Intestinal Digestion and Absorption of Carbs, Proteins and Fats

Question 1

What percentage of nutrients are already absorbed by the time the meal reaches the distal jejunum?

Answer 1

95%

Question 2

What are some structural and functional features of the duodenum and jejunum that facilitate digestion and absorption?

Answer 2

plicae circularis slow transit of food

villi increase surface area

segmenting contractions facilitate mixing

Question 3

What are the three general places within the small intestine where enzymatic hydrolysis of nutrients occurs?

Answer 3

1. in the lumen with pancreatic enzymes
2. at the microvillious membrane with brush border enzymes
3. in enterocytes - for the peptides at least

Question 4

What enzyme starts breaking down sugars first?

Answer 4

amylase

Question 5

Amylase blocker supplements have been suggested to promote weight loss. WHy don’t they really work?

Answer 5

they’re proteins that just get broken down in the gut

plus we have enzymatic activity ine xcess and undigested carbs can be broken down by bacteria leading to diarrhea and bloating

Question 6

What alpha-glucosidease inhibit has been used to treat type II diabetes?

Answer 6

acarbose

Question 7

Amylase products cannot be imediately absorbed and need further cleavage into which 3 absorbable monosaccharides at the brush border?

Answer 7

glucose

fructose and galactose

Question 8

Describe an effect of end-product inhibition that glucose has?

Answer 8

it can inhibit lactase activity

Question 9

How is the regulation of brush border hydrolases regulated temporally?

Answer 9

they are typically brokendown after a meal and then resynthesized before the next meal

Question 10

How are the brush border hydrolases regulated spatially?

Answer 10

they’re located on the villi and in higher concentrations in the proximal intestine than the distal intestine

Question 11

How do the brush border hydrolases avoid proteolytic cleavage?

Answer 11

they’re heavily glycosylated for protection

Question 12

What brush border hydrolase is an exception to the idea that htey don’t want to be cleaved?

Answer 12

sucrase-isomaltase is a single polypeptide that containes 2 active sites

tis’ cleaved into two pieces by trypsin

Question 13

how can you treat lactose intolerance if it’s due to a lack of lactase?

Answer 13

just supplement lactase with lactaid

Question 14

What transporter takes up glucose form the duodenum and jejunum? What else will it take?

Answer 14

SGLT1

also galactose

note - it’s secondary actieve transport so it requires a los intracellular Na concentration to make the gradient

Question 15

What transporter takes up fructose form the djodenum and jejunum?

Answer 15

GLUT5

Question 16

What basolateral transport will send all monosaccharides to the liver?

Answer 16

Glut2

Question 17

Digestion of proteins is initiated where? By what enzyme?

Answer 17

stomach with pepsin

Question 18

At what pH is pepsin inactivated?

Answer 18

over 4.5 - so in theory it should only be active in the stomach

Question 19

What enzymes take over for pepsin?

Answer 19

pancreatic proteases like endopeptidases and carboxypeptidases

Question 20

What brush border enzyme is required for activation of the pancreatis proteases?

Answer 20

enteropeptidase

Question 21

Proteins rich in what residue will be resistant to digestion?

Answer 21

proline

and of course anything glycosylated

Question 22

What is necessary for the apical membrane transporters to take up amino acids?

Answer 22

they are Na-dependent symporters, so you need the basolateral NaK pumps working

Question 23

What is the main peptide transporter? What does it cotransport peptides with?

Answer 23

PEPT1

cotransports the peptides wiht protons

Question 24

What supplies the luminal H ion for this?

Answer 24

the apical sodium/H exchanger which puts H into the lumen

supported by the NAK ATPase of course

Question 25

What happens to the peptides after they’re brought into the cell?

Answer 25

further digested by cytosolic proteases down to individual amino acids

Question 26

What is special about protein digestion and absorption during the first 6 months of life?

Answer 26

intact proteins can be absorbed by endocytosis - this is the mechanism of passive immunity in infants

Question 27

What cells will take up proteins form the gut to transfer them to lymphocytes and antigens (mediating a lot of food allergies)?

Answer 27

M cells

Question 28

Why don’t genetic disorders of apical amino acid transporters result in amino acid deficiencies?

Answer 28

Because you can still get the amino acids from the peptides that are brought into the cell and then further broken down

Question 29

Most of the dietary lipids are in what form? How about otherse?

Answer 29

90% are triglycerides

10% are cholesterol, phospholipids, and lipid soluble vitamins

Question 30

What cells secrete gastric lipase to initiate lipolysis?

Answer 30

cheif cells

Question 31

Gastric lipase is not required, but what population relies on it?

Answer 31

neonates because they have developmental delay of pancreatic enzyme expression

also in people with pancreatic insufficiency

Question 32

how is pancreatic lipase inhibited by bile acids?

Answer 32

they emulsify the fats and block lipase binding to the fat droplets

Question 33

What molecule is necessary to bind the bile acids, thus recruiting lipase to cleave the fatty acids?

Answer 33

colipase

Question 34

What other two pancreatic enzymes contribute to lipid digestion?

Answer 34

cholesterol esterase
phospholipase A2 (phospholipids)

Question 35

Why was olestra added to chips?

Answer 35

it’s a huge fat that’s too big to be absorbed. idea was that it would be effective for weight loss

turned out to be supeer side effecty

Question 36

What pancreatic lipas einhibitor is used to treat obestiy?

Answer 36

orlistat (xenecal)

Question 37

Does orlistat work?

Answer 37

it does have a modest effect, but many regain the weight after taken off the drug

does reduce risk of type 2 CM and lowers blood pressure but you get fatty stools, abdominal cramping, diarrhea and vat soluble vitamin deficiencies