Carbohydrate Metabolism

Question 1

What are the dietary sources of energy?

Answer 1

Carbohydrates –> Glucose
-polysaccharides
-disaccharides
-monosaccharides
Fats –> Fatty Acids
-mostly TAG
Proteins –> Amino Acids

Question 2

What are the 4 major fates of glucose?

Answer 2

-Synthesis of structural polymers : extracellular matrix and cell wall polysaccharides
-storage: glycogen, starch, sucrose
-oxidation via glycolysis: pyruvate
-oxidation via pentose phosphate pathway: ribose-5-phosphate

Question 3

what is a dietary polysaccharide?

Answer 3

-mostly starch
-digested by amylase to maltose and oligosaccharides
-saliva
-duodenum

Question 4

What is fiber?

Answer 4

-cellulose!
-B 1-4 linkages not hydrolyzed in vertebrates
-ruminants have microbes that can degrade cellulose
other types: pectin, lignin, etc.

Question 5

How are oligosaccharides and disaccharides digested?

Answer 5

digested to monosaccharides by various enzymes including: sucrase and B-galactosidase

Question 6

What is a monosaccharide?

Answer 6

mostly: glucose, galactose, fructose
taken up in then intestine by :
-Facilitated diffusion
-active transport

Question 7

What is glycolysis?

Answer 7

glucose taken up into cells baby facilitated diffusion, glucose (6 carbon) is split into 2 pryuvates, free energy conserved as ATP and NADH

Question 8

What are the 2 phases of glycolysis?

Answer 8

Preparatory: glucose is phosphorylated twice (2 ATP’s invested), split into two 3 carbon molecules
Payoff: leads to formation of pyruvate, generates 4 molecules of ATP and 2 of NADH per 6-carbon glucose

Question 9

What is the net energy of glycolysis?

Answer 9

2 ATP and 2 NADH per glucose

Question 10

What happens to the NADH from glycolysis overall?

Answer 10

-two molecules of NAD+ are reduced to NADH per molecule of glucose

Question 11

what happens to NADH under aerobic conditions?

Answer 11

-shuttle systems indirectly transfer cytosolic NADH into mitochondria
-glyercol phosphate shuffle
-malate-asparate shuttle
-shuttle electrons from cytoplasmic NADH to mitochondrial NADH or FADH2
-NADH and FADH2 are then oxidized through electron transfer and oxidative phosphorylation

Question 12

what happens to NADH under aerobic conditions?

Answer 12

-must oxidize the NADH to regenerate NAD+ for glycolysis to continue
-fermentation: lactic acid fermentation and ethanol fermentation

Question 13

What is the final product of glycolysis?

Answer 13

pyruvate!
still contains most of the energy in glucose
can be converted to acetyl-coA then:
oxidized to CO2 in the Krebs cycle, used for fat synthesis and other reactions, can be reduced to lactate or ethanol

Question 14

How is lactate formed in the body?

Answer 14

during vigorous muscle contraction
some tissues release lactate under aerobic conditions
converted back to glucose in the liver

Question 15

How is glucose taken into the liver?

Answer 15

regulated at the level of glucose transport
in the liver: high levels of GLUT2, essentially equilibrium step, allows uptake to increase as concentration in hepatic portal vein increases

Question 16

What is glucokinase?

Answer 16

major isoform in liver
much higher Km for glucose

Question 17

Why is glucose metabolism important?

Answer 17

blood glucose is the only source of energy for the brain, nervous system, red blood cells, testes, renal medulla

Question 18

What is gluconogenesis?

Answer 18

-occurs in the liver
-occurs when glycogen is depleted (fasting, between meals, after vigorous exercise)

Question 19

What are the precursors for gluconogenesis?

Answer 19

-primarily 3-carbon compounds
-lactate
-pyruvate
-glycerol
-certain amino acids

Question 20

What is the general concept of gluconogenesis?

Answer 20

mostly a reverse of glycolysis
problem!
three reactions in glycolysis essentially irreversible:
-hexokinase
-phosphofructokinase-1
-pyruvate kinase

Question 21

Why are those 3 steps not reversible?

Answer 21

-delta G is very far from equilibrium
-HUGE free energy change, this means we need alternative pathways to bypass

Question 22

What are the 3 reactions needed to bypass the irreversible ones?

Answer 22

-pyruvate –> PEP (2 rxns)
-fructose 1,6-bisphospate to fructose 6-phosphate
-glucose 6-phosphate to glucose

Question 23

What is the cori cycle?

Answer 23

when muscle is burning glucose and making lactate

Question 24

What is glycogen metabolism?

Answer 24

major carbohydrate store in humans
glycogen is a branched-chain polysaccharide

Question 25

How is glycogen synthesized?

Answer 25

-phosphoglucomutase
-UDP- glucose pyrophosphorylase
-glycogen synthase reaction (regulates glycogen synthesis)

Question 26

How is glycogen degraded?

Answer 26

-regulated by glycogen phosphorylase
-glycogen breakdown to glucose-1-phosphate
-converted to glucose 6-phosphate

Question 27

What are the fates of glucose-6-phosphate?

Answer 27

-muscle: enters glycolysis
-liver: enters glycolysis or converted to glucose and released into the bloodstream

Question 28

What occurs in the FED state?

Answer 28

glucose is converted to glycogen in muscle and liver
gluconeogenesis is suppressed
glycogen breakdown is suppressed

Question 29

What happens between meals?

Answer 29

-blood glucose largely derived from degradation of liver glycogen
-longer fasting, exercise- blood glucose increasingly derived from gluconeogensis