Carbohydrate metabolism

Question 1

glucose uptake

Answer 1

into tissues - used for energy in brain

Question 2

glycogen synthesis

Answer 2

triglycerides in adipose tissue

Question 3

glycogen breakdown

Answer 3

glycogenolysis

Question 4

glycolysis

Answer 4

glucose breakdown

Question 5

gluconeogenesis

Answer 5

lactate from liver used

Question 6

the pancreas

Answer 6

• Secretes hormone insulin into blood in response to an increase in blood glucose
• Exocrine - release digestive enzymes into ducts
• Endocrine - hormones - Islets of Langerhans - released into blood stream

Question 7

what happens when glucose is in the blood?

Answer 7

• Some glucose taken up by liver via facilitated transport (uptake) - regulate gluc absorption
• Insulin activates hexokinase (glucokinase) to store glucose as glycogen (glycogen synthesis)
• Liver hexokinase reaction reversible so glucose can also leave liver cell (glycogenolysis and gluconeogenesis) to maintain blood glucose
- Use when not taking carb in diet

Question 8

why do we need to maintain blood glucose?

Answer 8

• Glucose primary fuel source of brain cells (glycolysis), as well as turnover and neurotransmitter synthesis
• 2% of body weight, but consumes 20% of glucose, making it main consumer of glucose for body weight (6 mg/100 g tissue/min or 100-150 g/day)
• Normal diet 300 g/day, and liver stores around 100 g/day
- Astrocytes
- Not regulated by insulin

Question 9

astrocytes

Answer 9

support cells to help neuron function - store glyc

Question 10

how is most of the remaining glucose taken up

Answer 10

by skeletal muscle

Question 11

skeletal muscle glucose uptake

Answer 11

• Glucose enters muscle via facilitated transport (uptake)
• Hexokinase (glucokinase in liver) phosphorylates glucose to maintain gradient
• Unlike liver, glucose-6-phosphate cannot leave cell and enters glycolysis or glycogen synthesis (500 g whole body)
- Doesn’t store as much as liver - can be manipulated with nutrition and ex

Question 12

glucose transport via GLUT4

Answer 12

• GLUT4 translocated to plasma membrane from intracellular vesicles in response to insulin
• Muscle contraction also stimulates GLUT4 translocation to plasma membrane
• Calcium (Ca2+) thought to be main signal

Question 13

glycogen synthesis

Answer 13

• Glucose-6-phosphate (G6P) converted to glucose-1- phosphate (G1P) by phosphoglucomutase
• Uridine diphosphate glucose (UDP-glucose) is synthesised from G1P and uridine triphosphate UTP
• UDP-glucose binds to pre-existing glycogen molecule or glycogenin and UDP is released
• Glycogenin is a protein at the core of a glycogen molecule that has autocatalytic (add to itself) activity
• Glycogen synthase and branching enzymes then form proglycogen, which grows into macroglycogen - rate limiting step

Question 14

how does insulin stimulate glycogen synthesis?

Answer 14

• Switches protein phosphatase on
• Inactive as phosphorylated - blocks catalytic site
• Phosphatase removes phosphate

Question 15

muscle contraction/how does exercise reduce glycogen synthesis?

Answer 15

• Kinase - phoshphorylates

- No food = no insulin = no glyc

Question 16

how does insulin inhibit glycogen breakdown (glycogenolysis)?

Answer 16

• Glyc phosphorylase - rate limiting step

- Active = phosphorylated§

Question 17

muscle contraction/how does exercise stimulate glycogen breakdown?

Answer 17

Insulin levels decrease

Question 18

glycolysis preferential during exercise

Answer 18

• Calcium (Ca2+) and adrenaline main signals stimulating glycogenolysis
• Reduced insulin with exercise
• G6P derived from both
glycogenolysis and glucose uptake
• Enters glycolysis pathway (leaves liver by action of glucose-6- phosphatase)

Question 19

gluconeogenesis - liver

Answer 19

• Liver is first and foremost reserve for maintaining blood glucose in postabsorptive period and during fasting
• Once reserve is exhausted (overnight fast can almost fully deplete liver glycogen stores), glucose must be made from non-carbohydrate precursors
• Circulating fatty acids and glycerol increase during fasting
• Unfortunately fatty acids cannot be a precursor as the pyruvate dehydrogenase and pyruvate kinase reactions are irreversible (i.e. acetyl- CoA from fatty acids cannot form glucose)
• Reversal of glycolysis?
• Acetyl-CoA from fatty acids plays an important role in inhibiting pyruvate dehydrogenase and diverting pyruvate to oxaloacetate and malate
• Lack of insulin relieves inhibition of muscle proteolysis to release amino acids, such as alanine

Question 20

reversal of glycolysis

Answer 20

• Pyruvate kinase inhibited by alanine
• Glycerol is phosphorylated to glycerol-3-P via glycerol kinase
• Lactate produced at high rates in muscle during exercise, and also continuously in red blood cells

Question 21

different carbohydrate sources for exercise

Answer 21

liver

ingested

skeletal muscle

maintain glucose at 5 mmol/l

Question 22

liver carbohydrate source

Answer 22

o 100 mmol/kg
o 80-100g
o 320-400 kcals
o Replenished with just food

Question 23

ingested carbohydrate source

Answer 23

o Up to 1000g/day

Question 24

skeletal muscle carbohydrate source

Answer 24

o 50-100 mmol/kg
o 400-700g
o 1600-2800 kcals
o More than liver as muscles bigger

Question 25

blood glucose at rest

Answer 25

* 80-100g in liver * 0.1g/min * Sink maintained at 5 mmol/l * Glucose uptake in diff tissues at rest 0.1g/min * Brain is biggest consumer

Question 26

blood glucose during carbohydrate ingestion

Answer 26

Volume of sink increases

Question 27

skeletal muscle glucose uptake increases during exercise

Answer 27

* Muscle contraction also stimulates GLUT4 translocation to plasma membrane * Ca2+ main signal * Regulated by ex intensity

Question 28

blood glucose during prolonged exercise

Answer 28

Reduced volume of sink

Question 29

liver glycogenolysis and glucose output increased during prolonged exercise

Answer 29

Exercise increases activation of the sympathetic NS releases adrenaline inhibits insulin release inhibits glycogenolysis glucagon stimulates glycogenolysis

Question 30

carbohydrate oxidation during prolonged exercise

Answer 30

- Start using other fuels - muscle glyc | • Rate of carb ox cannot fuel muscle contraction - stop ex

Question 31

why does glycogenolysis increase as exercise intensity increases?

Answer 31

* Increased AMP * ATP only fuel used for muscle contraction * Higher muscle contraction = higher breakdown

Question 32

muscle and liver glycogen stores are finite and become depleted during exercise

Answer 32

can only be used for so long

Question 33

blood glucose response to exercise with water or 1.5/min carbohydrate ingestion

Answer 33

* Stimulus greater than use at start | * Didn't fatigue - maintain blood glucose and carb ox

Question 34

glucose oxidation response to exercise with water or 1.5/min carbohydrate ingestiom

Answer 34

* Stimulus greater than use at start | * Didn't fatigue - maintain blood glucose and carb ox

Question 35

slowing down the rate of muscle glycogen use during exercise with 0.5/min carbohydrate

Answer 35

* Running * ½ marathon pace * 0.5g/min - carb 5.5% electrolyte solution - 500ml with 30g carb - tested Lucozade * Spared glyc utilisation - run for another half hour - 33%

Question 36

which type of carb is best to ingest?

Answer 36

* 3g/min * Can't ox more than 1g/min * Rate of gluc ox stays the same regardless of amount digested * W/ fructose (absorbed diff) - can get more into system and oxidise

Question 37

how can glycogen loading increase exercise performance?

Answer 37

Ingest carb after ex - load up muscle

Question 38

what is glycogen loading caused by?

Answer 38

increase in sensitivity of muscle to insulin * Muscle contraction stimulates GLUT4 translocation to plasma membrane * End of ex TCA cycle, PDC and glycolysis flux return to near resting, but GLUT4 maintained for several hrs post ex * Glyc resynthesis highest in first few hrs post ex, but insulin sensitivity elevated for few days

Question 39

when is glycogen resynthesis highest?

Answer 39

in hours after exercise immediately after