The liver and glucose homeostasis

Question 1

List some main functions of the liver

Answer 1

Stores glycogen made from ingested carbohydrates and other non carbohydrate precursors
Breaks down and releases glucose when plasma glucose falls
Synthesise glucose from non carbohydrate precursors for the release to maintain plasma glucose
Synthesis of fatty acids from glucose and secretes them as triglycerides in the form of VLDL
Synthesis of ketone bodies

Question 2

State the 3 main bodies of fuels from which ATP can be made from

Answer 2

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

Question 3

Describe the two phases of assimilation

Answer 3

Immediate ‘absorptive’ events i.e. the immediate fate of the dietary compounds. Liver and adipose tissue mainly take up materials = import

Post-absorptive events i.e. between meals, the cells of the liver and adipose tissues = export

Question 4

How is hypoglycemia prevented by the body?

Answer 4

Glucose is a major energy substrate in the body and blood glucose is constantly replenished to prevent hypoglycaemia.

Question 5

Which organ is most at risk to hypoglycemia? Why is this?

Answer 5

The brain as cerebral cells derive their energy predominantly from aerobic metabolism of glucose

Question 6

What can the cerebral cells not do?

Answer 6

Store glucose in significant amounts or synthesise glucose

Metabolise substrates other than glucose or ketone bodies

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 7

What is gluconeogenesis?

Answer 7

Glucose synthesis in liver and kidneys from non-carbohydrate precursors e.g amino acids, glycerol, lactate

Question 8

What is glycogenolysis?

Answer 8

Mobilisation of liver glycogen stores

Question 9

What is glycolysis?

Answer 9

Oxidation of glucose by peripheral tissues

Question 10

What is glycogen and fat synthesis?

Answer 10

Conversion of glucose into glycogen and fat

Question 11

Which chemicals/molecules increase the entry of glucose into the blood?

Answer 11

Increased by glucagon, catecholamines, cortisol

and growth hormone

Question 12

Which chemicals/molecules increase the loss of glucose from the blood?

Answer 12

Insulin

Question 13

Give the maintained concentration of plasma glucose

Answer 13

2.5-8mM

Question 14

How long is glucose absorbed from the intestine following a meal?

Answer 14

2-3 hours

Question 15

How long do glycogen stores last?

Answer 15

12-24 hours

Question 16

During sleep what happens to glucose synthesis?

Answer 16

During sleep or during extended food deprivation there is a gradual dependence on de novo glucose synthesis by gluconeogenesis
Increases since the store has run out

Question 17

What is the major metabolic role of insulin?

Answer 17

Glucose entry into cells is the major and most important metabolic effect of insulin

Question 18

Describe how glucose enters cells in tissues

Answer 18

Glucose enters cells by facilitated diffusion i.e. it is a carrier-mediated process with glucose entering the cells down its concentration gradient.

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 19

Where is Glut 1 found?

Answer 19

Found in many tissues e.g. erythrocytes, muscle, brain, kidney, colon, placenta, foetal tissue.

Question 20

Where is Glut 2 found?

Answer 20

Found in liver, pancreatic beta cells

Question 21

Where is Glut 3 found?

Answer 21

Found in the brain

Question 22

Where is Glut 4 found?

Answer 22

Found in skeletal muscle, adipose tissue (insulin sensitive)

Question 23

Where is Glut 5 found?

Answer 23

Small intestine, fructose transporter

Question 24

Where are glucose transporters found?

Answer 24

All cells express at least one transporter isoform constitutively as a certain level of glucose uptake is an absolute necessity

Question 25

When will Glut 3 work?

Answer 25

All the time- does not matter what the concentration of the glucose is in the blood

Question 26

When will Glut 2 work?

Answer 26

When the glucose level is very high

Question 27

Why doesn’t Glut 2 work in low blood glucose levels?

Answer 27

So other organs can get the glucose instead

Question 28

Will insulin affect Glut 2?

Answer 28

No

Question 29

Which glucose transporter is sensitive to glucose?

Answer 29

Glut 4

Question 30

What happens to Glut 4 when insulin binds to a cell?

Answer 30

Effect of insulin binding to cell translocated the Glut 4 into the cell membrane from the cytoplasm

Question 31

State the immediate effects of insulin

Answer 31

Increases in the rate of glucose uptake in muscle and
adipocytes

Modulation of the activity of enzymes involved in
glucose metabolism

These effects:
occur within minutes
do NOT require protein synthesis
occur at insulin concentrations of 10-9 to 10-10 M

Question 32

State the longer lasting effects of insulin

Answer 32

Increase expression of liver enzymes that synthesize
glycogen
Increase expression of adipocyte enzymes that
synthesize triacylglycerols
Inhibits lipolysis in adipose tissue
Functions as a growth factor for some cells e.g. fibroblasts

These effects:
occur over several hours
require continuous exposure to insulin at around
10-8 M

Question 33

What is the pentose phosphate pathway?

Answer 33

Cytosolic pathway present in all cells

Branches from glycolysis at G-6-P

Question 34

What are the 2 products of the pentose phosphate pathway?

Answer 34

Two products of the pathway:
Ribose phosphate – used to synthesize RNA and DNA
NADPH – used for reductive biosynthesis and to maintain redox balance of the cell

Question 35

In which cells is the pentose phosphate pathway present?

Answer 35

Tissues involved in biosynthesis (e.g. liver, adipose tissue) are rich in PPP enzymes

In cells where biosynthetic processes are less active, PP intermediates are recycled back into glycolysis