Lecture 41 11/27/23 Flashcards
What is digestion?
breaking down complex nutrients to simple molecules
What is absorption?
transporting simple molecules across intestinal epithelium
Which three features increase intestinal surface area?
-plicae circulares
-villi
-brush border
What are the characteristics of enterocytes?
-epithelial cells covering villi and crypts
-apical membrane faces intestinal lumen and contains microvilli
-basolateral membrane allows nutrients to exit into the bloodstream
What are the characteristics of enterocyte tight junctions?
-connect the apical half of enterocytes
-allow passage of water and electrolytes
Which substances are found at the small intestinal surface/diffusion layer?
-mucus
-glycocalyx
-unstirred water layer
What are the functions of physical breakdown of food?
-allows food to flow through intestinal space
-enlarges surface area for chemical digestion
What is hydrolysis?
splitting of chemical bond through insertion of water
What are the characteristics of the luminal phase of digestion?
-occurs first
-enzymes originate from major GI glands
-mix with ingesta in gut lumen
-incomplete hydrolysis
What are the characteristics of the membranous phase of digestion?
-occurs second
-enzymes bound to epithelial surface
-complete hydrolysis
How do simple and complex carbohydrates differ?
-simple carbs have one molecular unit
-complex carbs have 2+ molecular units
What is a starch?
-polymeric carbohydrate
-large number of glucose units
-alpha [1,4] glycosidic bonds
What is a fiber?
-non-starch polysaccharide
-beta [1,4] glycosidic bonds
-undigestible by mammals
Which enzyme mediates luminal starch digestion?
alpha-amylase from the pancreas/salivary glands
Which type of breakdown happens in the luminal phase?
breakdown of starch by amylase
Which type of breakdown happens in the membranous phase?
breakdown of sugars into glucose by specific enzymes (ex. maltose broken down by maltase)
Which nutrient is provided by proteins?
amino acids
What are the characteristics of luminal protein digestion?
-HCl begins the process
-requires a large number of different enzyme types due to the different AA combinations
What are the characteristics of dietary fats?
-triglycerides are the most abundant type
-most digestion occurs in small intestine
What are the four phases of lipid assimilation?
-emulsification
-hydrolysis
-micelle formation
-absorption
What is emulsification?
physical breakdown of fat globules into a smaller size to promote contact with digestive enzymes
How is emulsification done in the stomach?
grinding and mixing
How is emulsification done in the intestines?
bile salts and phospholipids
How do bile salts and phospholipids emulsify fats?
-fat soluble ends break down the fat globule surface
-hydrophilic ends stick out and make the globules more water soluble/easier to break apart
Which enzyme is used to emulsify triglycerides?
pancreatic lipase
What does pancreatic lipase emulsify triglycerides into?
2-MAG and FFAs
What is the role of cholesterol esterase?
cholesterol digestion
What is the role of phospholipase A2?
phospholipid digestion
What are the characteristics of triglyceride breakdown?
co-lipase makes a path for lipase to penetrate bile-coated triglycerides
-lipase hydrolyzes the triglycerides into FFAs and 2-MAG
What is a micelle?
globular structure with polar head groups on the outside and hydrophobic fatty acyl chains on the inside
How are micelles formed?
bile salts bind with hydrolyzed lipid products to form micelles
What happens to dietary fat in the stomach?
warming and mixing to form liquid fat globules
What happens to dietary fat in the duodenum?
exposure to bile leads to production of emulsified fat droplets
What happens to dietary fat in the jejunum?
-lipase, co-lipase, and bile components lead to micelle formation
-diffusion of micelles through unstirred water layer allows direct transport of micelle components (except bile acids) into enterocytes
What happens to dietary fat in the ileum?
bile acid absorption occurs through sodium co-transport
What is the main function of micelle formation?
to allow small lipids to cross the unstirred water layer
What are the characteristics of bile acid absorption?
-occurs in ileum
-co-transport with sodium
-portal circulation to the liver allows for recycling
What are the characteristics of chylomicron formation?
-occurs within enterocyte
-micelle lipids go to ER and are reformed into triglycerides and phospholipids
-packaged with cholesterol and other proteins to form chylomicrons
Where are chylomicrons absorbed?
into lymphatic circulation
