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Terms in this set (38)
Most absorption of food occurs in the small intestine
-large intestine can absorb water and electrolytes
where does most absorption of the GI tract take place?
folding of intestinal mucosa. Most prominent in the jejunum. Increase surface area by 3
what is a Keckring's fold?
Finger-like protrusion (~1 mm) from Kerckring's folds. Increase surface area by 10
what is a villi?
Hair-like protrusion (~1 mm) from the surface of epithelial cells. Comprise "brush border". Increase SA 20 fold
what is a microvilli?
secreted into lumen or membrane bound
digestive enzymes may be either ________ or __________
hydrolytic enzymes bound in the brush border of the small intestine
where does membrane-bound digestion occur?
1. Unstirred water layer (hydrophilic)
2. two cell membranes (brush border and basolateral membrane; hydrophobic).
what layers must nutrients pass through before it can be absorbed?
Na+ is actively transported out, across the basement membrane. Thus, Na+ will diffuse into cell from lumen and bring water with it
how is sodium treated in the enterocyte? why?
starch (a polysacchartide) and a number of disaccharides
what is the most common carbohydrate that we eat?
what carbohydrate is not able to be digested in humans because we lack a certain enzyme?
glucose polymer that can either be branched or not
what is the structure of starch?
amylase digest into maltose and dextrin in mouth, which are further broken down by membrane bound brush border enzymes maltase and apha-dextrinase to glucose monomers
what enzyme breaks down starch?
lactose broken down to galactose and glucose in the brush border by lactase
where is lactose broken down? into what? by what enzyme?
sucrose broken down to fructose and glucose in the brush border by sucrase
where is sucrose broken down? into what? by what enzyme?
Na/K ATPase in basolateral membrane of enterocyte creates a concentration gradient that is later used as the driving force of glucose/galactose by SGLT-1. Two Na+ translocated for every glucose moved. Co-transport
how is glucose moved across the enterocyte for absorption?
no. fructose moves by facilitated diffusion (GLUT-5) that is energy and Na+ independent
does fructose require Na+ for absorption across the enterocyte membrane?
travel via GLUT-2
how are glucose, galactose and fructose moved across the basement membrane?
diffusion or GLUT-2
how are monosaccharides moved from inside of enterocyte to the plasma?
pepsin at stomach
where are proteins broken down to polypeptides?
SI by enzymes produced in pancreas and secreted to duodenum. These include trypsin, chymotrypsin, carboxypeptidase, elastase
where are polypeptides broken down?
digested into amino acids, dipeptides and tripeptides by peptidases within the brush border of intestine. These forms can be absorbed at the membrane of enterocytes
where are small peptides further broken down?
pH. neutral, acidic, and basic have different carriers
free amino acids account for what % of transport across enterocyte membrane? transport is based on what characteristic?
as di or tripeptides. Are then further broken down into free amino acids. Along with free amino acids directly transported, are moved across basolateral membrane into capillary
60% of amino acids are transported into the cell how? then what?
single amino acids. Small peptides can act as antigens
the vast majority (>99% of proteins are transported into the capillary in what form? what problems can small peptides cause?
decreasing the surface tension at the water-oil interface. It is an emulsifier. Neither dissolves the fat
what is the role of bile salt and lecithin in fat digestion?
emulsification of water insoluble fat globules to allow enzymatic hydrolysis of fat molecules and the subsequent absorption by epithelial cells of the jejunum
describe the general process of fat digestion? where does it occur?
Formation and solubilization of mixed micelles
how must digested fats be transported across the aqueous layer and enterocyte membrane?
the fatty acids and glycerides and re-esterified into triglycerides and transported into lacteals to eventually enter the systemic circulation
what happens to digested fat once it is within the enterocyte?
lingual lipase in mouth but PRIMARILY pancreatic lipase
from where are digestive lipases secreted?
Cholesterol ester hydrolase
the pancrease secretes the following lipases? (3)
1 and 3
pancreatic lipase is specific for _____ and ____ esther linkages?
Bile salts reach a critical concentration causing molecular aggregation
what causes the formation of micelles?
are amphipathic. fats are solubilized within the interior
are micelles hydrophobic or hydrophilic?
emulsion particles are much larger. Miscelles pass the static aqueous layer and at the brush border the lipids are released to diffuse passively across the membrane. The bile salts are released and will be recycled in the ileum
are miscelles or emulsion particles larger? how are miscelles absorbed?
a chylomicron is a lipoprotein particle that forms in the golgi apparatus. The size of chylomicrons is somwhere in between emulsion particles and micelles.
what is a chylomicron? where does it form?
mostly triglycerides and phospholipids, only 2% protein
what is the composition of chylomicrons?
no are too large. Instead, cross basolateral membrane and enter lacteals. Will drain into circulation via thoractic duct
do chylomicrons enter capillaries?
1. Small amount of lipids (triglycerides) are digested by lingual lipase and gastric lipase at stomach.
2. Fats are emulsified by bile to form smaller-sized droplets at intestine.
3. Pancreatic lipase digest triglycerides into monoglycerides and free fatty acids at intestine.
4. Monoglycerides and free fatty acids are packaged into micelle and transported to brush border membrane.
5. Monoglycerides and free fatty acids are diffused into the membrane of enterocytes.
review slide of complete FA digestion
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