liver & glucose homeostasis

Question 1

what are some functions of the liver?

Answer 1

• stores glycogen made from ingested carbohydrates and other non-carbohydrate precursors
• breaks down glycogen and releases glucose when plasma glucose falls
• synthesise glucose from non-carbohydrate precursors for release to maintain plasma glucose
• deaminates surplus amino acids and converts the amino groups and ammonia to ureas
• synthesises fatty acids from glucose and secretes hem as triacyglycerols in VLDL
• synthesises ketone bodies and secretes them into the blood as fuel for other tissues

Question 2

what are the 3 main fuel bodies that energy for synthesis of ATP is derived from?

Answer 2

• glucose (stored as glycogen)
• long chain fatty acids (stored as triacylglycerol)
• amino acids (mainly present in proteins)

Question 3

what are the 2 main steps in fuel metabolism?

Answer 3

1) store fuels when they are abundant
2) release fuels in a controlled way during the postabsorbative period, during exercise or other periods of increased demand

Question 4

what happens after a meal to dietary fuels?

Answer 4

1) immediate ‘absorptive’ events. liver and adipose tissue mainly take up materials = import
2) post-absorptive events. the excels of the liver and adipose tissues = export

Question 5

what are the main energy stores?

Answer 5

• liver
• muscle
• adipose tissues

Question 6

why is blood glucose constantly replenished?

Answer 6

to prevent hypoglycaemia

Question 7

why is the brain most vulnerable to hypoglycaemia?

Answer 7

cerebral cells derive their energy predominantly from aerobic metabolism of glucose

Question 8

what can’t cerebral cells do when it comes to glucose metabolism?

Answer 8

1) store glucose in significant amounts or synthesise glucose
2) metabolise substrates other than glucose or ketone bodies
3) extract sufficient glucose for their needs from the extracellular fluids at low concentrations because glucose entry into the brain is not facilitated by hormones

Question 9

what are some mechanisms of controlling blood glucose?

Answer 9

• glycogenolysis
• gluconeogenesis
• glycolysis
• glycogen and fat synthesis

Question 10

how do hormones help to control blood glucose?

Answer 10

• blood glucose varies relatively little throughout the day or night despite changes in food intake
• this is mainly controlled by fluctuations in the circulating levels of insulin and glucagon
• alterations in the ratio of insulin:glucagon within the blood are essential for the maintenance of blood glucose as shown in diabetes mellitus

Question 11

how does glucose enter into tissues?

Answer 11

• glucose enters cells by facilitates diffusion
• glucose entry involves a family of glucose transporter proteins (Gluts) which are structurally related but encoded by different genes that are expressed in tissue specific manner

Question 12

where are Glut 1 proteins found?

Answer 12

in many tissues

Question 13

where are Glut 2 proteins found?

Answer 13

liver, pancreatic beta cells

Question 14

where are Glut 3 proteins found?

Answer 14

in brain

Question 15

where are Glut 4 proteins found?

Answer 15

skeletal muscle, adipose tissue

Question 16

where are Glut 5 proteins found?

Answer 16

small intestine, fructose transporter

Question 17

what are the immediate effects on cells of insulin?

Answer 17

• increase in rate of glucose uptake in muscle and adipocytes
• modulation of activity of enzymes involved in glucose metabolism
• occur within minutes
• do not require protein synthesis
• occur at insulin concentrations of 10^-9 to 10^-10 M

Question 18

what are the longer lasting effects on cells of continued exposure to insulin?

Answer 18

• increased expression of liver enzymes that synthesis glycogen
• increased expression of adipocyte enzymes that synthesis triacylglycerols
• inhibits lipolysis in adipose tissue
• functions as a growth factor for some cells
• occur over several hours
• require continuous exposure to insulin at around 10^-8M

Question 19

what is the pentose phosphate pathway (PPP)?

Answer 19

• cytosolic pathway present in all cells
• branches from glycolysis at G-6-P
• tissues involved in biosynthesis are rich in PP enzymes
• in cells where biosynthesis process are less active, PP intermediates are recycled back into glycolysis

Question 20

what are the 2 products of the pentose phosphate pathway (PPP)?

Answer 20

1) ribose phosphate: used to synthesise RNA and DNA

2) NADPH: used for reactive biosynthesis and to maintain redox balance of the cell