Chapter #14: Catabolism of BIomolecules Flashcards
What must be degraded into small molecules?
Proteins, lipids and polysaccharides that must be degraded into small molecules for absorption and transport.
How are proteins, lipids, and polysaccharides degraded?
Environment and hydrolytic enzymes
True/False
The digestion and absorption of proteins, lipids, and carbohydrates require a combination of chemical, mechanical, and biological processes.
True
Where does digestion begin?
Digestion begins in the mouth where food is mechanically degraded. Chewing converts the meal into a slurry that is more readily attacked by hydrolytic enzymes
Where does protein digestion begin?
Protein digestion begins in the stomach (pH = 1 to 2) with the enzyme pepsin, producing large protein fragments and some free amino acids.
What is beneficial about the low pH of the stomach in terms of protein digestion?
A. The low pH of the stomach denatures proteins, facilitating digestion
B. The optimal pH for pepsin activity is around 1-2.
C. All of the above.
D. None of the above.
All of the above
How is protein digested in the small intestine?
Small intestine: Secretions from the pancreas are added.
sodium bicarbonate (NaHCO3) to neutralize the acid contents of the stomach and bring the pH above 7.
a variety of enzymes including trypsin, chymotrypsin, elastase, and carboxypeptidases that function at a more alkaline pH.
How do zymogens relate to the digestion of protein?
Small intestine: The proteolytic enzymes are initially secreted as inactive *zymogens, then converted to active species.
The zymogen trypsinogen is activated by enteropeptidase, which is found in the intestinal mucosa, to form trypsin.
What happens in the small intestine after the protein are semi-digested?
Small intestine: After these enzymes have had their effect, the result is an abundance of free amino acids and peptides of two to eight amino acids in length.
The primary use of amino acids obtained through digestion or degradation is as building blocks for the synthesis of proteins and other nitrogenous compounds such as nucleotide bases.
What happened to the free amino acids, di-, and tri- peptides?
Specific transporters are present on the intestinal cells to take up amino acids and the smallest peptides.
How does digestion start on carbohydrates?
α-Amylase initiates digestion by cleaving α-1,4 bonds, but not α-1,6 bonds.
Other enzymes, including maltase, α-glucosidase, and αdextrinase complete the digestion.
Sucrose and lactose, two common disaccharides, are digested by sucrase and lactase, respectively.
What is found on the outer surfaces of epithelial cells of the small intestine?
Sucrase, lactase, and maltase
How are glucose and galactose transported into the intestine?
Glucose and galactose are transported into the intestine by the sodium-glucose linked transporter (SGLT) and the transporter GLUT5 allows entry of fructose.
What is lactose intolerance?
- Loss of lactase is normal in mammals (90% during the first four years of life).
- Consuming too much lactose without enzyme creates digestive issues.
- Retaining lactase into adulthood is abnormal (“lactose persistence”)
- Does raw milk accommodate “lactose intolerance”?
- Certain human populations have undergone a mutation on chromosome 2 which results in a bypass of the common shutdown in lactase production, allowing members of these populations to continue consumption of fresh milk and other milk products throughout their lives.
What is the main challenge with lipid digestion?
their hydrophobicity
Where does digestion of lipids start and what happens?
Triacylglycerols from the diet form lipid droplets in the stomach. Bile salts, secreted by the gallbladder, insert into the lipid droplets, rendering them more accessible to digestion by lipases.
Bile salts, such as glycocholate, facilitate lipid digestion in the intestine.
Ionized fatty acids generated by the action of lipases readily form micelles, which are carried to the intestinal epithelium cells for absorption.
What is the second step of lipid digestion?
Lipases, secreted by the pancreas, convert the triacylglycerols into 2 fatty acids and monoacylglycerol.
Where are free fatty acids and monoglycerides absorbed?
Free fatty acids and monoacylglycerols are absorbed by intestinal epithelial cells. Triacylglycerols are resynthesized and packaged with other lipids and proteins to form chylomicrons, which are then released into the lymph system.
The chylomicrons eventually enter the blood so that the triacylglycerols can be absorbed by tissues.
What is the take home message of the catabolism of biomolecules?
- Digestion requires a combination of physical, chemical, and biological processes.
- Proteins are digested by proteases in the stomach and small intestine and are absorbed in the small intestine.
- Starch is the main source of carbohydrates and difficult to digest. Carbohydrases cleave large molecules into small ones (di, tri).
- The hydrophobic nature of lipids is a challenge during digestion. The formation of micelles is essential for their digestion and absorption by the small intestine
