Digestion and Absorption

Question 1

What is caviteal digestion?

Answer 1

Digestion by enzymes that are secreted into the lumen

Question 2

What is membranal digestion?

Answer 2

Digestion by enzymes bound to the intestinal wall

Question 3

What are the five layers that the digested material has to pass through?

Answer 3

Glycocalyx
Apical cell membrane
Intracellular space
Basolateral cell membrane
Basement membrane
Capillary endothelium

Question 4

What is the make up carbohydrates that are intaken?

Answer 4

50% starch
30% sucrose
6% lactose
2% maltose
13% mixture of other sugars

Question 5

What two enzymes facilitate caviteal digestion of carbohydrates?

Answer 5

Salivary amylase

Pancreatic alpha amylase

Question 6

What is the optimal pH for salivary amylase?

Answer 6

6.7-7.5

Question 7

What is the substrate and product of salivary amylase?

Answer 7

Starch –> Tri and disaccharides

Question 8

Where is salivary amylase denatured?

Answer 8

Stomach

Question 9

Where does pancreatic alpha amylase work?

Answer 9

Small intestine

Question 10

Which bonds does pancreatic alpha amylase break?

Answer 10

Alpha 1,4 bonds

Question 11

What are the three enzymes that facilitate membranal carbohydrate digestion?

Answer 11

Maltase
Sucrase
Lactase

Question 12

What are the substrate and products of maltase?

Answer 12

Maltose –> glucose + glucose

Question 13

What are the substrate and products of lactase?

Answer 13

Lactose –> glucose + galactose

Question 14

What are the substrate and products of sucrase?

Answer 14

Sucrose –> glucose + fructose

Question 15

Describe the process of glucose/galactose absorption?

Answer 15

Na/K ATPase sets up concentration gradient of sodium
Sodium moves in from the gut via SGLT1 transporter and brings glucose/galactose with it
The sugar then diffuses across the basolateral membrane via GLUT2 channel

Question 16

Describe the process of fructose absorption

Answer 16

Fructose diffuses across the apical membrane via GLUT5

Fructose diffuses out via GLUT2

Question 17

Which cells release pepsinogen?

Answer 17

Chief cells

Question 18

What converts pepsinogen to pepsin?

Answer 18

Hydrochloric acid

Question 19

What is the optimal pH for pepsin?

Answer 19

pH 1-3

Question 20

Where is pepsin active?

Answer 20

Stomach

Question 21

Where is pepsin denatured?

Answer 21

Duodenum

Question 22

What enzyme activates trypsin?

Answer 22

Enterokinase

Question 23

What enzymes does trypsin activate?

Answer 23

Chymotrypsin
Elastase
Carboxypeptidases

Question 24

Describe the process of protein absorption?

Answer 24

A sodium gradient is set up by Na/K/ATPase across the basolateral membrane
A hydrogen ion gradient is set up by Na/H exchanger
An amino acid, dipeptide or tripeptide is brought in via cotransport with a proton via PepT1 transporter
Any dipeptides or tripeptides are broken down by a peptidase intracellularly
The amino acids leave via the basolateral membrane via facilitated diffusion

Question 25

What enzymes are involved in caviteal digestion of lipids?

Answer 25

``` Lingual lipase Gastric lipase Pancreatic lipase Phospholipase Cholesterol esterase ```

Question 26

What happens to the fatty acids after lipid digestion?

Answer 26

Formation of miscelles

Question 27

Describe the process of fatty acid absorption

Answer 27

Fatty acids enter the enterocyte by passive diffusion The fatty acids are reformed into triglycerides at the smooth ER to maintain a concentration gradient The triglycerides are modified at the Golgi apparatus with cholesterol, proteins and phospholipids to form chylomicrons The chylomicrons are exocytosed into the interstitium and are taken up by lymphatic lacteals

Question 28

What are the two routes of fluid absorption and where do they occur?

Answer 28

Paracellular - duodenum and jejumum | Transcellular - Ileum

Question 29

Describe the process of transcellular fluid absorption

Answer 29

Na/K/ATPase sets up a sodium gradient across the basolateral membrane Sodium enters across the apical membrane via Na/H antiporter of Na/Cl cotransporter Osmolarity increases in the cells so water enters via osmosis from the lumen

Question 30

Which ions are secreted into the lumen by the enterocyte?

Answer 30

Carbonate and potassium

Question 31

Describe the role of carbonic anhydrase in ion secretion and reabsorption

Answer 31

Carbonic anhydrase converts water and carbon dioxide into a proton and carbonate The proton leaves via the apical membrane via Na/H antiport The carbonate leaves via the apical membrane via Cl/HCO3 antiport

Question 32

When is potassium secreted by the enterocytes?

Answer 32

When luminal concentration is <25mM

Question 33

How does calcium travel in the blood?

Answer 33

As free ions in the plasma or bound to albumin or citrate phosphate

Question 34

Describe the process of calcium absorption?

Answer 34

Taken up into the enterocyte by facilitated diffusion Sequestered into the endoplasmic reticulum Passes through the basolateral membrane via Ca/ATPase of Ca/Na transporter

Question 35

Describe the process of calcium absorption regulation

Answer 35

Parathyroid hormone is secreted in response to a low plasma calcium concentration Parathyroid hormone stimulates calcitriol production from vitamin D Calcitriol stimulates Ca/ATPase Ca/ATPase reduces the intracellular concentration which increases absorption

Question 36

What three forms of iron may be found in the lumen?

Answer 36

Fe2+ Fe3+ Bound to haem

Question 37

How is Fe2+ absorbed?

Answer 37

Cotransport with Fe/H transporter

Question 38

How is Fe3+ absorbed?

Answer 38

Conversion to Fe2+ by membranal reductase | Cotransport with Fe/H transporter

Question 39

How is haem absorbed?

Answer 39

Endocytosis of the haem molecule | Digestion by haem oxygenase to release the iron ions

Question 40

Which vitamins are fat soluble?

Answer 40

A, D, E and K

Question 41

How are fat soluble vitamins absorbed?

Answer 41

Dissolved in miscelles

Question 42

Which vitamins are water soluble?

Answer 42

B and C

Question 43

How are water soluble vitamins absorbed?

Answer 43

Sodium dependent active transport

Question 44

Describe the process of vitamin B12 absorption

Answer 44

Vitamin B12 is ingested bound to proteins The proteins are digested Vitamin B12 binds to R proteins R proteins are digested Vitamin B12 binds to protease resistant intrinsic factor IF/B12 complexes on the brush border trigger endocytosis Carried in blood via transcobalamin