Biochemistry: Metabolic Pathways Quiz

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Questions and Answers

What is the primary function of the Krebs cycle?

To breakdown glucose into pyruvate

To synthesize glycogen from glucose

To convert glucose into glycogen

To produce compounds for other metabolic pathways and consume compounds (correct)

What is the primary function of the pentose phosphate pathway?

To generate NADPH for anabolic reactions and as an antioxidant. (correct)

To synthesize fatty acids.

To produce ATP for cellular energy.

To break down glucose for energy production.

What compound initiates and regenerates within the Krebs cycle?

Acetyl-CoA

Oxalacetate (correct)

Pyruvate

Glucose-6-phosphate

Where does the pentose phosphate pathway take place in eukaryotic cells?

In the cytoplasm. Signup and view all the answers

Which of the following best describes the process of glycogenolysis?

The breakdown of glycogen into glucose Signup and view all the answers

Which hormone primarily stimulates glycogenolysis in the liver?

Glucagon Signup and view all the answers

Which of the following is NOT a characteristic of the pentose phosphate pathway?

It occurs primarily in skeletal muscle. Signup and view all the answers

What are the two main stages of the pentose phosphate pathway?

Oxidative stage and non-oxidative stage. Signup and view all the answers

What is the primary role of UDP-glucose in gluconeogenesis?

To be the active form of glucose used to incorporate into growing glycogen chain Signup and view all the answers

Which of the following best describes the Krebs cycle?

A series of metabolic reactions in the mitochondria that produce ATP and $CO_2$. Signup and view all the answers

Which of these is a key function of glycogen?

It acts as a storage mechanism for glucose Signup and view all the answers

What is the first step of the Krebs cycle?

Oxidation of pyruvate to acetyl-CoA. Signup and view all the answers

Which of the following hormones primarily regulates the conversion of liver glycogen into blood glucose?

Glucagon and adrenaline Signup and view all the answers

What is meant by the term 'amphibolic' in the context of the Krebs cycle?

It consumes and produces compounds . Signup and view all the answers

Which of the following enzymes are directly involved in glycogenolysis?

Glycogen phosphorylase and debranching enzyme Signup and view all the answers

Which of these is a characteristic of anaerobic respiration?

It takes place without consuming oxygen. Signup and view all the answers

What is the direct product of the enzyme glucose 6-phosphatase?

Free glucose Signup and view all the answers

Which of these is NOT a source of free amino acids in the cell?

Direct dietary glucose intake Signup and view all the answers

What is the primary function of transamination reactions?

Transfer of an α-amino group from an amino acid to an α-ketoacid Signup and view all the answers

Which coenzyme is essential for transaminase reactions?

Pyridoxal phosphate (vitamin B6) Signup and view all the answers

During transamination, what is initially formed when an amino acid binds to the transaminase enzyme?

A Schiff base Signup and view all the answers

Which of the following is NOT a common amino acid donor in transamination reactions?

Glycine Signup and view all the answers

Which enzyme catalyzes the reaction where glutamate donates its amino group to pyruvate?

GPT or ALT Signup and view all the answers

Where are GPT or ALT enzymes predominantly located in the cell?

Cytosol Signup and view all the answers

Why can't glucose-6-phosphate directly enter the circulation from muscle cells?

Muscle cells lack the enzyme glucose-6-phosphatase. Signup and view all the answers

What is the primary role of the Cori cycle under high intensity, limited oxygen conditions?

Enable rapid ATP production in muscle tissue. Signup and view all the answers

Which metabolic pathways are directly integrated through the Cori cycle?

Glycolysis, glycogenolysis, gluconeogenesis, and lactic acid fermentation. Signup and view all the answers

During the muscle phase of the Cori cycle, what is the fate of NADH produced during glycolysis?

It is used to convert pyruvate to lactate. Signup and view all the answers

How many net ATP molecules are produced per glucose molecule during the muscle phase of the Cori Cycle?

2 Signup and view all the answers

What enzyme is responsible for converting lactate back into pyruvate in the liver during the Cori cycle?

Lactate dehydrogenase (LDH). Signup and view all the answers

Which enzyme catalyzes the conversion of oxaloacetate to phosphoenolpyruvate (PEP) in the liver during gluconeogenesis?

Phosphoenolpyruvate carboxykinase (PEPCK) Signup and view all the answers

What is the role of fructose 1,6-bisphosphatase in the liver's phase of the Cori Cycle?

It converts fructose 1,6-bisphosphate to fructose 6-phosphate. Signup and view all the answers

In the transamination process involving glutamate and oxaloacetate, which molecule is formed when oxaloacetate accepts an amino group?

Aspartate Signup and view all the answers

What is the primary function of glutamate dehydrogenase?

Separating the nitrogen group from glutamate through oxidative deamination Signup and view all the answers

Which of the following molecules inhibits the activity of glutamate dehydrogenase?

ATP Signup and view all the answers

What is the primary source of NH3 produced by the body?

Tissues Signup and view all the answers

Why is high level of NH3 toxic to the body?

It is uncharged and can readily cross the blood-brain barrier. Signup and view all the answers

Which of the following organisms is classified as ureotelic?

Humans Signup and view all the answers

Where does the urea cycle take place in the body?

Liver Signup and view all the answers

What is the main rate-limiting step of the urea cycle?

The formation of carbamoyl phosphate Signup and view all the answers

Which enzyme catalyzes the first step of the urea cycle, requiring two ATP molecules?

Carbamoyl phosphate synthetase I Signup and view all the answers

What molecule allosterically activates carbamoyl phosphate synthetase I (CPS1)?

N-acetylglutamate Signup and view all the answers

During the urea cycle, which molecule donates its carbamoyl group to ornithine?

Carbamoyl phosphate Signup and view all the answers

Which enzyme is responsible for condensing citrulline with aspartate to form argininosuccinate?

Argininosuccinate synthetase Signup and view all the answers

What two substances are generated in the reaction catalyzed by argininosuccinase?

Arginine and fumarate Signup and view all the answers

Which enzyme hydrolyzes the guanidino group of arginine, releasing urea and ornithine?

Arginase Signup and view all the answers

Which form of nitrogen waste is primarily excreted by terrestrial vertebrates?

Urea Signup and view all the answers

What is a key outcome of regulated metabolic pathways?

Consistent supply of energy and building blocks Signup and view all the answers

Study Notes

Introduction to Metabolism

Metabolism is the sum of all chemical reactions occurring in cells.
It's a highly coordinated cellular activity with a specific purpose and direction, involving numerous enzyme systems.
It involves the exchange of matter and energy with the environment.
Specific functions include:
- Obtaining chemical energy from sunlight or food.
- Converting nutrients into cellular components.
- Assembling these components into the cell's own macromolecules.
- Forming and breaking down molecules for specialized cellular functions.

Digestion

Digestion breaks down carbohydrates, lipids, and proteins into absorbable compounds (glucose, fatty acids, and amino acids, respectively).

Absorption

Absorption involves the passage of digested products, along with vitamins, minerals, water, etc., through the digestive system into the organism.

Phases of Metabolism

Absorption: The phase where chemicals and energy from the environment enter the protoplasm.
Transformation: This involves all actions where the protoplasm changes the absorbed species and energy, including secretion, digestion, assimilation, and dissimilation.
Excretion: Removal of chemicals not incorporated into the protoplasm.

Stages of Metabolism

First stage: Breakdown of large molecules into monomers (e.g., polysaccharides to glucose, lipids to glycerol and fatty acids, proteins to amino acids). No usable energy is released in this stage.
Second stage: Further breakdown of the numerous small molecules from the first stage into a smaller set of central metabolic molecules, tending toward acetyl CoA. A small amount of ATP is generated in this stage.
Third stage: Oxidation of acetyl CoA into water and carbon dioxide. Most of the energy from food is produced as ATP in this stage.

Division of Metabolism: Anabolism and Catabolism

Anabolism: A constructive metabolic process where large molecules are built from smaller ones, consuming energy (ATP). It's used to build proteins from amino acids.
Catabolism: A degradative metabolic process where large molecules from food or the organism's own reserves are broken down into smaller ones. This process releases energy, some of which is used directly and the rest stored in special molecules for later use.
Amphibolic pathways: Mixed routes for interconversion of metabolic intermediates between the beginning of anabolic pathways and the end of catabolic pathways.

Energy

Energy is the ability to do work or cause change in matter. It can take the form of heat, light, electricity, and motion.

Metabolic Pathways

A series of ordered reactions where the product of one reaction becomes the substrate for the next.
Examples include glycolysis.
A metabolic pathway is a sequence of chemical reactions transforming an initial substrate into one or more final products through intermediary metabolites.

Types of Metabolic Pathways

Catabolic pathways: Oxidative routes releasing energy and reducing power, and synthesizing ATP. Examples include glycolysis and beta-oxidation.
Anabolic pathways: Reductive routes consuming energy (ATP) and reducing power. Examples include gluconeogenesis and the Calvin cycle.
Amphibolic pathways: Mixed catabolic and anabolic pathways like the Krebs cycle, which produces energy and reducing power, and precursors for biosynthesis.

Glycolysis

Glycolysis (or the Embden-Meyerhof pathway) is the metabolic pathway that oxidizes glucose to produce energy for the cell.
It comprises 10 enzymatic reactions that convert glucose into two pyruvate molecules which can participate in subsequent metabolic pathways to further produce energy.
Functions: produce ATP and NADH, form energy source molecules for aerobic respiration and fermentation.
Features:
- Takes place in the cytosol.
- Does not require oxygen.
- Starts with one glucose molecule (6C).
- Ends with two pyruvate molecules (3C each).
- Considered an amphibolic pathway.

Pentose Phosphate Pathway

An alternative glucose catabolic pathway also known as the phosphogluconate pathway.
Glucose is oxidized but no ATP is produced.
Primarily produces NADPH (nicotinamide adenine dinucleotide phosphate reduced form) in eukaryotic cells.
Occurs in the cytoplasm.
Consists of irreversible oxidation and reversible interconversions. -More complex than glycolysis.
Not present in skeletal muscle.
Functions include:
- Provide reducing power in the cytoplasm (NADPH), essential for anabolic reactions, and a potent antioxidant in some cells.
- Provide pentoses (ribose), necessary for nucleotide and many cofactor enzyme synthesis.
Divided into two phases:
- Oxidative phase: Produces NADPH + H+.
- Non-oxidative phase: Produces various monosaccharides; pentose(ribose) is one important product.

Krebs Cycle

Part of cellular respiration.
Involves reactions leading from glycolysis to the respiratory chain.
Occurs in mitochondria in three phases:
- Oxidation of pyruvate to acetyl CoA.
- Krebs cycle.
- Oxidative phosphorylation.

Cori Cycle

Cyclic circulation of glucose and lactate between muscle and liver.
Muscles use glucose from glycogen reserves or the bloodstream.
Intense muscle activity leads to substantial lactate production.
Lactate diffuses into the blood and travels to the liver.
In the liver, lactate is converted back to glucose by gluconeogenesis.
Glucose returns to muscles.
Objectives: provides quick energy for muscle in low oxygen conditions, prevents lactic acidosis in muscles.

Gluconeogenesis

Anabolic pathway creating glucose from non-carbohydrate precursors like amino acids, lactate, pyruvate, glycerol, and intermediates of the citric acid cycle.
Most important in liver and kidneys.
Enables organisms to produce glucose when it's not available from food.
Essential for maintaining blood glucose levels during fasting states.

Transamination

Transfer of an amino group from an amino acid to a keto acid.
Amino acid converts to a keto acid.
Keto acid accepts amino group to form corresponding amino acid.
Reversible reaction catalyzed by transaminases.
Uses pyridoxal phosphate (vitamin B6).

Oxidative Deamination

Removal of an amino group from an amino acid.
Catalyzed by glutamate dehydrogenase.
Uses NAD or NADP coenzymes.
An important reaction for ammonia (NH3) removal.
Occurs in the mitochondrial matrix.
Allosteric enzyme activated by ADP.

Urea Cycle

Metabolic process for processing protein derivatives and producing urea as the final product.
A hepatic process, classifying animals by nitrogen excretion methods:
- Ammonotelic: Excrete ammonia (aquatic animals, fish & amphibians).
- Ureotelic: Convert ammonia to urea (humans, mammals, adult amphibians & sharks).
- Uricotelic: Convert ammonia to uric acid (birds & reptiles).
Five reactions, two in mitochondria and three in the cytoplasm.
- Key steps outlined, including enzymes and substrates.

Glucogenic and Ketogenic Amino Acids

Glucogenic: Produce intermediates for gluconeogenesis (e.g., pyruvate, oxaloacetate).
Ketogenic: Produce ketone bodies (e.g., leucine, lysine).
Some amino acids are both glucogenic and ketogenic (e.g., phenylalanine, tyrosine, tryptophan, isoleucine).

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Description

Test your knowledge on key metabolic pathways including the Krebs cycle and the pentose phosphate pathway. This quiz covers the primary functions, characteristics, and processes involved in glycolysis and glycogenolysis. Perfect for students studying biochemistry concepts.