General Physiology Practice Questions

Q: Which of the following membranes has the highest protein content per gram of tissue?

Inner mitochondrial membrane. The protein-to-lipid ratio of biological membranes varies significantly based on their physiological function. Proteins are responsible for active processes like transport, enzymatic reactions, and energy production, while lipids provide structural insulation. ### **Explanation of the Correct Answer** **A. Inner Mitochondrial Membrane (IMM):** This membrane has the highest protein content in the body (approximately **75-80% protein** and 20-25% lipid). This high density is due to the presence of the **Electron Transport Chain (ETC)** complexes, ATP synthase, and various transport proteins required for cellular respiration. The IMM is also unique because it contains **cardiolipin**, which makes it impermeable to most ions, necessitating a high number of specific transporter proteins. ### **Analysis of Incorrect Options** * **B. Outer Mitochondrial Membrane:** While it contains proteins like porins, its protein-to-lipid ratio is roughly **50:50**, similar to many other organelle membranes. * **C. Plasma Membrane:** Most plasma membranes (e.g., in RBCs) have a balanced ratio of approximately **50% protein and 50% lipid**. * **D. Myelin Sheath:** This membrane has the **lowest protein content** (approx. **20% protein** and 80% lipid). Its primary function is electrical insulation to facilitate saltatory conduction, which requires a high lipid (sphingomyelin) content rather than metabolic machinery. ### **High-Yield Clinical Pearls for NEET-PG** * **Highest Protein Content:** Inner Mitochondrial Membrane (~80%). * **Highest Lipid Content:** Myelin Sheath (~80%). * **Cardiolipin:** A phospholipid found almost exclusively in the IMM; its deficiency is seen in **Barth Syndrome**. * **Marker Enzyme for Mitochondria:** Succinate dehydrogenase (Inner membrane) and Monoamine oxidase (Outer membrane).

Q: In an adult man weighing 70 kg, what is the approximate extracellular fluid volume?

14 L. **Explanation:** The distribution of body fluids is a fundamental concept in physiology based on the **"60-40-20 Rule."** In an average adult male, Total Body Water (TBW) constitutes approximately **60%** of the total body weight. 1. **Total Body Water (TBW):** 60% of 70 kg = **42 L**. 2. **Intracellular Fluid (ICF):** 2/3rd of TBW (40% of body weight) = **28 L**. 3. **Extracellular Fluid (ECF):** 1/3rd of TBW (20% of body weight) = **14 L**. Therefore, for a 70 kg man, the ECF volume is calculated as: $70 \times 0.20 = 14\text{ L}$. **Analysis of Options:** * **Option A (42 L):** This represents the **Total Body Water (TBW)**, not the ECF. * **Option B (25 L):** This is close to the **Intracellular Fluid (ICF)** volume (approx. 28 L). * **Option D (12 L):** While the Interstitial Fluid (a sub-component of ECF) is roughly 11–12 L (15% of body weight), it does not account for the total ECF. **Clinical Pearls & High-Yield Facts:** * **ECF Components:** ECF is further divided into **Interstitial Fluid (3/4th of ECF)** and **Plasma (1/4th of ECF)**. For a 70 kg man, Plasma volume is ~3.5 L. * **Measurement (Indicator Dilution Method):** * **TBW:** Measured using Tritiated water ($^3\text{H}_2\text{O}$) or Deuterium oxide ($^2\text{H}_2\text{O}$). * **ECF:** Measured using **Inulin** (Gold Standard), Mannitol, or Sucrose. * **Plasma Volume:** Measured using **Evans Blue dye** (T-1824) or Radio-iodinated Albumin. * **Gender/Age Variation:** TBW is lower in females (approx. 50%) due to higher subcutaneous fat and highest in newborns (approx. 75%).

Q: High anion gap acidosis is seen in all except?

Renal tubular acidosis. **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as: $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$. The normal range is 8–12 mEq/L. **Why Renal Tubular Acidosis (RTA) is the correct answer:** RTA is a classic cause of **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic metabolic acidosis. In RTA, there is either a failure to reabsorb bicarbonate (Type 2) or a failure to excrete hydrogen ions (Type 1/4). To maintain electroneutrality as bicarbonate is lost, the kidneys retain **Chloride ($Cl^-$)**. Since the increase in chloride offsets the decrease in bicarbonate, the anion gap remains within the normal range. **Analysis of Incorrect Options (Causes of HAGMA):** High Anion Gap Metabolic Acidosis (HAGMA) occurs when "unmeasured anions" (fixed acids) accumulate in the blood. * **Diabetic Ketoacidosis (DKA):** Accumulation of acetoacetate and beta-hydroxybutyrate. * **Lactic Acidosis:** Accumulation of lactate due to tissue hypoxia or sepsis. * **Methanol Poisoning:** Metabolism of methanol into formic acid. **NEET-PG High-Yield Pearls:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA:** **HARDUP** (Hyperalimentation, Acetazolamide, **RTA**, Diarrhea, Uretero-sigmoidostomy, Pancreatic fistula). * **Key Distinction:** Diarrhea and RTA are the two most common causes of NAGMA tested in exams. Diarrhea involves GI loss of $HCO_3^-$, while RTA involves renal loss/retention issues.

Q: What happens when sodium enters cells?

There is a spike in action potential.. ### Explanation The correct answer is **A. There is a spike in action potential.** **1. Why Option A is correct:** The generation of an action potential is primarily driven by the movement of ions across the cell membrane. When a stimulus reaches the threshold potential, **voltage-gated sodium (Na⁺) channels** open rapidly. Since Na⁺ concentration is much higher extracellularly, it rushes into the cell following its electrochemical gradient. This influx of positive charge causes **depolarization**, making the membrane potential more positive. This rapid upstroke is known as the **"spike"** of the action potential. **2. Why other options are incorrect:** * **B. Plateau in action potential:** This is characteristic of cardiac ventricular muscle cells (Phase 2), caused by the balanced influx of **Calcium (Ca²⁺)** through L-type channels and the efflux of Potassium (K⁺). * **C. Repolarization:** This occurs when the membrane potential returns to its resting state, primarily due to the **efflux of Potassium (K⁺)** out of the cell and the closure of Na⁺ channels. * **D. Hyperpolarization:** This happens when the membrane potential becomes more negative than the resting membrane potential, usually due to an **excessive efflux of K⁺** or an **influx of Chloride (Cl⁻)**. **3. NEET-PG High-Yield Pearls:** * **Tetrodotoxin (from Pufferfish) and Saxitoxin:** Block voltage-gated Na⁺ channels, preventing the spike/depolarization. * **Local Anesthetics (e.g., Lidocaine):** Work by blocking voltage-gated Na⁺ channels from the inside, inhibiting signal conduction. * **Overshoot:** The portion of the action potential where the membrane potential is positive (above 0 mV) is called the overshoot. * **Na⁺-K⁺ ATPase:** This pump does not create the action potential but maintains the ionic gradients necessary for it to occur (3 Na⁺ out, 2 K⁺ in).

Q: The sites where myosin heads bind to actin in skeletal muscles are covered by which of the following?

Tropomyosin. ### Explanation **Correct Answer: A. Tropomyosin** In skeletal muscle, the interaction between actin and myosin is the fundamental step for contraction. In a resting (relaxed) state, the **active sites on the F-actin strand**—where myosin heads must attach to form cross-bridges—are physically blocked. This masking is performed by **Tropomyosin**, a long, rod-like protein that wraps around the actin filament. As long as tropomyosin remains in this position, the myosin heads cannot bind to actin, preventing contraction. **Why other options are incorrect:** * **B. Troponin:** This is a complex of three subunits (I, T, and C). While it is attached to tropomyosin, its role is to act as a "switch." When calcium binds to Troponin C, it undergoes a conformational change that pulls the tropomyosin away from the binding sites. It does not cover the sites itself. * **C. Calcium ions:** Calcium is the trigger for contraction. It binds to Troponin C to *uncover* the binding sites; it does not cover them. * **D. Calmodulin:** This is the calcium-binding protein used in **smooth muscle** contraction (where troponin is absent). It does not play a structural role in masking actin sites in skeletal muscle. **High-Yield NEET-PG Pearls:** * **The Troponin Complex:** * **Troponin T:** Binds to **T**ropomyosin. * **Troponin I:** **I**nhibits the actin-myosin interaction. * **Troponin C:** Binds **C**alcium ions (up to 4 ions per molecule). * **The "Walk-along" Theory:** Contraction occurs when tropomyosin moves into the "groove" of the actin helix, exposing the sites. * **Clinical Correlation:** Cardiac Troponins (I and T) are highly specific biomarkers for myocardial infarction because they are released into the blood when cardiac myocytes are damaged.

Q: Ceruloplasmin binds which of the following?

Copper. **Explanation:** **Ceruloplasmin** is an alpha-2 globulin synthesized in the liver. It serves as the primary carrier protein for **Copper** in the plasma, binding approximately 95% of circulating copper. Each molecule of ceruloplasmin can bind six to eight copper atoms. Beyond transport, ceruloplasmin functions as a **ferroxidase enzyme**. It oxidizes ferrous iron ($Fe^{2+}$) to ferric iron ($Fe^{3+}$), which is a crucial step for iron to bind to transferrin for transport in the blood. Therefore, while it interacts with iron metabolism, its primary structural binding partner is copper. **Analysis of Incorrect Options:** * **Option A (Iron):** Iron is primarily transported by **Transferrin** and stored by **Ferritin**. Ceruloplasmin aids iron metabolism via its ferroxidase activity but does not "bind" it as a carrier. * **Option C (Zinc):** Zinc is mainly transported by **Albumin** (approx. 60%) and alpha-2 macroglobulin. * **Option D (Manganese):** Manganese is transported in the blood bound to **Transmanganin** (a beta-1 globulin) and albumin. **Clinical Pearls for NEET-PG:** * **Wilson’s Disease:** Characterized by a deficiency of ceruloplasmin due to a defect in the *ATP7B* gene. This leads to copper deposition in the liver (cirrhosis), brain (basal ganglia), and eyes (**Kayser-Fleischer rings**). * **Menkes Disease:** An X-linked recessive disorder (defect in *ATP7A*) resulting in impaired copper absorption and low ceruloplasmin levels, leading to "kinky hair" and neurological issues. * **Acute Phase Reactant:** Ceruloplasmin levels increase during inflammation, infection, or trauma.

Q: Which of the following cells are equivalent to macrophages?

Histiocytes. **Explanation:** The correct answer is **Histiocytes**. This question tests your knowledge of the **Mononuclear Phagocyte System (MPS)**, a collection of phagocytic cells derived from bone marrow monocytes that reside in various tissues to provide immune surveillance. **1. Why Histiocytes are correct:** Macrophages are mature monocytes that have migrated into tissues. When these cells reside specifically within **connective tissue**, they are termed **Histiocytes**. They function as professional phagocytes, engulfing cellular debris and pathogens, and act as antigen-presenting cells (APCs). **2. Analysis of Incorrect Options:** * **Keratinocytes (A):** These are the primary structural cells of the epidermis. While they can produce cytokines, their main role is forming the skin barrier, not phagocytosis. * **Mast cells (C):** These are myeloid cells found in connective tissue, but they are mediators of **Type I Hypersensitivity** reactions. They contain granules rich in histamine and heparin; they are not considered part of the macrophage lineage. * **Microphages (D):** This is an older term used to describe **Neutrophils**. While neutrophils are phagocytic, they are short-lived "first responders" and are distinct from the long-lived mononuclear macrophage system. **3. High-Yield NEET-PG Clinical Pearls:** To excel in General Physiology and Pathology, memorize the tissue-specific names of macrophages: * **Liver:** Kupffer cells * **CNS:** Microglia * **Lungs:** Alveolar macrophages (Dust cells) * **Bone:** Osteoclasts * **Kidney:** Mesangial cells * **Skin:** Langerhans cells (specifically APCs) * **Placenta:** Hofbauer cells **Key Concept:** All these cells originate from **CD14+ monocytes** in the blood.

Question 1

Which of the following membranes has the highest protein content per gram of tissue?

Accepted Answer

Inner mitochondrial membrane

Answer

Outer mitochondrial membrane

Answer

Plasma membrane

Answer

Myelin sheath

Question 2

In an adult man weighing 70 kg, what is the approximate extracellular fluid volume?

Accepted Answer

14 L

Answer

42 L

Answer

25 L

Answer

12 L

Question 3

High anion gap acidosis is seen in all except?

Accepted Answer

Renal tubular acidosis

Answer

Diabetic ketoacidosis

Answer

Lactic acidosis

Answer

Methanol poisoning

Question 4

What happens when sodium enters cells?

Accepted Answer

There is a spike in action potential.

Answer

There is a plateau in action potential.

Answer

There is repolarization.

Answer

There is hyperpolarization.

Question 5

The sites where myosin heads bind to actin in skeletal muscles are covered by which of the following?

Accepted Answer

Tropomyosin

Answer

Troponin

Answer

Calcium ions

Answer

Calmodulin

Question 6

Ceruloplasmin binds which of the following?

Accepted Answer

Copper

Answer

Iron

Answer

Zinc

Answer

Manganese

Question 7

Which of the following cells are equivalent to macrophages?

Accepted Answer

Histiocytes

Answer

Keratinocytes

Answer

Mast cells

Answer

Microphages

Question 8

Following a tetanizing stimulus to a muscle, there is a constant contraction. This is due to:

Accepted Answer

At high stimulus frequency all muscle fibers are contracting

Answer

Recruitment phenomenon

Answer

Contraction of different muscle fibers at different times

Answer

All of the above

Question 9

The equilibrium potential of the resting membrane for a given electrolyte is described by which equation?

Accepted Answer

Nernst equation

Answer

Goldman equation

Answer

Faraday's law

Answer

Donnan-Gibbs equation

Question 10

All of the following are true about cell membrane lipids except?

Accepted Answer

Exhibit symmetrical arrangement

Answer

Amphipathic in nature

Answer

Exhibit lateral diffusion along with proteins

Answer

Form bilayer

General Physiology — MCQs

General Physiology — MCQs

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