Questions: Nutrient Digestion and Absorption

Question 1

After enterocytes in the jejunum absorb long-chain fatty acids and monoglycerides, these lipids reach the bloodstream by:

• A. Entering the portal vein and traveling directly to the liver
• B. Being packaged into chylomicrons that enter lymphatic lacteals
• C. Binding to albumin and crossing the basolateral membrane into capillaries
• D. Being converted to glucose in the enterocyte and entering the portal circulation

Answer: (B)

Long-chain fatty acids are reassembled into triglycerides inside enterocytes and packaged with cholesterol and apolipoproteins into chylomicrons. Because chylomicrons are too large to cross capillary walls, they are secreted into lymphatic lacteals in the villus and travel via the thoracic duct to enter the bloodstream at the subclavian vein — bypassing the liver's first-pass metabolism. Short- and medium-chain fatty acids, by contrast, do enter portal circulation.

Question 2

Stomach acid (HCl) digests proteins through enzymatic hydrolysis of peptide bonds.

• T. True
• F. False

Answer: False

HCl does not break peptide bonds enzymatically. Its roles are to denature proteins (unfolding them so enzymes can access cleavage sites) and to convert the inactive zymogen pepsinogen into active pepsin by cleaving an inhibitory peptide. Pepsin — not acid — then performs enzymatic hydrolysis of peptide bonds. This distinction matters clinically: antacids neutralize acid but do not directly inhibit proteolysis.

Question 3

Why must lipids be emulsified by bile salts before pancreatic lipase can efficiently digest them in the small intestine?

Model answer: Lipids are hydrophobic and aggregate into large fat droplets in the aqueous intestinal environment, exposing only a small surface to the surrounding fluid. Bile salts are amphipathic — they have both hydrophobic and hydrophilic regions — and break large droplets into tiny micelles, dramatically increasing the total surface area available for pancreatic lipase to bind and act. Without emulsification, lipase can only access the outer layer of large globules, making digestion far too slow.

This is an application of surface chemistry: enzymatic reactions occur at interfaces, so surface area is the rate-limiting factor. Bile salts do not digest lipids themselves; they are a physical delivery system that optimizes the conditions for lipase activity. Absence of bile (e.g., bile duct obstruction) leads to fat malabsorption and steatorrhea even when pancreatic lipase secretion is normal.