S2 Flashcards
Which tissues have an absolute requirement for glucose?
red blood cells, neutrophils, kidney medulla cells, and cells in the lens of the eye
What range should plasma glucose be in?
Normal blood glucose concentration = 4.0-6.0 mmol/L.
What enables blood glucose to be kept at required levels?
A store of glucose= glycogen
What is the renal threshold?
Renal threshold = 10 mmol/L. This is the maximum amount of glucose that the body can handle before the kidney can no longer reabsorb glucose and appears in the urine. Renal threshold can be altered by physiological state (e.g. pregnancy) and other factors such as age.
How is glycogen stored?
In granules
Where are the main stores of glycogen?
Skeletal Muscle- 300mg of glycogen stored in muscles cannot be released into the blood and can only be used by the muscle itself
Liver-100mg of glycogen is stored in the liver and can be used to raise the blood concentration of glucose
Describe the structure of glycogen
Polymer consisting of chains of glucose molecules linked into straight chains by α1-4 glycosidic bonds.
highly branched structure formed by α1-6 glycosidic bonds every 8-10 residues
branched chains clump together to form granules inside certain cells, acting as a store of glucose.
What is the advantage of storing glucose in branches?
Branches give more points for enzymes to react and release more glucose simultaneously, safe storage without drawing lots of water into the cell
What is glycogenesis?
process of turning glucose into glycogen. four reactions, two of require ATP to occur. These reactions add glucose onto the end of one of the strands of a glycogen molecule.
What does glycogen synthase do?
form α1-4 glycosidic bonds with an existing branch
What does branching enzyme do?
forms α1-6 glycosidic bonds to begin a new branch
What enzymes are involved in glycogenolysis?
Glycogen phosphorylase breaks the α1-4 glycosidic bonds.
Debranching enzyme breaks the α1-6 glycosidic bonds.
Phosphoglucomutase converts glucose 1-phosphate to glucose 6 phosphate
What is liver and muscle glycogen used for?
Used by muscle for energy production- lacks glucose 6 phosphatase
Released by liver into blood for use by other tissues- buffer of blood glucose levels
What is the rate-limiting enzymes of glycogen metabolism?
Glycogen Synthase inhibited by glucagon and adrenaline, activated in activity by insulin
Glycogen Phosphorylase inhibited by insulin, activated when glucagon or adrenaline
What effect does glucagon have on muscle glycogen stores?
not respond to glucagon, because glucose produced from the breakdown of muscle glycogen cannot be used to raise blood glucose adenosine monophosphate (AMP) is the allosteric activator of Glycogen Phosphorylase. High [AMP] in the muscle, glucose is released to allow for the production and release of ATP
What are glycogen storage diseases?
a rare group of inborn errors of metabolism diseases that involve a deficiency in an enzyme involved in glycogen storage
Too much glycogen storage= damage tissues and can occur if the cells are unable to break down the glycogen.
When and where does gluconeogensis take place?
after 8 hours of fasting, when glycogen stores are being depleted and a new source of glucose is needed. It occurs mainly in the liver, but also in the kidney cortex.
What are the three major precursors for gluconeogensis?
Lactate from anaerobic glycolysis in exercising muscle and RBC
Glycerol released from adipose tissue breakdown of triglycerides
Amino acids- mainly alanine
Why can’t you synthesise glucose from acetyl-CoA?
Acetyl-CoA enters TCA and loses 2C
What are the key enzymes in gluconeogenesis and the control sites?
PEPCK (phosphoenolpyruvate carboxykinase) and Fructose 1,6-bisphosphate (which are both regulated)
Glucose-6-Phosphatase (which is not regulated).
The two key enzymes of gluconeogensis are regulated by hormones in response to?
Starvation/fasting
Prolonged exercise
Stress
How are lipids stored in the body?
Triacylglycerol for storage= hydrophobic, anhydrous form in adipose
Highly efficient energy store
When are lipid stores utilised?
Prolonged exercise, stress, starvation, during pregnancy
What is the diameter of a typical adipocyte?
~0.1mm – cells expand as more fat added
Can increase in size about fourfold on weight gain before dividing and increasing total number of fat cells
What is the typical weight of fat in an average adult?
~15kg
Describe dietary triacylglycerol metabolism
Lipids eaten in the diet are hydrolysed in the small intestine by pancreatic lipases
=transported across the brush border into the lacteals (lymphatic vessels which transport digested fat, all other substances are transported in the blood).
Lipids are transported in chylomicrons as TAGs and can be stored in adipose tissue if there is plenty of glucose
