Metabolic pathways and their interactions

Question 1

How many ATP molecules are generated by burning one molecule of glucose, and what percentage of energy is recovered?

Answer 1

~32 ATP molecules are generated, with approximately 38% energy recovery. The rest is lost as heat.

Question 2

What is the difference between glycogenesis, glycogenolysis, and gluconeogenesis?

Answer 2

GG- The process of synthesizing glycogen from glucose.
GY - The breakdown of glycogen into glucose.
GN - The metabolic pathway that generates glucose from non-carbohydrate precursors.

Question 3

How much ATP does the body generate daily, and where does it go?

Answer 3

The body generates approximately 55 kg of ATP daily, used in various metabolic processes, including glycogenesis, glycogenolysis, and gluconeogenesis.

Question 4

How is fat an efficient energy store compared to glycogen?

Answer 4

Fat is approximately 2x more efficient than glycogen in storing energy.

Question 5

Which components of fat can be used for energy?

Answer 5

Both glycerol and free fatty acid side chains can be used for energy, but only glycerol enters the glycolytic pathway.

Question 5

What happens to fatty acids during b-oxidation?

Answer 5

Fatty acids undergo b-oxidation, which removes 2 carbon “snips” and converts them to acetic acid, then to acetyl CoA for the Krebs cycle.

Question 6

Why can fatty acids not be used for gluconeogenesis?

Answer 6

Fatty acids cannot be used to generate glucose via gluconeogenesis because they do not produce glucose precursors.

Question 7

Why is ammonia converted to urea in the body?

Answer 7

Ammonia is a byproduct of deamination during amino acid metabolism and is toxic, so it is converted to urea for excretion.

Question 7

What happens to amino acids when there is excess?

Answer 7

Excess amino acids are either converted to fat or burned for energy. They undergo transamination and deamination to generate keto acids, which are used in the Krebs cycle for energy.

Question 7

What is the role of nutrient pools in metabolism?

Answer 7

Nutrient pools are dynamic and interchangeable. Fat and carbohydrates can be stored and directly used for energy, while amino acids are used indirectly and cannot be stored.

Question 8

What is the role of insulin during the absorptive state?

Answer 8

Insulin, produced by β cells in the pancreas, directs events during the absorptive state, promoting anabolic processes like protein synthesis and fat storage.

Question 8

What happens during the postabsorptive state?

Answer 8

In the postabsorptive (fasting) state, the GI tract is empty, and energy needs are met by the breakdown of food reserves.

Question 8

What happens during the absorptive state of metabolism?

Answer 8

During the absorptive state, anabolism exceeds catabolism, glucose is the major fuel, and dietary amino acids and fats are used for anabolic processes like making proteins and fat stores. Excess metabolites are used to make fat once energy demands are met.

Question 9

What stimulates insulin secretion?

Answer 9

Insulin secretion is primarily stimulated by a rise in blood glucose. It is also stimulated by increased blood amino acids, glucose-dependent insulinotropic peptide (GIP) from the GI tract, and parasympathetic stimulation.

Question 9

What is the role of insulin in the absorptive state?

Answer 9

Insulin directs events of the absorptive state, promoting glucose transport into cells, increasing protein synthesis, and storing energy as glycogen and triglycerides (TGs).

Question 9

What is the structure of insulin?

Answer 9

Insulin is a small 51 amino acid protein consisting of two amino acid chains connected by disulfide bonds. It is initially synthesized as proinsulin, and the middle portion is excised to activate the hormone before release.

Question 10

How does insulin affect glucose transport?

Answer 10

Insulin activates carrier-mediated facilitated diffusion, increasing glucose uptake into cells (especially skeletal muscle and adipose tissue) by 15-20 fold via glucose transporter translocation.

Question 10

What tissues do not require insulin for glucose entry?

Answer 10

The liver, kidney, brain, and intestinal epithelial cells absorb glucose without the need for insulin.

Question 11

What is the role of insulin in metabolism?

Answer 11

Insulin is an anabolic, hypoglycemic hormone that removes glucose from circulation, either using it for energy or converting it to storage forms like glycogen and fat.

Question 11

What other effects does insulin have besides glucose transport?

Answer 11

Insulin enhances glucose oxidation for energy, promotes glucose storage as glycogen and triglycerides (TGs), increases amino acid uptake and protein synthesis, and inhibits gluconeogenesis and glycogenolysis in the liver.

Question 12

What occurs during the postabsorptive state?

Answer 12

In the postabsorptive state, blood glucose drops, and the body starts breaking down stored fat, proteins, and glycogen to maintain blood glucose levels, especially for the brain, which requires glucose for energy.

Question 13

What is the primary goal of the postabsorptive state?

Answer 13

The primary goal is to spare glucose for the brain and make more glucose available to the circulatory system.

Question 14

How is glucose made available to the blood during the postabsorptive state?

Answer 14

Glucose can be sourced from stored glycogen, tissue proteins, and fat (via glycerol).

Question 15

How does glycogenolysis in the liver help maintain blood glucose?

Answer 15

The liver has about 100 g of glycogen reserves, which can maintain blood glucose for ~4 hours during the postabsorptive state.

Question 16

Why doesn’t skeletal muscle release glucose directly to the circulation during glycogenolysis?

Answer 16

Skeletal muscle glycogen is converted to pyruvic acid or lactic acid, which is sent to the liver and converted back into glucose. Skeletal muscle does not release glucose directly because it lacks the necessary enzymes to export glucose to the blood.

Question 17

Why can't muscle cells produce glucose for circulation?

Answer 17

Muscle cells lack glucose-6-phosphatase, so they cannot produce glucose for circulation. Instead, they metabolize glucose-6-phosphate (G-6-P) via glycolysis to pyruvate or lactic acid, which enter the circulation and are converted to glucose by the liver.

Question 17

What happens during lipolysis in adipose tissue and liver?

Answer 17

Lipolysis in adipose tissue and liver produces glycerol, which the liver converts to glucose via gluconeogenesis. Free fatty acids (FFAs) are used in the Krebs cycle for energy but cannot be converted to glucose.

Question 18

How does skeletal muscle contribute to blood glucose levels in the post-absorptive state?

Answer 18

Skeletal muscle indirectly contributes to blood glucose by converting G-6-P to pyruvate or lactic acid, which enter the blood and are converted to glucose by the liver.

Question 19

When is cellular protein catabolism used?

Answer 19

Cellular protein catabolism is a last resort, used during prolonged fasting or stress, and involves deamination of amino acids in the liver to generate keto acids, which are converted to glucose via gluconeogenesis.

Question 20

What is glucose sparing?

Answer 20

Glucose sparing refers to the use of non-carbohydrate fuels (especially triglycerides) to conserve glucose. During the post-absorptive state, most tissues use fats as their main energy source.

Question 21

How does the body use fat during the post-absorptive state?

Answer 21

Lipolysis in adipose tissue releases free fatty acids (FFAs), which are oxidized for energy via the Krebs cycle. The liver also converts FFAs into ketone bodies, which can be used by cells for energy.

Question 22

What triggers the post-absorptive state?

Answer 22

A drop in blood glucose triggers the post-absorptive state, leading to the inhibition of insulin secretion and the stimulation of glucagon secretion.

Question 22

How does the brain adjust during prolonged fasting (4-5 days)?

Answer 22

After several days of fasting, the brain begins to use ketone bodies for energy, in addition to glucose.

Question 23

What is the function of glucagon in the post-absorptive state?

Answer 23

Glucagon promotes glycogenolysis, gluconeogenesis (from lactic acid, glycerol, and amino acids), and lipolysis in adipose tissue to release free fatty acids and glycerol into the blood.

Question 24

What is the role of glucagon in regulating blood glucose?

Answer 24

Glucagon is a potent hyperglycemic agent that stimulates the release of glucose into the blood, helping to raise blood glucose levels when they drop.

Question 25

What happens when you eat a high-protein, low-carbohydrate meal?

Answer 25

A high-protein, low-carbohydrate meal can stimulate glucagon secretion, as it helps maintain blood glucose levels by promoting gluconeogenesis and lipolysis.

Question 26

How does the sympathetic nervous system respond to a drop in blood glucose?

Answer 26

Sympathetic stimulation increases epinephrine release, which triggers lipolysis, fat mobilization from adipose tissue, and glycogenolysis, similar to the effects of glucagon.

Question 27

What role does the adrenal gland play in the "fight or flight" response?

Answer 27

The adrenal medulla secretes epinephrine and norepinephrine, which stimulate glycogenolysis, lipolysis, fat mobilization, and gluconeogenesis. The adrenal cortex secretes cortisol, which helps maintain blood glucose levels during stress.

Question 28

What are the functions of cortisol in the body?

Answer 28

Cortisol stimulates lipolysis, gluconeogenesis, and protein catabolism, helping the body adapt to changes in food availability and stress.