Topic A7

Question 1

What conditions are required for each fate of glucose-6-phosphate in the liver?

Answer 1

1. Dephosphorylated by G6P to yield free glucose- occurs when [glucose] is low, glucagon is high and epinephrine is high
2. Made into liver glycogen by glycogen synthase- occurs when [glucose] is high, insulin is high
3. Enter glycolysis to make acetyl CoA to make fatty acids -> TAGs/phospholipids- occurs when [glucose] is high, insulin is high
4. Enter glycolysis to make acetyl CoA to make ATP via oxidative phosphorylation to provide ATP for the hepatocytes- only if [glucose] is high
5. Enter pentose phosphate pathway to yield NADPH and ribose-5-phosphate (for nucleotide synthesis)

Question 2

What are ketone bodies?

Answer 2

• A source of energy that is made from acetyl-CoA in the liver
• Can enter the brain (unlike fatty acids which are bounds to serum albumin)
• oxidised for energy in place of/or to supplement glucose
• e.g. acetone, B-hydroxybutyrate, acetoacetate

Question 3

What tissues is blood glucose essential?

Answer 3

• Brain (use ketones during starvation)
• Red Blood cells
• Bone marrow
• Renal medulla
• Peripheral nerves

Question 4

Describe liver metabolism in the well fed state:

Answer 4

• Blood glucose is high- Insulin is present
• gluconeogenesis decreases
• glycolysis increases
• liver converts acetyl CoA made by glycoglysis into FAs -> TAGs and transports them to adipocytes for longterm storage
• glycogen synthesis increases

Question 5

Describe liver metabolism is early starvation e.g. 12 hours after a meal:

Answer 5

• Blood glucose is low- glucagon is high
• gluconeogenesis increases (liver is making glucose for glucose dependent tissues)
• glycolysis decreases (glucose is not being broken down for energy/converted to FAs)
• glycogenolysis increases- produces glucose for other tissues
• fatty acids are being converted to acetyl CoA by B-oxidation for fuel for the liver and for production of ketones
• amino acids begin to be broken down from sources such as skeletal muscle protein
• adipocytes are cleaving TAGs

Question 6

Describe what occurs during prolonged starvation e.g. days-weeks:

Answer 6

• fatty acids are the primary fuel source of the body
• contrary to early starvation there is a conservation of muscle protein
• reduction of glucose production
• increased B-oxidation: to produce acetyl-CoA for ketone body formation and for an energy source for cells
• excess of acetyl CoA compared to oxaloacetate (as oxaloactetate is being used for gluconeogenesis)