Digestion and Absorption Flashcards
what are the stages of digestion and absorption
mouth
oesophagus
stomach
intestines
digestion definition
breakdown the large molecules into small/simpler molecules
absorption definition
small molecules pass through cell membranes into the blood or lymph capillaries
refresher for enzymes
typically proteins
increase the speed of most chemical reactions that take place within cells, catalyst
vital for life
important for digestion and metabolism
cleavage
what is cleavage
splitting of a large complex molecule into smaller or simpler molecules
models that demonstrate enzyme action
lock and key
induced fit
lock and key model
both enzyme and substrate have a specific configuration and unique shape that fit together
induced fit model
the substrate induces the enzyme to adopt the form of the substrate
enzymes are flexible and favour geometric adoption for binding and catalysis of the substrate
digestion product of proteins
amino acids
digestion product of fats
fatty acids and glycerol
digestion product of carbohydrates
monosaccharides
monosaccharides
glucose
galactose
fructose
where does digestion and absorption take place
mouth
oesophagus
stomach
small intestine
large instestine
components of the GI tract
mouth
oesphagus
stomach
small intestine
large intestine
anus
accessory organs associated with the alimentary tract
salivary glands
liver
gall bladder
pancreas
another word for chewing
mastication
types of digestion in the mouth
mechanical
chemical
mechanical digestion in the mouth
3 main functions
reduces the size of ingested particles
mixes food with saliva, lubricating it and exposing the food to digestive enzymes
increasing the surface area of ingested material
increasing the surface area of ingested material
increases the rate it can be digested
chemical digestion in the mouth
salvia contains 2 enzymes with fat and carb targets
salivary amylase
lingual lipase
starch chemical digestion in the mouth
alpha amylase (ptyalin) cleaves internal alpha-1,4-glycosidic bonds present in starch
produces maltose, maltotriose and alpha limit dextrins
fat chemical digestion in the mouth
lingual lipase
plays role in hydrolysis of dietary lipids
ph of saliva
6.2-7.6
eliminates acids from food, drink and bacteria
clinical importance of salivary amylase
infants have low levels of pancreatic enzymes in the first year
pancreatic insufficiency e.g. cystic fibrosis
what does lipase hydrolyse
triglycerides
uses 3 molecules of water to break 3 ester bonds
forms 1 glycerol molecule
3 fatty acid molecules
what does amylase break down
starch at alpha 1,4 linkages into polysaccharides and maltose by addition of water
maltose
2 glucose molecules
R group
any group in which a carbon or hydrogen atom is attached to the rest of the molecule
oesophagus
connection between the mouth and the stomach
25-30cm
divided into cervical,thoracic and abdominal parts
uses peristalsis to pass food to stomach
upper and lower oesphageal sphincters control the movement of food into and out of the oesophagus
production and secretion of gastric juice in stomach daily
2L per day
stomach ph
1-3.5
break down food
activates enzymes
denatures proteins
kills bacteria
how much can stomach hold
2-4L of food
what Lines the stomach walls
mucus
acts primarily as a lubricant
protects mucosa from injury
with bicarbonate mucus will neutralise acid and maintain the surface of the mucosa at nearly neutral ph
this is part of gastric mucosal barrier that protects stomach from acid and pepsin digestion
water in stomach
acts as the medium for the action of acid and enzymes
and solubilises many of the constituents of a meal
what is intrinsic factor
glycoprotein produced by the parietal cells in the stomach
vitamin B12 binds to IF to form IF-Cbl complex
needed for absorption in the small intestine
plays crucial role in transportation and absorption of the vital vitamin B12 by terminal ileum
what is vitamin b12 needed for
mature red blood cells
neurological functioning
5 constituents of gastric juice
intrinsic factor
hydrogen ion
pepsin
mucus
water
stomach acid secretion
enterochromaffin-like cells ECL
gastrin and acetylcholine activate phospholipase C
PLC catalyses the formation of inositol triphosphate, IP3
IP# causes release of intracellular calcium ions and activates calmodulin kinase
calmodulin kinases phosphorylate variety of proteins
leading to H+ secretion
ECL cells have cholecystokinin-2 CCK-2 receptors for gastrin
gastrin stimulates ECL cells to release histamine
histamine activates adenylate cyclase to form cyclic AMP
cyclic AMP activates protein kinase A
protein kinase A phosphorylate a variety of proteins leading to H+ secretion
acetylcholine
key neurotransmitter of parasympathetic nervous system
gastrin
peptide hormone secreted by G cells
phospholipase C
enzyme that produced second messengers
IP3
intracellular messenger
mediating hormonal mobilisation of intracellular calcium
calmodulin kinase enzymes
involved in phosphorylation
substrate-level phosphorylation is the direct production of ATP during the enzymatic oxidation of a substance
involves transfer of a phosphate group from a substrate to an ADP molecule
what type of receptors do ECL cells have for gastrin
colcecystokinin 2 receptors, CCK-2
don’t have acetylcholine receptors
transport processes involved in the secretion of Hal
H+ is pumped actively into the lumen by the H+, K+-ATPase,
Cl- enters the cell across the basolateral membrane In exchange for HCO3-
inhibition of proton pump
totally blocks gastric acid secretion
PPI’s are activated at low pH
bind irreversibly to sulfhydryl (Thiol) groups of pump
and inactivate the enzyme
leads to acid secretory inhibition
therefore omeprazole is useful in peptic ulcer treatment
inhibition of carbonic anhydrase CA enzyme
decreases rate of acid secretion but doesnt prevent
metabolism produces much of the CO2 used to neutralise OH- but CO2 from blood is also required
HCO3-diffusing into the blood
gastric venous blood has a higher pH than that of the arterial blood when the stomach is secreting
protein digestion in the stomach
protein stimulates G cells= enteroendocrine cells
they release gastrin into the blood
gastrin stimulates the enterochromafin-like cells to release histamine
histamine then stimulates acid producing parietal cells (oxynitic cells)
gastrin also stimulates parietal cells to release Hal and chief cells to release pepsinogen
pepsinogen converted to pepsin to cleave protein (by HCl)
what stimulates gastrin release
acetylcholine
where are ECL cells
lamina proprietor of gastric glands
what do parietal cells secrete
hydrochloric acid and intrinsic factor
pepsin ph efficacy
optimum 1-3
denatured higher than 5
pepsin information
endopeptidase with specificity for peptide bonds involving aromatic L amino acids
what is the gastric mucosa composed of
pits and glands
pits and surface are lined with mucous or surface epithelial cells
base of pits are openings of glands which project into the mucosa towards outside or serosa
oxyntic glands
contain parietal cells and peptic or chief cells
secrete enzyme precursor pepsinogen
pyloric glands
contain gastrin producing G cells and mucous cells
whcih also produce pepsinogen
mucous neck cells
present where glands open into the pits
gastric pits and glands
negative feedback of protein digestion in stomach
low antral pH means D cells release somatostatin
inhibits g cells
prevents over secretion of acid
cell types in gastric pit and their functions
digestion of dietary fats
gastric lipase is produced by chief cells of the fundus
stimulated by the neurohormonal stimuli, gastrin and cholinergic mechanisms
inhibited by cholecystokinin CCK
and glucagon like peptide ! GLP1
cholinergic neurons
synthesis, release,metabolise acetylcholine
digestion of carbohydrates stomach
salivary amylase inactivates due to low ph so there is little chemical digestion of carbohydrates in the stomach
mechanical breakdown of carbohydrates is ongoing
strong peristaltic contractions of stomach mix carbohydrates into more uniform mixture of chyme
mechanical digestion in the stomach
food is mixed with gastric juices and churned by contractions in stomach wall
mixing waves
propulsion
retropulsion
break food into smaller pieces
mixing was
unique style of peristalsis that mixes and softens the food with gastric juices to create chyme
chyme
acidic fluid mixture of gastric juices and partly digested food which passes from stomach to small intestine
propulsion q
forward movement
retropulsion
backwards movement
gastric emptying in the stomach
holds food for 4-6 hours
absorbs some non polar substances like alcohol and aspirin
pylorus acts as filter to duodenum
what actually is gastric emptying
as peristaltic contractions become stronger
some of liquid chyme travels past pyloric sphincter as it has decreased its muscle tone
and into the duodenum
carbohydrates empty the fasters after liquids
small intestine
connects with pylorus and leads to osmium ileale in the colon
3 sections of small intestine
duodenum
jejunum
ileum
duodenum
continuous with gastric pylorus
common bile duct and pancreatic duct lead into duodenum
jejunum
middle section constituting about 2/5s of small intestine
ileum
terminal section ending in right iliac fossa in colon
main cells types in the small intestine
enterocytes
goblet cells
enteroendocrine cells
paneth cells
enteroendocrine small intestine
release signalling molecules
hormones like CCK, motilin
they coordinate and regulate the activities of the digestive system
paneth cells small intestine
release bacteriotoxic peptides and some growth factors
integral part of innate immune system
enterocytes small intestine
absorptive cells
complete enzymatic digestion of nutrients in their brush border
release absorbed materials to the serosal/internal side
goblet cells small intestine
secrete mucus to the lumenal/external side
acts as protective barrier
when chyme enters the duodenum what occurs
intestinal receptors in the duodenum are activated
then triggers mechanisms inhibiting gastric emptying
mechanisms triggered to inhibit gastric emptying
CCK increases distentibilityof the orad stomach
acid inhibits motility and emptying
other hormones such as secretin and gastric inhibitory peptide also inhibit emptying
regulation of emptying
results from presence of receptors in the duodenum
respond to physical properties such as osmotic pressure
and chemical compotiiosn such as H+ and lipids of the intestinal contents
small intestine chemical digestion of carbohydrates
pancreatic amylase is released following the stimulus of secretin and CCK
starch is digested into maltose, maltotriose and alpha limit dextrins
oligosaccharides and disaccharides are digested at the brush border
brush border enzymes
lactase
sucrase
isomaltase
maltase
lactase function
lactose to glucose and galactose
sucrase function
sucrose to glucose and fructose
isomaltase function
alpha 1,6 bonds of limit dextrins
maltase function
maltose to glucose and glucose
alpha limit dextrin
branched amylopectin remnants
small intestine chemical digestion of fats
bile acts as an emulsifier to increase the surface area
lipase action= lipids to fatty acids and glycerides
bile salts envelop the fatty acids and monoglycerides to micelles
as the brush border of the small intestine the fatty acids and monoglycerides diffuse out of the micelle into the absorptive cells
small intestine chemical digestion of protein
proteases such as trypsin and chymotrypsin
brush border peptidases hydrolyses to dipeptides and amino acids for absorption
mechanical digestion small intestine
mechanical digestion: segmentation and peristalsis mix chyme with secretions
absorption small intestine
folds called place circulares
villi
microvilli
increase surface area
which is approx 300 m2
how do absorbed nutrients move into circulation
by blood capillaries and laterals
or lymph channels
absorption across the intestinal wall of monosaccharides
SGLT1 needed for glucose uptake
energised by the electrochemical Na+ gradient
maintained by the extrusion of Na+ by the Na-K pump by secondary active transport
facilitated diffusion mediated by GLUT5 is for fructose absorption
facilitated diffusion GLUT2
active transport
influx occurs against concentration gradient
uses ATP
secondary active transport
concentration gradient of one molecule provides energy for transport of another molecule against concentration gradient
therefore energy from moving first molecule is enough to move second molecule against its concentration gradient
absence of sodium transport through intestinal membrane
virtually no glucose can be absorbed because glucose absorption occurs in a co-transport mode with active transport of sodium.
Inhibition of the Na-K pump reduces active glucose absorption by decreasing the apical membrane Na + gradient and thus effecting facilitated diffusion and decreasing the driving force for glucose entry.
Therefore, it is the initial active transport of sodium through the basolateral membranes of the intestinal epithelial cells that provides the eventual force for moving glucose through the membranes as well.
galactose transport mechanic m
almost the same as glucose
fructose transport mechanism
by facilitated fission down concentration gradient
through intestinal epithelium
isn’t coupled with sodium transport
much of fructose upon entering cell becomes phosphorylated
then converted to glucose
transported in form of glucose through blood
examples of transport proteins
SGLT1
GLUT5
GLUT2
SGLT1
sodium ion coupled transporter that mediates the uptake of glucose or galactose
involved in secondary active transport
GLUT5
mediates facilitated diffusion of fructose into the enterocyte
GLUT2
mediates their efflux across the basolateral membrane into the interstitial space
large intestine general
produces no digestive enzymes
reclaiming water and maintaining water balance
recovery of electrolytes
storage/elimination of remaining waste
peristalsis pushes the waste material toward canal end of the tract
can take 36 hours for food to move through the entire colon
what occurs in the large intestine
non-digestible carbohydrates: bacterial flora aids in digestion
limited mainly to anaerobic fermentation by essential colonic bacteria
methane and hydrogen also produced= flatus
bacteria manufacture vitamin K which is absorbed
adequate dietary fibres
how does bacterial flora aid in digestion
digestion of dietary fibres and undigested carbohydrates to short chain fatty acids
10% starches, cellulose and hemicellulose form fruit and vegetables and monosaccharides lactulose sorbitol and surculose
vitamin K
importance for clotting
adequate dietary fibres
soluble, slows down transit time such as pectins from fruit/veg, hemicelluloses from cereals
insoluble: draws water into stool= soften and increase faecel weight= improved obwel movement consistency snd speed up transit time such as cellulose form plant material
GI health
fluids, low fluid= dry stools= constipation
optimal bacterial flora fibre helps
avoidance of antibiotics
regular movement- physical activity
common problems of large intestine
constipation
diarrhoea
diverticulitis
chrons
colon cancer
