Unit 2 - Digestion/ Absorption

Question 1

How are carbohydrates digested?

Answer 1

• polysaccharides digested by salivary amylase
• majority digested by pancreatic amylase - occurs in duodenum (breaks down into oligosachharides by splitting 1,4 glycosidic bond)
• brush border enzymes - breaks down into monosaccharides

Question 2

give example of brush border enzymes

Answer 2

maltase
sucrase
lactase
dextrinase

Question 3

if there is a low concentration of glucose, how are carbohydrates absorbed?

Answer 3

absorbed into enterocytes across apical surface by SGLT1 then across basolateral surface by GLUT2

Question 4

if there is a high concentration of glucose, how are carbohydrates absorbed?

Answer 4

SGLT1 are saturated so absorbed via GLUT2 through apical and basolateral surface

Question 5

how are monosaccharides transported to liver?

Answer 5

in blood via hepatic portal vein

Question 6

how are proteins broken down?

Answer 6

• gastric digestion by proteolytic enzymes which are secreted in their inactive form: pepsinogen and then activated in acidic environment (lumen of stomach) to pepsin
• pancreatic digestion: trypsinogen/ chymotrypsinogens which are activated in response to alkaline environment - converted by enterokinase
• further digestion by exopeptidases

Question 7

how are proteins absorbed?

Answer 7

by enterocytes at brush border via carrier protein then exported into extracellulr space across basolateral membrane
- M cells package proteins into vesicles

Question 8

how are lipids digested?

Answer 8

• emulsification by bile salts initially: hydrophilic regions of bile salts repel so droplets are formed
• lipolytic enzymes from pancreas digest lipids - secreted in their inactive form but activated by trypsin in duodenum

Question 9

give two examples of lipolytic enzymes

Answer 9

• phospholipase A2- cleaves fatty acids

- colipase - anchors lipase to micelles= maximal SA

Question 10

how are lipids absorbed?

Answer 10

micelles pass across brush border - re-esterified in endoplasmic reticulum and form chylomicrons - released by exocytosis and join lacteal duct

Question 11

what are the three stages of digestion?

Answer 11

cephalic
gastric
intestinal

Question 12

describe the cephalic phase

Answer 12

neurogenic signals from cortex/ appetite centres relayed to stomach via parasympathetic efferent responses - vagal stimulation increases salivary secretion and vasodilation

Question 13

describe the gastric phase

Answer 13

distension of stomach walls activates mechanoreceptors which activates local enteric reflexes and vago-vagal reflexes -which leads to release of gastrin from G cells -
= causing release of HCl acid by parietal cells
= release of pepsinogen by chief cells
= increased motility

Question 14

in the intestinal phase why is secretin released?

Answer 14

HCl acid from stomach passes into small intestine - secretin inhibits acidity and stimulates release of bicarbonate rich solution by acinar cells of duodenum

Question 15

what causes the release of CCK? and what is the effect of CCK?

Answer 15

fat in chyme
= inhibits stomach motility and release of gastrin
= stimulates release of pancreatic enzymes (lipase)
= causes contraction of gall bladder

Question 16

what is the effect of orexin?

Answer 16

increases appetite

Question 17

what is the effect of ghrelin?

Answer 17

stimulates food intake, appetite and promotes food storage

Question 18

what is the effect of leptin?

Answer 18

made by adipocytes

- controls energy expenditure in the long term - inhibits hunger

Question 19

what is PYY?

Answer 19

gut peptide - reduces food intake, decreases appetite