Digestion

Question 1

Digestion is when components of meals must be degraded into small molecules for

Answer 1

absorption by epithelial cells of the intestine and for transport in the blood

Question 2

Proteins are digested to _____ by _____, which is secreted by _____

Answer 2

Amino acids, proteolytic enzymes (proteases), stomach and pancras

Question 3

Polysaccharides (ie starch) are cleaved into ______ by ______, which is from _____

Answer 3

Monosaccaharides, alpha-amylase, pancreas and saliva (tiny bit)

Question 4

Lipids are converted into _______ by ______, which are secreted from ______

Answer 4

Fatty acids, lipases, pancreas

Question 5

All digestive enzymes are

Answer 5

hydrolases (cleave their substrates by addition of H2O molecule)

Question 6

All digestive enzymes (except alpha-amylase) are secreted as inactive forms called

Answer 6

Zymogens or proenzymes, which are activated by proteolytic cleavage of part of the inactive precursor

Question 7

Before secretion of zymogens, they exist in

Answer 7

granules near cell membrane

Question 8

In response to signals: granules

Answer 8

fuse with the cell membrane and expel their contents into the lumen of the intestine

Question 9

What is Pepsin?

Answer 9

Stomach enzyme secreted as zymogen = pepsinogen (can activate itself to some degree in acidic environment from its little enzymatic activity), active pepsin activates remaining pepsinogen

Question 10

Digestion begins in mouth with process of chewing

Answer 10

• Teeth/tongue/saliva make food = homogenous aqueous slurry that is easily attacked by digestive enzymes
• Saliva (from salivary glands): Aq soln of Na+, K+, Cl-, and HCO3- that contains mucoproteins, facilitates homogenization of food, lubricates slurry for swallowing

Question 11

Saliva also contains alpha-amylase which

Answer 11

cleaves alpha-1,4-glycosidic bonds, but short time in mouth = minimal polysaccharide breakdown in mouth

Question 12

Protein digestion begins in the stomach:

Answer 12

1) Proteins are denatured by acidic environment, caused by breakage of ionic and H-bonds due to low pH, renders protein better substrate for degradation
2) Protein degradation begins via action of pepsin -> yields protein fragments that are further degraded in intestine

Question 13

Protein digestion continues in the intestine

Answer 13

1) partly digested food enters small intestine from stomach = low pH -> causes secretion of hormone secretin
2) Secretin promotes release of NAHCO3 from pancreas to neutralize pH of food
3) Polypeptide products of pepsin digestion = release of cholecystokinin (CCK) by intestine = pancreas then releases tons of digestive enzymes
4) Pancreatic proteases hydrolyze proteins into small fragments = oligopeptides
5) Digestion completed by peptidases, which cleave oligopeptides into A.As and di/tripeptides that can be conveyed into intestinal cell by transporters
6) Absorbed A.As are released into blood by antiporters for use by other tissues

Question 14

Dietary carbs are digested by alpha-amylase

Answer 14

Most common end products are monosaccharides: glucose, fructose, and galactose. Primary source if complex carb: starch.

1) Starch digested by pancreatic enzyme alpha-amylase, which cleavy alpha-1,4 bonds but not alpha-1,6 = di/trisaccharides maltose and maltotriose and dextrin
2) alpha-glucosidase digests maltose, maltotriose, and any other ogliosaccharides that may have escaped alpha-amylase
3) alpha-dextrinase further digests dextrin into simple sugars

Question 15

Digestion of disaccharides are much simpler than complex carbs

Answer 15

1) Sucrose digested by sucrase
2) Lactase degrades lactose into glucose and galactose
3) Glucose and galactose transported into intestinal cells by secondary transport process carried out by Na-glucose linked transporter
4) Fructose diffuses across cell membrane through transport: GLUT5
5) GLUT2 transporter releases all 3 monosaccharides into bloodstream, travel to tissues for fuel usage

Question 16

Digestion of lipids is complicated by their hydrophobicity

Answer 16

1) After leaving stomach, emulsification is enhanced with aid of bile salts (amphiphatic molecules from cholesterol in liver, secreted by gallbladder in response to CCK)
2) Bile salts insert into lipid droplets, making triacylglycerols more digestable
3) Triacylglycerols are degraded into FFAs and monoacylglycerol by lipases (pancreas), which attach to surface of a lipid droplet
4) FFAs and monosacylglycerols carried in micelles to plasma membrane where they are absorbed, transported into cells via FABP (fatty-acid-binding-protein)
5) FABPs inside the cell transport them to cytoplasmic face of SER -> resynthesized into triacylglycerols
6) Triacylglycerols associate with proteins and small amount of phospholipid and cholesterol to form lipoprotein transport particles = chylomicrons
7) Chylomicrons released into lymph system then into blood
8) Chylomicrons bind to membrane-bound lipoprotein lipases (at adipose and muscle tissues) where triacylglycerols are degraded into FFAs for transport into tissue then resynthesizes and stored