Physiology - nutrient digestion

Question 1

3 types of carbohydrates

Answer 1

Monosaccharides
Disaccharides
Polysaccharides

Question 2

What are monosaccharides and 3 examples

Answer 2

6 carbon sugars (hexose sugars)

Glucose, galactose, fructose

Question 3

What are disaccharides and 3 examples

Answer 3

2 monosaccharides joined by glycosidic bond

Lactose, sucrose, maltose

Question 4

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

Answer 4

Glucose + galactose

Broken down by lactase

Question 5

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

Answer 5

Glucose + fructose

Broken down by sucrase

Question 6

What monomers are maltose made of

Answer 6

Glucose + glucose

Broken down by maltase

Question 7

What are polysaccharides

Answer 7

More than 2 monosaccharides joined together

Question 8

3 forms of polysaccharides

Answer 8

Starch
Cellulose
Glycogen

Question 9

2 forms of starch (structure)

Answer 9

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

Question 10

Cellulose structure

Answer 10

Makes up plant cell walls

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

Question 11

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

Answer 11

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

Question 12

Glycogen structure

Answer 12

Animal storage form of glucose

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

Question 13

Epithelial cells have 2 membranes called…

Answer 13

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

Question 14

What enzyme breaks down starch and glycogen

Answer 14

Amylase

Question 15

3 mechanisms of transport across intestinal epithelial cells + description

Answer 15

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

Question 16

Where are brush border enzymes located

Answer 16

Microvilli

Question 17

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

Answer 17

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

Question 18

What transporter moves glucose

• into epithelial cells
• out of epithelial cells

Answer 18

SGLT1

GLUT-2

Question 19

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

Answer 19

Inward Na gradient because always pumping sodium out

Inside negative potential difference

Question 20

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

Answer 20

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

Question 21

Are

• Na/K pump
• SGLT1

primary or secondary active transporters

Answer 21

Na/K pump - primary

SGLT1 - secondary because dependent on Na/K pump

Question 22

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

Answer 22

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

Question 23

Protein structure

Answer 23

Amino acids linked by peptide bonds

Question 24

Examples of post-translational modification of polypeptides

Answer 24

Addition of CHO - glycoprotein

Addition of lipid - lipoprotein

Question 25

Small protein (peptide) length + normal protein length

Answer 25

Peptides - 3-10 amino acids | Protein - 10+ amino acids

Question 26

What enzymes hydrolyse peptide bonds

Answer 26

Proteases/peptidases

Question 27

2 types of proteases/peptidases

Answer 27

Endopeptidase - acts on interior of protein | Exopeptidase - acts on terminal amino acids

Question 28

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

Answer 28

20 | Ones we don't produce in body, need to get through diet

Question 29

Is protein absorbed mainly as peptides or individual amino acids

Answer 29

Peptides

Question 30

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

Answer 30

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

Question 31

Describe dipeptide/tripeptide transport, e.g. penicillin (tripeptide) (2) - what cation is it coupled with - what transporter pumps them into epithelial cell

Answer 31

1. Di/tripeptide coupled with H+ | 2. PepT1 (proton coupled transporter transports di/tripeptides and H+ into epithelial cell

Question 32

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

Answer 32

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

Question 33

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

Answer 33

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

Question 34

What is triacylglycerol made up of

Answer 34

Glycerol + 3 fatty acids

Question 35

What enzyme digests fat in the small intestine

Answer 35

Pancreatic lipase

Question 36

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

Answer 36

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

Question 37

What does lipase break TAG into (the constituents)

Answer 37

Monoglyceride + 2 fatty acids

Question 38

2 general requirements for emulsification of ingested fats

Answer 38

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)

Question 39

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

Answer 39

Bile salts Phospholipids Secreted in bile from liver

Question 40

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

Answer 40

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

Question 41

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

Answer 41

Micelles

Question 42

What are micelles and what are they made up of

Answer 42

Tiny emulsion droplets | Bile salt + monogylcerides + fatty acids + phospholipids

Question 43

How do bile salts form emulsification droplets

Answer 43

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

Question 44

Function of micelles

Answer 44

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

Question 45

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

Answer 45

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

Question 46

A dynamic equilibrium exists between... & how does it work

Answer 46

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

Question 47

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

Answer 47

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

Question 48

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

Answer 48

Amphiphatic protein because no bile inside enterocytes, only outside

Question 49

Function of vesicles (in terms of fat digestion) - what are they formed from - what do they do to the emulsified TAG droplet

Answer 49

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

Question 50

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

Answer 50

Chylomicron

Question 51

What are chylomicrons + what is it made up of

Answer 51

Extracellular fat droplets - phospholipids + cholesterol + fat soluble vitamins

Question 52

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

Answer 52

Lymphatic system via lacteals between endothelial cells

Question 53

Examples of fat soluble vitamins

Answer 53

Vitamin A, D, E, K

Question 54

Examples of water soluble vitamins

Answer 54

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

Question 55

How are water soluble vitamins absorbed (2 mechanisms)

Answer 55

Passive diffusion | Carrier mediated transport

Question 56

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

Answer 56

Intrinsic factor | Forms a complex which then travels through SI to distal ileum where it's absorbed

Question 57

What does B12 deficiency lead to

Answer 57

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

Question 58

Examples of dietary minerals (6)

Answer 58

Na, K, Mg, Ca, Fe, Zn

Question 59

How is iron absorbed from diet

Answer 59

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

Question 60

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

Answer 60

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)

Question 61

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

Answer 61

Iron ions (Fe2+) incorporated into ferritin

Question 62

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

Answer 62

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)

Question 63

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

Answer 63

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

Question 64

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

Answer 64

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