Metabolism and Hormonal Responses

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Questions and Answers

What metabolic process is activated in the liver due to a low insulin to glucagon ratio?

Glycogenolysis (correct)

Lipogenesis

Glycogen synthesis

Glycolysis

How does the liver respond to increased glucagon levels?

Decreases fatty acid mobilization

Activates gluconeogenesis (correct)

Inhibits protein breakdown

Promotes glycolysis

What fuels gluconeogenesis during fasting?

Glucose from glycogen stores

Cholesterol from lipolysis

Amino acids from protein breakdown (correct)

Fatty acids from glycolysis

Which compound produced from fatty acid oxidation activates gluconeogenesis?

Citrate Signup and view all the answers

What is the primary effect of lipolysis in the context of energy production?

Mobilizes fatty acids from triglycerides (TGs) Signup and view all the answers

How does glucose primarily enter muscle tissue during the fed state?

Via insulin-stimulated Glut 4 transport system Signup and view all the answers

What is one of the effects of the fall in insulin levels during fasting on muscle tissue?

Reduces glucose entry Signup and view all the answers

What is the fate of fatty acids that enter muscle tissue from both the diet and the liver?

Oxidized via β-oxidation to produce ATP Signup and view all the answers

In adipose tissue during the fed state, how is glucose metabolized?

Converted into acetyl CoA and then to fatty acids Signup and view all the answers

During early fasting, what significant metabolic change occurs in the liver?

Switch to glucose production Signup and view all the answers

What effect does insulin have on lipoprotein lipase (LPL) activity in adipose tissue?

It increases LPL activity Signup and view all the answers

Which transporters are involved in glucose uptake by the brain?

Glut 1 & 3 Signup and view all the answers

What happens to glycerol released from triacylglycerols in adipose tissue?

It is returned to the liver for re-use Signup and view all the answers

What happens to glucose levels when plasma glucose falls in the liver during early fasting?

The affinity for glucose uptake decreases due to low Glut 2 activity Signup and view all the answers

What is a primary role of the liver during gluconeogenesis?

Produce ketone bodies for other tissues Signup and view all the answers

How does a low insulin to glucagon ratio affect glucose metabolic processes in the liver?

Inhibits glycogen synthesis and glycolysis Signup and view all the answers

During starvation, which fuel source does muscle primarily utilize?

Ketone bodies from the liver Signup and view all the answers

What effect do ketone bodies have on muscle during fasting?

Reduce proteolysis and conserve glucose Signup and view all the answers

What occurs to fatty acids in the liver during the production of ketone bodies?

Excess acetyl CoA is converted to ketone bodies Signup and view all the answers

What is the primary metabolic state in muscle when insulin levels drop significantly?

Shift to fatty acids as the primary fuel Signup and view all the answers

What triggers the mobilization of triglycerides (TGs) in adipose tissue during the early fasting state?

Reductions in insulin and glucagon ratios Signup and view all the answers

Which transport systems allow the brain to continue taking up glucose during fasting?

Glut 1 and Glut 3 Signup and view all the answers

What is the primary energy source the body switches to during the starved state?

Free fatty acids Signup and view all the answers

Why does glycogenolysis not occur in skeletal muscle?

Insufficient glucagon receptors Signup and view all the answers

How does the metabolism of glucose in adipose tissue during the early fasting state change?

Is severely inhibited due to lowered insulin Signup and view all the answers

What happens to glycerol during times of nutrient deprivation?

It gets recycled to the liver for gluconeogenesis Signup and view all the answers

What is primarily dependent on gluconeogenesis in the liver after glycogen stores are depleted?

Plasma glucose levels Signup and view all the answers

Why can't the brain switch to fatty acids as a fuel source during fasting?

Free fatty acids cannot cross the blood-brain barrier Signup and view all the answers

What does cAMP PK do to glycogen synthase?

It phosphorylates glycogen synthase, switching it OFF. Signup and view all the answers

Which enzyme phosphorylates phosphorylase, thus activating it?

Phosphorylase kinase. Signup and view all the answers

How is phosphorylase kinase activated allosterically?

By Ca2+ ions. Signup and view all the answers

What effect does insulin have on glycogen metabolism?

It activates protein phosphatase-1, switching OFF breakdown. Signup and view all the answers

In the liver, what second messenger does glucagon use to stimulate glycogenolysis?

cAMP. Signup and view all the answers

What is the role of phosphorylase kinase in regulating glycogen metabolism?

It links muscle contraction with glycogen breakdown. Signup and view all the answers

What is required for effective glycogen metabolism to prevent futile cycling of intermediates?

Coordinated regulation of phosphorylase and glycogen synthase activity. Signup and view all the answers

What triggers glycogen degradation in both muscle and liver during stress?

Glucagon and adrenaline. Signup and view all the answers

Study Notes

Metabolism in the Fed State

Glycerol Metabolism: Glycerol from peripheral tissues converts to triacylglycerols and enters the TCA cycle.
Amino Acid Fate: Excess amino acids from the gut convert to pyruvate, metabolized via the TCA cycle for energy or converted to triacylglycerols.

Muscle Metabolism During Fed State

Glucose Uptake: Insulin-mediated Glut 4 transport system facilitates glucose entry; glucose is converted to glycogen or metabolized via glycolysis and TCA cycle.
Fatty Acid Oxidation: Fatty acids from chylomicrons and VLDL oxidize via β-oxidation to acetyl CoA, producing ATP for muscle contraction.
Protein Synthesis: Amino acids are utilized for protein synthesis.

Adipose Tissue Metabolism During Fed State

Glucose Entry: Glucose enters adipose tissue through Glut 4, converting to acetyl CoA, then to fatty acids and triacylglycerols.
Fatty Acid Storage: Fatty acids from VLDL and chylomicrons convert into triacylglycerols; LPL activity increases while HSL is inhibited by insulin.
Glycerol Recycling: Glycerol released from triacylglycerols returns to the liver for reuse.

Brain Metabolism During Fed State

Glucose Uptake: Brain utilizes glucose through Glut 1 & 3 transporters and metabolizes it oxidatively via glycolysis and TCA cycle to produce ATP.

Early Fasting State

Liver Glucose Shift: Liver transitions from a glucose-utilizing to a glucose-producing organ; glycogen synthesis decreases while glycogenolysis and gluconeogenesis increase.
Insulin/Glucagon Ratio: A reduced insulin to glucagon ratio activates glycogenolysis and gluconeogenesis from lactate and alanine.

Liver Metabolism in Early Fasting State

Glucose Export: Glucose entry halts as Glut 2 transporter affinity decreases; liver exports glucose instead.
Lipolysis Activation: Mobilization of fatty acids from triacylglycerols occurs; fatty acids are oxidized for energy.
Amino Acid Breakdown: Proteins are broken down to amino acids, fueling gluconeogenesis with fatty acids providing additional energy.

Muscle Metabolism in Early Fasting State

Decreased Glucose Entry: A fall in insulin reduces glucose entry; glycogenolysis does not occur due to lack of glucagon receptors in muscle.
Fatty Acid Utilization: Muscle shifts energy source to fatty acid oxidation, inhibiting glucose use.
Proteolysis: Protein breakdown supplies amino acids for energy or for gluconeogenesis.

Adipose Tissue Metabolism in Early Fasting State

Reduced Glucose Entry: Insulin decrease limits glucose uptake via Glut 4; glycolysis of glucose is inhibited.
Triacylglycerol Mobilization: Triacylglycerol breakdown is activated by low insulin and noradrenaline; fatty acids contribute to energy production.
Glycerol Recycling: Glycerol is recycled to the liver for gluconeogenesis.

Brain Metabolism in Early Fasting State

Continued Glucose Uptake: High affinity of Glut 1 and 3 allows glucose uptake despite fasting.
Inefficiency with Fatty Acids: Brain cannot utilize fatty acids as fuel due to the blood-brain barrier; it relies solely on glucose metabolism.

Metabolism in the Starved State

Chronic Hormonal State: Low insulin and high glucagon predominance; thyroid hormones decrease, lowering metabolic rate.
Primary Energy Source: Free fatty acids become the main energy source, and ketone bodies are produced as an alternative fuel.

Liver Metabolism in Starved State

Depleted Glycogen: Liver glycogen stores deplete within 24 hours; plasma glucose relies on gluconeogenesis.
Amino Acids and Gluconeogenesis: Amino acids from protein breakdown support gluconeogenesis, and fatty acids from lipolysis provide energy for this process.
Ketone Body Production: Excess acetyl CoA from fatty acid oxidation is converted to ketone bodies for use by other tissues.

Muscle Metabolism in Starved State

Fatty Acids as Fuel: Glucose entry is minimal, and muscle switches to fatty acids for energy.
Utilization of Ketone Bodies: Ketone bodies are absorbed and used as additional fuel, reducing muscle wasting by conserving glucose.

Glucose-Fatty Acid Cycle

Energy Regulation: Fatty acid oxidation spares glucose; cAMP activates phosphorylase kinase, while glycogen synthase is inhibited to prevent futile cycling.

Glycogen Metabolism Regulation

Phosphorylation Mechanisms: Phosphorylase kinase activation leads to glycogen breakdown; muscle contraction is linked to glycogen breakdown via Ca2+ signaling.
Insulin Action: Insulin activates protein phosphatase-1, promoting glycogen synthesis and inhibiting breakdown.

Summary of Glycogen Regulation

Coordinated Control: Efficient glycogen metabolism requires regulated activity of phosphorylase and glycogen synthase to prevent futile cycling of intermediates.

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Description

This quiz explores the metabolic processes activated in the liver due to hormonal changes, particularly during fasting states. It covers how the liver responds to glucagon, the role of gluconeogenesis, and the effects of lipolysis and insulin on muscle tissue. Test your knowledge on these crucial biochemical pathways!