physiology of digestion and absorption

Question 1

what is digestion

Answer 1

the enzymatic conversion of complex dietary substances to a form that can be absorbed

Question 2

where do most digestive processes occur and how

Answer 2

in the small intestine by luminal digestion and membrane digestion

Question 3

what is absorption

Answer 3

the processes by which the absorbable products of digestion are transferred across both the apical and basolateral membrane of enterocytes

Question 4

assimilation

Answer 4

the overall process of digestion and absorption

Question 5

what are the digestible carbohydrates

Answer 5

polysaccharides
oligosaccharides
monosaccharides
*all dietary carbohydrates must be converted to monosaccharides for absorption

Question 6

what are the poly saccharides

Answer 6

> they are polymers of glucose
starch = amylose and amylopectin
animal = glycogen

Question 7

oligosaccharides

Answer 7

> sucrose

>lactose

Question 8

monosaccharides

Answer 8

> glucose

>fructose

Question 9

the sequence of carbohydrate digestion

Answer 9

starch > oligosaccharide by intraluminal hydrolysis > monosaccharides by membrane digestion&raquo_space; now ready for absorption

Question 10

what is a-amylase

Answer 10

> basically snips link in peptide chain
an endoenzyme
breaks down linear internal alpha 1,4 linkages but not terminal alpha 1,4 linkages hence there is no production of glucose
products are glucose oligomers and alpha-limit dextrins

Question 11

what are oligosaccharidases

Answer 11

> integral membrane proteins with catalytic domain that faces the lumen of the GI tract
ie lactase maltase sucrase isomaltase

Question 12

where do the oligosaccharides cleave

Answer 12

the terminal a-1,4 linkages of maltose, maltotriose and alpha-limit dextrins

Question 13

where does the absorption of the final products of carbohydrate digestion occur

Answer 13

duodenum and jejunum

Question 14

final products of carbohydrate process

Answer 14

> involves entry and exit form the enterocytes via the apical and basolateral membranes
glucose and galactose are absorbed by secondary active transport mediated by SGLT1; fructose by facilitated diffusion mediated by GLUT5
exit fro all monosaccharides is mediated by facilitated diffusion by GLUT2

Question 15

what is the mode of operation of SGLT1

Answer 15

1. sodium binds to SGLT1
2. affinity for glucose increases - glucose binds
3. sodium and glucose translocate from extracellular to intracellular
4. 2 sodium ions dissociate and so affinity for glucose falls
5. glucose dissociates
6. the cycle is repeated

Question 16

features of protein digestion

Answer 16

> must be digested too oligopeptides and amino acids for efficient absorption
there are 4 major pathways for this

**protein > peptides > amino acids > amino acid in enterocyte > amino acid in blood

Question 17

what happens in stomach digestion

Answer 17

> hydrochloric acid begins to denature proteins

>pepsin cleaves the proteins into peptides

Question 18

pepsin

Answer 18

> is an endopeptidase with preference for bonds between aromatic and larger neutral amino acids
not essential for protein digestion
pH optimum of 1.8-3.5

Question 19

what happens in duodenal digestion

Answer 19

> the five pancreatic proteases are secreted as proenzymes from the acing cells of the exocrine pancreas and converted to active form in the duodenum
function as either endopeptidases or exopeptidases
ie trypsin
chymotrypsin
elastase
procarboxypeptidase A&B

Question 20

what catalyses the hydrolysis of protein

Answer 20

exo- and ends- peptidases

Question 21

where are there additional proteases

Answer 21

• at the brush border

- within the cytoplasm of the enterocyte

Question 22

features of brush border peptidases

Answer 22

• numerous
• have affinity for larger oligopeptides
• either endo or exo peptidases

Question 23

features of cytoplasmic peptidases

Answer 23

• less numerous

- primarily hydrolyse dipeptide or tripeptides

Question 24

how are proteins absorbed at the brush border

Answer 24

> are several mechanisms for this
5 are sodium dependent co-transporters mediating uphill movement ie BAT1 system (dysfunction of this results in Hartnup disease)
2 are sodium independent mediating uptake of cationic amino acids (dysfunction results in cystinuria)

Question 25

how are proteins absorbed at the basolateral membrane

Answer 25

> 3 mechanisms mediate the efflux of amino acids and are sodium independent
2 mediate influx and are sodium dependent - the net movement is this bidirectional

Question 26

how are di tri and tetra peptides absorbed

Answer 26

> hydrogen ion mechanism - PepT1 at the brush border
>further hydrolysed to amino acids within the enterocyte
>sodium independent systems at the basolateral membrane

Question 27

summary

Answer 27

Protein is digested in the lumen to amino acids, or oligopeptides, by pepsin and the pancreatic proteases

Peptidases at the brush border further hydrolyse oligopeptides to amino acids

Amino acids are transported across the apical membrane via a variety of amino acid transporters, some of which are Na+-dependent and others Na+-independent

Oligopeptides are transported across the apical membrane by the H+/oligopeptide co-transporter, PepT1

Oligopeptides with the cytoplasm are hydrolysed to amino acids by peptidases within the enterocyte

Amino acids exit the enterocyte across the basolateral membrane by several, Na+-independent. transporters