Carbohydrate Metabolism Practice Questions

Q: Gluconeogenesis in the fasting state is indicated by:

Pyruvate Carboxylase activation by Acetyl CoA. ***Pyruvate Carboxylase activation by Acetyl CoA*** - In the **fasting state**, Acetyl CoA activates **pyruvate carboxylase**, shunting pyruvate away from glycolysis towards gluconeogenesis. - This activation is crucial for the synthesis of **oxaloacetate** from pyruvate, a key step in glucose production. *Citrate activation by acetyl-CoA carboxylase* - **Citrate** is an allosteric activator of **acetyl-CoA carboxylase**, which is involved in **fatty acid synthesis**, not gluconeogenesis. - This enzyme's activity is favored during states of **energy surplus**, not fasting. *Fructose 1,6 bisphosphate activates Pyruvate Kinase* - **Fructose 1,6-bisphosphate** activates **pyruvate kinase** as part of **glycolysis**, promoting glucose breakdown. - This regulatory mechanism operates during **fed states** to increase glucose utilization, which is opposite to the fasting state. *Fructose 2,6 bisphosphate activates PFK-1* - **Fructose 2,6-bisphosphate** activates **phosphofructokinase-1 (PFK-1)**, a key regulatory enzyme in **glycolysis**. - High levels of fructose 2,6-bisphosphate promote **glucose breakdown**, which is inhibited during the fasting state to conserve glucose.

Q: Which glycosaminoglycan is present in the cornea?

Keratan sulfate. ***Keratan sulfate*** - **Keratan sulfate** is a prominent **glycosaminoglycan (GAG)** found in the corneal stroma, playing a crucial role in maintaining corneal transparency. - Its unique structure and interaction with collagen fibrils contribute to the precise spacing and organization of the **corneal lamellae**. *Dermatan sulfate* - **Dermatan sulfate** is primarily found in the skin, blood vessels, and heart valves, but it is not a major GAG component of the cornea. - It is involved in regulating collagen fibril assembly and stability in these specific tissues. *Chondroitin sulfate* - **Chondroitin sulfate** is abundant in cartilage, bone, and connective tissues, providing resistance to compression. - While trace amounts might be present, it is not the primary GAG responsible for the unique properties of the cornea. *Hyaluronic acid* - **Hyaluronic acid**, a non-sulfated GAG, is found in the vitreous humor and synovial fluid, contributing to lubrication and hydration. - It is generally absent or present in very small quantities in the adult cornea under normal physiological conditions.

Q: Facilitative glucose transporters (GLUTs) comprise a large family. Which of the following is a glucose transporter in myocytes?

GLUT 4. ***GLUT 4*** - **GLUT 4** is the primary **insulin-dependent glucose transporter** found in **adipose tissue** and **striated muscle (skeletal and cardiac muscle)**, including myocytes. - Its translocation to the cell surface is stimulated by **insulin** and muscle contraction, facilitating glucose uptake into these cells. *GLUT 1* - **GLUT 1** is ubiquitously expressed in nearly all mammalian cells, providing a **basal level of glucose uptake**. - It is particularly abundant in **red blood cells** and the **blood-brain barrier**. *GLUT 2* - **GLUT 2** is a **low-affinity, high-capacity glucose transporter** found in the **liver, pancreatic beta cells, kidney, and small intestine**. - It plays a crucial role in **glucose sensing** and the release of insulin. *GLUT 3* - **GLUT 3** is primarily found in **neurons** and the **placenta**, where it is responsible for **high-affinity glucose uptake**. - It ensures a constant supply of glucose to these tissues, even at low blood glucose concentrations.

Q: Both the liver and muscle contain glycogen, yet, unlike the liver, muscle is not capable of contributing glucose to the circulation. What is the reason for this?

Does not have the enzyme glucose-6-phosphatase. ***Does not have the enzyme glucose-6-phosphatase*** - **Glucose-6-phosphatase** is the enzyme responsible for dephosphorylating **glucose-6-phosphate** to glucose, allowing it to exit the cell and enter the bloodstream. - Since muscle cells lack this enzyme, the glucose-6-phosphate produced from glycogenolysis is trapped within the muscle cell and used for its own energy needs. *Glycolytic activity consumes all of the glucose it generates, preventing release into circulation.* - While muscle does utilize the glucose it generates for its own energy via glycolysis, the fundamental reason for trapping glucose within the cell is the absence of **glucose-6-phosphatase**, not just the consumption itself. - If **glucose-6-phosphatase** were present, the muscle could still release glucose even if some was used for glycolysis, especially under conditions of high glycogenolysis. *Does not have the enzyme glucose-1-phosphatase.* - **Glucose-1-phosphatase** is not a commonly recognized enzyme in glucose metabolism; the conversion between glucose-1-phosphate and glucose-6-phosphate is catalyzed by **phosphoglucomutase**. - Therefore, the absence of an enzyme with this specific name is not the reason muscle cannot release glucose into circulation. *Does not have the enzyme glycogen phosphorylase.* - Muscle tissue readily expresses **glycogen phosphorylase**, which is the enzyme responsible for breaking down glycogen into **glucose-1-phosphate** during glycogenolysis. - If muscle lacked **glycogen phosphorylase**, it would not be able to break down glycogen at all, which is contrary to its role as an energy reserve.

Q: Source of ATP in RBCs is

Glucose. ***Glucose*** - Red blood cells (RBCs) lack mitochondria, so they cannot perform **aerobic respiration**, **fatty acid oxidation**, or utilize **ketone bodies** for ATP production. - Their sole source of ATP is through **anaerobic glycolysis**, which metabolizes glucose to produce a small amount of ATP. *Fatty acid* - Fatty acid oxidation, or **beta-oxidation**, occurs in the mitochondria. - Since mature **RBCs lack mitochondria**, they cannot metabolize fatty acids to produce ATP. *Amino acid* - Amino acid metabolism for energy primarily involves the **Krebs cycle** and **oxidative phosphorylation**, which also take place in the mitochondria. - **RBCs lack the necessary enzymatic machinery** and organelles for this process. *Ketone body* - Ketone bodies are metabolized for energy in mitochondria, particularly in tissues like the brain and muscle, through the **Krebs cycle**. - **RBCs do not have mitochondria** and thus cannot utilize ketone bodies as an energy source.

Q: What type of carbohydrate is inulin classified as?

Fructosan. ***Fructosan*** - **Inulin** is a naturally occurring **polysaccharide** composed primarily of **fructose** units. - As such, it is classified as a **fructosan**, a type of **fructan**, meaning its main monosaccharide component is fructose. *Glucosan* - A **glucosan** is a polysaccharide primarily made up of **glucose** units, such as **starch** or **glycogen**. - Inulin's monomeric units are predominantly fructose, not glucose. *Galactosan* - A **galactosan** is a polysaccharide primarily composed of **galactose** units. - Inulin does not primarily consist of galactose units. *Mannosan* - A **mannosan** is a polysaccharide primarily composed of **mannose** units. - Inulin's structure is based on fructose, not mannose.

Q: In glycolysis, which of the following enzymes is not involved?

Pyruvate dehydrogenase. ***Pyruvate dehydrogenase*** - **Pyruvate dehydrogenase** is a mitochondrial enzyme complex that converts **pyruvate** to **acetyl-CoA** in the link reaction, which occurs after glycolysis and prepares for the citric acid cycle. - It is not directly involved in the ten-step glycolytic pathway itself, which converts glucose to pyruvate. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is a key regulatory enzyme in glycolysis, catalyzing the phosphorylation of **fructose-6-phosphate** to **fructose-1,6-bisphosphate**. - This step is often considered the **rate-limiting step** of glycolysis. *Glucokinase* - **Glucokinase**, located primarily in the liver and pancreatic beta cells, phosphorylates glucose to **glucose-6-phosphate** in the first step of glycolysis. - It has a high **Km** (low affinity) for glucose, allowing it to respond to high glucose concentrations. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of glycolysis, transferring a phosphate group from **phosphoenolpyruvate (PEP)** to ADP to form **ATP** and **pyruvate**. - This is one of the **irreversible** steps in glycolysis and a point of regulation.

Q: Which of the following metabolic pathways in carbohydrate metabolism is required for the synthesis of nucleic acids?

HMP shunt. ***HMP shunt*** - The **hexose monophosphate (HMP) shunt**, also known as the **pentose phosphate pathway**, is crucial for producing **ribose-5-phosphate**. - **Ribose-5-phosphate** is a direct precursor for the synthesis of **nucleotides** and subsequently **nucleic acids** (DNA and RNA). *Gluconeogenesis* - This pathway is responsible for the synthesis of **glucose from non-carbohydrate precursors**, primarily to maintain blood glucose levels during fasting. - It does not directly produce components needed for nucleic acid synthesis. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down **glucose into pyruvate**, generating ATP. - While it produces intermediates, it is not directly involved in creating the specific pentose sugars required for nucleic acids. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen from glucose** for storage in the liver and muscles. - This pathway is focused on glucose storage and has no direct role in nucleic acid synthesis.

Question 1

Gluconeogenesis in the fasting state is indicated by:

Accepted Answer

Pyruvate Carboxylase activation by Acetyl CoA

Answer

Citrate activation by acetyl-CoA carboxylase

Answer

Fructose 1,6 bisphosphate activates Pyruvate Kinase

Answer

Fructose 2,6 bisphosphate activates PFK-1

Question 2

What happens to glucose consumed in a soft drink by a 19-year-old athlete during a track event?

Accepted Answer

Muscles will primarily use glucose for immediate energy production.

Answer

Hexokinase helps convert glucose to energy quickly during exercise.

Answer

The glucose from the soda will be stored as glycogen in the liver.

Answer

Glucose will be used by both muscles and liver.

Question 3

Which glycosaminoglycan is present in the cornea?

Accepted Answer

Keratan sulfate

Answer

Dermatan sulfate

Answer

Chondroitin sulfate

Answer

Hyaluronic acid

Question 4

Facilitative glucose transporters (GLUTs) comprise a large family. Which of the following is a glucose transporter in myocytes?

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Question 5

Both the liver and muscle contain glycogen, yet, unlike the liver, muscle is not capable of contributing glucose to the circulation. What is the reason for this?

Accepted Answer

Does not have the enzyme glucose-6-phosphatase

Answer

Glycolytic activity consumes all of the glucose it generates, preventing release into circulation.

Answer

Does not have the enzyme glucose-1-phosphatase.

Answer

Does not have the enzyme glycogen phosphorylase.

Question 6

Source of ATP in RBCs is

Accepted Answer

Glucose

Answer

Fatty acid

Answer

Amino acid

Answer

Ketone body

Question 7

What type of carbohydrate is inulin classified as?

Accepted Answer

Fructosan

Answer

Glucosan

Answer

Galactosan

Answer

Mannosan

Question 8

In glycolysis, which of the following enzymes is not involved?

Accepted Answer

Pyruvate dehydrogenase

Answer

Phosphofructokinase

Answer

Glucokinase

Answer

Pyruvate kinase

Question 9

Which of the following statements is true regarding the pentose phosphate pathway?

Accepted Answer

The pentose phosphate pathway is a direct oxidative pathway of glucose metabolism

Answer

Glucose is the only substrate that can enter this pathway

Answer

The pathway has only monophosphates as intermediates

Answer

None of the options

Question 10

Which of the following metabolic pathways in carbohydrate metabolism is required for the synthesis of nucleic acids?

Accepted Answer

HMP shunt

Answer

Gluconeogenesis

Answer

Glycolysis

Answer

Glycogenesis

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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