Lecture 2 GI

Question 1

Digestion: ______
Absorption: _______

Answer 1

Digestion: breakdown of macromolecules to constituents that can then be absorbed

Absorption: transport across the epithelium

Question 2

Absorption from the mouth and stomach

A. Absorption is by ____
B. Limited to ____
(solutes with some solubility in both water and fat) e.g. nitroglycerin, alcohol, and aspirin

Answer 2

A. Absorption is by simple diffusion

B. Limited to amphiphilic solutes
(solutes with some solubility in both water and fat) e.g. nitroglycerin, alcohol, and aspirin

Question 3

Most of the absorption of organic nutrients occurs in the ______.

Answer 3

Most of the absorption of organic nutrients occurs in the small intestine.

Question 4

Absorption in the small intestine

Expansion of the digestive/ absorptive surface

The ____ of the small intestine is specialized to provide a large surface area for ___ & ____

Answer 4

The lumenal surface of the small intestine is specialized to provide a large surface area for digestion and absorption.

Question 5

Specialized lumen of the small intestine -

Specializations include: 1) large folds of the ___ & ___
2) fingerlike projections of the ____

3) projections of the ____.

Together these increase the surface area ~ 600 fold to about 200 m2.

Answer 5

Specializations include: 1) large folds of the mucosa, muscularis and submucosa,

2) fingerlike projections of the mucosa (villi) and
3) projections of the lumenal membrane of enterocytes (microvilli).

Together these increase the surface area ~ 600 fold to about 200 m2.

Question 6

Localization of absorption in the small intestine

Some nutrients (e.g. carbohydrate, protein and
lipid) are efficiently absorbed along the entire
length of the small intestine with the greatest
absorption in the \_\_\_  followed by the \_\_\_\_ and the smallest absorption occurs in the \_\_\_\_\_

Answer 6

Some nutrients (e.g. carbohydrate, protein and
lipid) are efficiently absorbed along the entire
length of the small intestine with the greatest
absorption in the duodenum followed by the jejunum and the smallest absorption occurs in the ileum

Question 7

Localization of absorption in the small intestine-

Absorption of others (beside carbs, proteins, fats) is restricted to certain regions of the small intestine. For instance, specialized absorptive systems for iron are found only in the ___ while those for vitamin B12 and bile salts are restricted to the _____.

Answer 7

Absorption of others is restricted to certain regions of the small intestine. For instance, specialized absorptive systems for iron are found only in the duodenum while those for vitamin B12 and bile salts are restricted to the ileum.

Question 8

Digestion and absorption of carbohydrates-

Carbohydrates often account for as much as 50% of caloric intake. Complex carbohydrates (starches) and dietary disaccharides ( e.g. sucrose) must be digested to ____ in order to be absorbed

Answer 8

Carbohydrates often account for as much as 50% of caloric intake. Complex carbohydrates (starches) and dietary disaccharides ( e.g. sucrose) must be digested to monosaccharides in order to be absorbed

Question 9

Complex carbs are 1st digested in the lumen of the GI tract by ____ & ____ to produce small oligo saccharides, di- and tri- saccharides. These are then digested by a variety of ____ in the brush border membrane of enterocytes to produce the monosaccharides: __, ___, & ____

Answer 9

Complex carbs are 1st digested in the lumen of the GI tract by salivary amylase (limited) and pancreatic amylase to produce small oligo saccharides, di- and tri- saccharides. These are then digested by a variety of oligosaccharidases in the brush border membrane of enterocytes to produce the monosaccharides: glucose, galactose and fructose.

Question 10

Fructose is absorbed by _____.

Answer 10

Fructose is absorbed by facilitated diffusion.

Question 11

Glucose and galactose are absorbed across the brush border membrane by ____

Answer 11

Glucose and galactose are absorbed across the brush border membrane by Na+ -dependent secondary active transport.

Question 12

A “carrier” is located in the brush border membrane of enterocytes binds both ___ & ____. Na+ is transported down its electrochemical gradient into the cell and is coupled with, and provides the energy for_____. The Na+ that enters the cell is transported across the basolateral membrane by the _____ thereby maintaining the gradient for Na+.

Answer 12

A “carrier” is located in the brush border membrane of enterocytes binds both Na+ and sugar (glucose or galactose). Na+ is transported down its electrochemical gradient into the cell and is coupled with, and provides the energy for, transport of sugar across the brush border membrane into the enterocyte. The Na+ that enters the cell is transported across the basolateral membrane by the Na/K-ATPase thereby maintaining the gradient for Na+.

Question 13

Sugar that enters the cell is transported across the basolateral membrane by _____ to complete the absorptive process. The net result is the transport of sugar, and Na+, from the lumen to the ______.

Answer 13

Sugar that enters the cell is transported across the basolateral membrane by facilitated diffusion to complete the absorptive process. The net result is the transport of sugar, and Na+, from the lumen to the serosal interstitial fluid and blood.

Question 14

Digestion and Absorption of Proteins

Proteins must be digested to ___ & _____ in order to be absorbed

Answer 14

Proteins must be digested to dipeptides and amino acids in order to be absorbed

Question 15

Initially proteins are digested in the stomach by ___ (limited) and then by _____.

Answer 15

Initially proteins are digested in the stomach by pepsin (limited) and then by pancreatic proteases.

Question 16

Pepsin and pancreatic proteases are secreted in ___ and must be ____. Pepsinogen is activated in the ___ by ___. Once some pepsin is formed it can activate additional ____

Answer 16

Pepsin and pancreatic proteases are secreted in inactive form and must be activated. Pepsinogen is activated in the stomach by acid. Once some pepsin is formed it can activate additional pepsinogen

Question 17

The pancreatic protease trypsinogen is converted to the active form, ___, by the brush border enzyme ____. Trypsin can then activate more trypsinogen as well as the other proenzymes.

Answer 17

The pancreatic protease trypsinogen is converted to the active form, trypsin, by the brush border enzyme enteropeptidase (enterokinase). Trypsin can then activate more trypsinogen as well as the other proenzymes.

Question 18

Digestion of dietary proteins by pepsin (limited) and pancreatic proteases produces ___ & ____. The final step in digestion is accomplished by ____ in the brush border membrane that digest peptides to produce ___ & ___.

Answer 18

Digestion of dietary proteins by pepsin (limited) and pancreatic proteases produces peptides and free amino acids. The final step in digestion is accomplished by peptidases in the brush border membrane that digest peptides to produce amino acids and dipeptides.

Question 19

Both free amino acids and dipeptides are absorbed from the small intestine by ____.

Answer 19

Both free amino acids and dipeptides are absorbed from the small intestine by secondary active transport.

Question 20

Amino acids are absorbed by a___, and dipeptides are absorbed by a _____.

Answer 20

Amino acids are absorbed by a Na+-AA co-transport system, and dipeptides are absorbed by a H+-dipeptide co-transport system

Question 21

In the enterocyte, dipeptides are digested to produce ____. The free amino acids are transported across the basolateral membrane by _____ and enter the serosal interstitial fluid and then the____ completing the absorptive process.

Answer 21

In the enterocyte, dipeptides are digested to produce free amino acids. The free amino acids are transported across the basolateral membrane by facilitated diffusion and enter the serosal interstitial fluid and then the blood completing the absorptive process.

Question 22

Emulsification of fat

Most fats in the diet are ____ but ___ & ___ are also present.

Answer 22

Most fats in the diet are triglycerides (TG) but phospholipids (PLP) and cholesterol (CH) are also present. coalescing.

Question 23

Peristaltic contractions break up ____ in the stomach into ___, producing an ___. After emptying from the stomach into the small intestine, the emulsion is stabilized by ____ that coat the lipid droplets and prevent them from _____.

Answer 23

Peristaltic contractions break up large lipid droplets in the stomach into smaller lipid droplets producing an emulsion. After emptying from the stomach into the small intestine, the emulsion is stabilized by bile salts that coat the lipid droplets and prevent them from coalescing.

Question 24

The pancreas secretes ____ into the lumen of the small intestine.

Answer 24

The pancreas secretes pancreatic lipase into the lumen of the small intestine.

Question 25

Lipase digests TG to ____ and ____. In addition pancreatic cholesterol esterase digests ____ to cholesterol (CH) and FFA while pancreatic phospholipase digests phospholipids (PLP) to ___ & ____.

Answer 25

Lipase digests TG to monoglycerides (MG) and free fatty acids (FFA). In addition pancreatic cholesterol esterase digests cholesterol esters to cholesterol (CH) and FFA while pancreatic phospholipase digests phospholipids (PLP) to lysophospholipid and FFA.

Question 26

Lipid digestion products are solubilized by incorporation into ___. ____ are also incorporated into the micelles. The micelles carry the lipid digestion products to close proximity with the _____.

Answer 26

Lipid digestion products are solubilized by incorporation into mixed micelles. Fat-soluble vitamins are also incorporated into the micelles. The micelles carry the lipid digestion products to close proximity with the brush border membrane of enterocytes.

Question 27

Lipid digestion products are released from the micelle and then absorbed into the enterocyte by ______. The micelle releases additional digestion products to maintain a saturated solution of these products in the vicinity of the ____

Answer 27

Lipid digestion products are released from the micelle and then absorbed into the enterocyte by simple diffusion. The micelle releases additional digestion products to maintain a saturated solution of these products in the vicinity of the enterocyte.

Question 28

Lipid absorption - Most lipids are absorbed in the ____ while the bile salts are reabsorbed in the ___.

Answer 28

Most lipids are absorbed in the jejunum while the bile salts are reabsorbed in the ileum

Question 29

After entering the enterocyte, MG's are re-synthesis to ___, CH is ___ and lysophospholipids are converted back to PLP. -The TG's aggregate to form____ in the cell. -CH ___ in the lipid droplets as do fat soluble vitamins. -A coating of protein and PLP is added to form a ____.

Answer 29

After entering the enterocyte, MG's are re-synthesis to TG, CH is esterified and lysophospholipids are converted back to PLP. The TG's aggregate to form lipid droplets in the cell. CH dissolves in the lipid droplets as do fat soluble vitamins. A coating of protein and PLP is added to form a chylomicron.

Question 30

The chylomicron exits the enterocyte by ____, enters the central lacteal and is carried to the ____.

Answer 30

The chylomicron exits the enterocyte by exocytosis, enters the central lacteal and is carried to the circulatory system.

Question 31

Absorption of vitamins Fat-soluble vitamins are incorporated in ____, absorbed by ____ and incorporated into ____. Efficient absorption of fat-soluble vitamins requires ___.

Answer 31

Fat-soluble vitamins are incorporated in mixed micelles, absorbed by simple diffusion and incorporated into chylomicrons. Efficient absorption of fat-soluble vitamins requires bile salts. A, D, E, K

Question 32

Most water-soluble vitamins are absorbed, by ____, ____, or _____.

Answer 32

Most water-soluble vitamins are absorbed, by diffusion, facilitated diffusion, or secondary active mechanisms.

Question 33

Vitamin B12 binds to ____ secreted by gastric parietal cells. The IF-B12 complex remains in the lumen until it reaches the ___ where the complex binds to specific receptors on ileal brush border membranes. The IF-B12 complex enters the cell. In the cell B12 is released from IF and binds to ____ and is absorbed into the ____.

Answer 33

Vitamin B12 binds to intrinsic factor (IF) secreted by gastric parietal cells. The IF-B12 complex remains in the lumen until it reaches the ileum where the complex binds to specific receptors on ileal brush border membranes. The IF-B12 complex enters the cell. In the cell B12 is released from IF and binds to transcobalmin and is absorbed into the circulatory system.

Question 34

Failure to absorb Vitamin B12 results in ____

Answer 34

Failure to absorb Vitamin B12 results in pernicious anemia.

Question 35

Major electrolytes (ingested and secreted):

Answer 35

sodium, chloride, potassium, hydrogen and bicarbonate.

Question 36

Sources of ingested or secreted fluid ``` Food/Drink: Saliva: Gastric Juice: Pancreatic juice: Bile: Small intestine: ```

Answer 36

``` Food/Drink: 1.2 L/day Saliva: 1.5 L/day Gastric Juice: 2.0 L/day Pancreatic juice: 1.5 L/day Bile: 0.5 L/day Small intestine: 1.5 L/day ```

Question 37

Absorbed Fluid Small intestine: Colon:

Answer 37

Small intestine: 6.7 L/day | Colon: 1.4 L/day

Question 38

The intracellular concentration of Na+ is maintained at a low level by activity of the____ which transports ___out of the cell in exchange for ___. Na+ moves "downhill" across the brush border membrane into the cell. Na+ is then transported "uphill" across the basolateral membrane by the ____.

Answer 38

The intracellular concentration of Na+ is maintained at a low level by activity of the Na/K ATPase which transports Na+ out of the cell in exchange for K+. Na+ moves "downhill" across the brush border membrane into the cell. Na+ is then transported "uphill" across the basolateral membrane by the Na/K ATPase.

Question 39

Cl- is absorbed "downhill" in response to the potential difference generated by ____

Answer 39

Cl- is absorbed "downhill" in response to the potential difference generated by absorption of Na.

Question 40

Potassium is absorbed by ____.

Answer 40

Potassium | Potassium is absorbed by simple diffusion.

Question 41

Potassium 1) Initially K+ in the ____ equilibrates electrochemically with K+ in the ____. Subsequent to equilibration, further absorption of K+ is dependent upon absorption of ____. 2) Absorption of water reduces the volume of ___, increasing the concentration of____ so that _____ is absorbed. During diarrhea, the loss of water will inhibit the ability to ____ and individuals may become _____.

Answer 41

1) Initially K+ in the chyme equilibrates electrochemically with K+ in the serosal interstitial fluid. Subsequent to equilibration, further absorption of K+ is dependent upon absorption of water. 2) Absorption of water reduces the volume of chyme, increasing the concentration of K+ so that additional K+ is absorbed. During diarrhea, the loss of water will inhibit the ability to reabsorb K+ and individuals may become hypokalemic.

Question 42

H+ secreted by the stomach reacts with HCO3- secreted by the ___ to form ____. 2) H2CO3 dissociates to form __ & ___ that are then 3) absorbed ___.

Answer 42

H+ secreted by the stomach reacts with HCO3- secreted by the pancreas to form H2CO3. 2) H2CO3 dissociates to form CO2 and H2O that are then 3) absorbed passively.

Question 43

There are two mechanisms for absorption of Ca2+ from the small intestine, ___ & ___.

Answer 43

passive and active

Question 44

Ca2+ is passively absorbed along ___ via: ____ pathway (through tight junctions) due to the Ca2+ electrochemical across the epithelial cells.

Answer 44

Ca2+ is passively absorbed along the ENTIRE small intestine via: paracellular pathway (through tight junctions) due to the Ca2+ electrochemical across the epithelial cells.

Question 45

Ca2+ is actively absorbed in the ____ by an ____.

Answer 45

Ca2+ is actively absorbed in the duodenum by an active mechanism.

Question 46

Active Ca2+ absorption Ca2+ enters the epithelial cells though channels on the brush border due to its ____. The Ca2+ is bound to ___ to store and decrease the intracellular Ca2+ concentrations. Ca2+ dissociates from calbindin and is ____ across the basolateral membrane primarily by a _____ and secondarily though _____

Answer 46

Ca2+ enters the epithelial cells though channels on the brush border due to its high electrochemical gradient. The Ca2+ is bound to calbinidn to store and decrease the intracellular Ca2+ concentrations. Ca2+ dissociates from calbindin and is actively transported across the basolateral membrane primarily by a Ca2+- ATPase and secondarily though the Na+-Ca2+ secondary active counter-transporter (Na-Ca exchanger).

Question 47

The capacity to absorb Ca2+ is increased by a ______ (1,25- 2+ dihydroxycholecalciferol). 1,25 dihydroxycholecalciferol increases the synthesis of the _______

Answer 47

The capacity to absorb Ca2+ is increased by a metabolite of vitamin D (1,25- 2+ dihydroxycholecalciferol). 1,25 dihydroxycholecalciferol increases the synthesis of the Ca channels, calbindin and the Ca2+-ATPase.

Question 48

Only a small fraction (approximately 10%) of ingested iron is absorbed and most iron absorption occurs in the _____

Answer 48

Only a small fraction (approximately 10%) of ingested iron is absorbed and most iron absorption occurs in the duodenum.

Question 49

1) Iron enters the cells via a ____ 2) Some iron is bound to ___ and 3) immediately transported across the enterocyte. The iron dissociates from the ____ and 4) is transported across the ____ membrane.

Answer 49

1) Iron enters the cells via a H+-Fe2+ secondary active co-transporter. 2) Some iron is bound to mobilferrin and 3) immediately transported across the enterocyte. The iron dissociates from the mobilferrin and 4) is transported across the basolateral membrane.

Question 50

When iron enters the blood it binds to ___ and is transported in the blood to various tissues including bone for red blood cell formation. Iron remaining in the cell is bound to ___ to form ferritin.

Answer 50

When iron enters the blood it binds to transferrin and is transported in the blood to various tissues including bone for red blood cell formation. Iron remaining in the cell is bound to apoferritin to form ferritin.

Question 51

If iron status is poor, much of this iron will be transported to the ____. If the iron is not needed it will remain stored as ___ until it is needed or the cell is exfoliated.

Answer 51

If iron status is poor, much of this iron will be transported to the blood. If the iron is not needed it will remain stored as ferritin until it is needed or the cell is exfoliated.

Question 52

The small intestine is very permeable to ___. Movement of water across the GI tract occurs in response to_____.

Answer 52

The small intestine is very permeable to water. Movement of water across the GI tract occurs in response to osmotic gradients across the epithelium.

Question 53

When a hypotonic meal empties from the stomach into the small intestine, an osmotic gradient is established so that water is ____ and the volume of the chyme is _____.

Answer 53

When a hypotonic meal empties from the stomach into the small intestine, an osmotic gradient is established so that water is absorbed and the volume of the chyme is reduced.

Question 54

emptying of a hypertonic meal into the small intestine will establish an osmotic gradient resulting in ____ thereby increasing the volume of the____.

Answer 54

emptying of a hypertonic meal into the small intestine will establish an osmotic gradient resulting in secretion of water thereby increasing the volume of the chyme.

Question 55

After adjustment of the chyme to isotonicity absorption of water is secondary to, and dependent upon, _____.

Answer 55

After adjustment of the chyme to isotonicity absorption of water is secondary to, and dependent upon, absorption of solute.

Question 56

Absorption of solutes reduces the osmolarity of the ___ and increases the osmolarity of the ____. Water is absorbed by ___ in response to the osmotic gradient. The net result is ___ of an isotonic fluid and reduction of the volume of ____.

Answer 56

Absorption of solutes reduces the osmolarity of the chyme and increases the osmolarity of the interstitial fluid. Water is absorbed by osmosis in response to the osmotic gradient. The net result is absorption of an isotonic fluid and reduction of the volume of chyme.

Question 57

Absorption of NaCl is particularly important for _____.

Answer 57

Absorption of NaCl is particularly important for absorption of water.

Question 58

The colon absorbs ___ & ____ reducing the chyme to -150 ml and forming the final product to be evacuated

Answer 58

The colon absorbs residual fluid and electrolytes reducing the chyme to -150 ml and forming the final product to be evacuated

Question 59

Solids - approximately 25-50 g of solids and 100 - 200 ml of water Bacteria = Dietary Fiber = Fat = Inorganic ions =

Answer 59

Solids - approximately 25-50 g of solids and 100 - 200 ml of water Bacteria = 30%-50% Dietary Fiber = 10%-50% Fat = 10%-20% Inorganic ions = 10%-20%