Carbohydrate Metabolism Practice Questions

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: 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 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: 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: 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: 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: Which of the following tissues relies EXCLUSIVELY on anaerobic glycolysis for ATP production?

Mature RBCs (exclusively anaerobic). ***Mature RBCs (exclusively anaerobic)*** - **Mature red blood cells** lack mitochondria, making them incapable of **oxidative phosphorylation** and thus relying entirely on **anaerobic glycolysis** for ATP. - This pathway produces **2 net ATP** molecules per glucose molecule, which is sufficient for their metabolic needs like maintaining ion gradients. *Skeletal muscle during exercise (anaerobic)* - While skeletal muscle can perform **anaerobic glycolysis** during intense exercise when oxygen supply is limited, it is not an exclusive reliance. - Skeletal muscle also utilizes **aerobic respiration** and **creatine phosphate** for ATP production depending on activity level and oxygen availability. *Cardiac muscle (primarily aerobic)* - **Cardiac muscle** has a very high metabolic demand and is rich in **mitochondria**, relying almost exclusively on **aerobic respiration** for ATP production. - It uses fatty acids, glucose, and lactate as fuel sources, producing a large amount of ATP efficiently with oxygen. *Liver hepatocytes (primarily aerobic)* - **Liver hepatocytes** are highly metabolically active and primarily rely on **aerobic respiration** for ATP production, performing diverse functions such as gluconeogenesis, glycogenolysis, and detoxification. - Although the liver can perform some anaerobic glycolysis under hypoxic conditions, it is not its exclusive or primary mode of ATP synthesis.

Q: Which of the following is NOT required for gluconeogenesis from lactate?

Transamination of pyruvate to alanine. ***Transamination of pyruvate to alanine*** - While **alanine** can be a substrate for gluconeogenesis, **lactate** is directly converted to pyruvate, which then enters the gluconeogenesis pathway. **Transamination to alanine** is not a required intermediate step for lactate-derived glucose production. - The direct conversion of **lactate to pyruvate** by **lactate dehydrogenase** is the key initial step, not its conversion to alanine. *Transport of lactate from muscle to liver* - **Lactate** produced in muscles (e.g., during intense exercise) must be transported to the **liver** via the bloodstream to be used for **gluconeogenesis** in the **Cori cycle**. - This transport is essential for clearing lactate from the periphery and supplying the liver with a gluconeogenic precursor. *Conversion of lactate to pyruvate* - **Lactate dehydrogenase** catalyzes the reversible conversion of **lactate to pyruvate**, which is the critical first step in converting lactate into a gluconeogenic substrate. - This reaction regenerates **NAD+** (not NADH), which is necessary for glycolysis to continue in muscle tissue. *None of the above* - This option is incorrect because there IS a step listed above that is not required: **transamination of pyruvate to alanine** is indeed not necessary for gluconeogenesis from lactate, making Option A the correct answer to this "NOT required" question.

Q: Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?

Fructose 2,6-bisphosphate. ***Fructose 2,6-bisphosphate*** - **Fructose 2,6-bisphosphate** is a potent **allosteric activator** of **phosphofructokinase-1 (PFK-1)**, increasing its affinity for fructose 6-phosphate and overcoming ATP inhibition. - Its synthesis is regulated by **insulin** (stimulating) and **glucagon** (inhibiting), linking glucose availability to glycolytic flux. *2,3-Bisphosphoglycerate (2,3-BPG)* - **2,3-BPG** is an important regulator of **hemoglobin oxygen affinity** in red blood cells. - It is not an allosteric activator of **PFK-1**; its primary role is in oxygen delivery. *Glucokinase* - **Glucokinase** is an **enzyme** in glycolysis, specifically catalyzing the phosphorylation of glucose to glucose 6-phosphate in the liver and pancreatic beta cells. - It is not an allosteric activator of **PFK-1** but rather an upstream enzyme in the pathway. *Phosphoenolpyruvate (PEP)* - **PEP** is an intermediate in glycolysis, formed from 2-phosphoglycerate and converted to pyruvate by pyruvate kinase. - It acts as an **allosteric inhibitor** of phosphofructokinase-1, signaling high energy status and slowing down glycolysis.

Question 1

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 2

Which of the following processes primarily utilizes lactate produced anaerobically?

Accepted Answer

Cori cycle

Answer

Gluconeogenesis

Answer

TCA cycle

Answer

Glycolysis

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 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 5

Enzyme deficient in Hers disease -

Accepted Answer

Liver phosphorylase

Answer

Muscle phosphorylase

Answer

Acid maltase

Answer

Debranching enzyme

Question 6

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 7

Which of the following is not a substrate for gluconeogenesis?

Accepted Answer

Leucine

Answer

Lactate

Answer

Propionate

Answer

Glycerol

Question 8

Which of the following tissues relies EXCLUSIVELY on anaerobic glycolysis for ATP production?

Accepted Answer

Mature RBCs (exclusively anaerobic)

Answer

Skeletal muscle during exercise (anaerobic)

Answer

Liver hepatocytes (primarily aerobic)

Answer

Cardiac muscle (primarily aerobic)

Question 9

Which of the following is NOT required for gluconeogenesis from lactate?

Accepted Answer

Transamination of pyruvate to alanine

Answer

Transport of lactate from muscle to liver

Answer

Conversion of lactate to pyruvate

Answer

None of the above

Question 10

Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?

Accepted Answer

Fructose 2,6-bisphosphate

Answer

2,3-Bisphosphoglycerate (2,3-BPG)

Answer

Glucokinase

Answer

Phosphoenolpyruvate (PEP)

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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