Cellular Physiology Practice Questions

Q: In the physiologic system, nitric oxide is known to act through which second messenger system?

Cyclic GMP. **Explanation:** Nitric Oxide (NO), also known as Endothelium-Derived Relaxing Factor (EDRF), is a potent vasodilator that functions as a gaseous signaling molecule. Unlike most hormones, NO is lipophilic and diffuses directly across cell membranes. **Why Cyclic GMP is Correct:** Once NO enters the target cell (typically vascular smooth muscle), it binds to and activates the enzyme **soluble Guanylyl Cyclase (sGC)**. This enzyme catalyzes the conversion of GTP to **cyclic GMP (cGMP)**. Increased levels of cGMP activate **Protein Kinase G (PKG)**, which leads to the dephosphorylation of myosin light chains and sequestration of intracellular calcium, resulting in smooth muscle relaxation and vasodilation. **Why Other Options are Incorrect:** * **Cyclic AMP (cAMP):** This is the second messenger for hormones like Glucagon, Epinephrine (via $\beta$ receptors), and PTH. While cAMP also causes vasodilation (e.g., via $\beta_2$ receptors), it is not the pathway for NO. * **Calcium ions:** Calcium usually acts as a messenger for contraction or secretion (via the $IP_3/DAG$ pathway). NO actually works to *decrease* cytosolic calcium to induce relaxation. * **Prostacyclins:** These are arachidonic acid derivatives ($PGI_2$) that act as local hormones. While they also cause vasodilation, they do so by increasing **cAMP**, not cGMP. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Nitrates:** Drugs like Nitroglycerin work by being converted into NO, thereby increasing cGMP. * **Sildenafil (Viagra):** Inhibits **Phosphodiesterase-5 (PDE-5)**, the enzyme that breaks down cGMP. This prolongs the effect of NO in the corpus cavernosum. * **NO Synthase (NOS):** NO is synthesized from the amino acid **L-Arginine** by the enzyme Nitric Oxide Synthase. * **ANP/BNP:** Atrial Natriuretic Peptide also uses cGMP as a second messenger, but it activates a *membrane-bound* (particulate) guanylyl cyclase rather than the soluble form used by NO.

Q: Meiotic crossover occurs in which of the following stages?

Pachytene. ### Explanation The question refers to the stages of **Prophase I** of Meiosis I, which is the most critical phase for genetic diversity. **Correct Answer: C. Pachytene** Crossing over (recombination) is the exchange of genetic material between non-sister chromatids of homologous chromosomes. This process occurs specifically during the **Pachytene** stage. It is mediated by the formation of **recombination nodules** on the synaptonemal complex. By the end of this stage, the homologous chromosomes are linked at the points of crossing over, known as chiasmata. **Analysis of Incorrect Options:** * **A. Leptotene:** This is the first stage where chromatin begins to condense into visible threads ("thin thread" stage). No pairing or crossing over occurs here. * **B. Zygotene:** This stage is characterized by **Synapsis**, the physical pairing of homologous chromosomes to form a bivalent or tetrad. While the synaptonemal complex forms here, actual crossing over has not yet begun. * **D. Diplotene:** In this stage, the synaptonemal complex dissolves, and homologous chromosomes begin to separate. They remain attached only at the **Chiasmata** (the physical manifestation of previous crossing over). **High-Yield NEET-PG Pearls:** * **Sequence mnemonic:** **L**ittle **Z**ebra **P**lays **D**irty **D**ogs (**L**eptotene, **Z**ygotene, **P**achytene, **D**iplotene, **D**iakinesis). * **Oocyte Arrest:** Primary oocytes are arrested in the **Diplotene** stage (specifically the Dictyotene stage) from fetal life until ovulation. * **Synaptonemal Complex:** Forms in Zygotene, is fully functional in Pachytene, and disappears in Diplotene. * **Nondisjunction:** Most chromosomal errors (like Down Syndrome) occur due to failure of proper separation during Meiosis I.

Q: Which of the following statements about facilitated diffusion is FALSE?

It can occur against an electrical gradient.. ### Explanation **Facilitated diffusion** is a form of passive transport that utilizes specific carrier proteins or channels to move substances across the cell membrane. **Why Option C is the Correct (False) Statement:** Facilitated diffusion is strictly a **passive process**. By definition, passive transport moves solutes down their **electrochemical gradient** (a combination of both concentration and electrical gradients). It cannot move a substance against an electrical gradient; doing so would require the input of metabolic energy (ATP), which would classify the process as **active transport**. **Analysis of Incorrect Options:** * **Option A:** Facilitated diffusion occurs **down the concentration gradient** (from high to low concentration). Unlike simple diffusion, it is limited by the number of available carrier proteins (Vmax/saturation). * **Option B:** Since it is a passive process driven by kinetic energy and entropy, it **does not require metabolic energy (ATP)**. * **Option D:** The rate and direction of facilitated diffusion for ions are heavily **influenced by the charge** of the molecule and the membrane potential, as these factors constitute the electrical component of the electrochemical gradient. **NEET-PG High-Yield Pearls:** * **Key Examples:** Glucose transport via **GLUT** transporters (e.g., GLUT4 in muscle/adipose) and water transport via **Aquaporins**. * **Kinetics:** Unlike simple diffusion, facilitated diffusion shows **saturation kinetics** (Vmax) and **specificity**. * **Competitive Inhibition:** Because it uses carriers, it can be inhibited by structurally similar molecules (e.g., galactose can inhibit glucose transport). * **Insulin's Role:** Insulin increases glucose uptake in specific tissues by increasing the number of GLUT4 transporters via facilitated diffusion.

Q: According to Fick's law of diffusion, particle flux will decrease if there is an increase in the:

Thickness of the membrane. ### Explanation Fick’s Law of Diffusion describes the rate at which molecules move across a biological membrane. The mathematical expression for the law is: **$J = \frac{D \cdot A \cdot \Delta C}{\Delta X}$** Where: * **$J$** = Rate of diffusion (Flux) * **$D$** = Diffusion coefficient (influenced by temperature and molecular size) * **$A$** = Surface area of the membrane * **$\Delta C$** = Concentration gradient * **$\Delta X$** = **Thickness of the membrane** (Diffusion distance) #### Why the Correct Answer is Right: According to the formula, the rate of diffusion ($J$) is **inversely proportional** to the thickness of the membrane ($\Delta X$). Therefore, as the thickness increases, the resistance to movement increases, causing the particle flux to **decrease**. #### Analysis of Incorrect Options: * **A. Concentration difference ($\Delta C$):** Diffusion is directly proportional to the gradient. A higher difference "pushes" more particles across, increasing flux. * **C. Area of the membrane ($A$):** A larger surface area provides more space for molecules to pass through simultaneously, increasing flux. * **D. Temperature:** Increasing temperature increases the kinetic energy of particles (increasing the diffusion coefficient $D$), which increases flux. #### Clinical Pearls & High-Yield Facts: * **Pulmonary Edema/Fibrosis:** In clinical practice, conditions like interstitial lung disease (fibrosis) or pulmonary edema increase the thickness of the blood-gas barrier. This decreases the diffusion of oxygen, leading to hypoxemia. * **Emphysema:** This condition decreases the **surface area** ($A$) available for gas exchange due to alveolar wall destruction, also leading to decreased flux. * **Lipid Solubility:** The diffusion coefficient ($D$) is also affected by the lipid solubility of the substance; CO₂ is 20 times more soluble than O₂, explaining why it diffuses faster despite a smaller pressure gradient.

Question 1

In the physiologic system, nitric oxide is known to act through which second messenger system?

Accepted Answer

Cyclic GMP

Answer

Cyclic AMP

Answer

Calcium ions

Answer

Prostacyclins

Question 2

Meiotic crossover occurs in which of the following stages?

Accepted Answer

Pachytene

Answer

Leptotene

Answer

Zygotene

Answer

Diplotene

Question 3

Transcytosis occurs in which of the following cells?

Accepted Answer

M cells of the intestine

Answer

Neuroglia

Answer

Pneumocytes

Answer

Axonlemma

Question 4

All of the following transport processes follow 'saturation kinetics' except:

Accepted Answer

Simple diffusion

Answer

Facilitated diffusion

Answer

Na+-Ca2+ exchanger

Answer

Na+-coupled active transport

Question 5

Which of the following statements about facilitated diffusion is FALSE?

Accepted Answer

It can occur against an electrical gradient.

Answer

It occurs down the concentration gradient.

Answer

It does not require metabolic energy.

Answer

It is influenced by the charge of the molecule.

Question 6

According to Fick's law of diffusion, particle flux will decrease if there is an increase in the:

Accepted Answer

Thickness of the membrane

Answer

Particle's concentration difference across the membrane

Answer

Area of the membrane

Answer

Temperature of the solution

Question 7

At equilibrium, the concentrations of Cl- inside and outside a cell are 8 mmol/L and 120 mmol/L, respectively. Calculate the equilibrium potential for Cl- at 37oC.

Accepted Answer

-71.7 mV

Answer

+4.07 mV

Answer

-4.07 mV

Answer

+71.7 mV

Question 8

Gap junctions are found in which of the following tissues?

Accepted Answer

Cardiac muscles

Answer

Intestine

Answer

Brain

Answer

Kidney

Question 9

The ligand-receptor complex dissociates in the endosome because:

Accepted Answer

Of the acidic pH of the vesicle

Answer

Of its large size

Answer

The vesicle loses its clathrin coat

Answer

Of the basic pH of the vesicle

Question 10

Among the listed ions, which one has the highest equilibrium potential?

Accepted Answer

Ca++

Answer

Na+

Answer

K+

Answer

H+

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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