Biology Class 11 Science Online Notes – Pancreas
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You can visit biology notes for class 11 Digestion in Stomach
The pancreas is an abdominal organ that is located behind the stomach and is surrounded by other organs, including the spleen, liver and small intestine. The pancreas is about 6 inches (15.24 centimeters) long, oblong and flat. It is located on the back of the abdomen, behind the stomach. The pancreas is connected to the duodenum (the first section of the small intestine) through a small tube called the pancreatic duct. It is an endocrine gland producing several important hormones including insulin, glucagon, somatostatin, and a pancreatic polypeptide which circulate in the blood. The pancreas is also an important digestive organ, secreting pancreatic juice containing bicarbonate to neutralize the acidity of chyme (The partly digested food mixed with gastric juices) which is expelled from the stomach, as well as digestive enzymes that assist digestion and absorption of nutrients in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme. The pancreas is also known as a mixed gland (it acts as an exocrine as well as endocrine glands).
The pancreas acts both as an endocrine and exocrine gland. The exocrine part of the gland consists of acini (singular is acinus, the acinus is a small sac-like cavity in a gland, surrounded by secretory cells). The endocrine part includes many cells known as islets of Langerhans. The acinar cells discharge their secretions into the main duct of the gland that communicates with the bile duct and releases their secretion into the duodenum through the sphincter of Oddi.
The exocrine pancreas produces 1200 to 1500 ml of pancreatic juice per day. This juice is colorless and has a pH of about 8. The pancreatic juice contains 1-2% of the organic material and 1% of the inorganic material. The organic matter is mainly protein in the form of enzymes. The most important inorganic component is sodium bicarbonate which makes the pancreatic secretion alkaline. Sodium bicarbonate is secreted from the epithelial cells lining the small pancreatic ducts. The acinar cells secrete several enzymes that help in digestion. One volume of pancreatic juice can neutralize equal volume of acidic gastric content. When the acidic gastric content passes through the pylorus (part of the stomach) into the duodenum, they are partially neutralized by the bicarbonate in the pancreatic juice for the effective enzyme action.
Digestive Enzymes in Pancreatic Juice
The pancreas secretes a magnificent battery of enzymes that collectively have the capacity to reduce virtually all digestible macromolecules into forms that are capable of, or nearly capable of being absorbed. Three major groups of enzymes are critical to efficient digestion:
- Proteases: Digestion of proteins is initiated by pepsin in the stomach, but the bulk of protein digestion is by the pancreatic proteases. Several proteases are synthesized in the pancreas and secreted into the lumen of the small intestine. The two major pancreatic proteases are trypsin and chymotrypsin, which are synthesized and packaged into secretory vesicles as the inactive proenzymes trypsinogen and chymotrypsinogen. Once trypsinogen and chymotrypsinogen are released into the lumen of the small intestine, they must be converted into their active forms in order to digest proteins. Trypsinogen is activated by the enzyme enterokinase, which is present in the intestinal juice. Once trypsin is formed it activates chymotrypsinogen, as well as additional molecules of trypsinogen.
Active Trypsin and Chymotrypsin acts on the native proteins and converts them to polypeptides and amino acids. Trypsin hydrolyzes the peptide linkages involving the carboxyl groups of arginine and lysine, while chymotrypsin acts on peptide bonds involving tyrosine and phenylalanine.
Trypsin and chymotrypsin digest proteins into peptides and peptides into smaller peptides, but they cannot digest proteins and peptides to single amino acids. Some of the other proteases from the pancreas, for instance, carboxypeptidase, have that ability, but the final digestion of peptides into amino acids is largely the effect of peptidases on the surface of small intestinal epithelial cells. Carboxypeptidase (is activated by both trypsin and enterokinase) helps in breaking the polypeptides into simpler peptides and amino acids by successive removal of amino acids containing free carboxyl group at the end of the polypeptide chain. This enzyme does not act upon dipeptides.
- Pancreatic Lipase
The fat in our diet is present mainly in the form of triglyceride, or neutral lipid. A triglyceride molecule cannot be directly absorbed across the intestinal mucosa. It is digested into a 2-monoglyceride and two free fatty acids. The hydrolysis of triglycerides is catalyzed by pancreatic lipase, which is delivered into the lumen of the gut as a constituent of pancreatic juice.
In addition to pancreatic enzymes, the bile salts (produced by the liver) must also be present in the lumen of the intestine in order for lipase to work efficiently in the digestion of dietary triglyceride and for the resulting fatty acids and monoglyceride to be absorbed. This means that normal digestion and absorption of dietary fat is critically dependent on secretions from both the pancreas and liver.
The major dietary carbohydrate for many species is starch, a storage form of glucose in plants. Amylase (technically alpha-amylase) is the enzyme that hydrolyzes starch to maltose (a glucose-glucose disaccharide), as well as the trisaccharide maltotriose and small branch points fragments called limit dextrins. The major source of amylase in all species is pancreatic secretions, although amylase is also present in the saliva of some animals, including humans.
Other Pancreatic Enzymes
In addition to the proteases, lipase, and amylase, the pancreas produces a host of other digestive enzymes, including ribonuclease, deoxyribonuclease, gelatinase, and elastase.
Regulation of pancreatic enzymes
When the acidic chyme enters the duodenum, the walls of duodenum secrete certain hormones into the blood, these hormones are secretin and cholecystokinin. The release of these hormones initiates the pancreas to produce the digestive enzymes. The secretion of digestive enzymes by the pancreas is also regulated by the autonomic nervous system.
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