GI PHYSIOLOGY IV - ABSORPTION

Question 1

TRUE/FALSE: absorption of the products of digestion for all nutrients always require a driving force

Answer 1

true: there’s always a driving force to help the nutrients move whether through the cells or in between the cells

Question 2

briefly describe the general pathway of absorption

Answer 2

GI lumen —— lining of the intestine —–> interstitial fluid —> blood/lymph

Question 3

what are the sites of absorption across the GI tract, what do they absorb and

Answer 3

Mouth, esophagus, stomach: minimal of lipid soluble substances

Small intestine: 90% of water and sodium + all nutrients

Large intestine:9% of water and sodium

Question 4

what are the factors that affect the absorption?

Answer 4

• motility
• SA available for absorption
• transportation across epithelium
• reduction in size of chemical digestion
• membrane transporters
• removal from interstitial fluid

Question 5

what are the 2 things tha mobility affect absorption?

Answer 5

• peristalsis: correct rate of propulsion and storage
• segmentation for exposure of products

Question 6

what are the anatomical adaptations that maximises the surface area

Answer 6

• length of intestine
• plicae circulares
• villi
• microvilli

Question 7

since the lumen of the intestine is continuous with the outside world, the intestinal epithelium therefore a barrie, but there are 2 pathways of transporting substances through the layer, what are they?

Answer 7

• paracellular: between cells
• transcellular: across the cell membrane and through the cytoplasm

Question 8

what are the traits of paracellular pathway and why is it easy to paracellularly transport?

Answer 8

• only barrier is the tight junction binding the cells together which makes it relatively non-selective because any small enough molecule would be able to cross
• passive: requires a gradient

Question 9

what are the traits of the transcellular pathway and why is it harder?

Answer 9

• solutes must cross 2 cell membrane
• the membrane is made of phospholipid –> if solute is not lipid soluble than it requires a transport protein

Question 10

what are the 2 mechanisms that help maximise the absorption?

Answer 10

• reduce the nutrient into smallest possible units
• transport protein

Question 11

what is interstitial fluid?

Answer 11

the fluid that fills the space between cells

Question 12

what are the substances that we absorb everyday?

Answer 12

• water
• sodium
• carbs
• proteins
• lipids
  -bile salts
• vitamins

Question 13

what is the amount of water that is delivered to small intestine per day?

Answer 13

9-10L –> replacement of the loss and reabsorption of secreted water is very important

Question 14

what is the mechanism of water absorption?

Answer 14

osmosis by the paracellular pathway

Question 15

what are the 2 mechanisms of Na+ absorption

Answer 15

• passive movement via paracellular pathway
• active transport via transcellular pathway

Question 16

how does Na+ transport into the cells?

Answer 16

• Na+ transport alone from high to low concentration
• Na+ transport coupled to monosaccharides
• Na+ transport coupled to amino
• Na+ channels or Na+/H+ exchanger

Question 17

how does Na+ transport out of the cells?

Answer 17

active transport: use energy - ATP with Na+/K+ ATP

Question 18

why do we remove substances from the interstitial fluid and how do we remove it?

Answer 18

• to prevent build up in interstitial fluid
• using blood vessels or lymphatics (lacteal)

Question 19

what are the 2 mechanisms for carbohydrate absorption? describe?

Answer 19

Passive and active

• Passive: monosaccharides passively diffuse via concentration gradient

Active -driving force: Na+/K+ ATPase:
- Co-transport with Na+ via cellular pathway
- Monosaccharides
- Apical membrane: Na+ glucose cotransporter
- Basolateral membrane: glucose carrier
- Driving force: Na+/K+ ATPase

Question 20

what are the 2 mechanisms for amino acid absorption?

Answer 20

Passive and active

Passive : amino acids diffuse down concentration gradient via paracellular pathway

Active -driving force: Na+/K+ ATPase:
- Co-transport with Na+ via cellular pathway (same as glucose)
- Apical membrane: Na+ amino acid cotransporter
- Basolateral membrane: amino acid carrier

Question 21

how are small peptides absorbed?

Answer 21

Active transport across cell membrane with Na+/K+ ATPas as driving force:
- Apical membrane: H+/peptide cotransporter
- Cytoplasmic peptidases digest small peptides into AA’s
- Basolateral membrane has amino acid carrier

Question 22

what are the 3 stages of products of fat digestion absorption?

Answer 22

Lumen and apical membrane
Free fatty acids and monoglycerides move by simple diffusion from micelle into the cell

Intracellular
- Transported to the ER where they are synthesised to triglycerides to maintain a gradient from apical diffusion.
- These are then secreted from the golgi as chylomicrons and sent to the basolateral membrane

**Basolateral membrane **
Exocytosed as chylomicrons, then these enter lymph via lacteals of villi

Question 23

what is chylomicrons?

Answer 23

droplets of triglyceride and other small lipids enclosed by membrane from

Question 24

TRUE/FALSE: absorption of the products of fat digestion does not require a driving force

Answer 24

false
- absorption of products of fat digestion does indeed REQUIRES a driving force (triglyceride gets taken into ER, the reasembly and driving away of the molecules means there’s always a concentration –> there’s always driving force)
- however, we absorption of products of fat digestion occurs by passive diffusion

Question 25

define apical and basolateral membrane?

Answer 25

• apical membrane facing an “outside” lumen
• a basolateral membrane facing neighboring cells and the basal lamina

Question 26

when does the bile salts in micelles absorbed and where does that occurs?

Answer 26

• after fat absorption complete
• happens in the ileum: active transport (apical Na+ dependent bile acid cotransporter)
• happens in colon: passive absorption

Question 27

Fat soluble vitamins are absorbed with what?

Answer 27

fats

Question 28

Water soluble vitamins (B group and C) are absorbed using what mechanism?

Answer 28

Na+ dependent

Question 29

What is the special case of vitamin absorption?

Answer 29

Vitamin B12:
- binds to the intrinsic factor in stomach
- there is specific transporter for intrinsic factor vitamin B12 in the ileum

Question 30

how is the residues of digestions be repulsed?

Answer 30

form faeces and being transfered to recum via peristaltic waves and eliminated from the body by defecation reflex