digestion Flashcards
digestion
breakdown of complex molecules into simple molecules
absorption
transporting simple molecules across intestinal epithelium
in order for nutrients to be absorbed in SI much be transported across apical (ie luminal) surface of
enterocytes
then across basolateral surface of cell
apical (luminal) surface of enterocytes covered in
microvilli
before nutrients can reach the apical surface of enterocytes of SI, what do they have to diffuse through
glycocalyx; jelly-like layer of glycoprotein
what is attached to microvilli of enterocytes
digestive enzymes and proteins; project into glycocalyx
goblet cells between enterocytes produce
mucous
fluid flows slower where
closer to intestinal surface; ie unstirred water layer (think like a river, faster in middle, slower at banks)
what is the diffusion barrier nutrients must pass through before getting absorbed
unstirred water layer, mucous and glycocalyx
why is physical digestion (breakdown of particle size) important
- allows food to pass through GI tract
- increases SA exposed to digestive enzymes
what does physical digestion begin and end
mastication
ends in distal stomach by grinding action near pylorus (aided by breakdown of connective tissue by HCl and pepsin)
chemical digestion of each major nutrient is achieved through the process of
hydrolysis; splitting of a chemical bond by the insertion of a water molecule
what bond is hydrolyzed in digestion of carbs
glycosidic bond
what bond is hydrolyzed in digestion of proteins
peptide bond
what bond is hydrolyzed in digestion of fats
ester bond
hydrolysis of triglyceride molecules
2 ester bonds are hydrolyzed to make a monoglyceride and 2 free fatty acids
2 general classes of enzymes that catalyze hydrolysis
luminal and membrane-bound
luminal enzymes that catalyzes hydrolysis, act in lumen and are secreted by
major GIT glands (salivary, gastric, pancreas)
luminal phase digestion results in _____ hydrolysis
incomplete
forms short-chain polymers
membrane-bound enzymes that catalyze hydrolysis act where
at the membrane surface of the epithelium of the small intestine
membranous-phase of digestion completes the
hydrolytic process: breaks short-chain polymers into monomers that can be absorbed
3 types of carbs
fibers
sugars
starches
carbs are nutrients containing
carbon, hydrogen and oxygen
fibers (carbs)
structural part of plants
not digested by mammalian enzymes
sugars (saccharides)
(carbs)
what are the 2 types
- Simple (monosaccharides), e.g., glucose, galactose and fructose
- Complex (polysaccharides) e.g., disaccharides such as lactose and sucrose, trisaccharides, oligosaccharides
starches (carbs)
glucose polymers with 2 major forms:
- amylose (straight-chain)
-amylopectin (branched-chain)
luminal phase digestion digests what type of carbs
starches only
what are starches digested by in luminal digestion
alpha-amylase
alpha amylase comes from
pancreas
digests starches
how does alpha-amylase digest starches
The linkages within the long-chain polymers
of glucose of amylose and amylopectin are attacked by alpha-amylase. It cleaves the starch chains in the middle, resulting in the production of polysaccharides of
intermediate chain length.
These intermediates are known as dextrins in amylose, and “limit dextrins” in amylopectin .
These chains continue to be attacked and further broken down until disaccharide (maltose) and trisaccharide (maltotriose)
units are formed. The end result of luminal-phase carbohydrate digestion is the creation of lots of disaccharides, trisaccharides, and oligosaccharides from large
starch molecules.
dextrins and limit dextrins
when alpha-amylase digests starch it created intermediates
These intermediates are known as dextrins in amylose, and “limit dextrins” in amylopectin
membranous phase digestion of starch digests what kind of starches
complex carbs as well as polysaccharides from luminal phase starch digestion
each complex carb has specific membrane-bound
enzyme in glycocalyx: sucrase, maltase, lactase
membrane phase digestion of carbs; carbs are digested to _____ before being absorbed
monosaccharides
how is protein digested in luminal phase digestion and the membranous phase digestion
large molecular proteins broken down into small peptide chains
These peptide chains are then digested to individual amino acids during membranous-phase digestion.
are some amino acids also released during luminal phase digestion
yes due to number of diff enzymes involved
proteolytic enzymes
aka peptidases; enzymes that break down proteins
where are proteolytic aka peptidases released
from the gastric glands and the pancreas as zymogens, which are then activated in the gut lumen
most peptidases are
endopeptidases, which break proteins at internal points along the amino acid chains
exopeptidases
type of peptidase that release single amino acids from ends of peptide chains
activation of zymogens that digest proteins occurs in
the gut lumen
in the stomach pepsinogen and chymosinogen are activated by
pepsinogen also activated by pepsin
hydrochloric acid
*pepsinogen also activated by pepsin
trypsinogen form pancreas is activated by
trypsin and enterokinase in duodenum
trypsin activates (other than trypsinogen)
other pancreatic enzymes in lumen: Chymotrypsin, proelastase, carboxypeptidases A and B, as well as additional trypsin
luminal phase digestion of proteins begins where
in the stomach
gastric digestion of protein facilitated by
HCl (hydrochloric acid)
why is gastric hydrolysis of protein important
most connective tissue in animals is proteins
membranous phase digestion of protein occurs by
peptide-digesting enzymes (peptidases) in glycocalyx
Yields free amino acids as well as di- and tripeptides that can be absorbed
lipids are water-
insoluble
in order to digest lipids what 3 things must occur
1) emulsify
2) hydrolyze
3) form micelles
emulsification of lipids
reducing size of lipid droplets to form stable suspension in water
where does emulsification of lipids start
stomach: lipids warmed to body temp and mechanical digestion breaks lipid gobules into droplets that pass to SI
emulsification of lipids in stomach
lipids warmed to body temp and mechanical digestion breaks lipid gobules into droplets that pass to SI
emulsification of lipid in SI
- Bile acids and phospholipids act as detergents
- Reduce surface tension of the lipids
- Droplets further decrease in size
hydrolysis of lipids; lipid droplets are coated by
bile products
how does hydrolysis of lipids occur
- Lipid droplets coated by bile products
- Enzyme ‘co-lipase’ clears a path through bile products
- Allows pancreatic lipase to hydrolyse triglyceride molecules into 2 FFA’s and a monoglyceride molecule
- Other lipid-digesting pancreatic enzymes include cholesterol esterase and phospholipase
how are micelles formed
The products of hydrolytic lipid digestion – the free fatty acids and monoglycerides –
and other products of lipid digestion combine with bile acids and phospholipids to
form micelles.
micelles are water-
soluble
how do lipids being turned into micelles allow them to be digested
Micelles are small and water-soluble (unlike lipids). The soluble micelles allow the lipids to diffuse through the gut lumen into the unstirred water layer, and into close contact with the absorptive surface of the apical membrane
Membranous-phase digestion occurs in microenvironment of
glycocalyx, mucous and an
unstirred water layer, with digestive enzymes in the intestinal surface membrane
Luminal-phase carbohydrate digestion
gives short-chain polysaccharides. This only applies to starches !
Sugars are digested in membranous-phase digestion. There is a separate enzyme for each
type of polysaccharide to break down it down to monosaccharides
Proteins are digested by a
variety of luminal-phase enzymes
Complete digestion of peptides to free amino amino acids occurs on the
enterocyte surface, and
within the enterocyte
Detergent action and enzymatic action are both necessary for digestion and absorption of
fats
Micelles are formed with bile acids to transport luminal fats to the
cell surface
Which of the following molecules is consumed during the process of hydrolytic digestion?
Water
Which of the following statements regarding the digestion of carbohydrates is INCORRECT?
A.
All carbohydrates are digested to monosaccharides before being absorbed by the enterocytes
B.
The luminal phase digestion of carbohydrates only applies to starches, because sugars are digested in the membranous phase
C.
Fibres are broken down during membranous-phase digestion
D.
Each complex carbohydrate has a specific membrane-bound enzyme, which projects into the glycocalyx
C.
Fibres are broken down during membranous-phase digestion
Which of the following statements regarding the digestion of protein is CORRECT?
A.
Exopeptidases cleave proteins at internal points along amino acid chain
B.
All peptides are digested to free amino acids before being absorbed by the enterocytes
C.
Large molecular proteins are broken down into small peptide chains during luminal digestion, via the action of a single proteolytic enzyme which breaks peptide bonds
D.
The autocatalytic action of trypsin on trypsinogen forms a positive feedback loop, ensuring the rapid and complete activation of trypsinogen in the gut
D.
The autocatalytic action of trypsin on trypsinogen forms a positive feedback loop, ensuring the rapid and complete activation of trypsinogen in the gut
Finding triglycerides and starch in the faeces of a dog with a poor body condition score, that has a normal feed intake would suggest:
A.
Malabsorption
B.
Maldigestion
B.
Maldigestion
