GI Physiology: Digestion and Absorption

Question 1

Where in the GI tract does most digestion occur?

Answer 1

the small intestine!

Question 2

Is there bacteria in stool?

Answer 2

Yeah a ton!

Question 3

How much water is lost to stool per day? How much does the colon absorb?

Answer 3

100ml is lost per day. 1L is absorbed by the colon

Question 4

What cell types are primarily found in the crypts of the small intestine? Name the function of each.

Answer 4

1. Enteroendocrine cells - secrete gastrin, CCK, or secretin.
2. Stem cells
3. Paneth cells - make lysozyme

Question 5

What cell types are primarily found in the middle or at the top of small intestinal villi? What does each do?

Answer 5

1. Enterocytes - absorb nutrients and express membrane enzymes.
2. Goblet cells - mucin secretion

Question 6

Where in the small intestinal villi does DNA synthesis occur? Where does protein synthesis occur? Where is the disaccharidase and peptidase activity found?

Answer 6

DNA synthesis happens in CRYPTS ONLY.

Protein synthesis happens in the middle of the villi as the cells are differentiating.

Disaccharidase and peptidase enzymes are expressed at the top (tips) of the villi.

Question 7

How often is the epithelial lining of the small intestine replaced?

Answer 7

every 5-6 days

Question 8

Do stem cells divide and move up villi like a conveyor belt as they differentiate?

Answer 8

Yeah!

Question 9

Name two important things that are absorbed mainly in the distal ileum.

Answer 9

1. B12

2. Bile acids for recycling

Question 10

What is the purpose of having folds, villi, and microvilli in the small intestine?

Answer 10

To increase surface area!

Question 11

Name three major membrane bound enzymes found in the small intestine.

Answer 11

1. Enterokinase
2. Disaccharidases (there are 5 subtypes)
3. Peptidases (2 subtypes)

Question 12

Trypsinogen can be cleaved to its active form, trypsin, by the membrane-bound enzyme called ________ in the small intestine. Trypsin can then _________ many of the other pancreatic proteases.

Answer 12

enterokinase

activate/cleave

Question 13

Why do you need to use so many different proteases?

Answer 13

To cleave proteins at different AAs!

Question 14

Can carbs, protein, lipids, sodium, and water be absorbed pretty much anywhere along the small intestine?

Answer 14

Yeah - although absorption in the duodenum is greatest.

Question 15

Can the body store vitamins?

Answer 15

Just the fat-soluble ones. Water soluble ones are used or excreted in urine.

Question 16

Is starch a polymer?

Answer 16

Yeah

Question 17

Amylase begins digesting __________ (a macromolecule) in the mouth, and in the small intestine numerous ___________ (class of enzymes) finish the job.

Answer 17

carbohydrates in the mouth

numerous oligosaccharidases in the small intestine

Question 18

Are the oligosaccharidases in the small intestine secreted, or are they membrane bound?

Answer 18

Membrane bound

Question 19

Which two disaccharides are made of something other than glucose?

Answer 19

lactose (glucose + galactose)

sucrose (glucose + fructose)

Question 20

How are the three monosaccharides transported into enterocytes? What happens after they get into the enterocytes?

Answer 20

Glucose and galactose are transported by a Na+ dependent co-transporter into enterocytes.

Fructose uses a non-Na+ dependent transporter.

After, they get into the capillaries via facilitated diffusion and go to the liver.

Question 21

Describe how proteins are digested and get into enterocytes. What happens after they get into enterocytes?

Answer 21

Pancreatic proteases break proteins down to smaller bits: large protein bits, small, protein bits, and free AAs.

Free AAs get into enterocytes using Na+ dependent transporters.

Small proteins (3 AAs) get in using a H+ ion dependent co-transporter.

Large proteins don’t really get in, they gotta be broken up more.

After getting into the enterocyte they go to the capillary with the help of both co-transporters and facilitated transporters.

Question 22

What happens if you can’t make bile salts or can’t get them into your small intestine?

Answer 22

Steatorrhea, fat malabsorption and fat-soluble vitamin deficiency.

Question 23

What is the major enzyme that is made by the pancreas (secreted into the duodenum) that works with micelles to aid in free fatty acid absorption?

Answer 23

Lipase

Question 24

What happens after free fatty acids enter enterocytes?

Answer 24

They are packaged into chylomicrons and exocytosed into the lymphatics (the lacteal in the villi)

Question 25

Isn’t the colon awesome at absorbing water, considering its relative surface area?

Answer 25

Yeah