of these substances to the brush border, the bile salts are again released back into the chime to be used again and again for ferrying process.
Digestion of Cholesterol Esters and PhospHolipids: -
Most of the cholesterol in the diet is in the form of cholesterol esters, which are combinations of free cholesterol and one molecule of fatty acid. Phospholipids also contain fatty acid chains within their molecules. Both the cholesterol esters and the phospholipids are hydrolyzed by two other lipases in the pancreatic secretion that free the fatty acids-the enzyme cholesterol ester hydrolase to hydrolyse the cholesterol ester and phospholipase A2 , to hydrolyse the phospholipids.
The bile salts micelles play the same role in ferrying free cholesterol and the remaining portions of the digested phospholipids molecules as they play in ferrying monoglycerides and free fatty acids. Indeed this role of micelles is essential to the absorption of cholesterol because essentially no cholesterol can be absorbed without the function of the micelles. On the other hand, as much as 60% of the triglycerides can be digested and absorbed even in the absence of bile salts micelles.
Absorption of Facts: -
Monoglycerides and the fatty acids are carried to the surfaces of the microvilli in the brush border of small intestine. Here both of them diffuse immediately through the enterocyte cell membrane to the interior of the enterocyte, this is possible because these lipids are as soluble in the enterocytes membrane as in the micelles. This leaves the bile acid micelles still in the chime and absorb still more monoglycerides and free fatty acids and similarly carry these also to the epithelial cells. Thus the micelles perform a ferrying function, which is highly important for fat absorption. In the absence of abundance of bile acid micelles, about 97%of the fat is absorbed, in the absence of bile acids, only 40 to 50 % is normally absorbed.
The undigested triglycerides as well as the diglycerides are highly soluble in the lipid memebrane of the intestinal enterocyte. Even so only small quantities of them are normally absorbed because the bile acid micelles will not dissolve either triglycerides or diglycerides and therefore will not ferry them to the enterocyte membrane.
After entering the enterocyte, the fatty acids and monoglycerides are taken up by the smooth endoplasmic reticulum, and here they are mainly recombined to form new triglycrides. A few of the monoglycerides are further digested into glycerol and fatty acids by an intracellular lipase. Then these free fatty acids, too, are reconstituted by the smooth endoplasmic reticulum into triglycerides, using for this purpose new glycerol that is synthesized de novo from alpha-glycerophosphate, this synthesis requiring both energy from adenosine triphosphate and a complex of enzymes to catalyze the reactions.
Formation of Chylomicrons: -
Once formed, the reconstituted triglycerides aggregate first within the endoplasmic reticulum and then in the golgi apparatus into globules that contain absorbed cholesterol, absorbed phospholipids and small amounts of newly synthesized cholesterol, and phospholipids. The phospholipids arrange themselves in these globules with the fatty protions of the phospholipids toward the center and the polar portions located on the surface. This provides an electrically charged surface that makes these globules miscible with the fluids of the cell in addition, small amount of several types of apoprotein,also synthesized by the endoplasmic reticulum, coat part of the surface of each globule. In this form, the globules are released from the golgi apparatus and excreted by cellular exoytosis into the basolateral spaces around the cell; from there, they pass into the lymph in the central lacteal of the villus. These globules are then called chylomicrons.
The apoproteins are essentionl for cellular exocytosis of the chylomicrons to occur.
Especially apoprotein B, because they provide a means for attaching the fatty globule to the cell membrane before it is extruded. In people who have a genetic inability to form apoprotein B, the cannot proceed the rest of the way to be absorbed.
Transport of the Chylomicrons in the Lymph: -
From the basolateral surface of the enterocytes, the chylomicrons wend their way the central lacteals of the villi from here are propelled, along with the lymph, by the lymphatic pump upward through the thoracic duct to be emptied into the great veins of the neck. Between 80 and 90 per cent of all fat absorbed from the gut is absorbed in this manner and transported to the blood by way of the thoracic lymph in the form of chylomicrons.
Direct Absorption of Fatty Acids into the Portal Blood: -
Small quantities of short and medium chain fatty acids such as those from butter fat are absorbed directly into the portal blood rather than being converted into triglycerides and absorbed into the lymphatics. The cause of this difference between shorter –chain fatty acids are more water-soluble and mostly are not reconverted into triglycerides by the endoplasmic reticulum. This allows direct diffusion of these fatty acids from the epithelial cells into the capillary blood of the villi.
Q.8 Describe the process of digestion of proteins in alimentary canal. (2007, 09)
Related Question -
Q. Give an account of digestion of Proteins. (2016)
Q. Describe the digestion and absorption of proteins in the alimentary canal of human being. (2019)
Ans. Proteins of the Diet: -
The dietary proteins are formed of long chain of amino acids bound together by peptide linkages.
The characteristics of each type of protein are determined by the types of amino acids in the protein molecule and by the arrangement of these amino acids.
Digestion of Proteins in the Stomach: -
Pepsin the important peptic enzyme of the stomach is most active at a ph of about 2 to 3 and is inactive at pH 5.Consequently for this enzyme to cause any digestive action on protein, the stomach juices must be acidic. As such gastric glands secrete a large quantity of hydrochloric acid. This acid is secreted by the parietal cells at a pH of about 0.8, but by the time it is mixed with the stomach contents and with the secretions from the non parietal cells of stomach the pH ranges around 2 to 3, a highly favourable range of acidity for pepsin activity.
One of the important features of pepsin digestion is its ability to digest collagen, an albuminoid that is affected little by other digestive enzymes. Collagen is a major constituent of the intercellular connective tissue of meats, therefore for the digestive enzymes of the digestive tract to penetrate meats and digest the cellular proteins, it is first necessary that the collagen fibres be digested. Consequently, in people who lack peptic activity in stomach the ingested meats are less well penetrated by the digestive enzymes and therefore may be poorly digested.
Pepsin only begins the process of protein digestion, usually providing only 10 to 20 % of the total protein digestion. This splitting of proteins is a process of hydrolysis that occurs at the peptide linkages between the amino acids.
Digestion of Proteins by Pancreatic Secretion: -
Most protein digestion occurs principally in the upper small intestine ,in the duodenum and jejunum, under the influence of proteolytic enzymes of the pancreatic secretion. When the proteins leave the stomach. They ordinarily are in the form of proteoses, peptones, and large polypeptides. Immediately on entering the small intestine the partial breakdown products are attacked by the major proteolytic enzymes trypsin, chymotrypsin, carboxypolypeptidase and proelastase. Both trypsin and chymotrypsin can split protein molecules into small polypeptides, carboxypolypeptidase then cleaves individual amino acids from the carboxyl ends of the polypeptides. Proelstases give rise to elastase that in turn digests the elastin fibres that hold meats together. Only a small percentage of the proteins are digested all the way to their constituent amino acids by the pancreatic juices. Most remain as dipeptides, tripeptide and some even larger.
Digestion of Peptides by Peptidases in the Enterocytes that Line the Small Intestine: -
The last digestion of proteins in the intestinal lumen is achieved by the enterocytes that line the villi of the small intestine, mainly in the duodenum and jejunum. These cells have the brush border that consists literally of hundreds of microvilli projecting from the surface of each cell. In the cell membrane of each of these microvilli are multiple peptidases that protrude through the membranes to the exterior, where they come in contact with the intestinal fluids. Two types of peptidase enzymes are especially important, aminopolypeptidase and several dipeptidases. They succeed in splitting the remaining larger polypeptides into tripeptides and dipeptides and a few all the way to aminoacids. Both the amino acids and dipeptides and tripeptides are easily transported through the microvilli membrane to the interior of the enterocyte.
Finally inside the cytosol of the enterocyte are multiple other peptidases that are specific for the remaining types of linkages between the amino acids. Within minutes virtually all the last dipeptides and tripeptides are digested to the final stage of simple amino acids, they then pass through the opposite side of the enterocyte into the blood.