Carbohydrate Metabolism Practice Questions

Q: Which of the following is NOT a function of glycosaminoglycans?

Transport of lipids in the bloodstream. ***Transport of lipids in the bloodstream*** - Glycosaminoglycans (GAGs) generally do not play a direct role in the **transport of lipids** in the bloodstream. Lipid transport is primarily mediated by **lipoproteins** (e.g., chylomicrons, VLDL, LDL, HDL). - While some GAGs might interact with lipoproteins in the extracellular matrix, their fundamental role is not lipid transport but rather structural and signaling functions. *Lubrication of joints* - This is a well-established function of GAGs, particularly **hyaluronic acid**, which contributes to the **viscoelastic properties of synovial fluid**, reducing friction in joints. - Hyaluronic acid helps maintain the **hydration** and **shock-absorbing capacity** of articular cartilage. *Wound healing process* - Glycosaminoglycans, especially **hyaluronic acid** and **heparin sulfate**, are crucial in **wound healing** processes, where they modulate inflammation, cell migration, and tissue remodeling. - They provide a **scaffold for cell proliferation** and differentiation in the wound bed. *Anticoagulant activity* - **Heparin**, a highly sulfated glycosaminoglycan, is a potent **anticoagulant** that works by activating **antithrombin III**, thereby inhibiting various coagulation factors like thrombin. - Other GAGs, like **heparan sulfate** found on cell surfaces, also exhibit mild anticoagulant properties.

Q: Which of the following is not a substrate for gluconeogenesis?

Leucine. ***Leucine*** - **Leucine** is an exclusively **ketogenic amino acid**, meaning its breakdown products can only be converted into **ketone bodies** or fatty acids, not glucose. - It does not have a carbon skeleton that can be directly converted into **pyruvate** or **oxaloacetate**, which are key intermediates in gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise or fasting. - It is converted to **pyruvate** by **lactate dehydrogenase**, and pyruvate can then enter the gluconeogenic pathway. *Propionate* - **Propionate** is a fatty acid with an odd number of carbon atoms, primarily derived from the catabolism of odd-chain fatty acids or from bacterial fermentation in the colon. - It can be converted into **succinyl CoA**, an intermediate of the citric acid cycle, which can then be used for gluconeogenesis. *Glycerol* - **Glycerol**, released during the breakdown of triglycerides, is an important substrate for gluconeogenesis. - It is phosphorylated to **glycerol-3-phosphate**, which is then oxidized to **dihydroxyacetone phosphate (DHAP)**, an intermediate in glycolysis and gluconeogenesis.

Q: What is the respiratory quotient of carbohydrates?

1. ***Option: 1 (Correct Answer)*** - The **respiratory quotient (RQ)** is the ratio of **carbon dioxide produced to oxygen consumed** during metabolism. - For carbohydrates, complete oxidation yields equal moles of CO2 and O2, resulting in an **RQ of 1.0**. - Example: C6H12O6 + 6O2 → 6CO2 + 6H2O, giving RQ = 6CO2/6O2 = 1.0 - This value reflects that carbohydrates are highly oxygenated molecules, requiring less external oxygen for their oxidation relative to the CO2 produced. *Option: 0.5* - An RQ of 0.5 is not observed for any major macronutrient during complete oxidation. - This value would imply significantly lower CO2 production relative to O2 consumption, which doesn't match any physiological substrate metabolism. *Option: 0.8* - An RQ of approximately 0.8 is characteristic of a **mixed diet** or the average value sometimes cited for **protein metabolism**. - Protein RQ typically ranges from 0.8-0.85, as proteins require more oxygen for their oxidation compared to the CO2 produced. - The exact RQ can vary depending on the specific amino acids being metabolized. *Option: 0.75* - An RQ around 0.7-0.75 may represent **fat-predominant metabolism** or a mixed diet with fats and carbohydrates. - Pure **fat metabolism** has an RQ of approximately **0.7**, as fats require substantial oxygen for oxidation due to their lower oxygen content relative to carbon and hydrogen. - Fats contain many C-H bonds and few C-O bonds, necessitating more oxygen for complete combustion.

Q: Which of the following statements about NADP is correct?

Involved in HMP shunt. ***Involved in HMP shunt*** - **NADPH**, the reduced form of NADP+, is primarily generated in the **hexose monophosphate shunt (HMP shunt)**, specifically during the oxidative phase. - The NADPH produced in the HMP shunt is crucial for **reductive biosynthesis** reactions and maintaining the **redox balance** of the cell. *Acts as a coenzyme form of Riboflavin* - **NADP is derived from Niacin (Vitamin B3)**, not Riboflavin (Vitamin B2). - **Flavin adenine dinucleotide (FAD)** and **flavin mononucleotide (FMN)** are the coenzyme forms of Riboflavin. *Involved in glycolysis* - **NADP is not directly involved in glycolysis**; instead, **NAD+** is the primary coenzyme that accepts electrons in glycolysis, specifically during the oxidation of glyceraldehyde-3-phosphate. - While some enzymes in glycolysis can interact with NADP+ under specific conditions, its main role is not within the glycolytic pathway. *Involved in fatty acid oxidation* - **Fatty acid oxidation (beta-oxidation)** primarily utilizes **NAD+** and **FAD** as electron acceptors. - **NADP+** is not a direct participant in the electron transport chain during fatty acid breakdown.

Q: Enzyme deficient in Hers disease -

Liver phosphorylase. ***Liver phosphorylase*** - Hers disease, also known as Glycogen Storage Disease Type VI, is specifically caused by a deficiency of **liver phosphorylase**. - This enzyme is crucial for the breakdown of **glycogen in the liver**, leading to an inability to release glucose into the bloodstream during fasting. *Muscle phosphorylase* - Deficiency of **muscle phosphorylase** (myophosphorylase) causes **McArdle disease** (Glycogen Storage Disease Type V), which primarily affects muscle energy. - Patients typically present with exercise intolerance, muscle pain, and cramps, not the hepatic symptoms seen in Hers disease. *Acid maltase* - Deficiency of **acid maltase** (also known as alpha-glucosidase) is responsible for **Pompe disease** (Glycogen Storage Disease Type II), a lysosomal storage disorder. - This enzyme deficiency leads to glycogen accumulation in lysosomes in various tissues, including muscle, liver, and heart, causing muscle weakness and cardiomyopathy. *Debranching enzyme* - A deficiency in the **debranching enzyme** (amylo-1,6-glucosidase) causes **Cori disease** or **Forbes disease** (Glycogen Storage Disease Type III). - This results in the accumulation of abnormally structured glycogen with short outer branches in the liver, muscle, and heart.

Q: Most important carbohydrate store for maintaining blood glucose homeostasis -

Hepatic glycogen. ***Hepatic glycogen*** - The liver contains **100-120g of glycogen**, which is the most crucial carbohydrate store for **maintaining blood glucose homeostasis**. - **Hepatic glycogen** can be mobilized and released as glucose into the bloodstream to supply all body tissues, especially during fasting. - Although muscle glycogen is quantitatively larger (~400-500g), it cannot contribute to blood glucose as muscle lacks glucose-6-phosphatase. - The liver's unique ability to release free glucose makes hepatic glycogen the **most metabolically important** carbohydrate store. *Blood glucose* - **Blood glucose** (~5g total in circulation) represents carbohydrates available for immediate energy, not a storage form. - This is far too small to be considered a major carbohydrate reserve. *Glycogen in adipose tissue* - **Adipose tissue** primarily stores **fat (triglycerides)**, with negligible glycogen content. - Adipose tissue plays virtually no role in carbohydrate storage. *None of the options* - This is incorrect because **hepatic glycogen** is indeed the most important carbohydrate store for glucose homeostasis.

Q: Which of the following processes primarily utilizes lactate produced anaerobically?

Cori cycle. ***Cori cycle*** - The **Cori cycle** (lactic acid cycle) involves the transport of **lactate** produced during anaerobic metabolism in muscles to the liver. - In the **liver**, this lactate is then converted back to **glucose** via gluconeogenesis, which can be returned to the muscles. *Gluconeogenesis* - **Gluconeogenesis** is the synthesis of glucose from non-carbohydrate precursors, one of which is lactate. - While it uses lactate, it is only one component of the broader **Cori cycle**, which describes the inter-organ cooperation. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down glucose into pyruvate, which can then be converted to lactate under anaerobic conditions. - This process *produces* lactate but does not *utilize* it, acting upstream of lactate production. *TCA cycle* - The **TCA cycle** (Krebs cycle) is a central part of aerobic respiration that oxidizes acetyl-CoA to produce ATP, NADH, and FADH2. - It does not directly utilize lactate; instead, lactate is typically converted to pyruvate before potentially entering the TCA cycle under aerobic conditions.

Q: Which of the following statements about glycolysis is incorrect?

Two carbon end product is formed. ***Two carbon end product is formed*** - Glycolysis breaks down one molecule of **glucose (a 6-carbon sugar)** into two molecules of **pyruvate**, which is a **3-carbon compound**. - Therefore, the end product of glycolysis is a **3-carbon molecule**, not a 2-carbon molecule. *Provide nutrition to cancer cells* - Many cancer cells exhibit increased rates of glycolysis, even in the presence of oxygen, a phenomenon known as the **Warburg effect**. - This increased glycolysis provides necessary **ATP and metabolic intermediates** for rapid cell proliferation. *Substrate level phosphorylation at pyruvate kinase* - **Pyruvate kinase** catalyzes the transfer of a phosphate group from **phosphoenolpyruvate (PEP)** to ADP, forming ATP and pyruvate. - This is a classic example of **substrate-level phosphorylation** within glycolysis. *NADPH is formed by glyceraldhyde-3-phosphate dehydrogenase* - During the oxidation of **glyceraldehyde-3-phosphate** to **1,3-bisphosphoglycerate** by glyceraldehyde-3-phosphate dehydrogenase, **NAD+ is reduced to NADH**, not NADPH. - **NADPH** is primarily generated in the **pentose phosphate pathway** and is used for reductive biosynthesis, while NADH is used in the electron transport chain for ATP production.

Q: What is the main enzyme involved in glycogen breakdown (glycogenolysis)?

Glycogen phosphorylase. ***Glycogen phosphorylase*** - This is the **rate-limiting and primary enzyme** for **glycogenolysis**, the breakdown of glycogen into glucose units. - It cleaves **α-1,4-glycosidic bonds** in glycogen, releasing **glucose-1-phosphate** units. - Regulated by both **allosteric mechanisms** and **hormonal control** (epinephrine, glucagon). - Works until it reaches 4 glucose residues from a branch point, where debranching enzyme takes over. *Glycogen synthase* - This is the main enzyme for **glycogenesis** (glycogen synthesis), not breakdown. - It catalyzes formation of α-1,4-glycosidic bonds to build glycogen chains. - This is the opposite direction of metabolism from what the question asks about. *Glucose-6-phosphatase* - This enzyme is involved in **gluconeogenesis** and the final step of converting **glucose-6-phosphate to free glucose**. - It is NOT directly involved in glycogen breakdown itself, but rather in the subsequent conversion pathway. - Found primarily in **liver and kidney** to release free glucose into blood. *Hexokinase* - This enzyme phosphorylates free glucose to **glucose-6-phosphate** (opposite direction). - It is involved in **glucose utilization**, not glycogen breakdown. - It traps glucose inside cells for metabolism or glycogen synthesis.

Q: Which metabolic pathway provides instant energy to muscles?

Embden-Meyerhof pathway. ***Embden-Meyerhof pathway*** - This pathway, also known as **glycolysis**, rapidly breaks down glucose into pyruvate to produce **ATP without oxygen**, providing instant energy to muscles during high-intensity activity. - Generates a net of **two ATP molecules** per glucose molecule, which is crucial for quick bursts of energy. *HMP shunt* - The **hexose monophosphate shunt** primarily produces **NADPH** for reductive biosynthesis and **ribose-5-phosphate** for nucleotide synthesis, not immediate large-scale ATP for muscle contraction. - Plays a role in protecting cells from **oxidative stress** and synthesizing precursors for DNA and RNA. *Cori cycle* - The **Cori cycle** involves the recycling of **lactate** produced in muscles back to glucose in the liver, which is a slower process for maintaining glucose homeostasis rather than providing instant muscle energy. - It helps prevent **lactic acidosis** during strenuous activity but is not a primary pathway for rapid ATP generation. *TCA cycle* - The **TCA cycle (Krebs cycle)** is part of **aerobic respiration** and produces a significant amount of ATP, but it is a slower, more sustained energy production pathway that requires oxygen. - Primarily active during **lower-intensity**, longer-duration activities when oxygen supply is adequate.

Question 1

Which of the following is NOT a function of glycosaminoglycans?

Accepted Answer

Transport of lipids in the bloodstream

Answer

Lubrication of joints

Answer

Wound healing process

Answer

Anticoagulant activity

Question 2

Which of the following is not a substrate for gluconeogenesis?

Accepted Answer

Leucine

Answer

Lactate

Answer

Propionate

Answer

Glycerol

Question 3

What is the respiratory quotient of carbohydrates?

Accepted Answer

1

Answer

0.5

Answer

0.8

Answer

0.75

Question 4

Which of the following statements about NADP is correct?

Accepted Answer

Involved in HMP shunt

Answer

Involved in fatty acid oxidation

Answer

Involved in glycolysis

Answer

Acts as a coenzyme form of Riboflavin

Question 5

Enzyme deficient in Hers disease -

Accepted Answer

Liver phosphorylase

Answer

Muscle phosphorylase

Answer

Acid maltase

Answer

Debranching enzyme

Question 6

Most important carbohydrate store for maintaining blood glucose homeostasis -

Accepted Answer

Hepatic glycogen

Answer

Blood glucose

Answer

Glycogen in adipose tissue

Answer

None of the options

Question 7

Which of the following processes primarily utilizes lactate produced anaerobically?

Accepted Answer

Cori cycle

Answer

Gluconeogenesis

Answer

TCA cycle

Answer

Glycolysis

Question 8

Which of the following statements about glycolysis is incorrect?

Accepted Answer

Two carbon end product is formed

Answer

Provide nutrition to cancer cells

Answer

Substrate level phosphorylation at pyruvate kinase

Answer

NADPH is formed by glyceraldehyde-3-phosphate dehydrogenase

Question 9

What is the main enzyme involved in glycogen breakdown (glycogenolysis)?

Accepted Answer

Glycogen phosphorylase

Answer

Glycogen synthase

Answer

Glucose-6-phosphatase

Answer

Hexokinase

Question 10

Which metabolic pathway provides instant energy to muscles?

Accepted Answer

Embden-Meyerhof pathway

Answer

HMP shunt

Answer

Cori cycle

Answer

TCA cycle

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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