Carbohydrate Metabolism

Question 1

Describe the energy cycle in life.

Answer 1

Question 2

Describe the relationships between gluconeogenesis, pentose phosphate pathway, glycogen metabolism and oxidative phosphorylation.

Answer 2

Question 3

Describe the role of oxygen in harvesting food energy.

Answer 3

Most eukaryote cells make ATP in the mitochondria.
Not all cells have mitochondria. e.g., red blood cells.

Question 4

Describe the metabolism of glucose.

Answer 4

Question 5

How does glycolysis alone make some ATP?

Answer 5

Question 6

What is the glycolysis reaction?

Answer 6

1. the uptake of glucose by the enzyme hexokinase.
2. hexokinase has high affinity for glucose (low km)
3. glucose is phosphorylated to G-6-Phosphate.
4. Glucose is trapped in cell as now negatively charged (ATP is hosphate donor).
5. ATP is used during investment phase (energetically unfavourable reactions).
6. Electrons are released during payoff.
7. Electronsa re transferred to NAD and makes NADH
8. Usually referred to as substrate level phosphorylation (not oxidative as glycolysis does not require oxygen).

Question 7

how does hormones and enzymes regulate glycolysis?

Answer 7

1. the regulation of glycolysis by activation / inhibition of rate-limiting enzymes is short term (minutes-hours).
2. Glycolysis also regulated by hormones-insulin.
3. Hormonal control is slower and often more pronounced by influencing the amount of enzyme synthesised (10-20 x increase in enzymes over hours-days).

Question 8

What are some alternative fates of pyruvate?

Answer 8

1. if glycolysis continues, electrons released will eventually “saturate” all the available NAD converting it to NADH. Glycolysis would stop = fatal!
2. Regeneration of NAD occurs by making ethanol (fermentation) or lactic acid.
3. oxidative phosphorylation -> much greater levels of ATP produced and requires oxygen.

Question 9

Krebs Cycle Overview

Answer 9

1. AKA - tricarboxylic or citric acid cycle.
2. oxidation of acetyl-CoA to CO2 & H2O.
3. occurs in mitochondrial matrix - electron transport chain (ETC).
4. number of roles in metabolism.
5. important in anabolism - glucose, ketones, AAs & heme synthesis.

Question 10

Krebs Cycle - regualtion

Answer 10

1. availability of ADP -> ATP production matches that consumption - e.g., resting Vs exercise
2. Activation and inhibition of allosteric enzyme activities.

Question 11

Pentose Phosphate Pathway - Overview

Answer 11

1. Occurs in cytosol
2. no ATP consumed or produced.
3. G-6P <-> Ribose 5-P + CO2 + 2NADPH
4. Rate and direction of reactions determined by intermediates NADPH (biochemical reductant):
   - steroid synthesis (Adrenal cortex)
   - reduction of reactive oxygen intermediates
   - Detoxification (liver cytochrome P-450)
   - respiratory burst in phagocytic cells
5. Ribose 5-P -synthesis of nucleotides

Question 12

Pentose Phosphate Pathway - Reactions

Answer 12

Question 13

Glycogen metabolism - overview

Answer 13

1. Human require a constant supply of BG
   - preferred energy source for brain
   - energy source for cells with few or no mitochondria e.g., RBCs
   - energy source for exercising muscle (anaerobic glycolysis)
2. Blood glucose obtained from:
   - diet: sporadic and not reliable
   - gluconeogenesis: can supply sustained synthesis, but slow to respond to decreased blood glucose level (BGL).
   - glycogen: glucose from liver glycogen (maintains BGL)
   - muscle glycogen releases glucose during exercise for the muscle.

liver glycogen increase during well-fed state, and decreases during fast.
muscle glycogen only moderate depleted during prolonged fasting.

Question 14

Glycogen metabolism - synthesis

Answer 14

Question 15

glycogen metabolism - breakdown

Answer 15

Question 16

Glycogen metabolism - regulation

Answer 16

1. liver (maintenance of BGL)
   - G synthesis - insulin high (well-fed state)
   - G breakdown - Glucagon high (during fast)
2. Skeletal muscle
   - G breakdown - epinephrine (during exercise)
   - G synthesis - insulin high (rest)
3. Regulation is complex - controlled by enzyme activation and hormones.

Question 17

Describe insulin’s function in carbohydrate metabolism.

Answer 17

• produced in pancreas -> decreased BG (normal range 3.3 - 7 mM)
• balance maintained between glucose utilisation and production.
• all subject to hormonal control -> the insulin / glucagon ratio is critical in maintainance of BG level.
• Diabetes: failure of control mechanisms -> characterised by abnormal BG level & intolerance to ingested glucose. Type 1 - loss of insulin secretion by pancreas.

Question 18

What are the relationships between gluconeogenesis, glycogen metabolism, pentose phosphate pathway and oxidative phosphorylation.

Answer 18

Question 19

Why is Gluconeogenesis important and when is it used?

Answer 19

1. Constant supply of glucose required by brain, RBC, kidney medulla, lens & cornea, exercising muscle.
2. if no dietary CHO, liver glycogen maintains BG for ~ 10~18 hrs.
3. Gluconeogenesis signfiicant during prolonged fast (glycogen stores depleted).
4. Glucose may be formed from: lactate, pyruvate, glycerol (backbone of triacylglycerol) and alpha-keto acids (derived from amino acids).
5. Gluconeogenesis occurs in liver (90%) - kidneys (10%)
6. requires large energy input, used to maintain life during starvation!

Question 20

What are the reactions involved in gluconeogenesis?

Answer 20

Question 21

How is gluconeogenesis regulated?

Answer 21

Question 22

How is metabolism of other carbohydrates established? Flow chart

Answer 22

Question 23

How is metabolism of other carbohydrates established?

Answer 23

1. although glucose is the most common monosaccharide by humans, fructose and sucrose occur in significant amounts in diet -> energy.
2. Fructose - by product of sucrose catabolism.
3. corn syrup = fructose - also found in fruits, vegetables and honey.
4. Uptake of fructose into muscle and liver cells is not insulin dependent & does not illicit insulin secretion.