Carbohydrate Metabolism Practice Questions

Q: Hexokinase is inhibited by?

Glucose-6-phosphate (G6P). ***Glucose-6-phosphate (G6P)*** - Hexokinase is subject to **feedback inhibition** by its product, **glucose-6-phosphate**, preventing the accumulation of high levels of G6P inside the cell. - This regulatory mechanism ensures that glycolysis does not proceed unchecked when energy needs are met or when G6P levels are already sufficient. *Glucagon* - **Glucagon** is a hormone that generally promotes **glucose production** and release, primarily by stimulating gluconeogenesis and glycogenolysis, rather than directly inhibiting hexokinase. - Its effects on glucose metabolism are more about increasing blood glucose levels than directly regulating the initial step of glycolysis in most tissues. *Glucose* - **Glucose** is the **substrate** for hexokinase, meaning it is the molecule that hexokinase acts upon to convert it into glucose-6-phosphate. - Therefore, glucose does not inhibit hexokinase; instead, its presence is necessary for the enzyme's activity. *Insulin* - **Insulin** is a hormone that promotes **glucose uptake** and utilization by cells, often by increasing the number of glucose transporters on cell surfaces. - While insulin can indirectly influence glycolysis by increasing glucose availability, it does not directly inhibit hexokinase; rather, it generally supports cellular glucose metabolism.

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: 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: Glucagon stimulates

Gluconeogenesis. ***Gluconeogenesis*** - **Glucagon** is a hormone that primarily acts to raise **blood glucose levels** by stimulating the production of glucose from non-carbohydrate sources. - This process, **gluconeogenesis**, occurs mainly in the liver and is initiated by glucagon to counteract hypoglycemia. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen** from glucose and is primarily stimulated by insulin when blood glucose levels are high. - Glucagon's role is to *inhibit* glycogen synthesis and instead promote glycogen breakdown. *Fatty acid synthesis* - **Fatty acid synthesis** is an anabolic process that primarily occurs when there is an excess of energy and glucose, often stimulated by **insulin**. - Glucagon generally has an **inhibitory effect** on fatty acid synthesis, as its main goal is to mobilize energy stores, not create them. *Glycolysis* - **Glycolysis** is the breakdown of glucose to produce energy, and it is stimulated when glucose is abundant and energy is needed. - Glucagon primarily acts to *inhibit* glycolysis in the liver, thereby conserving glucose for use by other tissues and promoting its release into the bloodstream.

Q: Which of the following is an aldose?

Glucose. ***Glucose*** - An **aldose** is a monosaccharide containing an **aldehyde group** (—CHO) in its open-chain form. - **Glucose** possesses an aldehyde group at carbon-1 and is therefore classified as an aldose. *Fructose* - **Fructose** is a **ketose**, meaning it contains a **ketone group** (C=O) in its open-chain structure, typically at carbon-2. - While it is a monosaccharide, its functional group differentiates it from aldoses. *Erythrulose* - **Erythrulose** is a **ketotetrose**, meaning it is a four-carbon sugar with a **ketone group**. - Unlike aldoses, which have an aldehyde group, erythrulose's defining characteristic is its ketone functional group. *None of the options* - This option is incorrect because **Glucose** is indeed an aldose, fitting the definition of having an aldehyde functional group. - Therefore, there is a correct option provided among the choices.

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 transporter is responsible for the transport of glucose in the pancreas?

GLUT 2. ***GLUT 2*** - **GLUT2** is a **low-affinity, high-capacity** glucose transporter primarily found in the **pancreatic beta cells**, liver, small intestine, and kidneys. - In pancreatic beta cells, GLUT2 allows rapid entry of glucose for metabolism, leading to **insulin secretion** in response to elevated blood glucose levels. *GLUT 1* - **GLUT1** is a **ubiquitous glucose transporter** found in most tissues, including red blood cells and the blood-brain barrier. - It has a high affinity for glucose, ensuring **basal glucose uptake** even at low concentrations. *GLUT 3* - **GLUT3** is a **high-affinity glucose transporter** concentrated in **neurons** and the brain. - Its efficient glucose uptake is critical for the constant and high energy demands of the central nervous system. *GLUT 4* - **GLUT4** is an **insulin-dependent glucose transporter** primarily found in **adipose tissue** and **striated muscle (skeletal and cardiac muscle)**. - Insulin stimulates the translocation of GLUT4 to the cell membrane, facilitating glucose uptake from the blood after a meal.

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 is the major glycosaminoglycan of synovial fluid?

Hyaluronic acid. ***Hyaluronic acid*** - **Hyaluronic acid** is the primary glycosaminoglycan in **synovial fluid**, providing its characteristic **viscosity** and **lubricating properties**. - It plays a crucial role in maintaining **joint health** by reducing friction and acting as a shock absorber. *Chondroitin sulfate* - **Chondroitin sulfate** is abundant in **cartilage**, contributing to its **compressive strength**. - While present in connective tissues, it is not the major glycosaminoglycan of synovial fluid. *Dermatan sulfate* - **Dermatan sulfate** is primarily found in **skin**, **blood vessels**, and **heart valves**. - Its main roles involve tissue structure and repair, not lubrication of synovial fluid. *Heparan sulfate* - **Heparan sulfate** is found on **cell surfaces** and in the **extracellular matrix**, especially in the **basement membranes**. - It regulates cell growth, adhesion, and signaling, and is not a major component of synovial fluid viscosity.

Question 1

Hexokinase is inhibited by?

Accepted Answer

Glucose-6-phosphate (G6P)

Answer

Glucose

Answer

Insulin

Answer

Glucagon

Question 2

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 3

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 4

Fructose intolerance is due to deficiency of which enzyme?

Accepted Answer

Aldolase B

Answer

Aldolase A

Answer

Fructokinase

Answer

Triokinase

Question 5

Glucagon stimulates

Accepted Answer

Gluconeogenesis

Answer

Glycogenesis

Answer

Fatty acid synthesis

Answer

Glycolysis

Question 6

Which of the following is an aldose?

Accepted Answer

Glucose

Answer

Fructose

Answer

Erythrulose

Answer

None of the options

Question 7

The energy for glycogenesis is provided by -

Accepted Answer

UTP

Answer

GTP

Answer

GDP

Answer

AMP

Question 8

Which transporter is responsible for the transport of glucose in the pancreas?

Accepted Answer

GLUT 2

Answer

GLUT 1

Answer

GLUT 3

Answer

GLUT 4

Question 9

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 10

Which of the following is the major glycosaminoglycan of synovial fluid?

Accepted Answer

Hyaluronic acid

Answer

Chondroitin sulfate

Answer

Dermatan sulfate

Answer

Heparan sulfate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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