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Question 1

Absorption of the products of digestion for all nutrients always requires a driving force : Tr u e or False?

Answer 1

T

Question 2

What is absorbtion

Answer 2

Absorption is the (Net) passage of
substances from the GI lumen across
the lining of the intestine into the
interstitial fluid and then into the blood
or lymph

Question 3

Answer 3

Question 4

Sites of absorption - how much is absorbed in each place and what is absorbed?

Answer 4

Mouth, esophagus, stomach
- Minimal absorption
- Lipid soluble substances

Small intestine
- Main site of absorption
- 90% of water and sodium
- All nutrients

Large intestine
- 9% on water and sodium

Question 5

Factors affecting absorption

Answer 5

• Motility
• Surface area available for absorption
• Transport across epithelium
• Reduction in size (chemical digestion)
• Membrane transporters
• Removal from interstitial fluid

Question 6

Affect of motility on absorption
- propulsion

Answer 6

Correct rate of propulsion to allow
- digestion (storage - having enough time to digest)
- absorbtion (peristalsis - having enough time to absorb)

Question 7

Affect of motility on absorption
- segmentation

Answer 7

• exposure of products of digestion to absorb active surface

Question 8

Rate of absorption proportional to

Answer 8

Surface area
- Greater the surface area the faster the rate of absorption

Question 9

Anatomical adaptations maximizes
surface area

Answer 9

• Length of intestine (6m)
• Circular folds (plicae circulares)
• Villi
• Microvilli

Question 10

Transport of molecules across epithelium - what’s the prob??

Answer 10

• The lumen of the intestine is continuous
  with the outside world
• The intestinal epithelium is a barrier

Question 11

Two pathways for Transport of molecules across epithelium
- BLOOD and LYMPH

Answer 11

• Paracellular
  
  • Between cells
• Transcellular
  
  • Across the cell membranes
  • Through the cytoplasm

Question 12

Paracellular pathways – gaps between cells
- FEATURES

Answer 12

• Solutes do not cross cell membranes
• Only barrier is tight junctions binding cells together
• Relatively non-selective
  
  • If the solute is small enough it can get across
• Passive
  
  • Requires a gradient
  • (blood vessels on other side of epithelia help with the
    driving force)

Question 13

Transcellular pathway features

Answer 13

• Solutes must cross two cell membranes
• Cell membranes are lipid bilayers
  
  • If solute is not lipid soluble it requires a transport protein

Question 14

Two ways to maximise absorption across available surface area

Answer 14

• chemical digestion
  
  • reduces nutrients into smallest unit possible
• Specific transport protein (need one in both membranes)
  
  • absorb what is required
  • allows active transport
    
    • against a gradient

Question 15

Removal of substance from interstitial fluid
- how it affects absorbtion

Answer 15

There is a large blood flow to intestine

Arrangement in vili of blood vessels and lacteals prevent the build up in interstitial fluid

Question 16

How much water do we absorb each day? - kinda YAPPY

Answer 16

Drink ≈1.5 L per day
- Replaces water lost sweat, urine & faeces and as we
breathe

But how much is secreted into the GI tract?
- Salivary secretion ≈1.5 L per day
- Gastric secretion ≈ 3 L per day
- Pancreatic secretion ≈ 1.5 L per day
- Biliary (Liver) secretion ≈ 0.5 L per day
- Small intestinal secretion ≈1.5L per day

Total water delivered to small intestine ≈ 9-10 L per day
- If we do not replace the losses and reabsorb the secreted water – major problem

Question 17

Mechanism of water absorption

Answer 17

Osmosis
- Passive movement of water from lumen into blood
- Osmotic gradient (driving force) set up by absorption of salts and nutrients
- Mostly by the paracellular pathway

Question 18

Sodium (Na+) absorption

Answer 18

Passive movement via paracellular pathway
OR
Active transport via the cells
- Transcellular
- Requires transporters to cross the cell membranes

Mechanism:
- Na+ transport alone
- Na+ transport coupled to monosaccharides (e.g.,
glucose, galactose)
- Na+ transport coupled to amino acids

Question 19

Absorption of Na+ alone

Answer 19

• Na+ moves into cell down its gradient
  
  • High to low concentration
  • Transport proteins in apical membrane
    
    • Na+ channels
    • Na+/H+ exchangers
      -To get out of cell it needs to go from low to high concentration using active transport (Uses energy – ATP - Na+,K+-ATPase)

Question 20

What are the 2 Mechanisms
Of carbohydrate absorbtion

Answer 20

Passive and active absorption

Question 21

Carbohydrate Passive absorption via paracellular path

Answer 21

Monosaccharides
- Glucose
- Galactose
- Fructose

Diffuse down concentration gradient via
paracellular pathway

Question 22

Carbohydrate absorption - active absorbtion

Answer 22

Cotransport with Na+
- Via cellular pathway

Monosaccharides
- E.g. Fructose, Glucose Galactose

Transported across cell membrane
- Driving force = Na+/K+ATPase
- on Apical membrane - Na+ glucose cotransporter
- on Basolateral membrane - Glucose carrier

Question 23

Absorption of products of protein digestion – Passive

Answer 23

Passive absorption via paracellular path
Amino acids Diffuse down concentration gradient via
paracellular pathway

Question 24

Absorption of products of protein digestion – active absorption of amino acids

Answer 24

Active absorption
Cotransport with Na+
- Via cellular pathway (Similar to glucose absorption)

Transported across cell membrane
- Driving force from Na+/K+ATPase in basolateral membrane
- In Apical membrane - Na+ amino acid cotransporter
- In Basolateral membrane - Amino acid carrier

Question 25

Absorption of products of protein digestion – Small Peptides (di and tri peptides)

Answer 25

- Active absorption - Cotransport with H+ - Via cellular pathway - Transported across cell membrane - Driving force is Na+/K+ATPase in basolateral membrane (maintains –ve membrane potential for H+ movement) - in Apical membrane - H+ / Peptide cotransporter - Cytoplasmic peptidases - Digest small peptides to AA’s - in Basolateral membrane there is an Amino acid carrier

Question 26

Absorption of products of fat digestion

Answer 26

Lumen and Apical membrane: - Free fatty acids and monoglycerides move by simple diffusion from micelle into cell. - Micelle not absorbed. - Bile salts absorbed in ileum. Intracellular: - Transported to the endoplasmic reticulum. - Resynthesized to triglycerides to maintain gradient for apical diffusion. - Secreted from golgi as chylomicrons. - Targeted to basolateral membrane Basolateral membrane: - Exocytosed as chylomicrons. - Enter lymph via lacteals of villi.

Question 27

Bile salt absorption

Answer 27

- The bile salts in the micelles are eventually absorbed - Occurs after fat absorption complete In the ileum: Active transport process - Apical Na+ dependent bile acid cotransporter In the colon - Passive absorption - Absorbs 95% of bile salts

Question 28

Vitamin absorption - fat soluble vitamins

Answer 28

V i t a m i n s A , D, E , K - Absorbed with fats

Question 29

Vitamin absorbtion - Water soluble vitamins

Answer 29

- B group and vitamin C - Na+ dependent - Very similar to monosaccharide and amino acid absorption.

Question 30

Vitamin absorbtion - vitamin B12

Answer 30

- Absorbed in the ileum - Binds to intrinsic factor produced in stomach - Specific transporter for Intrinsic factor vitamin B12 in the ileum

Question 31

We do not absorb everything - ELMINATION

Answer 31

Expulsion of residues of digestion - Faeces formed in large intestine - Transferred to rectum via peristaltic waves (Mass movements) - Elimination from body by defecation reflex