Question

17. Describe the following events in the large intestine: motility, secretion, absorption, bacterial activity, and feces production. 18. Name the sphincters of the GI tract and describe their locations. 19. Compare the digestion of carbohydrates, fats and proteins in the small intestine. 20. Compare the absorption of carbohydrates, fats and proteins in the small intestine. 21. Make a table of the major regions of the digestive tract and the foodstuffs digested there.
Question “17”

Answer 1

17. ANS: Large intestine motility:

The large intestine uses Haustral contractions and mass movements for its motility. Haustral contractions are similar to the segmentation found in the small intestine (in that they both involve contractions of ring like muscles that just move material back and-forth), but the contractions in the large intestine occur much slower so as to provide additional lime for reabsorption to occur. The Haustral contractions are controlled by reflexes involving the intrinsic Nexuses.

Mass movements involve simultaneous contractions of ascending and transverse colons that act to move the digested food significant length along the large intestine, and occur rather quickly mass movement are controlled by the gastrocolic reflex (which occurs in response to the stomach receiving food), which is itself mediated by gastrin and extrinsic autonomic nerves. The movements ultimately move the feces into the rectum, where it waits to be expelled.

Large intestine secretion:

The large intestine secretes mucus from goblet cells both on their interior surface and buried crypts. This mucus lubricates the colon and provides cohesion to feces and also secretes bicarbonate ions to neutralize acids made by intestinal bacteria.

Large intestine absorption:

Much of the water in chyme is reabsorbed as material passes through the large intestine. Colic bacteria produce vitamin K, which the intestinal epithelium then absorbs Sodium and chloride are absorbed into these cell as well.

Large intestine bacteria and feces:

Large intestine good bacteria can create an acidic environment in the colon and this acidic environment inhibits bad bacteria like salmonella and E. coli from growing. Beneficial Bacteria help with the Production of the Lactase enzyme which is needed in order to break down food. Lactase is needed to digest milk products and without the enzyme, people will be intolerant. The colon bacteria digest small amounts of cellulose. Vitamin K, riboflavin, and thiamin are formed from the presence of bacteria.

During its passage through the colon, digested materials are processed into feces from reacting with the gut bacteria. The feces are received by the ascending colon which absorbs water and pushes the feces to the descending colon. The gut bacteria break down the fiber and release acetate, and butyrate. These chemicals are absorbed by the cell lining of the colon for nourishment.

18. ANS: Sphincters in the GI Tract

Sphincter                                                                             Location

1. Upper Esophageal Sphincter                     between esophagus and pharynx

2. Lower Esophageal (Cardiac) Sphincter    between low-end of esophagus and stomach

3. Pyloric Sphincter                                          between stomach/duodenum (Small Intestine)

4. Sphincter of Oddi                                         between duodenum/ducts of liver and pancreas

5. Ileocecal Sphincter                                        between the small/large intestine (Ileum and Cecum)

6. Internal Sphincter                                        Present in the rectum (Involuntary)

7. External Sphincter                                        Anal opening (Voluntary)

19, 20. ANS: Small intestine digestion and absorption:

Carbohydrates:

Digestion of carbohydrates means breaking down the complex starches into simple sugar units that are small enough to get readily absorbed. The disaccharides are broken down to monosaccharides in the walls of small intestines by specialized enzymes. Lacatase breaks down lactose to glucose and galactose. Sucrose is broken down by sucrase to produce glucose and fructose. In the small intestines the pancreatic amylase and dextrinase breakdown the polysaccharide from starch in to disaccharide.

The digested monosaccharides are absorbed and transported to liver through portal vein. The soluble fiber is metabolized to acids and gases in the large intestine through the activity of the bacteria. The insoluble fiber adds bulk to the feces and is excreted.

Fats:

Fat digestion mostly occurs in the small intestines by the release of hormone called cholecystokinin (CCK) from the intestinal cells. The CCK stimulates the gall bladder to release the bile and the pancreas to release the lipase. These are delivered to small intestines through the common bile duct. Bile acts as an emulsifier and breaks the fat into small droplets called micelles.

Due to emulsification the surface area of lipids is increased and the pancreatic lipase is allowed to break down the triglycerides into monoglycerides and free fatty acids efficiently. The fat digestion occurs rapidly due to release of increased amount of pancreatic lipase and colipase helps the lipase to act upon the micelles.

Proteins:

The enzymatic digestion of proteins begins in the stomach by secreting the hydrochloric acid. The action of pepsin is incomplete because it can break a few peptide bonds of large poly peptide.The partially digested proteins in the stomach enter the duodenum. The hormone secretin and cholecystokinin are released from the walls of the small intestine and these trigger the pancreas to release enzymes called proteases or protein splitting enzymes such as trypsin, chymotrypsin and carboxypeptidase. These enzymes are together break down the poly peptides into short peptides and amino acids.

The digested proteins amino acids are actively absorbed by cells of the small intestines.These amino acids reach the liver through portal vein and are used in protein synthesis, energy conversion and transported into blood to reach other cells.

The carbohydrates breakdown into simple glucose that enters the glycolysis. The fats are broken down to fatty acids that are subjected to beta oxidation in mitochondria to provide energy. The proteins are broken down to amino acids that can be converted to glucose through gluconeogenesis in liver.

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