Carbohydrate Metabolism Practice Questions

Q: The energy for glycogenesis is provided by -

UTP. ***UTP*** - **Uridine triphosphate (UTP)** is essential for **glycogenesis** as it activates glucose by forming **UDP-glucose** from glucose-1-phosphate. - The reaction (Glucose-1-P + UTP → UDP-glucose + PPi) creates a **high-energy intermediate** that drives glycogen synthesis. - The subsequent hydrolysis of pyrophosphate (PPi) makes this activation step **irreversible**, and the energy stored in UDP-glucose is used for **glycosidic bond formation** when glucose is added to the growing glycogen chain. *GTP* - **Guanosine triphosphate (GTP)** is primarily involved in **protein synthesis**, G-protein signaling, and the citric acid cycle. - It is not used for glucose activation in glycogenesis; that role is specific to **UTP**. *GDP* - **Guanosine diphosphate (GDP)** is a product of GTP hydrolysis and functions in regulatory processes. - It does not serve as an energy donor for glycogen synthesis. *AMP* - **Adenosine monophosphate (AMP)** is a low-energy signal molecule that indicates cellular energy depletion. - High AMP levels **inhibit glycogenesis** and activate glycogenolysis through allosteric regulation of key enzymes. - It does not provide energy for anabolic pathways like 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.

Q: Which of the following tests is most commonly used to detect glucose in urine?

c) Glucose-oxidase test. ***Glucose-oxidase test*** - The **glucose-oxidase test** is a highly specific and sensitive enzymatic test used to detect **glucose** in urine. - It uses the enzyme glucose oxidase which specifically catalyzes the oxidation of glucose to gluconic acid and hydrogen peroxide, which then produces a color change. - This is the **most commonly used method** in modern clinical practice for detecting glucosuria due to its **high specificity for glucose** and ease of use (dipstick method). - It is the preferred test for **monitoring diabetes** and screening for hyperglycemia. *Benedict's test* - **Benedict's test** is a general chemical test for **all reducing sugars** (glucose, fructose, galactose, lactose, maltose), not specifically glucose. - It works by reducing copper sulfate (Cu²⁺) to copper oxide (Cu⁺) in an alkaline solution, forming a colored precipitate (green, yellow, orange, or brick-red depending on sugar concentration). - While it can detect glucose, it **lacks specificity** and can give false positives with other reducing substances (vitamin C, certain drugs), making it less suitable for routine clinical testing. *Fehling's solution* - **Fehling's solution** is also a general chemical test for **reducing sugars** based on copper reduction, similar to Benedict's test. - It consists of two solutions mixed before use and detects various reducing sugars, not just glucose. - It is **not commonly used in clinical urine analysis** due to lack of specificity and the need for heating and mixing two solutions, making it impractical compared to the simple glucose-oxidase dipstick. *None of the above* - This option is incorrect because the **glucose-oxidase test** is indeed the most commonly used test for detecting glucose in urine in modern clinical practice.

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

Occurs mainly in the liver. ***Occurs mainly in the liver*** - The **liver** is the primary site for **gluconeogenesis**, responsible for maintaining blood glucose levels during fasting. - The kidneys also contribute, especially during prolonged fasting, but to a lesser extent. *It uses exactly the same enzymes as glycolysis in reverse* - While gluconeogenesis shares some enzymes with glycolysis, there are **three irreversible steps in glycolysis** that require different enzymes in gluconeogenesis to bypass them. - Key bypass enzymes include **pyruvate carboxylase**, **phosphoenolpyruvate carboxykinase (PEPCK)**, **fructose-1,6-bisphosphatase**, and **glucose-6-phosphatase**. *It only occurs during fed state when insulin levels are high* - **Gluconeogenesis is activated during fasting or starvation** when blood glucose levels are low, and it is largely **inhibited by high insulin levels**. - Its purpose is to produce new glucose to prevent hypoglycemia, not to store excess glucose. *Fatty acids are the primary substrate for gluconeogenesis* - **Fatty acids cannot be directly converted to glucose** in significant amounts in humans because they are broken down into acetyl-CoA, which cannot be used for net glucose synthesis. - Primary substrates include **lactate**, **amino acids** (from protein breakdown), and **glycerol** (from triglyceride breakdown).

Q: Which of the following statements about GLUT 2 transporters is correct?

Insulin independent. ***Insulin independent*** - GLUT2 transporters facilitate glucose transport into cells **regardless of insulin levels**, making them crucial for basal glucose sensing and transport functions. - This **insulin independence** is vital for organs like the liver and pancreatic beta cells to respond to varying glucose concentrations. *Insulin dependent* - **Insulin-dependent** transporters, such as **GLUT4**, respond to insulin by relocating to the cell membrane to increase glucose uptake. - This characteristic applies to tissues like **skeletal muscle** and **adipose tissue**, not GLUT2. *Found in cardiac muscle* - **Cardiac muscle** primarily utilizes **GLUT4** for glucose uptake, which is insulin-dependent. - While other GLUT transporters might be present in cardiac tissue, **GLUT2** is not the primary mechanism for glucose transport here. *Found in brain* - The **brain** predominantly uses **GLUT1** and **GLUT3** for glucose transport, which have **high affinity** for glucose to ensure constant supply. - **GLUT2** is not a primary transporter of glucose in the brain.

Q: Gluconeogenesis occurs in all except:

Muscle. ***Muscle*** - **Muscle tissue** lacks the enzyme **glucose-6-phosphatase**, which is essential for releasing free glucose into the bloodstream during gluconeogenesis. - While muscle can store glycogen, it primarily uses glucose for its own energy needs and does not contribute significantly to systemic glucose homeostasis through gluconeogenesis. *Liver* - The **liver** is the primary site of **gluconeogenesis**, producing glucose to maintain blood glucose levels during fasting and starvation. - It contains all the necessary enzymes, including **glucose-6-phosphatase**, to convert precursors like lactate, amino acids, and glycerol into glucose. *Kidney* - The **kidney** becomes a significant site of **gluconeogenesis** during prolonged fasting, contributing up to 10-20% of the body's glucose production. - Renal gluconeogenesis primarily utilizes **lactate** and **glutamine** as substrates. *Gut* - The **small intestine (gut)** has been identified as a site of **gluconeogenesis**, particularly following a meal rich in protein. - Its contribution is relatively smaller compared to the liver but plays a role in **postprandial glucose homeostasis**.

Q: Inhibition of glycolysis by increased supply of O2 is called ?

Pasteur effect. ***Pasteur effect*** - The **Pasteur effect** describes the phenomenon where the rate of **glycolysis** is inhibited when **oxygen** is available (aerobic conditions). - This inhibition occurs because **oxidative phosphorylation** is more efficient at generating ATP, leading to reduced reliance on glycolysis for energy production. *Crabtree phenomenon* - The **Crabtree phenomenon** is the opposite of the Pasteur effect, where high concentrations of **glucose** inhibit oxygen consumption in the presence of oxygen. - This is primarily observed in some **cancer cells** and yeast, leading to increased glycolysis even under aerobic conditions. *Lewis phenomenon* - The **Lewis phenomenon** (also known as the hunting reaction) refers to the cyclical vasodilation and constriction of peripheral blood vessels in response to **cold exposure**. - It's a physiological response to protect tissues from **frostbite** and is not related to glycolysis or oxygen supply. *None of the options* - This option is incorrect as the phenomenon described, inhibition of glycolysis by increased O2, is a well-established biochemical process known as the **Pasteur effect**.

Q: Fructose intolerance is due to deficiency of which enzyme?

Aldolase B. ***Aldolase B*** - **Hereditary fructose intolerance** is a genetic disorder caused by a deficiency in the enzyme **aldolase B**. - This deficiency leads to an accumulation of **fructose-1-phosphate** in the liver, kidneys, and small intestine, causing **hypoglycemia**, **vomiting**, and **liver damage** upon exposure to fructose. *Fructokinase* - A deficiency in **fructokinase** causes **essential fructosuria**, a benign metabolic disorder. - This condition is asymptomatic because **fructose** simply accumulates in the blood and urine without causing significant clinical problems. *Triokinase* - **Triokinase**, also known as **glycerol kinase**, is involved in glycerol metabolism, converting glycerol to **glycerol-3-phosphate**. - Its deficiency is not directly linked to fructose intolerance and typically presents with **hyperglycerolemia**. *Aldolase A* - **Aldolase A** is one of the three aldolase isoenzymes (A, B, and C) and is primarily involved in **glycolysis**, specifically in the breakdown of **fructose-1,6-bisphosphate**. - A deficiency in aldolase A can lead to **hemolytic anemia** and **myopathy**, not directly fructose intolerance.

Q: What is the maximum value on the glycemic index scale that classifies a food as low glycemic index?

55. ***55*** - A food is classified as having a **low glycemic index (GI)** if its GI value is **55 or less**. - The GI scale classifies foods as: **low GI (≤55), medium GI (56-69), and high GI (≥70)**. - This classification indicates that the food causes a slower and lower rise in blood glucose levels compared to high or medium GI foods. *25* - This value is well below the threshold for a low GI food and is not the maximum value for this classification. - While a food with a GI of 25 would indeed be considered low GI, the question asks for the **maximum value** that still falls within this category. *45* - This value is still within the low GI range, but it is not the maximum value for this classification. - Foods with a GI up to 55 are considered low GI. *65* - A GI value of 65 falls into the **medium glycemic index** category (GI 56-69). - Therefore, this value classifies a food as medium GI, not low GI.

Question 1

The energy for glycogenesis is provided by -

Accepted Answer

UTP

Answer

GTP

Answer

GDP

Answer

AMP

Question 2

Which metabolic pathway provides instant energy to muscles?

Accepted Answer

Embden-Meyerhof pathway

Answer

HMP shunt

Answer

Cori cycle

Answer

TCA cycle

Question 3

Which of the following tests is most commonly used to detect glucose in urine?

Accepted Answer

c) Glucose-oxidase test

Answer

a) Benedicts test

Answer

b) Fehling solution

Answer

d) None of the above

Question 4

Which of the following statements about gluconeogenesis is correct?

Accepted Answer

Occurs mainly in the liver

Answer

It uses exactly the same enzymes as glycolysis in reverse

Answer

It only occurs during fed state when insulin levels are high

Answer

Fatty acids are the primary substrate for gluconeogenesis

Question 5

Which of the following statements about GLUT 2 transporters is correct?

Accepted Answer

Insulin independent

Answer

Insulin dependent

Answer

Found in cardiac muscle

Answer

Found in brain

Question 6

Gluconeogenesis occurs in all except:

Accepted Answer

Muscle

Answer

Kidney

Answer

Gut

Answer

Liver

Question 7

Inhibition of glycolysis by increased supply of O2 is called ?

Accepted Answer

Pasteur effect

Answer

Crabtree phenomenon

Answer

Lewis phenomenon

Answer

None of the options

Question 8

Fructose intolerance is due to deficiency of which enzyme?

Accepted Answer

Aldolase B

Answer

Aldolase A

Answer

Fructokinase

Answer

Triokinase

Question 9

What is the maximum value on the glycemic index scale that classifies a food as low glycemic index?

Accepted Answer

55

Answer

25

Answer

45

Answer

65

Question 10

Which one of the following statements concerning gluconeogenesis is correct?

Accepted Answer

It is important in maintaining blood glucose during the normal overnight fast.

Answer

It occurs primarily in the liver.

Answer

It is stimulated by elevated levels of acetyl CoA.

Answer

It is primarily inhibited by insulin.

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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