NEET-PG 2015

1414 Questions — Page 32 of 142

Biochemistry

9 questions

Q311

What is the respiratory quotient of carbohydrates?

Q312

Transport of lipids from the intestine to other tissues is by -

Q313

Energy source used by brain in later days of starvation is

Q314

Which method is used to separate a mixture of lipids?

Q315

Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?

Q316

Bile acids are synthesized from ?

Q317

Pyridoxine is required in -

Q318

Most abundant source of fuel in starvation -

Q319

Which fat-soluble vitamin is most classically known for its steroid hormone-like action through nuclear receptors?

NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs

Question 311: What is the respiratory quotient of carbohydrates?

A. 0.5
B. 0.8
C. 0.75
D. 1 (Correct Answer)

Explanation: ***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.

Question 312: Transport of lipids from the intestine to other tissues is by -

A. Chylomicrons (Correct Answer)
B. LDL
C. HDL
D. VLDL

Explanation: ***Chylomicrons*** - **Chylomicrons** are the **largest lipoprotein particles** that transport **dietary (exogenous) lipids** from the **intestine** to peripheral tissues - They are synthesized in **intestinal enterocytes** after fat absorption and enter the bloodstream via the **lymphatic system (thoracic duct)** - They carry **triglycerides (85-95%), cholesterol, phospholipids, and fat-soluble vitamins** (A, D, E, K) - **Apolipoprotein B-48** is the characteristic structural protein of chylomicrons - After delivering triglycerides to tissues (via lipoprotein lipase), chylomicron remnants are taken up by the **liver** *LDL (Low-Density Lipoprotein)* - LDL transports **cholesterol from the liver to peripheral tissues** (not from intestine) - It carries **endogenous cholesterol**, not dietary lipids from the intestine - Often called "**bad cholesterol**" due to its role in atherosclerosis - Contains **Apolipoprotein B-100** *HDL (High-Density Lipoprotein)* - HDL performs **reverse cholesterol transport** - moving excess cholesterol from peripheral tissues **back to the liver** - It does **not transport lipids from the intestine** to tissues - Called "**good cholesterol**" for its protective cardiovascular role - Contains **Apolipoprotein A-I and A-II** *VLDL (Very-Low-Density Lipoprotein)* - VLDL is synthesized in the **liver** (not intestine) and transports **endogenous triglycerides** to peripheral tissues - It carries lipids **from the liver**, not from the intestine - VLDL is converted to IDL and then LDL after losing triglycerides - Contains **Apolipoprotein B-100**

Question 313: Energy source used by brain in later days of starvation is

A. Glucose
B. Ketone bodies (Correct Answer)
C. Glycogen
D. Fatty acids

Explanation: ***Ketone bodies*** - During **prolonged starvation**, the liver produces **ketone bodies** (acetoacetate and β-hydroxybutyrate) from fatty acid breakdown. - The brain adapts to utilize these ketone bodies as a primary energy source, reducing its reliance on **glucose**. *Glucose* - While **glucose** is the primary energy source for the brain under normal conditions, its availability diminishes significantly during prolonged starvation. - The brain attempts to conserve glucose for essential functions by switching to alternative fuels. *Glycogen* - The brain stores very limited amounts of **glycogen**, which are rapidly depleted within minutes of glucose deprivation. - It is not a sustainable or significant energy source during extended periods of starvation. *Fatty acids* - **Fatty acids** cannot directly cross the **blood-brain barrier** to a significant extent, thus they are not a direct fuel source for brain cells. - They are, however, used by the liver to synthesize ketone bodies, which then serve as brain fuel.

Question 314: Which method is used to separate a mixture of lipids?

A. Electrophoresis
B. Chromatography (Correct Answer)
C. Isoelectric focusing
D. PAGE

Explanation: ***Chromatography*** - **Chromatography** (e.g., thin-layer chromatography, gas chromatography, high-performance liquid chromatography) is widely used to separate lipids based on differences in their **polarity**, **molecular weight**, or **solubility** in various solvents. - This method allows for the isolation and identification of different lipid classes and individual lipid species from a complex mixture. *Electrophoresis* - **Electrophoresis** separates molecules based on their **charge** and **size** in an electric field, making it more commonly used for proteins and nucleic acids. - Lipids are generally **uncharged** or have very low charge, which makes them poorly suited for separation by standard electrophoretic methods without modification. *Isoelectric focusing* - **Isoelectric focusing** is a type of electrophoresis that separates molecules based on their **isoelectric point (pI)**, which is the pH at which a molecule has no net charge. - This technique is primarily used for **proteins** and **peptides**, as lipids typically lack ionizable groups necessary for establishing a distinct pI. *PAGE* - **PAGE** (Polyacrylamide Gel Electrophoresis) is a common method used to separate **proteins** and **nucleic acids** based on their size and charge. - Lipids are **hydrophobic** and do not readily migrate through an aqueous polyacrylamide gel matrix, making PAGE unsuitable for their direct separation.

Question 315: Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?

A. Vitamin K
B. Vitamin D (Correct Answer)
C. Vitamin A
D. Biotin

Explanation: ***Vitamin D*** - The skin synthesizes vitamin D (specifically **cholecalciferol**) upon exposure to **ultraviolet B (UVB) radiation** from sunlight. - This endogenous production can be sufficient to meet physiological needs under adequate sun exposure, making it conditionally non-essential in the diet. *Vitamin K* - While **intestinal bacteria synthesize some vitamin K (K2)**, it is generally not in sufficient quantities to meet all physiological needs, especially for blood clotting. - Dietary intake of **vitamin K1 (phylloquinone)** from leafy green vegetables is critical. *Vitamin A* - **Vitamin A (retinol)** is obtained primarily from the diet, either directly from animal sources or from carotenoid precursors (like **beta-carotene**) in plants. - The body cannot synthesize vitamin A de novo; it relies on dietary intake and conversion from precursors. *Biotin* - Although the **gut microbiota can synthesize biotin**, the amount produced is generally considered insufficient to meet the body's requirements. - Therefore, biotin is primarily obtained through dietary intake, functioning as a coenzyme in various metabolic reactions.

Question 316: Bile acids are synthesized from ?

A. Heme
B. Ribulose
C. Arachidonic acid
D. Cholesterol (Correct Answer)

Explanation: ***Cholesterol*** - **Bile acids** are derivatives of **cholesterol**, synthesized in the liver through a multi-step enzymatic pathway. - The conversion of cholesterol to bile acids is a primary mechanism for the excretion and transport of cholesterol from the body. *Heme* - **Heme** is a component of hemoglobin and myoglobin, primarily involved in oxygen transport and storage. - Its degradation product is **bilirubin**, which forms part of bile but is distinct from bile acids. *Ribulose* - **Ribulose** is a 5-carbon sugar, playing a key role in the **pentose phosphate pathway** and the **Calvin cycle** in photosynthesis. - It is not a precursor for bile acid synthesis. *Arachidonic acid* - **Arachidonic acid** is a polyunsaturated fatty acid that serves as a precursor for **eicosanoids** (prostaglandins, thromboxanes, and leukotrienes). - These molecules are involved in inflammation and immune responses but are unrelated to bile acid synthesis.

Question 317: Pyridoxine is required in -

A. Glycolysis
B. TCA cycle
C. Glycogenesis
D. Transamination (Correct Answer)

Explanation: ***Transamination*** - **Pyridoxal phosphate (PLP)**, the active form of pyridoxine (vitamin B6), is an essential **coenzyme for aminotransferases (transaminases)** - Transamination reactions involve the transfer of an **amino group** from an amino acid to a keto acid, which is crucial for amino acid metabolism - This is the classic biochemical function of vitamin B6 and a frequently tested concept *Glycolysis* - Glycolysis is a metabolic pathway that breaks down glucose into pyruvate - Key cofactors for glycolysis include **NAD+ and ATP**, not vitamin B6 - Does not require pyridoxine as a coenzyme *TCA cycle* - The **TCA cycle (Krebs cycle)** is a central metabolic pathway for energy production - Uses enzymes that require cofactors such as **NAD+, FAD, and Coenzyme A** (derived from pantothenic acid) - Pyridoxine is not directly involved as a coenzyme in TCA cycle reactions *Glycogenesis* - Glycogenesis is the process of synthesizing **glycogen from glucose** - Primarily involves enzymes like **glycogen synthase** and **branching enzyme** - Requires **UTP and glucose-1-phosphate**, not pyridoxine

Question 318: Most abundant source of fuel in starvation -

A. Liver glycogen
B. Muscle glycogen
C. Adipose tissue (Correct Answer)
D. Blood glucose

Explanation: ***Adipose tissue*** - **Adipose tissue** stores **triglycerides**, which are hydrolyzed into fatty acids and glycerol to serve as the body's primary energy source during prolonged starvation. - The energy reserve in adipose tissue is significantly larger than glycogen stores, providing **sustained fuel** for days or weeks. *Liver glycogen* - **Liver glycogen** is a readily available source of glucose but is rapidly depleted within **12-24 hours** during starvation. - Its primary role is to maintain **blood glucose levels** for glucose-dependent tissues like the brain. *Muscle glycogen* - **Muscle glycogen** is used primarily for **muscle contraction** and cannot be directly released into the bloodstream to maintain blood glucose levels. - While it's a significant energy reserve for working muscles, it does not contribute to systemic fuel needs during starvation. *Blood glucose* - **Blood glucose** is the immediate circulating fuel, but it is tightly regulated and its levels decrease during starvation as glycogen stores are depleted. - It is not an abundant stored source of fuel but rather a transport form of energy.

Question 319: Which fat-soluble vitamin is most classically known for its steroid hormone-like action through nuclear receptors?

A. Vitamin K
B. Vitamin D (Correct Answer)
C. Vitamin A
D. Vitamin E

Explanation: ***Correct Answer: Vitamin D*** - **Vitamin D** (specifically its active form, **calcitriol** or **1,25-dihydroxyvitamin D₃**) is the **most classically recognized** fat-soluble vitamin that functions as a **steroid hormone** - It binds to the **vitamin D receptor (VDR)**, which is a member of the **nuclear receptor superfamily** - This VDR-calcitriol complex acts as a transcription factor, regulating gene expression involved in **calcium and phosphate homeostasis**, bone metabolism, skeletal development, and immune function - The mechanism is analogous to classic steroid hormones like cortisol, estrogen, and testosterone *Incorrect: Vitamin A* - **Vitamin A** (as **retinoic acid**) also interacts with nuclear receptors (**retinoic acid receptors - RARs** and **retinoid X receptors - RXRs**) to regulate gene transcription - However, Vitamin A is **most classically associated** with vision (rhodopsin in retinal photoreceptors), epithelial cell differentiation, embryonic development, and immune function - While it does have nuclear receptor-mediated actions, **Vitamin D is more prominently described** as having steroid hormone-like activity in standard medical education *Incorrect: Vitamin K* - **Vitamin K** functions primarily as a **cofactor for γ-glutamyl carboxylase**, an enzyme that catalyzes post-translational modification of glutamate residues to γ-carboxyglutamate (Gla) - Essential for the synthesis of **clotting factors** (II, VII, IX, X, protein C, protein S) and bone proteins (osteocalcin) - Does **not** act through nuclear receptors or function as a steroid hormone *Incorrect: Vitamin E* - **Vitamin E** (α-tocopherol) is a powerful **lipid-soluble antioxidant** that protects cell membranes from oxidative damage by scavenging free radicals - Functions primarily through its **antioxidant properties**, not through nuclear receptor binding - Does **not** have steroid hormone-like actions

Physiology

1 questions

Q311

Which of the following cell types is considered insulin-dependent?