Human Physiology, 14th edition (2016)

The Digestive System 655

| CHECKPOINT

16a. List the enzymes involved in carbohydrate digestion,

indicating their origins, sites of action, substrates,

and products.

16b. List each enzyme involved in protein digestion,

indicating its origin and site of action. Also, indicate

whether the enzyme is an endopeptidase or

exopeptidase.

17a. Describe how bile aids both the digestion and

absorption of fats. Explain how the absorption of

fat differs from the absorption of amino acids and

monosaccharides.

17b. Trace the pathway and fate of a molecule of

triglyceride and a molecule of cholesterol in a

chylomicron within an intestinal epithelial cell.

17c. Cholesterol in the blood may be attached to any of

four possible lipoproteins. Distinguish among these

proteins in terms of the origin and destination of the

cholesterol they carry.

George’s heartburn was caused by GERD—gastroesopha-

geal reflux disease—that probably resulted from his obesity.

This reflux caused Barrett’s esophagus, where columnar

epithelial cells replaced a stratified squamous epithelium

and increased his risk of adenocarcinoma. Omeprazole is

a proton pump inhibitor that reduces the ability of parietal

cells to pump H^1 into gastric juice, decreasing the acidic

damage it can cause. His vertical sleeve gastrectomy

allowed him to lose weight rapidly, which would relieve the

pressure on his stomach and perhaps his GERD. His first

bout of abdominal pain may have been caused by irritable

bowel syndrome, but does not appear to be due to inflam-

matory bowel disease, because his colonoscopy revealed

no structural changes. His diarrhea and constipation may

then have been caused by neural dysfunction that increased

or decreased his intestinal motility. Gallstones accounted

for his later abdominal pain, and probably also caused him

to develop acute pancreatitis, which was revealed by ele-

vated levels of the pancreatic enzymes amylase and lipase

in his blood. Removal of his gallbladder (cholecystectomy)

resolved these issues.

See the additional chapter 18 Clinical Investigations on Peptic

Ulcer and Celiac Sprue in the Connect site for this text.

Clinical Investigation SUMMARY

Lipoprotein Class Origin Destination Major Lipids Functions

Chylomicrons Intestine Many organs Triglycerides, other

lipids

Deliver lipids of dietary

origin to body cells

Very-low-density

lipoproteins (VLDLs)

Liver Many organs Triglycerides,

cholesterol

Deliver endogenously

produced triglycerides to

body cells

Low-density lipoproteins

(LDLs)

Intravascular removal

of triglycerides from

VLDLs

Blood vessels, liver Cholesterol Deliver endogenously

produced cholesterol to

various organs

High-density lipoproteins

(HDLs)

Liver and intestine Liver and steroid-hormone-

producing glands

Cholesterol Remove and degrade

cholesterol

Table 18.8 | Characteristics of the Lipid Carrier Proteins (Lipoproteins) Found in Plasma