Metabolism

Question 1

What is the absorptive state, and when does it occur?

Answer 1

The absorptive state occurs during and shortly after eating, lasting about 3-4 hours post-meal; focuses on anabolism and energy storage.

Question 2

What happens to carbohydrates during the absorptive state?

Answer 2

1. Glucose is absorbed and used as the primary energy source.
2. Excess glucose is converted to glycogen in the liver and muscles (glycogenesis).
3. Remaining glucose is converted to triglycerides for fat storage.

Question 3

How is glucose transported into cells during the absorptive state?

Answer 3

Glucose transport into cells is stimulated by insulin, which promotes glycolysis, glycogenesis, and lipogenesis.

Question 4

What happens to proteins during the absorptive state?

Answer 4

1. Amino acids are used for protein synthesis in tissues.
2. Excess amino acids are converted into glucose or fatty acids for storage via deamination in the liver.

Question 5

How are lipids metabolized in the absorptive state?

Answer 5

1. Dietary fats are packaged into chylomicrons and transported via the lymphatic system.
2. Lipoprotein lipase breaks triglycerides into fatty acids and glycerol for cellular uptake.
3. Fatty acids are stored in adipose tissue or used for energy.

Question 6

What is the role of the liver in the absorptive state?

Answer 6

The liver stores glucose as glycogen, converts excess glucose to triglycerides, and deaminates amino acids for energy or storage.

Question 7

What is the post-absorptive state, and when does it occur?

Answer 7

Occurs when the digestive tract is empty, typically 4-12 hours after eating; focuses on catabolism to maintain blood glucose levels for energy.

Question 8

What happens to glycogen during the post-absorptive state?

Answer 8

Glycogen is broken down into glucose (glycogenolysis) in the liver to maintain blood glucose levels.

Question 9

What process provides glucose from non-carbohydrate sources in the post-absorptive state?

Answer 9

Gluconeogenesis produces glucose from amino acids, glycerol, and lactate in the liver and kidneys.

Question 10

How are lipids used for energy in the post-absorptive state?

Answer 10

1. Stored triglycerides in adipose tissue are broken into glycerol and fatty acids (lipolysis).
2. Fatty acids undergo beta-oxidation to produce ATP.
3. Glycerol can enter gluconeogenesis to form glucose.

Question 11

What are ketone bodies, and when are they produced?

Answer 11

Ketone bodies are produced during prolonged fasting or low glucose availability when the liver converts fatty acids into an alternative energy source for tissues like the brain.

Question 12

What is the role of proteins during the post-absorptive state?

Answer 12

Proteins are broken down into amino acids, which are then used for gluconeogenesis or energy production.

Question 13

What hormones regulate the absorptive state?

Answer 13

Insulin is the primary hormone in the absorptive state, promoting glucose uptake, glycogenesis, lipogenesis, and protein synthesis.

Question 14

What hormones regulate the post-absorptive state?

Answer 14

1. Glucagon stimulates glycogenolysis, gluconeogenesis, and lipolysis.
2. Adrenaline enhances glycogenolysis and lipolysis during stress or exercise.
3. Cortisol promotes protein catabolism and gluconeogenesis during prolonged fasting.

Question 15

How does the sympathetic nervous system influence metabolism?

Answer 15

The sympathetic nervous system stimulates glycogenolysis and lipolysis through adrenaline during stress or energy demands.

Question 16

What is the primary energy source in the absorptive state vs. the post-absorptive state?

Answer 16

• Absorptive state: Glucose is the primary energy source.
• Post-absorptive state: Fatty acids and ketone bodies become the main energy sources, with glucose reserved for critical tissues like the brain.

Question 17

How is the liver central to both the absorptive and post-absorptive states?

Answer 17

• In the absorptive state, the liver stores glycogen and synthesises triglycerides.
• In the post-absorptive state, the liver produces glucose via glycogenolysis and gluconeogenesis, and ketone bodies during prolonged fasting.

Question 18

Define a lipoprotein.

Answer 18

A fat + apoprotein

Question 19

What are the 4 major types of apoproteins?

Answer 19

1. Chylomicrons
2. VLDL
3. LDL
4. HDL

Question 20

How are free fatty acids transported?

Answer 20

Bind to albumin

Question 21

What happens to chylomicron remnants?

Answer 21

1. Taken up by the liver (rich in cholesterol).
2. Cholesterol is used for bile acid synthesis or repackaged into very low-density lipoproteins (VLDL).

Question 22

What happens to VLDL after secretion from the liver?

Answer 22

1. VLDL delivers triglycerides to tissues via LPL, becoming intermediate-density lipoproteins (IDL).
2. IDL is either taken up by the liver or further metabolized to low-density lipoproteins (LDL).

Question 23

What is the fate of LDL?

Answer 23

1. LDL primarily delivers cholesterol to peripheral tissues.
2. Excess LDL is taken up by the liver via LDL receptors.

Question 24

What role does high-density lipoprotein (HDL) play in lipid metabolism?

Answer 24

1. HDL collects excess cholesterol from tissues and returns it to the liver.
2. Facilitates cholesterol esterification.

Question 25

How are glycogen stores different in the liver/skeletal muscle?

Answer 25

In post-absorptive state skeletal muscle also starts glycogenolysis BUT can’t dephosphorylate glucose phosphate hence free glucose can’t be transported into blood stream.

Question 26

How does skeletal muscle release glucose into blood?

Answer 26

1. Instead glucose oxidised to pyruvate / lactate.
2. Converted to glucose by liver and then can enter blood stream.
3. This cycling referred to as Cori cycle.

Question 27

Describe the role of the 4 lipoproteins.

Answer 27

1. Chylomicrons transport lipids absorbed from the small intestine.
2. VLDL transport lipids from the liver to other tissues in the body.
3. LDL transport cholesterol to other tissues in the body.
4. HDL transport cholesterol from other tissues in the body back to the liver. They also transfer apoproteins from the liver to chylomicrons and VLDL.