Chapter 46: Nutrition and Digestion (Part 3, Week 12) Flashcards
[Start 46.4 Mechanisms of Digestion and Absorption in Vertebrates]
What are most of the ingested carbohydrates in omnivores such as humans?
Polysaccharides (polymers of glucose) starch and cellulose from plants and glycogen from animals.
The remainder consists of simple carbohydrates, such as the monosaccharides fructose and glucose in fruit, and disaccharides (combinations of two monosaccharides), such as lactose in milk. Humans also add the disaccharide sucrose (table sugar) to their food. Certain other animals consume sucrose from sources such as maple sap and sugarcane.
Why does almost all polysaccharide digestion take place in the small intestine?
Due to the action of amylase secreted into the instestine by the pancreas.
The products of starch digestion via amylase are molecules of the disaccharide maltose. Maltose and any ingested sucrose and lactose are broken down into monosaccharides—fructose, glucose, and galactose—by enzymes located on the brush border of the small intestine epithelial cells.
T/F A large amount of polysaccharide is digested in the mouth by salivary amylase.
False. Only a very small amount.
What is the product created from the digestion of starch?
Molecules of the disaccharide maltose.
What monosaccharides are available after the the breakdown of the disaccharides of Maltose and ingested sucrose and lactose?
Fructose, glucose, and galactose by enzymes located on the brush border of the small intestine epithelial cells.
The monosaccharides are then absorbed into the epithelial cells. Fructose crosses the apical membrane (the surface facing the lumen) of the epithelial cells by facilitated diffusion, whereas glucose and galactose undergo secondary active transport coupled to Na+.
How do the monosaccharides then leave the epithelial cells?
Facilitated diffusion through transporters located in the basolateral membrane of the epithelial cells and enter the blood.
The circulatory system distributes the monosaccharides and other absorbed nutrients to the cells of the body.
T/F Most adult cats are lactose-intolerant.
True. It is a misconception.
Some visits to the veterinarian are related to GI symptoms caused by well-meaning owners who regularly feed milk to their adult pets.
Why do people and/or animals lose the ability to digest lactose?
Because the only dietary source for lactose is milk, it is not surprising that older mammals lose the ability to digest this disaccharide.
This occurs because the gene that encodes the enzyme lactase is shut off at the age of weaning or shortly thereafter.
The developmental mechanisms that turn off lactase production and activity are not firmly established, but they are known to involve decreased transcription of the lactase-encoding gene.
What happens if an adult mammal were to drink milk and it is lactose intolerant?
As a result, water that normally would be absorbed by osmosis with the digested monosaccharides formed from lactose would also remain in the intestine.
Farther along the alimentary canal, microorganisms in the large intestine digest some of the lactose for their own use and, in the process, release by-products such as hydrogen and other gases.
The combination of water retention and bacterial action results in GI symptoms such as diarrhea, gas, and cramps.
What is lactose intolerance an example of?
A human polymorphism.
A genetic trait that varies among people.
Surprisingly, the coding regions of this gene and its core promoter are identical whether individuals are lactose-tolerant or lactose-intolerant.
Recently, however, Finnish investigators uncovered two single-nucleotide changes located in presumed regulatory sites that control the expression of the lactase-encoding gene. These changes are associated with prolonged expression of the gene, allowing it to continue to beactive after weaning.
People carrying these mutations are lactose-tolerant and can consume milk products through adulthood. Bycomparison, adults who lack these mutations—and are thereforelactose-intolerant—can consume dairy products only in smallamounts or not at all. However, their ability to do so can begreatly improved if the product has been commercially treatedwith purifi ed lactase to predigest the lactose.
How are proteins broken down to polypeptide fragments in the stomach? (2)
By pepsin and in the small intestine by proteases such as trypsin.
Why does the pancreas secrete its enzymes as inactive precursors?
This prevents the active enzymes from digesting the pancreas itself.
How is the inactive form of trypsin, called trypsinogen, activated in the small intestine?
The enzyme has two names.
It enters the small intestine, and is cleaved into the active molecule by the enzyme enterokinase AKA enteropeptidase.
The active site of which is located on the apical surfaces of intestinal epithelial cells.
T/F Trypsin activates the inactive forms of other pancreatic enzymes.
True.
How the small polypeptides from the broken down from polypeptides and proteins further digested and then absorbed?
The active site of which is located on the apical surfaces of intestinal epithelial cells breaks them down to amino acids.
Individual amino acids, coupled to Na+, then enter the epithelial cells by secondary active transport.
Amino acids leave these cells and enter the blood by facilitated diffusion across the basolateral membrane. Like carbohydrates, proteins are almost completely digested and absorbed in the duodenum of the small intestine.
How are individual amino acids cleaved by the proteases?
Together, these proteases cleave off one amino acid at a time from the N-terminus and C-terminus of polypeptide fragments.
What are most lipids in the form of and where does almost entirely all digestion occur?
Triglycerides
The small intestine.
What is the major fat-digesting enzyme, secreted by the pancreas?
Lipase
