Lecture 27: GI Digestion & Absorption

What chemical process is responsible for breaking down nutrients during digestion?

Hydrolysis

Which three locations contribute enzymes for luminal digestion?

Salivary glands, stomach, pancreas

Where are brush‑border enzymes located?

On the microvilli of enterocytes

Which type of digestion handles di‑ and tri‑peptides inside the cell?

Intracellular digestion

Which salivary enzyme begins carbohydrate digestion?

α‑amylase (ptyalin)

Which enzyme begins lipid digestion in the mouth?

Lingual lipase

Which pancreatic enzyme digests starch?

Pancreatic amylase

Which brush‑border enzyme activates trypsinogen?

Enteropeptidase

Which region of the GI tract absorbs most nutrients, vitamins, ions, and water?

Duodenum and jejunum

Which part of the small intestine absorbs bile salts and vitamin B12?

Ileum

Which part of the GI tract absorbs most water and electrolytes?

Colon

Which macronutrient digestion produces mostly glucose as the final product?

Carbohydrates

Which transporter absorbs glucose and galactose at the apical membrane?

SGLT1

Which transporter absorbs fructose at the apical membrane?

GLUT5

Which transporter exports all monosaccharides across the basolateral membrane?

GLUT2

Which gastric enzyme begins protein digestion?

Pepsin

Which pancreatic enzymes digest internal peptide bonds?

Endopeptidases (trypsin, chymotrypsin, elastase)

Which transporter absorbs di‑ and tri‑peptides?

PepT1

What percentage of luminal protein digestion products are small peptides?

About 60%

What is the major dietary lipid class (≈90%)?

Triacylglycerols (TAGs)

Digestion of nutrients uses ______ to break chemical bonds.

water (hydrolysis)

Brush‑border digestion occurs on the ______ of enterocytes.

microvilli

Lactose is digested by the enzyme ______.

lactase

Sucrose is digested by the enzyme ______.

Sucrase

Amino acids enter enterocytes via ______‑dependent secondary active transport.

Na+

Di‑ and tri‑peptides enter enterocytes via ______‑dependent cotransport.

H+

Bile salts form ______ that ferry lipids to the brush border.

mixed micelles

Long‑chain fatty acids are re‑esterified into TAGs in the ______.

Smooth endoplasmic reticulum (SER).

Chylomicrons are released from enterocytes via ______.

Exocytosis

Chylomicrons enter the ______ rather than blood capillaries.

Lacteals (lymphatics)

Vitamin B12 is absorbed in the ileum only when bound to ______.

intrinsic factor

Aldosterone increases intestinal absorption of ______.

Na+

Water absorption in the small intestine follows the ______ gradient.

osmotic

Calcium and iron absorption are ______ regulated.

actively

T/F: All carbohydrate digestion occurs in the stomach.

False: most occurs in the small intestine via pancreatic amylase and brush-border enzymes

T/F: GLUT2 is located on the apical membrane for glucose uptake.

False: GLUT2 is basolateral; SGLT1 is apical.

T/F: Pepsin is active at acidic pH.

True

T/F: All proteins must be digested to amino acids before absorption.

False: di‑ and tri‑peptides are absorbed via PepT1

T/F: Short‑chain fatty acids enter lymphatics as chylomicrons.

False: SCFAs enter directly into blood capillaries.

T/F: Bile salts are required for digestion of lipids.

True

T/F: The small intestine absorbs most dietary nutrients.

True

T/F: Lactase deficiency increases breath hydrogen levels.

True

T/F: Enteropeptidase activates all pancreatic enzymes directly.

False: it activates trypsinogen; trypsin activates the rest.

T/F: Chylomicrons enter the bloodstream directly from enterocytes.

False: they enter lymph first, then the bloodstream

What enzyme splits α‑limit dextrins?

Isomaltase (part of sucrase‑isomaltase).

In the monosaccharide absorption diagram, which transporter uses Na⁺ cotransport?

SGLT1

Which enzymes digest internal peptide bonds?

Endopeptidases (trypsin, chymotrypsin, elastase)

Which structure transports chylomicrons?

Lacteal (lymphatic capillary).

What molecule forms the outer shell of mixed micelles?

Bile salts

Which enzyme begins starch digestion in the mouth?

α‑amylase (ptyalin).

What percentage of starch digestion can occur in the stomach?

Up to 40%

Which enzyme continues starch digestion in the small intestine?

Pancreatic amylase

Which brush‑border enzymes complete starch digestion?

Maltase, sucrase‑isomaltase, α‑dextrinase

Which disaccharide digestion occurs ONLY at the brush border?

Lactose, maltose, and sucrose.

What are the three monosaccharide end‑products of carbohydrate digestion?

Glucose, galactose, fructose

Which transporter is competitively shared by glucose and galactose?

SGLT1

What test shows elevated breath hydrogen in lactase deficiency?

Lactose breath test

Which gastric enzyme digests collagen?

Pepsin

Which pancreatic enzyme activates other proteolytic enzymes?

Trypsin

Which enzyme activates trypsinogen?

Enteropeptidase

Which enzymes digest internal peptide bonds?

Endopeptidases (trypsin, chymotrypsin, elastase).

Which enzymes digest terminal amino acids from peptides?

Exopeptidases (carboxypeptidase, aminopeptidase).

What ion drives PepT1 transport?

H+

What ion drives amino acid transporters?

Na+

Which age group absorbs intact proteins via endocytosis?

Infants (up to 6 months)

Which intestinal cell type transports intact antigens to lymphocytes?

M cells (in Peyer’s patches)

What percentage of protein entering the small intestine is endogenous?

50%

Pancreatic lipase requires the cofactor ______ to function.

Colipase

Chylomicrons are packaged in the ______ before exocytosis.

Golgi apparatus

Chylomicrons enter the ______ rather than blood capillaries.

lacteals

Short‑chain fatty acids enter the ______ directly.

blood capillaries

Sodium absorption is increased by the hormone ______.

Aldosterone

Calcium absorption occurs primarily in the ______.

Duodenum

Iron absorption occurs primarily in the ______.

Duodenum

Folate absorption occurs primarily in the ______.

Duodenum

The hepatic portal vein carries absorbed nutrients to the ______.

liver

Fat‑soluble vitamins are absorbed along with ______.

lipids

T/F: Pancreatic amylase can digest α‑1,6 bonds in starch.

False: it digests α‑1,4 bonds; α‑1,6 bonds require isomaltase.

T/F: All fatty acids enter lymph as chylomicrons.

False: short‑ and medium‑chain fatty acids enter blood directly.

T/F: Water absorption in the small intestine is an active process.

False: water follows osmotic gradients created by solute absorption

What effect does emulsification have on fat droplets?

Decreases droplet size and increases surface area

Which pancreatic enzyme hydrolyzes TAGs into 2‑MAG and FFAs?

Pancreatic lipase

Which transporter assists long‑chain fatty acid uptake at the apical membrane?

FAT/CD36

Which lipoprotein is the largest and least dense?

Chylomicron

Which lipoprotein is secreted by enterocytes into lymph along with chylomicrons?

VLDL

Which lipid digestion products enter blood directly instead of lymph?

Glycerol, short‑chain fatty acids, medium‑chain fatty acids.

What is the “acid microclimate” at the brush border important for?

Releasing lipids from micelles for absorption

Which enzyme is responsible for splitting α‑limit dextrins?

Isomaltase

What is the major cause of increased breath hydrogen in lactase deficiency?

Colonic bacterial fermentation of undigested lactose

What is the advantage of peptide absorption over amino acid absorption?

Faster absorption and higher capacity

Which specialized cells transport antigens to lymphoid tissue?

M cells

What percentage of protein entering the small intestine is endogenous?

50%

What is the turnover time of intestinal epithelial cells?

6 days

Which vitamins require fat for absorption?

Vitamins A, D, E, and K

Emulsification of fats is performed by bile salts and ______.

Lecithin

Pancreatic lipase converts TAGs into FFAs and ______.

2‑monoacylglycerol (2‑MAG).

Mixed micelles are composed of bile salts, FFAs, cholesterol, and ______.

2-MAG

Amino acids cross the basolateral membrane via ______ diffusion.

faciliated

T/F: GLUT5 transports fructose.

True

Which organelle synthesizes apolipoproteins?

Rough ER