Metabolic Control in Tissues

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Questions and Answers

What percentage of the body's solid mass is made up of skeletal muscles?

One half (correct)

Three fifths

Three quarters

One quarter

What is the primary source of energy for resting muscles?

Amino acids

Ketone bodies

Glucose

Fatty acids (correct)

What is the purpose of creatine phosphate in exercising muscles?

To increase ATP production

To decrease ATP degradation

To maintain ATP levels (correct)

To increase lactate production

What is the name of the cycle that occurs when lactate produced by exercising muscles is converted back into glucose in the liver?

Cori cycle Signup and view all the answers

What happens to skeletal muscle proteins during prolonged fasting and starvation?

They are broken down to provide carbon skeletons for gluconeogenesis Signup and view all the answers

What is the result of increased amino acid degradation in skeletal muscles during prolonged fasting and starvation?

The glucose-alanine cycle Signup and view all the answers

How much glycogen is stored in skeletal muscles in the average adult male?

250 grams Signup and view all the answers

What is the main difference between the metabolic patterns of short duration, vigorous exercise and low power-output, long distance running?

Energy source used Signup and view all the answers

What is the primary source of ATP in mammalian erythrocytes?

Anaerobic glycolysis Signup and view all the answers

What is the purpose of lactate produced in RBCs?

To be used as a precursor for gluconeogenesis in other tissues Signup and view all the answers

What is the primary function of ATP in RBCs?

To maintain electrolyte balance via Na+/K+ ATPase Signup and view all the answers

What is the primary fuel stored in adipose tissues?

Triacylglycerols Signup and view all the answers

Why do adipocytes use glucose for the provision of dihydroxyacetone phosphate?

Because they lack the enzyme glycerol kinase Signup and view all the answers

What percentage of body weight is stored as triacylglycerols in an average male?

12% Signup and view all the answers

What happens to the brain's glucose consumption after 24 hours of starvation?

It decreases significantly Signup and view all the answers

Why is the adaptation of the brain to use ketone bodies crucial during starvation?

To preserve skeletal muscle proteins Signup and view all the answers

What type of muscle fibers do sprinters possess?

White muscle fibers Signup and view all the answers

During the first seconds of a 100-meter sprint, what is the primary source of energy for ATP resynthesis?

Phosphocreatine Signup and view all the answers

What is the primary source of energy for ATP synthesis in marathon running?

Aerobic oxidation of fatty acids Signup and view all the answers

What is unique about the heart's metabolism?

It is completely aerobic Signup and view all the answers

What is the primary source of fuel for the brain?

Glucose Signup and view all the answers

What happens when the blood glucose falls below 2 mm in the brain?

Serious irreversible brain damage may occur Signup and view all the answers

What is the primary source of energy generation in the body after depletion of whole body TAG reserve?

Skeletal muscle protein degradation Signup and view all the answers

What is the result of blocked blood vessels in the heart?

The supplied tissue of the heart dies Signup and view all the answers

What is the role of glucokinase in the liver during a well-fed state?

Catalyzes the phosphorylation of glucose to G-6-P Signup and view all the answers

What is the primary function of the liver in regulating blood sugar levels?

Control of blood glucose levels Signup and view all the answers

What is the role of ketone bodies in the brain?

They are used in prolonged fasting or starvation Signup and view all the answers

What is the ultimate consequence of skeletal muscle protein degradation as a sole source of energy generation?

Cardiac and respiratory failure Signup and view all the answers

What is the estimated daily glucose uptake by the brain after three weeks of starvation?

35 grams/day Signup and view all the answers

What is the primary role of the liver in steroid metabolism?

Synthesis of steroids Signup and view all the answers

What determines the total period of time a person can survive without food intake?

Size of the fat deposits Signup and view all the answers

What is the fate of glucose-6-phosphate in the liver during a well-fed state?

Glycogenesis Signup and view all the answers

What is the primary fate of amino acids arriving at the liver in the fed state?

Used for protein synthesis Signup and view all the answers

What is the primary source of energy for hepatocytes during fasting?

Fatty acids oxidation Signup and view all the answers

What is the result of glucagon stimulation of its receptor on hepatocytes during fasting?

Glucose release into the blood Signup and view all the answers

What is the primary function of the hexose monophosphate shunt?

Pentose phosphate synthesis Signup and view all the answers

What is the primary fate of acetyl-CoA in the liver during the fed state?

Used for synthesis of TAG and phospholipids Signup and view all the answers

What is the primary hormone that stimulates glucose release into the blood during fasting?

Glucagon Signup and view all the answers

What is the byproduct of glycolysis that can be converted into acetyl-CoA?

Pyruvate Signup and view all the answers

What is the primary function of glucose 6-phosphatase in the liver and kidney?

Dephosphorylation of glucose Signup and view all the answers

Study Notes

Tissues that Regulate Overall Body Metabolism

Liver, muscle, brain, and adipose tissues are the vital tissues that regulate overall body metabolism.

Metabolism in Skeletal Muscles

Skeletal muscles make up about half of the body's solid mass and dominate energy metabolism.
Muscles can use a wide variety of fuels, including glucose, fatty acids, and ketone bodies.
Muscle energy demand varies with activity, with resting muscles depending mainly on fatty acid oxidation.
Glucose derived from muscle glycogenolysis is primarily used by muscles only (no G-6-Phosphatase).
Characteristics of energy metabolism in skeletal muscles:
- Store about 250 grams of glycogen in the average adult man.
- Lactate produced by exercising muscles diffuses to blood and is converted back into glucose in the Cori cycle.
- Prolonged fasting and starvation cause increased rate of degradation of skeletal muscle proteins to provide carbon skeleton for gluconeogenesis.
- Increased amino acid degradation results in the glucose-alanine cycle.
- Creatine phosphate is necessary for maintaining ATP levels during exercise.

Differences between Sprinters and Marathon Runners

Sprinters have large muscle mass, mainly consisting of white muscle fibers with high anaerobic capacity, and store considerable amounts of glycogen and creatine phosphate.
Marathon runners have muscles consisting mainly of highly aerobic red muscles, densely packed with mitochondria and myoglobin.
During the first seconds of a 100-meter sprint, ATP is rapidly reformed at the expense of phosphocreatine, catalyzed by creatine kinase.
In marathon running, ATP synthesis is mainly powered by aerobic oxidation involving an increased rate of fatty acid mobilization and acetyl-CoA oxidation.

Metabolism in the Heart

Heart muscles have a regular rhythm of contraction and relaxation.
Heart has completely aerobic metabolism at all times.
Mitochondria are more abundant in heart muscles than in skeletal muscles.
Heart uses blood glucose, ketone bodies, and fatty acids, which are oxidized aerobically.
There is also phosphocreatine in the heart.
When blood vessels are blocked, the supplied tissue of the heart dies, leading to myocardial infarction or heart attack.

Metabolism in the Brain

The brain of adult mammals normally uses only glucose as fuel.
It has very active aerobic metabolism.
It uses oxygen up to 20% of total consumption at rest.
It does not store glycogen but depends on fresh glucose from the blood.
When blood glucose falls below 2 mm (hypoglycemia), serious irreversible brain damage may occur.
The brain does not use fatty acids from the blood but can use ketone bodies in prolonged fasting or starvation, which reserves muscle proteins from degradation.

Metabolism in Blood Cells

Mammalian erythrocytes contain no mitochondria, thus are exclusively dependent on anaerobic glycolysis.
They produce lactate, which diffuses to plasma, and can be used as a precursor for gluconeogenesis.
RBCs use ATP for maintaining electrolyte balance via the action of Na+/K+ ATPase.

Metabolism in Adipose Tissues

Adipocytes are the main fuel-storing tissues in the body, storing triacylglycerols that can support energy generation for several weeks.
Because adipocytes lack the enzyme glycerol kinase, they use glucose for the provision of dihydroxyacetone phosphate required for the formation of glycerol-3-phosphate.
Body response to starvation:
- The total body reserve of fuel is sufficient to support energy demand for several weeks.
- The average man possesses about 12% of his body weight as triacylglycerols, while females may store up to 25% of their body weight as fat.
- The amount of carbohydrates stored is only about 2% of the fuel reserve.
- The liver glycogen will be exhausted just a few hours after a meal.
- The human body adapts metabolically during starvation by increasing the use of fuels other than carbohydrates.

Liver Metabolism

The liver is a central organ in metabolism, containing all enzymes of major metabolic pathways.
Unique pathways in the liver include:
- Control of blood glucose levels.
- Regulation of circulating fatty acids, amino acids, and other metabolites.
- Steroid synthesis, bile acids, and vit-D activation.
- Synthesis of most plasma proteins.
- Disposal of toxic metabolites such as ammonia and bilirubin.
- Inactivation and excretion of hormones and drugs.
- Synthesis and export of essential nutrients and precursors.

Fate of Different Substances in the Liver

In a well-fed state, glucose is taken up by hepatocytes and phosphorylated to G-6-P in a reaction catalyzed by glucokinase.
Fates of glucose-6-P:
- Glycogenesis.
- G-6-P may be oxidized via glycolysis.
- Pyruvate is the end product of aerobic glycolysis.
- About 15% of G-6-P may be oxidized via the hexose monophosphate shunt.
- Pyruvate may be converted into acetyl-CoA by PDHC.
- Very little of acetyl-CoA derived from glycolysis in the liver is oxidized in the citric acid cycle for energy generation.

Fate of Absorbed Amino Acids and Lipids in the Liver

Amino acids arriving at the liver in the fed state condition will primarily be used for protein synthesis.
The liver is constantly turning over its protein to adjust its metabolic activities.
In the fed state condition, the liver may actively use acetyl-CoA for synthesis of TAG and it also synthesizes phospholipids and cholesterol.

Fuel Metabolism in the Liver during Fasting

2-3 hours after the last meal, the levels of blood glucose start to decline, and insulin secretion decreases with a concomitant increase in glucagon release.
This hormonal change will switch on and off the major metabolic pathways in the liver to conserve blood glucose concentration and promote the use of stored fuels.
The glucagon stimulation of its receptor on hepatocytes will promote glucose release into blood by activating the enzymes of glycogenolysis and gluconeogenesis.
The fatty acids oxidation becomes the principle source of energy in hepatocytes during fasting to synthesize ketone bodies (the acetoacetate and β-hydroxybutyrate).

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Description

This quiz covers the vital tissues that regulate overall body metabolism, including liver, muscle, brain, and adipose tissues. It also discusses metabolism in skeletal muscles.