Digestion and Absorption in the GI tract

Question 1

Where does carbohydrate digestion begin?

Answer 1

in the mouth - ptyalin, an alpha amylase

Question 2

How much carbohydrate digestion is done by ptyalin?

Answer 2

<5% in the mouth, but it will continue to work in the stomach –> leading to 30-40% of starch digestion to maltose

Question 3

How is carbohydrate digested in the stomach?

Answer 3

The action of pytalin continues for about an hour, before it’s inhibits by the gastric acid

Question 4

Where/how is the majority of the carbohydrate digested?

Answer 4

In the duodenum, by pancreatic enzymes
- pancreatic alpha amylase is very similar to the salivary one, but much more powerful
- will digest all the starch

Question 5

How is carbohydrate absorbed?

Answer 5

After digestion by pancreatic amylase, the intestinal epithelial enzymes @ the brush border will break it down disaccharides and small glucose to monosaccharides

Question 6

What are the breakdown products of lactose, sucrose, and maltose?

Answer 6

Lactose = galactose + glucose
Sucrose = fructose + glucose
Maltose = glucose

Question 7

Where does protein digestion begin?

Answer 7

It begins in the stomach.

Question 8

What happens in the stomach re: protein digestion?

Answer 8

Pepsin –> breaks down the collagen fibers, so proteolytic enzymes can penetrate

Question 9

Where is the majority of the protein digested?

Answer 9

Duodenum, for pancreatic proteolytic enzymes

Question 10

What are the actions of pancreatic proteolytic enzymes?

Answer 10

Breaks protein down to di- , tri- peptides and some larger peptides

Question 11

Where does the majority of the protein gets broken down into amino acids?

Answer 11

At the brush border, this is done by peptidases in the enterocytes that line the villi

Question 12

What are some pancreatic proteolytic enzymes and their actions?

Answer 12

• trypsin & chymotrypsin: split in to small polypeptides
• carboxypeptidases: cleaves amino acids at the carboxyl tail of polypeptides
• proelastase: elastase, digests elastin fibers that hold meat together

Question 13

What happens to protein at the enterocytes?

Answer 13

• brush border: aminopolypeptidase and dipeptidases –> split protein into tri-, di-peptides and some amino acids –> then transported into the enterocytes
• inside the enterocytes: amino acid linkage specific enzymes breakdown protein into amino acid, then it enters blood

Question 14

Where is fat digested?

Answer 14

in the duodenum - needs emulsification by bile acid and lecithin

Question 15

What’s emulsification?

Answer 15

Process which fat globules are broken into smaller pieces by detergent (bile acids, esp lecithin) –> increases the total surface area
- lipases = water soluble, can only attack fat at their surface

Question 16

How is triglycerides digested?

Answer 16

triglyceride is digested by pancreatic lipase
- can be digested into free fatty acids within minutes after emulsification

Question 17

What’s the role of micelle in fat digestion?

Answer 17

Formation of micelle from bile acids accelerates fatty acid digestion.
- hydrolysis of triglycerides = highly reversible, so if there is free fatty acids in the proximity, the reaction will stop
- so micelle packs the fatty acids and monoglycerides to the brush borders of enterocytes

Question 18

How does the absorptive surface of the small intestine increase?

Answer 18

• fold of kerckring (x3)
• villi (x10)
• microvilli (x100)

Question 19

How is water absorbed in the intestines?

Answer 19

• by osmosis
• Na+ in the paracellular space creates osmotic pool –> water absorbed in the tight junctions in the apical borders, then into circulating blood of the villi

Question 20

How is Na+ absorbed?

Answer 20

It is actively transported (Na+/K+ pump) from inside the enterocyte to the paracellular space in the basolateral membranes
- this provides a steep electrochemical gradient for Na+ from chyme to move through the brush border into the epithelial cell cytoplasm

Question 21

How does aldosterone influence water absorption?

Answer 21

Aldosterone (stimulated by dehydration, secreted by the adrenal gland) enhances water absorption, along with other ions such as Cl-
- most prominent at the colon

Question 22

What happens in diarrhea?

Answer 22

In the large intestines, toxins from cholera and other bacteria can stimulate secretion of large amounts of NaCl and water

Question 23

How is calcium absorbed?

Answer 23

It’s actively absorbed in the intestine, with the aid of vitamin D
- parathyroid hormone will activate vitamin D at the kidneys; activated vitamin D enhances Ca2+ absorption

Question 24

How is carbohydrate absorbed?

Answer 24

once it’s broken down into monosaccharides –> most abundant = glucose
- glucose: active co-transport with Na+ via the SGLT1 from lumen into the enterocyte, then it’s facilitated diffusion via GLUT2 from inside the cell through the basolateral membranes
- galactose is absorbed the same way as glucsoe
- fructose has it’s own facilitated diffusion via GLUT5 from lumen into the cell, but also uses GLUT2 to exit into the paracellular space
- some fructose is converted to glucose in the cytoplasm

Question 25

How is protein absorbed?

Answer 25

Protein, once is broken down to di- , tri-peptide and free amino acids, uses the same mechanism as glucose transport
- Na+ co-transport
- a few amino acids has their own special membrane transport similar to fructose

Question 26

How is fat absorbed into the enterocyte?

Answer 26

Fats = soluble in the enterocyte border
- some are recombined into triglycerides in cytoplasm
- a few are further digested into glycerol and fatty acids
- triglycerides cannot freely pass through the enterocytes

Question 27

How is fat absorbed into the body?

Answer 27

Triglycerides will enter the Golgi apparatus –> aggregates as globules (cholesterol and phospholipids)
- re-package themselves so the fatty heads are in the middle with the polar tails on the surface –> makes the globules miscible with water
- then globules released from the Golgi apparatus and exocytosed into the basolateral membrane
- enter the lymphatic system via central lacteal of villi
- once in the lymphatics, it’s call a chylomicron
- drains into the thoracic duct