Lecture #26: Digestive System--Small Intestine

Question 1

What are the major parts of the small intestine?

Answer 1

Duodenum
Jejunum
ileum

Question 2

What are the functions of the villi and microvilli?

Answer 2

• Increase the surface area for absorption
• Cell division within intestinal glands produces new cells that move up to replace old cells that have been lost.

• Brush border enzymes found on the surfaces of microvilli participate in chemical breakdown of carbohydrates,
proteins, and nucleosides.
– Pancreatic juice is present in the lumen with
chime, and it contains amylase, proteases,
lipase and nucleases.
– No brush border lipases or nucleases present

Question 3

What type of digestion of carboyhydrates does the mouth have?

Answer 3

salivary amylase

Question 4

What type of digestion of carboyhydrates does the stomach have?

Answer 4

Mechanical only

Question 5

What type of digestion of carbohydrates does the pancreas have?

Answer 5

Pancreatic amylase in pancreatic juice

Question 6

What types of digestion of carbohydrates does the small intestine have?

Answer 6

Brush border, enzymes maltase, sucrase, and lactase:

act on disaccharides and produce monosaccharides (fructose, glucose, & galactose)

Question 7

What is lactose intolerance?

Answer 7

those without lactase will have bacteria

ferment lactose, and the result will be gas and diarrhea.

Question 8

What happens in the absorption of monosaccharides?

Answer 8

1. Absorption into epithelial cells
   – glucose & galactose - sodium symporter (secondary active
   transport)
   – Fructose - facilitated diffusion
2. Movement out of epithelial cell into bloodstream
   – by facilitated diffusion

Question 9

What happens in the digeston of proteins in the stomach?

Answer 9

HCl denatures or unfolds proteins, and pepsin breaks proteins into smaller polypeptides (smaller chains of amino acids)

Question 10

What happens in the digestion of proteins in the pancreas?

Answer 10

Pancreatic enzymes (proteases) in pancreatic juice continue to split peptide bonds between amino acids

Question 11

What happens in the digestion of proteins in the small intestine?

Answer 11

Brush border enzymes aminopeptidase and dipeptidase (split off amino acids at the amino end of polypeptides (aminopeptidase) ) and (split apart two-amino acid chains (dipeptidase))

Question 12

What happns with the absorption of amino acids and dipeptides?

Answer 12

1. Absorption into epithelial cells of duodenum and
jejunum
– active transport with Na+ or H+
ions (symporters using
secondary active transport)

2. Movement out of epithelial cell into blood
   – by facilitated diffusion

Question 13

What happens with the digestion of lipids in the mouth?

Answer 13

Lingual lipase

Question 14

What happens with the digestion of lipids in the stomach?

Answer 14

Gastric lipase–important in infants

Question 15

What happens with the digestion of lipids in the pacreas?

Answer 15

Pancreatic lipase in pancreatic juice continues to split triglycerides into fatty acids and monoglycerides

Question 16

What happens with the digestion of lipids in the liver?

Answer 16

Bile emulsifies lips to form tiny misclles

Question 17

What happens with the digestion of lipids in the small intestine?

Answer 17

No digestion! No liapse enzymes in the membranes of brush broder cells

Question 18

What happens in the absorption of lipids?

Answer 18

1. Short-chain fatty acids are absorbed by simple diffusion.
2. Long-chain fatty acids and monoglycerides (products of lipases) are
   sequestered within tiny micelles after emulsification by bile; will also enter cells by simple diffusion, leaving bile salts behind.
   • Bile salts are also reabsorbed & later recycled (into bile) by the liver.
   • Fat-soluble vitamins (A, K, D3, E) are able to enter cells w/ lipids
3. Inside epithelial cells fats are rebuilt and coated with protein to form chylomicrons
4. Chylomicrons leave intestinal cells by exocytosis into interstitial fluid where they can only enter lacteals (too big for capillaries)
   – travel in lymphatic system to reach subclavian veins
   – removed from the blood by the liver and fat tissue

Question 19

What happens in the absorption of electrolytes?

Answer 19

• Sources of electrolytes
– GI secretions, ingested foods and liquids

• Enter epithelial cells by diffusion and secondary
active transport
– sodium and potassium move by active transport
– chloride, iodide and nitrate follow passively
– iron, magnesium and phosphate ions use active transport

• Intestinal Ca2+ absorption requires vitamin D and
parathyroid hormone

Question 20

What are fat soluble vitamins and how do they travel?

Answer 20

• Fat-soluble vitamins (A, K, D3
, E)
– travel in micelles & are absorbed by simple diffusion with lipids

Question 21

What are water soluble vitamins?

Answer 21

Absorbed by diffusion

Question 22

What must the vitamin B12 be combined with?

Answer 22

Vitamin B12 must combine with intrinsic factor from
the stomach parietal cells before it is transported
into the cells of the ileum
– absorbed by receptor mediated endocytosis

Question 23

What happens with the absorption of water?

Answer 23

9L fluid enters the GI tract
per day.

• Small intestine absorbs 8L

• Large intestine absorbs 90%
of the last liter

• Absorption of is
accomplished by osmosis –
through cell membranes
into cells and then from the
cells into the capillaries
inside the intestinal villi.