Digestion, Absorption, food intake: Johnson

Question 1

Know the sources, types, and function of the primary digestive enzymes.

Answer 1

Salivary glands:
amylase
lingual lipase

Stomach:
pepsin

Pancreas:
lipase-colipase*- triglycerides—> free FAs
protease
trypsin- converts other zymogen proteases to active form
chymotrypsin
amylase
carboxypeptidase
elastase
phospholipase A- lecithin—>lysolecithin
cholesterol esterase- removes fatty acid group from chole.

Intestine: (membrane-bound enzymes, not secreted)
enterokinase- trypsinogen–> trypsin
disaccharidases
peptidases

*colipase is NOT an enzyme, but is needed to free fat droplets from bile acids and make them available to lipase for breakdown to FFAs –> storage in micelles.

Question 2

Understand the process involved in the digestion and absorption of carbohydrates.

Answer 2

::Luminal digestion::
Amylase hydrolyzes starches/glycogen to smaller sugars (maltotriose, maltose, a-dextrin)

\::Membrane digestion:: 
isomaltase (primarily), sucrase, glucoamylase break:
a-dextrin, maltotriose, maltose---> glucose
lactase:
lactose-->glucose, galactose
sucrase:
sucrose--> glucose, fructose
Trehalase:
Trehalose--> glucose

Glucose req’s active transport (aerobic)

Question 3

Describe the digestion and absorption of proteins.

Answer 3

Pepsin and pancreatic proteases hydrolyze proteins–> large peptides, di/tripeptides (60-80%), and free AAs.
Membrane-bound peptidases further break down larger peptides to di/tris and free AAs.

Membrane carriers transport free AAs and di/tripeptides into enterocytes.
Cytoplasmic peptidases breakdown di/tris–> free AAs

Question 4

Indicate the sources of fluid and the sites of its absorption in the GI tract.

Answer 4

Secretion:
9L of fluid passes through gut/day. 2L from diet, 7L secreted by gut epithelium.
1L-saliva
2L-gastric
1L-bile
2L-pancreatic
1L-small intestine

Absorption:
4L-duodenum, jejunum
3.5- Ileum
1.4- Colon
.1-.2L- excreted

*further down GI tract you go (duo–>colon), tight junctions get tighter = less water absorbed.
Most water absorbed transcellularly.

Question 5

Know the mechanisms and the location of the sites of absorption of Na+, Cl-, K+, HCO3-, and H2O.

Answer 5

Na+ channel - electrogenic
Na+ coupled transport w/ AAs, glucose, galactose- electrogenic
NaCl cotransporter- electroneutral
Na+/H+ exchanger (creates pH gradient across enterocytes)- electroneutral

Colon: exchange of Cl- (in) for HCO3- (out)

K+ absorbed paracellularly in bulk flow as NaCl is absorbed and water follows.
K+ secreted in colon in exchange for Na+

further down GI tract you go (duo–>colon), tight junctions get tighter = less water absorbed.
Most water absorbed transcellularly.

Question 6

Understand the mechanism and significance of the intestinal secretion of fluid and electrolytes.

Answer 6

• Secretion by crypts of small bowel.
• Depends upon NaK/2Cl- cotransporter
• Cl- channel (CFTR) secreted Cl- into gut lumen. Activated by secretin and VIP and Ach

Question 7

Describe the different mechanisms that result in diarrhea.

Answer 7

2 types: 
Osmotic 
-impaired absorption
- accumulation of solutes
- lack of enzymes, decr. absorptive surface, precipitation of bile salts, hypermotility

Secretory

• incr. secretion by crypt cells
• bacteria (cholera, incr. secretory hormones)
• incr. adenylate cyclase (activates CTFR)
• activation of apical Cl- channels (bacterial toxins)

Question 8

Describe the basic processes that regulate food intake.

Answer 8

From arcuate nucleus:
POMC—> aMSH —I MC4R –> food intake, ^ metabolism
NYP—> Y1R–> ^ appetite, v metabolism
AgRP—I MC4R (normally–> satiety)–> ^ food intake

CCK—I gastric emptying–> feeling of fullness
PYY, leptin—I NPY

Question 9

Discuss the digestion and absorption of various lipids including the roles of micelles and chylomicrons.

Answer 9

• Micelles in intestinal lumen carry FFAs, monoglycerides, and cholesterol
• Chylomicrons are made up of phospholipids (7%), TGs (90%), cholesterol (2%), and apoproteins (1%)
• Chylomicrons enter lacteals and are transported through lymph vessels and dumped into venous circulation.

Question 10

Say you have digested fat in a stool sample, where is the problem?

What if there is non-digested fat in a stool sample?

Answer 10

Inadequate E.H.C. causes digested fat in stool.

Inadequate pancreatic secretion of lipase results in non-digested fat in stool.