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GI Anatomy > Physiology - nutrient digestion > Flashcards

Physiology - nutrient digestion Flashcards

1
Q

3 types of carbohydrates

A

Monosaccharides
Disaccharides
Polysaccharides

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2
Q

What are monosaccharides and 3 examples

A

6 carbon sugars (hexose sugars)

Glucose, galactose, fructose

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3
Q

What are disaccharides and 3 examples

A

2 monosaccharides joined by glycosidic bond

Lactose, sucrose, maltose

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4
Q

What monomers are lactose made of and what is it broken down by

A

Glucose + galactose

Broken down by lactase

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5
Q

What monomers are sucrose made of and what is it broken down by

A

Glucose + fructose

Broken down by sucrase

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6
Q

What monomers are maltose made of

A

Glucose + glucose

Broken down by maltase

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7
Q

What are polysaccharides

A

More than 2 monosaccharides joined together

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8
Q

3 forms of polysaccharides

A

Starch
Cellulose
Glycogen

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9
Q

2 forms of starch (structure)

A

a-amylose: glucose linked in straight chains by a-1,4-glycosidic bonds
amylopectin: glucose chains highly branched with a-1,6-glycosidic bonds forming the branches

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10
Q

Cellulose structure

A

Makes up plant cell walls

Unbranched, linear chains of glucose monomers linked by b-1,4-glycosidic bonds

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11
Q

Why can’t cellulose be broken down by amylase + what breaks it down instead

A

Amylase can’t digest its b-1,4-glycosidic bonds so require bacteria in the colon that express CELLULASE to break it down

HOWEVER AMYLASE CAN DIGEST THE B-1,4-GLYCOSIDIC BONDS OF STARCH

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12
Q

Glycogen structure

A

Animal storage form of glucose

Glucose monomers linked by a-1,4-glycosidic bonds

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13
Q

Epithelial cells have 2 membranes called…

A

Apical (faces the GI tract lumen) and basolateral (faces blood side)

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14
Q

What enzyme breaks down starch and glycogen

A

Amylase

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15
Q

3 mechanisms of transport across intestinal epithelial cells + description

A

Transcellular - pass straight through epithelial cell
Paracellular - pass between 2 epithelial cells (through tight junction)
Vectorial transport - if molecule is water soluble then need transport protein

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16
Q

Where are brush border enzymes located

A

Microvilli

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17
Q

Describe glucose/galactose (‘/’ because they’re absorbed by same mechanism) transport across epithelial cells (5)

  • coupled to transport of what cation
  • what transporter involved to pump into epithelial cell from GI tract lumen
  • what then happens to the cation
  • what then happens to the glucose/galactose + what transporter moves it out the cell into blood
A
  1. Coupled to sodium
  2. When Na and glucose/galactose present, SGLT1 transporter recognises them and binds them to it
  3. SGLT1 undergoes conformational change, delivering them into the cell
  4. When inside cell, Na pumped straight out basolateral membrane by Na/K pump into blood
  5. Once glucose/galactose concentration inside the epithelial cell exceeds that in the blood, a concentration gradient (high to low) is created allowing glucose/galactose to move out the cell into the blood via GLUT-2 transporter
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18
Q

What transporter moves glucose

  • into epithelial cells
  • out of epithelial cells
A

SGLT1

GLUT-2

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19
Q

What gradient does the Na/K pump produce + what membrane potential is created by it

A

Inward Na gradient because always pumping sodium out

Inside negative potential difference

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20
Q

What kind of gradient does sodium being pumped across epithelial cells into the blood create

A

Osmotic gradient - drawing water through tight junctions between epithelial cells (passes paracellularly)

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21
Q

Are

  • Na/K pump
  • SGLT1

primary or secondary active transporters

A

Na/K pump - primary

SGLT1 - secondary because dependent on Na/K pump

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22
Q

Describe fructose transport (4)

  • what transporter moves fructose from GI tract lumen into cells
  • is energy required
  • what transporter moves fructose out of cells into blood
A
  1. GLUT5 transporter binds fructose, undergoes conformational change and delivers it inside cell
  2. No energy required for GLUT5 transporter because we don’t have a blood fructose concentration , just gets metabolised straight away
  3. GLUT2 transporter then moves fructose out of epithelial cell into blood
  4. NO Na TRANSPORT SO NO WATER DRAWN IN
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23
Q

Protein structure

A

Amino acids linked by peptide bonds

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24
Q

Examples of post-translational modification of polypeptides

A

Addition of CHO - glycoprotein

Addition of lipid - lipoprotein

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25
Q

Small protein (peptide) length + normal protein length

A

Peptides - 3-10 amino acids

Protein - 10+ amino acids

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26
Q

What enzymes hydrolyse peptide bonds

A

Proteases/peptidases

27
Q

2 types of proteases/peptidases

A

Endopeptidase - acts on interior of protein

Exopeptidase - acts on terminal amino acids

28
Q

How many essential amino acids do we need + what are essential amino acids

A

20

Ones we don’t produce in body, need to get through diet

29
Q

Is protein absorbed mainly as peptides or individual amino acids

A

Peptides

30
Q

Describe amino acid transport (4)

  • what cation is it coupled to
  • what transporter pumps the amino acid and the cation into the epithelial cell from GI tract lumen
  • what then happens to the cation + what then happens to the amino acid
  • what gradient is produced
A
  1. Amino acid coupled with sodium, bind to sodium coupled amino acid transporter (SAAT1), undergoes conformational change and delivers them inside epithelial cell
  2. Na gets pumped out through Na/K pump in basolateral membrane into blood
  3. Amino acid pumped out basolateral membrane via another transport protein (no specific name) into blood
  4. Na coming into blood creates osmotic gradient drawing water into cell then into blood
31
Q

Describe dipeptide/tripeptide transport, e.g. penicillin (tripeptide) (2)

  • what cation is it coupled with
  • what transporter pumps them into epithelial cell
A
  1. Di/tripeptide coupled with H+

2. PepT1 (proton coupled transporter transports di/tripeptides and H+ into epithelial cell

32
Q

What is the PepT1 transporter (di/tripeptide transporter) dependent on

A

Acid microclimate on the epithelial surface (maintained at low pH) because acidic environment produces higher abundance of H+ which drives the transport of di/tripeptides into the cell

33
Q

What other transport protein is involved in di/tripeptide transport (located next to PepT1 in apical membrane)

A

Sodium hydrogen exchanger (NHE3) - takes H+ back out in exchange for Na coming into cell

34
Q

What is triacylglycerol made up of

A

Glycerol + 3 fatty acids

35
Q

What enzyme digests fat in the small intestine

A

Pancreatic lipase

36
Q

Since lipase is water soluble, digestion of lipid droplets (TAG) takes place where on the droplet

A

Surface of the lipid droplet because it’s hydrophilic on the surface

37
Q

What does lipase break TAG into (the constituents)

A

Monoglyceride + 2 fatty acids

38
Q

2 general requirements for emulsification of ingested fats

A

Mechanical disruption of large lipid droplets into smaller ones - achieved by smooth muscle contraction in stomach

Emulsifying agent - prevents small droplets reforming into large droplets (prevents re-aggregation)

39
Q

What are the 2 emulsifying agents of fat and how are they secreted

A

Bile salts
Phospholipids
Secreted in bile from liver

40
Q

What type of molecules are bile salts and phospholipids and how does that property allow emulsification of fat

A

Amphiphatic (have both polar and non-polar portions)
Non-polar portion associates with hydrophobic core of lipid droplet and polar portion associates with hydrophilic surface

41
Q

Absorption of lipase digestion products is very slow so it is enhanced by…

A

Micelles

42
Q

What are micelles and what are they made up of

A

Tiny emulsion droplets

Bile salt + monogylcerides + fatty acids + phospholipids

43
Q

How do bile salts form emulsification droplets

A

Bile salts combine with phospholipids and lipid droplets to form emulsification droplet

44
Q

Function of micelles

A

Transporters - gets the droplets (of monoglycerides + fatty acids) close enough to the small intestine epithelial cell to be absorbed

45
Q

How do micelles actually transport lipid droplets into the small intestine (related to pH)

A

Micelles are stable at high pH but when it reaches the apical surface of the enterocytes where there’s a low pH (Acid microclimate), the fatty acids in the micelle pick up H+ ions and become uncharged which destabilises micelle structure, allowing contents to be expelled into enterocyte

46
Q

A dynamic equilibrium exists between… & how does it work

A

Micelle breakdown and micelle reformation
Every now and then, micelles come into contact with acid microclimate, releasing its contents and anything that doesn’t get absorbed by enterocytes get reformed into micelle

47
Q

When fatty acids + monoglycerides enter enterocyte (released from the micelle), what are they reformed into and why do they need to be reformed into this

A

TAG - because the free fatty acids + monoglycerides in solution would clog up the epithelial cell so need to be re-emulsified

48
Q

Once inside enterocytes, what are TAG droplets emulsified with (not bile salts + phospholipids)

A

Amphiphatic protein because no bile inside enterocytes, only outside

49
Q

Function of vesicles (in terms of fat digestion)

  • what are they formed from
  • what do they do to the emulsified TAG droplet
A

Vesicles formed from smooth endoplasmic reticulum form around the emulsified TAG droplet (with amphiphatic protein within the enterocyte) and buds off the ER, moving to and fusing with basolateral membrane of the enterocyte and exocytosing out of it into the blood

50
Q

Once TAG droplet is exocytosed out of enterocyte, what is it referred to as

A

Chylomicron

51
Q

What are chylomicrons + what is it made up of

A

Extracellular fat droplets - phospholipids + cholesterol + fat soluble vitamins

52
Q

Because chylomicrons are too big to squeeze between tight junctions, they pass into what instead

A

Lymphatic system via lacteals between endothelial cells

53
Q

Examples of fat soluble vitamins

A

Vitamin A, D, E, K

54
Q

Examples of water soluble vitamins

A

B group vitamins (e.g. folic acid), vitamin C

55
Q

How are water soluble vitamins absorbed (2 mechanisms)

A

Passive diffusion

Carrier mediated transport

56
Q

What does vitamin B12 bind to in stomach and what does it form

A

Intrinsic factor

Forms a complex which then travels through SI to distal ileum where it’s absorbed

57
Q

What does B12 deficiency lead to

A

Pernicious anaemia (Failure of RBC maturation) because vitamin B12 needed for RBC maturation

58
Q

Examples of dietary minerals (6)

A

Na, K, Mg, Ca, Fe, Zn

59
Q

How is iron absorbed from diet

A

Iron is transported across brush border membrane (apical membrane) via DMT1 transporter into duodenal enterocytes

60
Q

What does the DMT1 (Divalent metal 1) transporter transport into intestinal cells + how does it work

  • coupled to what cation
  • dependent on what environment
  • what other transporter does it work together with
A

It’s a hydrogen ion coupled transporter dependent on acid microclimate that moves IRON into intestinal cells

Works alongside NHE3 transporter (exchanges H+ out for Na+ coming in)

61
Q

How is iron stored in the body once absorbed into small intestine

A

Iron ions (Fe2+) incorporated into ferritin

62
Q

What happens to the iron that doesn’t incorporate with ferritin within the duodenal enterocytes

A

It’s transported across basolateral membrane into blood and binds to transferrin which is then transported from gut to liver where it gets incorporated into haemoglobin (enriching blood with iron)

63
Q

What happens to ferritin expression if we have hyperaemia (high iron in blood)

A

Gut increases ferritin expression in duodenum so more iron can be bound to ferritin

64
Q

What happens to ferritin expression if we have anaemia (low iron in blood)

A

Gut decreased ferritin expression in duodenum so more iron can be released to blood

GI Anatomy (21 decks)

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