2. GI Part 6 Flashcards
where does 85% of water absorption take place
small intestine
55% in jejunum, 30% in ileum
14% in large intestine
requisites for an efficient absorption (4)
- increasing resorption surface
- mucosa uptake mechanisms
- high blood perfusion
- permeability
why is Na transport efficient
it represents the driving force for most transport processes
how is Cl absorbed
by carriers as well as passive through paracellular pathway
how is K absorbed
in small intestine through paracellular pathway
what does calcitriol stimulate (3)
- opening of apical membrane Ca channels
- synthesis of calbindin
- increase in Ca ATPase
how is Mg absorbed
through Mg channels and paracellularly
how is P absorbed
through Na/phosphate symporter
what is calbindin and what does it do
it is a calcium binding proteins that takes Ca to the basolateral membrane
describe the absorption of vitamin B12 (5 steps)
- vitamin B12 binds to HC
- Trypsin removes vitamin B12 from HC
- vitamin B12 binds to IF
- receptor separates vitamin B12 and IF
- IF gets recycles and vitamin B12 goes into blood
what is HC and what does it do
HC – haptocorrin, aka transcobalamin I
protects vitamin B12 from stomach acid
what is IF and why is it important
IF - intrinsic factor
the component in the stomach that is needed for vitamin B12 absorption
describe absorption of iron in the small intestine (2)
- Fe3+ cannot be absorbed –> ferrireductase changes it to Fe2+
- ferroportin sends Fe2+ to blood, but it needs to be oxidized to Fe3+ in order for it to be bound to apotransferrin to then change apotransferrin to transferrin
describe absorption of vitamins in the small intestine (3)
- carrier protein that takes up Vitamin Bs (B1, B2, B6)
- sodium co-transporter (secondary active transport) –> vitamin C, biotin
- cotransport with proteins –> folic acid
define catabolic pathways
energy capture (ATP) as a result of degradation of energy rich molecules
define anabolic pathways
what does it require
combine small molecules (amino acids) to form more complex molecules (proteins)
requires energy (ATP –> ADP) and often chemical reductions (NADH)
what are different energy sources in living organisms (5)
- glucose
- fatty acids
- amino acids
- ketone bodies
- volatile fatty acids
what are the 3 phases of energy metabolism
- absorptive phase
- post-absorptive phase
- prolonged energy deficiency
when does the absorptive phase of energy metabolism take place
during active digestion and absorption of nutrients from the gut
what happens during the absorptive phase of energy metabolism (4)
- insulin is released
- glucose is taken up by the liver and converted to glycogen and fatty acids
- fatty acids are sent out of the liver in VLDL to adipose tissue or muscle
- amino acids are used for protein synthesis or are deaminated for gluconeogenesis
when does the post absorptive phase of energy metabolism take place
where do nutrients go
between meals
nutrients are being mobilized from storage pools to tissues
what happens during the post absorptive phase of energy metabolism (3)
- glucagon is released
- glycogenolysis and gluconeogenesis are stimulated to increase glucose
- amino acids are mobilized from muscle
when does the prolonged energy deficiency phase take place in energy metabolism
food deprivation
what happens during prolonged energy deficiency phase of energy metabolism (3)
- glucose and amino acids are conserved
- fatty acids are mobilized in the form of non esterified fatty acids (NEFA)
- formation of ketone bodies in liver mitochondria
define glycolysis
the breakdown of glucose by enzymes, releasing energy and pyruvic acid
what happens to glucose once it gets into the portal blood
once in the portal blood, glucose will reach the liver
what mediates glucose transport into cells
glucose transport into cells in mediated by GLUT
hat is the net gain of glycolysis
2 pyruvate
2 NADH
2 ATP
what happens to pyruvate and NADH produced from glycolysis
pyruvate used in mitochondria
NADH goes through electron transport chain and comes out as NAD+
describe anaerobic glycolysis
2 ATP generated for each molecule of glucose converted to 2 molecules of lactate
no net production of NADH
describe aerobic glycolysis
direct consumption and formation of ATP is the same as in anaerobic glycolysis
pyruvate imported to mitochondria to produce acetyl CoA which then enters the Krebs cycle
NADH can be oxidized during electron transport chain (regeneration of NAD+)
during electron transport chain, 3 ATP produced for each NADH molecule oxidized
describe the TCA cycle
final pathway where carbs, amino acids, and fatty acids converge
energy provided by TCA is essential for most animals and humans
occurs close to electron transport chain
process is aerobic – oxygen used as electron acceptor
delivers reduced NADH and FADH2
define gluconeogenesis
production of glucose from non sugar molecules such as amino acids, lactate, glycerol
why do we need gluconeogenesis
during a prolonged fast, hepatic glycogen stores are depleted, glucose is then formed from precursors other than carbs
is gluconeogenesis a reverse glycolysis
where are enzymes it uses located
no
it is a special pathway that requires enzymes localized in the mitochondria and the cytosol
what are important tissues for gluconeogenesis
liver
kidney
important substrates for gluconeogenesis (3)
- glycerol –> glycerol phosphate
- lactate –> pyruvate
- amino acids –> TCA cycle –> oxaloacetate
define glycogenesis
mechanism to store glucose as glycogen in order to mobilize glucose in absence of a dietary source
main glycogen stores in the body
skeletal muscle
liver
define glycogenolysis
process by which glucose is mobilized from glycogen granules in order to be sent into the blood and to other tissues
where does the pentose phosphate cycle occur
cytosol
is ATP consumed or generated in the pentose phosphate pathway
no ATP is consumed or generated
what does the pentose phosphate pathway produce
it produces a major portion of the NADPH used in the body as well as pentose ribose 5-phospahte
functions of NADPH in physiological processes (5)
- important source of electrons (reductases in the body)
- carrying electrons to electron transport chain complexes
- reducing enzyme cytochrome P450
- respiratory burst in phagocytic cells (NADPH oxidase produces ROS to kill bacteria)
- synthesis of NO
what is cytochrome P450 involved in (3)
- steroid hormone synthesis
- bile acid synthesis
- detoxification
fate of short an medium chain fatty acids in the body
get into portal circulation (bound to albumin) and reach the liver
fate of chylomicrons in the body
TAGs will be converted into free fatty acids and glycerol by lipoprotein lipase
where is lipoprotein lipase expressed
capillaries of skeletal muscle, adipose tissue, heart, lung, kidney, liver
what happens to FFA (3)
can be stored as TAG (adipocytes)
can be used to produce energy (in other cell)
can remain in the blood (bound to plasma proteins)
what are chylomicron remnants
what happens to them
cholesteryl esters, phospholipids, lipoproteins, fat soluble vitamins
will be endocytosed by liver cells (receptor mediated)
what can lipids be used for
energy
structural components
hormone precursors
energy reserves
relevance of fatty acids – energy
during a fast period fatty acids are bound to albumin in plasma (FFA) on the way to tissues (coming form adipose tissue) –> oxidation (energy production in most tissues)
relevance of fatty acids – structural components
phospholipids and glycolipids in the plasma membrane
relevance of fatty acids – hormone production
prostaglandins
relevance of fatty acids – energy reserves
TAG in adipose tissues
what do you get from beta oxidation of fatty acids
net energy gain
from 1 palmitoyl CoA that has been oxidized to CO2 and H2O –> 8 ACoA, 7 NADH, 7 FADH2
from these molecules 131 ATP can be generated
subtract 2 ATP needed for process = 129 ATP
how are ketone bodies formed
the mitochondria in liver can convert ACoA from fatty acid oxidation into ketone bodies which are important sources of energy during fasting periods
examples of ketone bodies
acetoacetate
3-hydroxybutyrate (B-)
acetone
what happens during a prolonged fast to promote ketone body formation
fatty acids mobilized from adipose tissue come to the liver yielding much more ACoA than necessary
fatty acid oxidation also produces high amounts of NADH, which shift OAA to malate
result is the utilization of excess ACoA for ketone bodies formation
what happens to ketone bodies in peripheral tissues
they are converted to ACoA which enters TCA cycle
fate of absorbed amino acids in the liver
57% – urea
14% – liver proteins
6% – plasma proteins
23% – systemic circulation
important molecules in physiology that are derived from amino acids (4) and kinda what they do?
- hydroxylation of tryptophan yields serotonin (neurotransmitter and paracrine hormone)
- acetylation and methylation of serotonin –> melatonin (hormone which influences reproductive activity)
- hydroxylation of tyrosine yields dopa, which is subsequently decarboxylated to form neurotransmitter dopamine (in some neurons dopamine is hydroxylated to form norepinephrine)
- decarboxylation of histidine yields histamine (mediator of allergic reactions)
peptides of physiological importance (4)
- oxytocin – produced in hypothalamus (uterine contraction and milk secretion)
- ADH – produced in hypothalamus, essential for maintenance of water balance
- bradykinin – vasoactive substance
- angiotensin II – potent vasoconstrictor
polypeptides of physiological relevance (4)
- gastrin – stomach hormone, stimulates secretion of gastric glands
- CCK – stimulates pancreas and liver secretion
- glucagon – produces by alpha cells of pancreas
- atrial natriuretic peptide (ANP) – produced in heart (atria), essential for regulation of blood volume and pressure
describe glucogenic amino acids
those that produce TCA intermediates and can thereby enter the TCA cycle, yielding oxaloacetate (a glucose precursor)
list glucogenic amino acids (14)
- alanine
- arginine
- asparagine
- aspartate
- cysteine
- glutamate
- glutamine
- glycine
- proline
- serine
- histidine
- methionine
- threonine
- valine
describe branch chain amino acids
serve as sources of energy in msucle cells
list BCCA (3)
- valine
- leucine
- isoleucine
what happens to BCCA
after deamination, BCAA are converted to a-ketoacids which enter TCA
at same time pyruvate serves as an acceptor of BCAA’s amino group yielding to the formation of alanine which leaves the msucle and is used by the liver for gluconeogenesis
the comings and goings of BCAAs (9)
- BCAA come from blood
- enter muscle cell
- deaminate into a-ketoacids
- produce energy
- make pyruvate
- changes into alanine
- goes into blood
- goes to liver
- produce glucose
what is a way to get alanine
BCAA metabolism
why is alanine important
alanine from BCAA metabolism represents an important way to eliminate ammonia (NH3) from the body through urea formation in the liver (only occurs in liver)
urea then goes to kidney to be released as urine
