GI 8 Flashcards
digestion
physical and chemical breakdown of ingested foods into absorbable molecules
Primary Site of Digestion?
Small intestine
Where do most enzymes in the small intestine come from?
the pancreas
absorption
movement of neutrients, water, and electrolytes from the lumen of the intestine into the bloodstream
3 Components of the Intestine that increase Surface Area
- Villi
- Microvilli
- Glycocalyx
glycocalyx
intestinal brush border on surface of microvilli, holds enzymes secreted from intestinal epithelial cells
2 Major Forms of Starch
- Amylose
- Amylopectin
Major Action of digestive enzymes?
hydrolysis
2 Phases of Digestion
- Luminal phase
- Membrane phase
Explain starch digestion by hydrolysis.
enzymes break bonds of the starches during the luminal phase into smaller molecules, but complete hydrolysis occurs during the digestive phase into singular monomers to be absorbed by the intestinal epithelium
Luminal phase of digestion
incomplete hydrolysis of nutrients
Membrane (brush border) phase of digestion
generates absorptive molecules or very short oligomers
What is a possible third step in digestion?
cytosolic proteolysis
Cytosolic proteolysis
short peptides undergo further digestion within the intestinal epithelial cell (cytosol) by intracellular hydrolases
2 Paths for Absorption
- Transcellular Path
- Paracellular Path
transcellular path for absorption
nutrient moves through intestinal cell into the interstitial fluid
paracellular path for absorption
nutrients move across tight junctions between the intestinal epithelial cells
Most common path for nutrient absorption?
transcellular path
4 Transcellular Transport Mechanisms
- Passive transport (facilitated diffusion)
- Primary active transport
- Secondary active transport
- Tertiary active transport
How are the active transport mechanisms named?
based on where the energy comes from
Primary Active Transport
direct consumption of metabolic energy, move across a membrane d/t presence of electrical or chemical gradient
2 Examples of Primary Active Transport Pumps
- Sodium-Potassium-ATPase pump
- Proton pump on parietal cells of gastric acid secretion
On which aspect of the cell will you find your primary active transport pumps?
basolateral side
Sodium Potassium ATPase Pump
pumps 3 Na+ in and 2 K+ out to create a gradient within the cell and generate energy for use by other transporters
Two Types of Secondary Active Transporters
- Sodium co-transporters
- Exchangers (countertransport, antiport)
Sodium CoTransport Protein
has binding sites for Na+ and other specific molecules (glucose, galactose, AAs, vitamins, bile acids)
Secondary Active Transport
transport using the potential energy of a sodium gradient (rather than a direct use of metabolic energy)
Exchangers
binding sites for two different ligands on opposite sides of the plasma membrane
Tertiary Active Transport
driven by electrochemical gradients that were established by secondary active transport
Of these pumps, label what type of transport each is.
Red = Primary
Purple = Secondary (exchanger)
Green = Secondary (sodium cotransporter)
Yellow = Tertiary
What is a starch?
glucose polymer
Three Forms of Plant Carbohydrates
- Starches
- Sugars
- Fibers
How does the body digest fiber?
it is dependent on bacteria
End product of carbohydrate digestion? (3)
Monosaccharides!
1. Glucose
2. Fructose
3. Galactose
Fructose absoprtion is by what transport mechanism?
facilitated diffusion
Glucose and galactose are absorbed through the apical surface by what transport mechanism?
Secondary active transport by cotransporters (specifically SGLT-1, sodium ______ linked tranporters)
How are monosaccharides absorbed on the basolateral side?
facilitated diffusion (all three)
oligomer
short peptide
monomer of protein
amino acid
Can carbs be digested with cytosolic proteolysis?
no
5 Pancreatic Proteases
- Trypsinogen
- Chymotrypsinogen
- Proelastase
- Procarboxypeptidase A
- Procarboxypeptidase B
Are any of the proteases secreted in active form?
no, all as zymogens from the pancreas
Which protease is secreted the most?
trypsinogen (40% of all proteases)
Where do you find pepsinogen and when is it activated?
in the stomach; activated as long as pH in the small intestine remains below 4.5
What enzyme in the small intestine converts trypsinogen to its active form? Where is this found?
enterokinase, in the brush border (glycocalyx)
What activates the remaining proteases in the small intestine?
trypsin
Can trypsin act on trypsinogen?
yes
Is protein digestion by pepsin in the stomach essential?
no, it is non-essential
Peptidases
additonal enzymes present in the membrane phase of digestion to continue to break down proteins into monomers and oligomers
Mechanism of amino acid transport across the apical membrane?
Secondary active (sodium cotransporter)
Mechanism of dipeptide and tripeptide absorption across the apical membrane?
PEPT-1 (peptide transporter 1), cotransporter with H+ ion
Mechanism by which amino acids cross the basolateral membrane?
facilitated diffusion
Are lipids water soluble?
no
3 Dietary Lipids
- Triglycerides
- Cholesterol
- Phospholipids
amphipathic
hydrophilic and hydrophobic components
Emulsification
bile salts surround lipids (preventing them from joining as one large glob, increasing surface area) and form micelles
Micelle
lipid droplet surrounded by bile salts carried to the intestinal epithelial cells before the hydrophillic side dissolves inthe aqueous part
Are all enzymes for lipid digestion secreted as zymogens?
No, some are secreted in active form (such as those for cholesterol esters or triglycerides (lipases))
5 Steps of Lipid Absorption
- Form micelles in the lumen
- Diffuse across apical membrane
- Re-esterification
- Packaged into chylomicrons (vesicles)
- Excytosed across basolateral membrane into lymph (thoracic duct), and eventually to blood
Where are bile salts absorbed?
ileum
What % of lipid digestion products are re-esterified inside the cell?
70
Are fat soluble vitamins absorbed the same way as lipids?
yes
Mechanism of water soluble vitamin transport?
Soium-dependent cotransporters (secondary active transport)
(just like glucose, galactose, and AAs)
Vitamin B12 absorption?
dependent on the intrinsic factor released by parietal cells, happens in the ileum
