Flashcards in Chapter 15: Carbs + Proteins Deck (44):
What are examples of macronutrients?
Carbohydrates, proteins, lipids, water
Are carbohydrates essential nutrients?
No, but they supply energy
What are the functions of carbohydrates?
Glucose = major energy substrate
What are major sources of carbohydrates?
Fruits and vegetables
Milk + milk products
Differentiate amongst monosaccharides, disaccharides, and polysaccharides?
Monosaccharides = 1 unit (glucose, galactose, fructose)
Disaccharides = 2 sugar units
Polysaccharides = chains = starch or glycogen
What is starch?
A mixture of large polymers composed of alpha-linked glucose molecules in plants
Define dietary fiber, added/functional fiber, and total fiber.
Dietary fiber = nondigestible carbohydrates + lignin that are intrinsic and intact in plants
Added/functional fiber = isolated nondigestible carbohydrates that have proven beneficial physiological effects in humans
Total fiber = sum of dietary + added fiber
Differentiate between soluble and insoluble fiber and give examples of foods that contain each
Soluble fiber = found in foods like oat bran, barley, nuts, seeds, beans, fruits, strawberries
Insoluble fiber = whole wheat and grain products, vegetables and wheat bran
What are two principles of carbohydrate digestion?
1) intestine can only absorb monosaccharides
2) enzymatic digestion by glucosidases occurs in two phases:
Luminal digestion by salivary + pancreatic enzymes
Brush-border digestion via membrane-bound enzymes
What pH inactivates amylases?
(Especially salivary vs. pancreatic amylase)
Which amylase is the major digestive enzyme for starch?
What is the MOA of amylase?
Chews up polysaccharide into maltose, maltotriose, and alpha-limit dextrin
Where is the brush border located?
Apical membrane of small intestinal epithelial cells
What are substrates of brush border?
Linear glucose oligomers
Describe the brush border hydrolysis of glucose oligomers.
Glucoamylase and sucrase/isomaltase enzymes hydrolyze short chain glucose (example: maltose, maltriose) to glucose
Describe brush border hydrolysis of alpha-limit dextrins
Sucrase/isomaltase (one enzyme w/ two activities) can hydrolyse both alpha-1,6 and alpha-1,4 bonds, allowing digestion of alpha-limit dextrins
Describe brush border digestion of lactose
Cleaved by a single, brush border enzyme called lactase (or LPH) to yield glucose and galactose
Low activity of lactase
What is the RLS for digestion and absorption of lactose?
Malabsorption is common = lactose intolerance
Describe brush border digestion of sucrose
Cleaved by brush border enzyme sucrase/isomaltase to yield glucose and fructose
Rate of hydrolysis >>> rate of uptake (RLS)
Name 3 monosaccharide transporters in the intestine
Sodium-glucose cotransporter = SGLT-1
Glucose/hexose transporter = GLUT-2
Glucose/hexose transporter = GLUT-5
What is sucrose?
= glucose + fructose
What is lactose?
Glucose + galactose
What is maltose?
Product of starch digestion
Glucose + glucose
What transporter brings in glucose after brush border hydrolysis of glucose oligomers?
What transporter brings in glucose and galactose after brush border digestion of lactose?
What transporters bring in fructose and glucose after brush border digestion of sucrose?
Symporter = secondary active transport -Glucose and galactose
Cotransport of sodium and monosaccharide
Expressed at apical surface of small intestinal epithelial cells
Uniporter for transport of glucose and fructose (basolateral side)
When there's High glucose/fructose levels = inserts on apical side
Uniporter for fructose
Where is GLUT-2 found?
Intestine, liver, pancreas, kidney
Where is GLUT-5 found?
What is villous atrophy?
Loss of brush border hydrolases = intestinal malabsorption
What features of protein digestion and absorption are similar to carbohydrates digestion?
Luminal digestion to oligopeptides with limited digestion to free amino acids
Brush border digestion to yield tri- and dipeptides and free amino acids
Pancreatic digestive enzymes quantitatively most important
What general features about protein digestion/absorption are different from carbohydrate absorption?
Oligomers (tri- and dipeptides) can be transported into enterocytes
Cytotoxic digestion occurs
Greater diversity of substrates and products requires many more peptidases and transporters
Additional digestive enzymes from stomach (pepsin), not salivary glands
Describe features of gastric proteolysis
Mediated by pepsin
Products mainly large, non-absorbable peptides = help to stimulate gastrin secretion by G cells in Antrim
Not essential for normal protein digestion
What are features of pepsin?
+ stored and secreted by chief cells as inactive precursor (called pepsinogen)
+ cleavage of peptide at acid pH yields active form
+ inactivated at pH >4.5 (in small intestine)
What are the most abundant peptidases in the small intestine's lumen?
Describe alterations of protein digestion and absorption
Pancreatic insufficiency: cystic fibrosis, pancreatitis, malignancy
Intestinal malabsorption: villous atrophy
Genetic defects: deficiencies in AA transporters
Compare monomers of carbs vs. proteins
Glucose, galactose, fructose
Compare luminal hydrolysis in carbs vs. proteins
Salivary + pancreatic amylase
Gastric pepsin, pancreatic proteases (trypsin...)
Compare bush border digestion in carbs vs proteins
Lactase, sucrase/isomaltase, glucoamylase
Multiple endo and ecto peptidases
Compare uptake characteristics in carbs vs. proteins
Monomers and oligomers (di and tripeptides)
Compare apical epithelial uptake in carbs vs. proteins
SLGT -1 (glucose, galactose)
***conditionally GLUT2 (glucose, fructose)
Multiple AA transporters