Cellular Physiology Practice Questions

Q: A bilipid layer is most permeable to which of the following substances?

Urea. **Explanation:** The permeability of a cell membrane (a phospholipid bilayer) is determined by the **physicochemical properties** of the solute. The lipid bilayer is hydrophobic; therefore, substances that are small, non-polar, or uncharged cross most easily via simple diffusion. **Why Urea is Correct:** Urea is a **small, uncharged polar molecule**. While it is polar, its small molecular size and lack of net charge allow it to penetrate the lipid bilayer significantly more easily than ions or large polar molecules. In the hierarchy of membrane permeability, small uncharged molecules (like Urea, $H_2O$, and $CO_2$) have higher permeability coefficients than larger molecules or charged ions. **Analysis of Incorrect Options:** * **Sodium ($Na^+$) and Potassium ($K^+$):** These are small but carry a **net charge**. Charged ions are surrounded by a hydration shell, making them highly hydrophilic. They cannot dissolve in the hydrophobic fatty acid tails of the bilayer and require specific channels or transporters to cross. * **Glucose:** While uncharged, glucose is a **large, polar molecule**. Its size prevents it from squeezing through the lipid bilayer, necessitating facilitated diffusion via GLUT transporters. **NEET-PG High-Yield Pearls:** 1. **Permeability Hierarchy:** Hydrophobic molecules ($O_2, N_2, \text{benzene}$) > Small uncharged polar molecules ($\text{Urea}, H_2O, \text{Glycerol}$) > Large uncharged polar molecules ($\text{Glucose}$) > Ions ($H^+, Na^+, K^+, Cl^-$). 2. **Overton’s Rule:** The more lipid-soluble a substance is (higher oil-water partition coefficient), the greater its permeability. 3. **Clinical Note:** While urea can cross the bilayer slowly by simple diffusion, specialized transporters (**UT-A and UT-B**) exist in the kidneys to facilitate rapid movement, essential for the countercurrent multiplier system.

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.

Q: Which of the following is NOT a second messenger?

Guanylyl cyclase. In cellular signaling, **second messengers** are small intracellular molecules that relay signals received by cell-surface receptors to target molecules inside the cell [1]. **Why Guanylyl Cyclase is the correct answer:** Guanylyl cyclase is an **enzyme**, not a second messenger [1]. It catalyzes the conversion of Guanosine Triphosphate (GTP) into **cyclic GMP (cGMP)**. While cGMP acts as a second messenger, the enzyme responsible for its production (Guanylyl cyclase) does not. It exists in two forms: membrane-bound (activated by Atrial Natriuretic Peptide) and soluble (activated by Nitric Oxide) [1]. **Analysis of incorrect options:** * **Cyclic AMP (cAMP):** One of the most common second messengers, produced from ATP by the enzyme Adenylyl cyclase [1]. It primarily activates Protein Kinase A (PKA). * **Diacylglycerol (DAG):** A lipid-derived second messenger produced by the cleavage of PIP₂ by Phospholipase C [1]. It remains in the plasma membrane to activate Protein Kinase C (PKC). * **Inositol triphosphate (IP₃):** Also produced from PIP₂ cleavage [1]. It is water-soluble and diffuses to the endoplasmic reticulum to trigger the release of **Calcium ions** (another vital second messenger) [1]. **High-Yield NEET-PG Pearls:** * **The "Big Five" Second Messengers:** cAMP, cGMP, IP₃, DAG, and Ca²⁺. * **Nitric Oxide (NO):** Acts via soluble guanylyl cyclase to increase cGMP, leading to smooth muscle relaxation (vasodilation) [1]. * **G-Protein Coupled Receptors (GPCRs):** Gs and Gi proteins regulate cAMP levels, while the Gq protein activates the PLC-IP₃-DAG pathway [1].

Question 1

A bilipid layer is most permeable to which of the following substances?

Accepted Answer

Urea

Answer

Potassium

Answer

Sodium

Answer

Glucose

Question 2

Regarding the transport of calcium (Ca++) across a membrane, which of the following statements is true?

Accepted Answer

It requires hydrolysis of ATP.

Answer

It is a passive transport.

Answer

It is a symport.

Answer

It maintains a high intracellular Ca++ concentration.

Question 3

Meiotic crossover occurs in which of the following stages?

Accepted Answer

Pachytene

Answer

Leptotene

Answer

Zygotene

Answer

Diplotene

Question 4

Transcytosis occurs in which of the following cells?

Accepted Answer

M cells of the intestine

Answer

Neuroglia

Answer

Pneumocytes

Answer

Axonlemma

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

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

Accepted Answer

Ca++

Answer

Na+

Answer

K+

Answer

H+

Question 9

Which of the following is NOT a second messenger?

Accepted Answer

Guanylyl cyclase

Answer

Cyclic AMP

Answer

Diacylglycerol

Answer

Inositol triphosphate

Question 10

How are proteins transported into the cell?

Accepted Answer

Pinocytosis

Answer

Osmosis

Answer

Active transport

Answer

Passive diffusion

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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