Textbook Chapter 14: Digestion Flashcards
Digestion
the biochemical set of reactions by which food is converted into molecules that will be further manipulated to yield biologically useful energy and biosynthetic building blocks.
Proteins are digested to amino acids by —— secreted by ——-.
- proteolytic enzymes (proteases)
- the stomach and pancreas
Polysaccharides such as starch are cleaved into
monosaccharides by —— from the ——-
- a-amylase
- pancreas and to a lesser extent in saliva
Lipids are converted into fatty acids by —– secreted by the ——.
- lipases
- pancreas
All of the digestive enzymes are —–; that is, they cleave their substrates by ——.
- hydrolases
- the addition of a molecule of water
With the exception of —— all digestive enzymes are
secreted as inactive forms, called ——-, which are subsequently activated by —-
- a-amylase
- zymogens or proenzymes
- proteolytic cleavage
Explain how Zymogens are excreted
- Before their secretion, zymogens exist in granules near the cell membrane. In response to signals discussed below, the granules fuse with the cell membrane, expelling their contents into the lumen of the intestine.
The enzyme enteropeptidase (also called enterokinase)
is anchored to the epithelial cells of the small intestine, activates the pancreatic zymogen trypsinogen to form trypsin, which in turn activates the remaining pancreatic zymogens
The stomach enzyme pepsin (3)
- itself secreted as a zymogen called pepsinogen.
- Pepsinogen has a small amount of enzyme activity and can activate itself to some degree in an acidic environment.
- The active pepsin activates the remaining pepsinogen
Digestion begins in the mouth with the process of chewing, where …..
- teeth, tongue, and saliva are employed to homogenize a bite of pizza, converting it into an aqueous slurry that is more readily attacked by digestive enzymes than a piece of poorly chewed food would be.
Saliva, secreted by the salivary glands, is an aqueous solution of ……. These components…..
- Na+, K+, Cl- and HCO3- and that contains
mucoproteins. - These components facilitate the homogenization of the food and lubricate the resulting slurry for swallowing. Saliva also contains a-amylase which cleaves a-1,4 glycosidic bonds. Because of the short duration of time that food is in the mouth, polysaccharide breakdown in the mouth is minimal.
Subsequent to homogenization, the food passes into the stomach, where two principal activities take place:
- First, the proteins are denatured by the acidic environment of the stomach, where the PH is maintained at values ranging from 1-2. This denaturation, caused
by the breakage of ionic and hydrogen bonds due to the low PH renders protein a better substrate for degradation. - Second, the process of protein degradation begins in the stomach with the action of the proteolytic enzyme pepsin, as discussed above. The action of pepsin yields protein fragments that will be further degraded by the proteases of the intestine.
How is the acid environment of the stomach generated?
Specialized cells lining the stomach contain the membrane protein H+-K+ ATPase (gastric proton pump) that pumps protons into the stomach in exchange for K+ at the expense of ATP hydrolysis
The partly digested proteins as well as carbohydrates and lipids move from the acidic environment of the stomach to the beginning of the small intestine. Explain what happens in between (2):
- The low of the food PH stimulates the cells of the small intestine to release the hormone secretin. Secretin, in turn, promotes the release of sodium bicarbonate NaHCO3 from pancreatic cells, which neutralizes the PH of the food as it exits the stomach.
- The polypeptide products of pepsin digestion also
stimulate the release of the hormone cholecystokinin (CCK) by intestinal cells. The pancreas responds to CCK by releasing a host of digestive enzymes into the intestine, where the digestion of proteins continues and the digestion of lipids and carbohydrates begins.
Cholecystokinin (CCK)
is secreted by specialized intestinal cells and causes the secretion of bile salts from the gallbladder and digestive enzymes from the pancreas
Secretin
stimulates sodium bicarbonate secretion from the pancreas, which neutralizes the stomach acid
The pancreatic proteases and peptidases difference in role
The pancreatic proteases hydrolyze the proteins into small fragments called oligopeptides, but digestion is completed by enzymes called peptidases that are attached to the external surfaces of the intestinal cells
peptidases (2)
- Enzymes that cleave the oligopeptides into amino acids and di- and tripeptides that can be conveyed into an intestinal cell by transporters. At least seven different transporters exist, each specific to a different group of amino acids.
- attached to the external surfaces of the intestinal cells
The absorbed amino acids after being absorbed into the inestinal cells from transporters are subsequently released into the blood by a number of —–
for use by other tissues
antiporters
Protein digestion is primarily a result of the activity of —-
- enzymes secreted by the pancreas
Glutens
storage proteins in plants, providing amino acids as well as carbon, nitrogen, and sulfur for growth and development
Complex carbohydrate starch/branched homopolymer of glucose is digested primarily by
the pancreatic enzyme α-amylase
α-Glucosidase, α-Dextrinase (2)
location?
- digests maltose, maltotriose, and any other oligosaccharides that may have escaped digestion by the amylase/ further digests limit dextrin (a carbohydrate rich
in a-1,6 bonds) into simple sugars - on the surfaces of the intestinal cells
How are Glucose, Galactose and fructose transported into the cell?
Glucose and galactose are transported into the intestinal
epithelial cells by a secondary active-transport process carried out by the sodium–glucose linked transporter while fructose diffuses across the cell membrane through a transporter called GLUT5
GLUT 2
Transporter that releases all three monosaccharides into the bloodstream, where they can travel to other tissues to be used as fuel.
Lipids are prepared for digestion in the …… because …..
- stomach
- The grinding and mixing that takes place in the stomach converts lipids into an emulsion, a mixture of lipid droplets and water
After the lipids leave the stomach, emulsification is enhanced with the aid of —-
bile salts
bile salts (2)
- amphipathic molecules synthesized from cholesterol in the liver and secreted from the gallbladder in response to cholecystokinin
- break down larger fat globules in food into small droplets of fat
Triacylglycerols are degraded to free fatty acids and
monoacylglycerol by ……., which attach to the surface of a lipid droplet.
enzymes secreted by the pancreas called lipases
The final digestion products, free fatty acids and monoacylglycerol, are then ……
carried in micelles to the plasma membrane of the intestinal epithelial cells where they will subsequently be absorbed
The fatty acids and monoacylglycerol are transported into
the intestinal cells by membrane proteins such as ……
the fatty-acidbinding protein (FABP)
Explain what happens when the fatty acids and monoacylglycerol are transported into the intestinal cells (3)
- Once inside the cell, fatty-acid-transport proteins (FATP) carry them to the cytoplasmic face of the smooth endoplasmic reticulum (SER), where the triacylglycerols are resynthesized from fatty acids and monoacylglycerol.
- After transport into the lumen of the SER, the triacylglycerols associate with specific proteins and a small amount of phospholipid and cholesterol to form lipoprotein transport particles called chylomicrons
- The chylomicrons are released into the lymph system and then into the blood
How does the body use lipids after a high fat meal?
After a meal rich in lipids, the blood appears milky because of the high content of chylomicrons. These particles bind to membrane-bound lipoprotein lipases, primarily at adipose tissue and muscle, where the triacylglycerols are once again degraded into free fatty acids and monoacylglycerol for transport into the tissue. The triacylglycerols are then resynthesized and stored. In the muscle and other tissues, they can be oxidized to provide energy.
