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glucose is a six
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carbon molecule that's used to make
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energy in the form of adenosine
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triphosphate
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or atp glucose is such an important
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energy source that our body stores
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excess glucose in skeletal muscle cells
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and liver cells in the form of glycogen
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glycogen is basically an enormous
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molecule or polymer
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that's made up of glucose molecules
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linked together by glycosidic bonds
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you can think of glycogen having a main
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chain and there being multiple branches
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sprouting off of it
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these branches allow glycogen to be
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compact and capable of rapid addition
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and removal of glucoses
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it's kind of like growing a plum tree in
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a tiny house with a short ceiling
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the short ceiling limits the tree's
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vertical growth but the tree is able to
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branch off
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so that it can still grow and produce
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many plums into tight space
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now let's say that you just wrapped up a
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delicious lunch you had tacos
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glucose is absorbed from the intestine
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and that causes our blood sugar to go up
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the pancreas responds to high blood
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sugar by secreting insulin
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insulin axon glucose transporters on the
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cell membrane
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which are called glutes and makes them
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bring more glucose into all cells in our
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body
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inside the cell an enzyme called
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hexokinase adds a phosphate group to its
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sixth carbon
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creating glucose 6-phosphate then
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glucose-6-phosphate is broken down
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during glycolysis
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making atp as a by-product over time atp
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levels start to rise and that inhibits
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certain enzymes in glycolysis
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when that happens the extra glucose
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6-phosphate can be used to make glycogen
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that usually takes place in the liver
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and muscle cells
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there are four main steps in glycogen
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synthesis
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first is attaching a uridine diphosphate
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or udp molecule to glucose
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second is attaching the glucose part of
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the udp glucose molecule to a glycogen
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primer called glycogenin
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forming a short linear glycogen chain
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which serves as a primer
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third is adding more glucose molecules
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to the primer
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a bit like forming a conga line
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[Music]
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and fourth is adding branches to the
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glycogen molecule
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so starting with step one to make udp
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glucose
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an enzyme called phosphoglucomutase
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moves the phosphate from the sixth
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carbon of glucose six phosphate to the
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first carbon
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creating glucose one which uniquely
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comes in the form of uridine
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triphosphate
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or utp in the presence of glucose
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1-phosphate and utp
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an enzyme called udp-glucose
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pyrophosphorylase cuts
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phosphate molecules off of utp which
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give the energy necessary to complete
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this reaction
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so only one phosphate remains attached
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to uridine
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and then glucose one phosphate is added
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to it
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that makes two phosphates so the
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resulting molecule is called udp
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glucose
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once many glucose molecules are
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converted into udp glucose molecules
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we're ready to create glycogen an enzyme
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called glycogen synthase
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catalyzes the attachment of the glucose
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part of udp glucose to another glucose
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residue at the end of the glycogen
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branch
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forming an alpha 1 4 glycosidic bond
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it's almost as if the glucose molecules
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are holding hands
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and in addition to prolonging the
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glycogen chain another byproduct of this
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reaction is udp
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but it turns out that glycogen synthase
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can only elongate an already existing
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glycogen chain that's at least four
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glucose molecules long
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so if there aren't at least four glucose
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molecules linked up together already
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then glycogen synthesis requires a
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protein called glycogenin
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glycogenin plays the role of fooling
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glycogen synthase by catalyzing the
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attachment of four glucoses to itself
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creating a short chain connected with
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alpha one four glycosidic bonds
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by doing that it's able to tell glycogen
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synthase hey we have a chain here that
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looks kind of like an old glycogen
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molecule
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and glycogen synthase falls for it
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and elongates this short chain on
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glycogenin by attaching lots of glucose
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molecules to it through alpha-1-4
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glycosidic bonds
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this elongates the chain and creates a
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new linear glycogen molecule
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next an enzyme called the branching
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enzyme goes to the end of the chain and
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cuts off a chain of about six to eight
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glucose residues in length
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the branching enzyme then attaches that
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chain to the side of the linear glycogen
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strand by creating an alpha 1
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6 glycosidic bond so there's now a bond
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between the first carbon of the glucose
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on the small cleaved segment
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and the sixth carbon of a glucose that's
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part of the linear chain
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and as soon as you've shortened the
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linear chain glycogen synthase will
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elongate it once again
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this happens over and over again
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resulting in a branched glycogen tree to
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serve as stored energy
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now let's say it's been a couple of
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hours since those tacos
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and you decide to go for a run because
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you're fasting your blood glucose levels
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take a dip
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in response the pancreas secretes the
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hormone glucagon
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and the adrenal glands secrete
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epinephrine to increase your heart rate
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it turns out that glucagon tells the
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liver cells to break glycogen down into
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individual glucose molecules
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and epinephrine tells skeletal muscle
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cells to do the same thing
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in both the liver and skeletal muscle
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cells glycogen breakdown starts with the
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branches
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first an enzyme called glycogen
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phosphorylase cleaves the alpha 1
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4 bonds between individual glucose
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residues and catalyzes the transfer of a
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phosphate group to the free
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glucose the result is that the enzyme
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releases one glucose one phosphate
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molecule at a time
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it keeps on doing this until exactly
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four glucose molecules were made on the
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branch
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next a de-branching enzyme literally
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cuts off glycogen branches
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it has a component called four alpha
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glucanotransferase
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which transfers three out of the four
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glucose molecules off of the branch and
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reattaches them to the linear glycogen
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chain instead
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extending it as a result the same
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debranching enzyme has another component
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known as alpha 1-6
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glucosidase which cleaves off the
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alpha-1-6 glycosidic bond and releases a
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free glucose
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so for each glucose that's removed via
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phosphorylysis
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there's a glucose 1-phosphate that gets
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liberated and it's converted to
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glucose-6-phosphate by
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phospho-glucomutase
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the difference between glycogen
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breakdown in the liver and what goes on
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in the muscles
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results from different enzymes in those
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two tissues
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in liver cells glucose 6-phosphatase
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removes the phosphate off of the sixth
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carbon
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releasing free glucose into the
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bloodstream for other organs and tissues
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to use
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skeletal muscle doesn't have this enzyme
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so it simply uses the glucose
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6-phosphate by sending it into the
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glycolysis pathway to make energy
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that can help you with that run
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glycogen metabolism is primarily
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regulated by two pancreatic hormones
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insulin and glucagon now a general rule
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of thumb
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is that glycogen synthase is active when
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it doesn't have a phosphate
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whereas glycogen phosphorylase is active
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when it does have a phosphate attached
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to it
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so in liver and skeletal muscle cells
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insulin binds to a tyrosine kinase
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receptor on the cell surface
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and that ultimately activates a protein
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phosphatase which goes around removing
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phosphates from glycogen synthase
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making it active as well as from
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glycogen phosphorylase
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making it inactive this promotes
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glycogen synthesis
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and decreases its breakdown
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on the other hand glucagon in the liver
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cells bind to a g-protein-coupled
00:08:02
receptor on the cell surface
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which activates adenolyl cyclase which
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converts atp to cyclic amp
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or camp camp then activates protein
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kinase a
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which adds a phosphate to glycogen
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phosphorylase kinase
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which activates it glycogen
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phosphorylase kinase adds a phosphate to
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glycogen phosphorylase
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increasing its activity and promoting
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glycogen breakdown
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it also adds a phosphate glycogen
00:08:30
synthase decreasing its activity and
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therefore decreasing glycogen synthesis
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all right as a quick recap glycogen is a
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multi-branched compact structure that's
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made of alpha-1-4 glycosidic bonds
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between the glucose molecules
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and alpha-1-6 bonds at the branching
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points
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glycogen is considered the major form of
00:08:50
glucose storage in the body
00:08:52
and it's primarily stored in the liver
00:08:54
cells and skeletal muscle cells
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after a meal high insulin levels promote
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glycogen synthesis
00:09:01
whereas during fasting high glucagon and
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epinephrine levels promote glycogen
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breakdown
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you
