H 2 Digestion

H2-1 State that digestive juices are secreted into the alimentary canal by glands including salivary glands, gastric glands in the stomach wall, pancreas, and wall of the small intestine

• In humans, 3 pairs of salivary glands secrete saliva into the mouth through ducts in the mouth’s mucosal lining • Saliva moistens and lubricates food to make it easier to swallow and keep it from scratching the esophageal tissue as it moves toward the stomach

• Saliva also contains the enzyme, salivary amylase, which starts the breakdown of carbohydrates (starch) in the mouth into maltose • Salivary gland secretion is controlled by the nervous system, maintaining a constant level of secretion to keep the mouth moist • Food in the mouth triggers increased secretion via taste-sensitive neurons which send impulses to the brain to stimulate the salivary glands • In the stomach, there is an extra layer of smooth muscle for churning food and mixing it with gastric juice

• Gastric juice is an acidic secretion of the tubular gastric glands of the mucosa • These exocrine glands contain parietal cells, which secrete hydrochloric acid, and chief cells which secrete pepsinogen, a protein-digesting enzyme that is activated by the low pH of the stomach which converts pepsinogen to pepsin, the active form of the enzyme • Having an inactive form of the enzyme (pepsinogen) which gets converted to the active form (pepsin) outside of the chief cells keeps the chief cells from digesting themselves • In the stomach, only proteins are partially digested

• The parietal cells also secrete intrinsic factor which is a polypeptide necessary for the absorption of Vitamin B12 which is required for the production of rbcs • Individuals who lack sufficient amounts of intrinsic factor develop a type of anemia called pernicious anemia

• The pancreas is an accessory organ that contributes secretions to the digestive tract • Pancreatic fluid is released into the duodenum via the pancreatic duct, therefore, the pancreas is an exocrine gland • Pancreatic fluid contains trypsin and chymotrypsin which digest proteins, pancreatic amylase which digests starch, and lipase which digests fat; pancreatic fluid is released by secretory cells called acini • These enzymes are inactive as they are released into the duodenum and are called zymogens • Zymogens are activated by the brush border enzymes of the small intestine

• The microvilli of the small intestine participate in digestion because a number of digestive enzymes are embedded within the epithelial cells’ plasma membrane, with their active sites exposed to the chyme • These brush border enzymes include those that hydrolyze the disaccharides lactose and sucrose, as well as others • The brush border enzymes complete the digestive process that started with the pancreatic enzymes released into the duodenum

H2.2 Explain the structural features of exocrine glands

• Multicellular exocrine glands are structurally complex with 2 parts: an epithelium-derived duct and a secretory unit consisting of secretory cells called acini • Supportive connective tissue surrounds the secretory unit and supplies it with blood vessels and nerve fibers • The connective tissue often forms a fibrous capsule that extends into the gland proper and divides the gland into lobes

• The pancreas, most sweat glands, and salivary glands are merocrine exocrine glands and release their products by exocytosis as they are produced, with the secretory cells not being altered in any way

• The secretory cells of a gland produce the product and secrete it (into acini). • The acini collect the product and pass it into the duct which leads into a lumen or out of the body

H2.3 Compare the composition of saliva, gastric juice and pancreatic juice

Saliva • Saliva is 99.5% water • Inorganic constituents include salts (chlorides, carbonates, phosphates, sulfates) and dissolved gases • Organic constituents include the enzymes amylase and lysozyme, proteins (mucin, albumin, and globulins), small amounts of urea, and unusual waste products such as acetone • Epithelial cells and leukocytes are also present

Gastric juice • The hormone gastrin increases the secretion of gastric juice • Pepsin digests proteins to peptones (secondary proteins; a water soluble compound formed by partial hydrolysis of a protein by an acid or enzyme during digestion) • Hydrochloric acid converts pepsinogen to active pepsin • Intrinsic factor in gastric juice combines with vitamin B12 (extrinsic factor) to prevent its digestion and promote its absorption in the small intestine • Gastric juice also contains mucus and the enzyme rennin (chymosin)

Pancreatic juice • Contains sodium bicarbonate which neutralizes the acidity of chyme • The enzyme trypsinogen is converted to the active trypsin by enterokinase, and trypsin in turn converts chymotrypsinogen to the active chymotrypsin (pancreatic proteases) • Amylase hydrolyzes starch to maltose and lipase digests emulsified fats to fatty acids and glycerol

H2.4 Outline the control of digestive juice secretion by nerves and hormones using the example of secretion of gastric juice

• The presence of food in the mouth triggers the production of the hormone gastrin by cells in the wall of the duodenum as well as cells in the part of the stomach nearest the duodenum • Secretions of the stomach are increased by parasympathetic impulses (vagus nerve) after sight or smell of food, and there is sustained release as stomach distention is detected by stretch receptors • Gastrin will stimulate acid production, and as pH falls too low in the stomach, gastrin release is inhibited by the hormones secretin and somastostatin

H2.5 Outline the role of membrane-bound enzymes on the surface of epithelial cells in the small intestine in digestion

• The villi and microvilli of the small intestine greatly increase the surface area of the small intestine • It is over this surface area that products of digestion are absorbed • The microvilli participate in digestion because a number of digestive enzymes are embedded within the epithelial cells’ plasma membranes, with their active sites exposed to the chyme

• These brush border enzymes include those that hydrolyze the disaccharides lactose and sucrose as well as others and complete the digestive process that started with the action of pancreatic enzymes released into the duodenum • Some digestive enzymes, such as maltase, are immobilized in the surface membrane of cells on the surface of intestinal villi. • These enzymes continue working even if the cell is rubbed off the villus and mixed into the intestinal contents

H2.6 Explain the reasons for cellulose not being digested in the alimentary canal

• Humans lack the enzyme necessary to digest cellulose (cellulase), the carbohydrate that functions as the chief structural component of plants • Much of the food value of plants is tied up in cellulose and the digestive tract of many animals harbor colonies of cellulose-digesting microorganisms • The relationships between these microorganisms and their animal hosts are mutually beneficial and provide an excellent example of symbiosis

• Animals that exhibit these symbiotic relationships include cows and sheep with cellulose-digesting bacteria and termites with an intestinal protozoan • Intestinal microorganisms also produce molecules such as vitamin K that are important to the well-being of their vertebrate hosts

H2.7 Explain why pepsin and trypsin are initially synthesized as inactive precursors, and how they are subsequently activated

• Chief cells which are found in exocrine glands of the stomach mucosa secrete pepsinogen, a weak protease that requires a very low pH to be active • The low pH is provided by the HCl which is also present in the stomach • Activated pepsinogen molecules then cleave one another at specific sites, producing a much more active protease, pepsin • The process of secreting an inactive enzyme which is then converted into one that is more active outside the cell prevents the chief cells from digesting themselves

• In humans, only proteins are partially digested in the stomach • Pancreatic fluid contains trypsin which along with chymotrypsin, digest proteins in the small intestine • Released primarily as the inactive enzymes, trypsinogen and chymotrypsinogen, they are activated by brush border enzymes of the intestine. • Trypsinogen is activated by enterokinase (a brush border enzyme) to trypsin, which in turn activates chymotrypsinogen • This prevents the pancreas from self-digestion

H2.8 Discuss the roles of gastric acid and Heliobacter pylori in the development of stomach ulcers and stomach cancers

• A stomach ulcer is an open sore in the stomach wall where digestive juices (acid and the enzyme pepsin) have begun to eat away at the stomach lining • Twenty years ago, it was believed that stress contributed to stomach ulcers and that cigarette smoking, alcohol abuse, and eating spicy foods were contributing factors

• It is now recognized that about 80% of ulcers are caused by infection from the bacterium called Heliobacter pylori • The remaining 20% are caused by overuse of anti-inflammatory drugs such as aspirin and ibuprofen• H. pylori survives in the stomach by producing the enzyme urease which neutralizes stomach acid and allows the bacterium to colonize the stomach’s mucous lining• This opens up the stomach wall to attack from digestive fluids• Infection with H. pylori is now considered as a primary cause of peptic ulcers and their recurrence

• The changes caused in the stomach lining by the bacterium link it as a strong risk factor for stomach cancer. • When an H. pylori infection is identified, it can be treated with antibiotics. • A blood test to check for H. pylori antibodies is available and can be easily done.

H2.9 Explain the problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this problem

• The main exocrine secretion of the liver is bile which is a fluid mixture consisting of bile pigments and bile salts • Bile salts play a very important role in the digestion of fats • Fats are insoluble in water making them difficult to digest and enter the intestine as large drops within the watery chyme

• Fat molecules are only accessible to the enzyme lipase at the lipid-water interface and can only work on the surface of the lipid sphere making digestion very slow • Lipase has an active site to which the hydrophobic lipid molecule can bind • The bile salts are lipid-soluble and partly water-soluble • They work like detergents and disperse the large fat drops into a fine suspension of smaller droplets

• This process, called emulsification, produces a greater surface area of fat upon which lipase can act, and allows the digestion of fats to occur more rapidly

• Fats are pretreated with bile salts • Lipase attaches at the water-lipid interface • The polar parts of the bile salt face the aqueous phase with the non-polar part clinging to the fat globule • The polar parts repel each other • They interact with water causing fatty globule to be broken up into smaller fat droplets forming an emulsion

Chemical digestionUsing a range of specialised enzymes that work in different conditions of the different compartments of the organ-system.