IAS12 Flashcards
carb metabolism significance
ensure constant energy supply to cells
glucose is primary source of energy
functions of carbs beyond energy production
support brain function as brain relies on glucose
contribute to nucleic acid formation
involve in immune system function, connective tissue structure
glycosidic linkage
1-4: no branch / 1-6: branch
affect branching, shape & chars
amylase v amylopectin
both are glucose polymers & starch
amylase: 1-4 glycosidic links only
amylopectin: branched, 1-4 & 1-6 bonds
alpha-amylase
cleave 1-4 glycosidic linkage in amylose & amylopectin releasing single glucose, eventually forms glucose, maltose, isomaltose, maltotriose & alpha limit dextrins (short chain branched amylopectin remnant)
glycosidase
break down disaccharides into monosaccharides for absorption:
lactase, maltase, sucrase
carb absorption
only absorb monosaccharides
hence single units of glucose must be cleaved from polymers in GI tract
carb digestion: overview
mouth: salivary alpha amylase digests alpha-1,4 bonds in amylase
-> inactivated in stomach
sucrose & lactose digested in stomach
intestine: pancreatic alpha amylase further cleaves 1,4 bonds in maltase, etc. to release free glucose, further cleavage of branched forms in intestinal wall to form mono-
undigested carbs go to ileum, fermented in bacteria & egested in feces
fate of sugars after absorption
absorbed via hepatic portal vein
stored in liver or transported to cells
intestinal villi structure
basal side connected to many blood & lymph vessels
brush borders face lumen
digestive enzymes attach to brush borders i.e. membrane lining of enterocytes
digestion at villi
disaccharides digested to monosaccharides by glycosidases (small intestinal disaccharidases, produced by enterocytes) at brush borders
monosaccharide absorption at villi & why
water-soluble mono cannot directly transport through hydrophobic plasma membrane (repelled)
glucose & galactose uptake by symporter SGLT1 against conc. grad. by coupling with transport of Na
fructose uptake by GLUT5, less efficient in large amts since uptake not coupled to Na+
monosaccharide exiting cell
some retained for metabolic uses
fructose transport to capillary through GLUT5 in basolateral membrane
glucose, galactose, fructose transport out thru GLUT2 in basolateral membrane
types of cellular glucose transport
active: SGLT (intestine, kidney tubule, choroid plexus) (IKC)
facilitated & insulin insensitive
GLUT4: (my friend) muscle & fat cells
facilitated & insulin sensitive:
GLUT1: (bumble bee’s best) almost all tissues but highly abundant in blood cell, blood-brain barrier, baby / fetus
GLUT2: (last known phrase is) liver, kidney, pancreas, intestine // also bidirectional
GLUT3: (no problem): neurons, placenta
sites that use glucose as major or sole fuel
sole: RBC & brain (almost, can use KB)
RBC: glucose converted to lactate
major: muscle (also use FA & KB)
glucose phosphorylation
entry mediated by GLUT1
after entry, converted irreversibly to G6P, traps G6P in cell