Physiology - nutrient digestion Flashcards
3 types of carbohydrates
Monosaccharides
Disaccharides
Polysaccharides
What are monosaccharides and 3 examples
6 carbon sugars (hexose sugars)
Glucose, galactose, fructose
What are disaccharides and 3 examples
2 monosaccharides joined by glycosidic bond
Lactose, sucrose, maltose
What monomers are lactose made of and what is it broken down by
Glucose + galactose
Broken down by lactase
What monomers are sucrose made of and what is it broken down by
Glucose + fructose
Broken down by sucrase
What monomers are maltose made of
Glucose + glucose
Broken down by maltase
What are polysaccharides
More than 2 monosaccharides joined together
3 forms of polysaccharides
Starch
Cellulose
Glycogen
2 forms of starch (structure)
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
Cellulose structure
Makes up plant cell walls
Unbranched, linear chains of glucose monomers linked by b-1,4-glycosidic bonds
Why can’t cellulose be broken down by amylase + what breaks it down instead
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
Glycogen structure
Animal storage form of glucose
Glucose monomers linked by a-1,4-glycosidic bonds
Epithelial cells have 2 membranes called…
Apical (faces the GI tract lumen) and basolateral (faces blood side)
What enzyme breaks down starch and glycogen
Amylase
3 mechanisms of transport across intestinal epithelial cells + description
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
Where are brush border enzymes located
Microvilli
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
- Coupled to sodium
- When Na and glucose/galactose present, SGLT1 transporter recognises them and binds them to it
- SGLT1 undergoes conformational change, delivering them into the cell
- When inside cell, Na pumped straight out basolateral membrane by Na/K pump into blood
- 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
What transporter moves glucose
- into epithelial cells
- out of epithelial cells
SGLT1
GLUT-2
What gradient does the Na/K pump produce + what membrane potential is created by it
Inward Na gradient because always pumping sodium out
Inside negative potential difference
What kind of gradient does sodium being pumped across epithelial cells into the blood create
Osmotic gradient - drawing water through tight junctions between epithelial cells (passes paracellularly)
Are
- Na/K pump
- SGLT1
primary or secondary active transporters
Na/K pump - primary
SGLT1 - secondary because dependent on Na/K pump
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
- GLUT5 transporter binds fructose, undergoes conformational change and delivers it inside cell
- No energy required for GLUT5 transporter because we don’t have a blood fructose concentration , just gets metabolised straight away
- GLUT2 transporter then moves fructose out of epithelial cell into blood
- NO Na TRANSPORT SO NO WATER DRAWN IN
Protein structure
Amino acids linked by peptide bonds
Examples of post-translational modification of polypeptides
Addition of CHO - glycoprotein
Addition of lipid - lipoprotein
Small protein (peptide) length + normal protein length
Peptides - 3-10 amino acids
Protein - 10+ amino acids
What enzymes hydrolyse peptide bonds
Proteases/peptidases
2 types of proteases/peptidases
Endopeptidase - acts on interior of protein
Exopeptidase - acts on terminal amino acids
How many essential amino acids do we need + what are essential amino acids
20
Ones we don’t produce in body, need to get through diet
Is protein absorbed mainly as peptides or individual amino acids
Peptides
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
- Amino acid coupled with sodium, bind to sodium coupled amino acid transporter (SAAT1), undergoes conformational change and delivers them inside epithelial cell
- Na gets pumped out through Na/K pump in basolateral membrane into blood
- Amino acid pumped out basolateral membrane via another transport protein (no specific name) into blood
- Na coming into blood creates osmotic gradient drawing water into cell then into blood
Describe dipeptide/tripeptide transport, e.g. penicillin (tripeptide) (2)
- what cation is it coupled with
- what transporter pumps them into epithelial cell
- Di/tripeptide coupled with H+
2. PepT1 (proton coupled transporter transports di/tripeptides and H+ into epithelial cell
What is the PepT1 transporter (di/tripeptide transporter) dependent on
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
What other transport protein is involved in di/tripeptide transport (located next to PepT1 in apical membrane)
Sodium hydrogen exchanger (NHE3) - takes H+ back out in exchange for Na coming into cell
What is triacylglycerol made up of
Glycerol + 3 fatty acids
What enzyme digests fat in the small intestine
Pancreatic lipase
Since lipase is water soluble, digestion of lipid droplets (TAG) takes place where on the droplet
Surface of the lipid droplet because it’s hydrophilic on the surface
What does lipase break TAG into (the constituents)
Monoglyceride + 2 fatty acids
2 general requirements for emulsification of ingested fats
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)
What are the 2 emulsifying agents of fat and how are they secreted
Bile salts
Phospholipids
Secreted in bile from liver
What type of molecules are bile salts and phospholipids and how does that property allow emulsification of fat
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
Absorption of lipase digestion products is very slow so it is enhanced by…
Micelles
What are micelles and what are they made up of
Tiny emulsion droplets
Bile salt + monogylcerides + fatty acids + phospholipids
How do bile salts form emulsification droplets
Bile salts combine with phospholipids and lipid droplets to form emulsification droplet
Function of micelles
Transporters - gets the droplets (of monoglycerides + fatty acids) close enough to the small intestine epithelial cell to be absorbed
How do micelles actually transport lipid droplets into the small intestine (related to pH)
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
A dynamic equilibrium exists between… & how does it work
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
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
TAG - because the free fatty acids + monoglycerides in solution would clog up the epithelial cell so need to be re-emulsified
Once inside enterocytes, what are TAG droplets emulsified with (not bile salts + phospholipids)
Amphiphatic protein because no bile inside enterocytes, only outside
Function of vesicles (in terms of fat digestion)
- what are they formed from
- what do they do to the emulsified TAG droplet
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
Once TAG droplet is exocytosed out of enterocyte, what is it referred to as
Chylomicron
What are chylomicrons + what is it made up of
Extracellular fat droplets - phospholipids + cholesterol + fat soluble vitamins
Because chylomicrons are too big to squeeze between tight junctions, they pass into what instead
Lymphatic system via lacteals between endothelial cells
Examples of fat soluble vitamins
Vitamin A, D, E, K
Examples of water soluble vitamins
B group vitamins (e.g. folic acid), vitamin C
How are water soluble vitamins absorbed (2 mechanisms)
Passive diffusion
Carrier mediated transport
What does vitamin B12 bind to in stomach and what does it form
Intrinsic factor
Forms a complex which then travels through SI to distal ileum where it’s absorbed
What does B12 deficiency lead to
Pernicious anaemia (Failure of RBC maturation) because vitamin B12 needed for RBC maturation
Examples of dietary minerals (6)
Na, K, Mg, Ca, Fe, Zn
How is iron absorbed from diet
Iron is transported across brush border membrane (apical membrane) via DMT1 transporter into duodenal enterocytes
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
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)
How is iron stored in the body once absorbed into small intestine
Iron ions (Fe2+) incorporated into ferritin
What happens to the iron that doesn’t incorporate with ferritin within the duodenal enterocytes
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)
What happens to ferritin expression if we have hyperaemia (high iron in blood)
Gut increases ferritin expression in duodenum so more iron can be bound to ferritin
What happens to ferritin expression if we have anaemia (low iron in blood)
Gut decreased ferritin expression in duodenum so more iron can be released to blood
