Cellular Physiology Practice Questions

Q: True about caveolin?

All of the above. **Explanation:** Caveolins are integral membrane proteins (specifically **Caveolin-1, 2, and 3**) that are the primary structural components of **caveolae**—small, flask-shaped invaginations of the plasma membrane. These structures are specialized types of lipid rafts. * **Transcytosis (Option A):** Caveolae are essential for the vesicular transport of macromolecules (like albumin and insulin) across endothelial cells. They facilitate the movement of substances from the apical to the basolateral membrane. * **Cholesterol Regulation (Option B):** Caveolins are cholesterol-binding proteins. They play a critical role in transporting newly synthesized cholesterol from the endoplasmic reticulum to the plasma membrane and are involved in cellular cholesterol homeostasis. * **Signal Transduction (Option C):** Caveolae act as "signaling scaffolds." They concentrate various signaling molecules (such as G-proteins, Src-family kinases, and Nitric Oxide Synthase) within their domain, facilitating efficient cell signaling. Since caveolin is involved in all these physiological processes, **Option D (All of the above)** is the correct answer. **High-Yield Facts for NEET-PG:** * **Caveolin-3** is specifically expressed in muscle cells; mutations in the CAV3 gene lead to **Limb-Girdle Muscular Dystrophy (LGMD1C)**. * Caveolae are characterized by the presence of the protein **Cavin** in addition to Caveolin. * They are particularly abundant in adipocytes, endothelial cells, and muscle cells. * Unlike clathrin-coated pits, caveolae are considered **clathrin-independent** endocytic pathways.

Q: The magnitude of the electrical potential difference that exists across a membrane can be determined by which of the following equations?

Nernst equation. ### Explanation **Correct Answer: C. Nernst equation** The **Nernst equation** is the fundamental formula used to calculate the **equilibrium potential** (electromotive force) for a single ion across a semi-permeable membrane. It determines the electrical potential difference required to exactly balance the concentration gradient of that ion, such that there is no net movement. * **Formula:** $E = \frac{RT}{zF} \ln \frac{[Ion]_{out}}{[Ion]_{in}}$ (Simplified at body temperature: $E = \pm 61 \log \frac{[Ion]_{out}}{[Ion]_{in}}$). * In physiology, it explains why the resting membrane potential is close to the equilibrium potential of Potassium ($K^+$), as the membrane is most permeable to it at rest. **Analysis of Incorrect Options:** * **A. Gibbs-Donnan equation:** Describes the behavior of charged particles near a semi-permeable membrane when one ion is non-diffusible (e.g., plasma proteins). It explains the resulting unequal distribution of diffusible ions, not the magnitude of the potential itself. * **B. Henderson-Hasselbalch equation:** Used in acid-base physiology to calculate the **pH** of a buffer solution or the ratio of bicarbonate to carbon dioxide in the blood. * **D. Fick’s law:** Relates to the **rate of diffusion** of a gas or solute across a membrane. It states that the flux is proportional to the concentration gradient and surface area. **High-Yield Clinical Pearls for NEET-PG:** * **Goldman-Hodgkin-Katz (GHK) Equation:** While Nernst is for a *single* ion, the GHK equation calculates the *resting membrane potential* by considering the permeability and concentration of *all* major ions ($Na^+, K^+, Cl^-$). * **Standard Potentials:** $E_{K^+} \approx -90\text{ mV}$, $E_{Na^+} \approx +60\text{ mV}$. * **Hypokalemia** makes the resting membrane potential more negative (hyperpolarization), making cells less excitable.

Q: Transfer of solutes between cells occurs via which of the following structures?

Gap Junctions. **Explanation:** The correct answer is **Gap Junctions**. **1. Why Gap Junctions are correct:** Gap junctions (communicating junctions) are specialized intercellular connections composed of clusters of channels called **connexons**. Each connexon is formed by six protein subunits called **connexins**. These channels create a direct physical bridge between the cytoplasm of adjacent cells, allowing the passage of ions, small water-soluble molecules, and secondary messengers (like cAMP or $Ca^{2+}$). This facilitates **electrical and metabolic coupling**, which is essential for coordinated activities like the rhythmic contraction of cardiac muscle and smooth muscle. **2. Why other options are incorrect:** * **Desmosomes (Macula Adherens):** These are "spot welds" that provide mechanical strength by anchoring intermediate filaments (keratin) of adjacent cells. They prevent cells from pulling apart but do not allow solute transfer. * **Fascia Adherens:** Found in the intercalated discs of cardiac muscle, these anchor actin filaments and help transmit contractile forces between cells. * **Zonula Occludens (Tight Junctions):** These are "sealing junctions" located at the apical end of epithelial cells. Their primary function is to prevent the paracellular movement of solutes and maintain cell polarity, acting as a barrier rather than a channel. **High-Yield Clinical Pearls for NEET-PG:** * **Connexin 26:** Mutations are a leading cause of congenital non-syndromic deafness. * **Connexin 32:** Mutations are associated with Charcot-Marie-Tooth disease (X-linked). * **Cardiac Synctium:** Gap junctions are the structural basis for the functional syncytium of the heart, allowing rapid spread of action potentials.

Q: cAMP acts by:

Increasing Cl- secretion. **Explanation:** The correct answer is **C. Increasing Cl- secretion.** Cyclic adenosine monophosphate (cAMP) is a vital second messenger that regulates ion transport across epithelial membranes. In the intestinal and respiratory mucosa, an increase in intracellular cAMP activates **Protein Kinase A (PKA)**. PKA subsequently phosphorylates the **Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)**, which is a cAMP-gated chloride channel located on the apical membrane. This phosphorylation opens the channel, leading to a massive efflux of Cl- ions into the lumen. Sodium (Na+) and water follow passively due to the electrochemical gradient and osmotic pull, resulting in secretory activity. **Analysis of Incorrect Options:** * **Options A, B, and D:** While Na+ and K+ movements often accompany Cl- secretion to maintain electroneutrality and osmotic balance, cAMP does not primarily or directly act by increasing their secretion. Na+ secretion is typically a passive process following the active transport of anions like Cl-. **Clinical Pearls for NEET-PG:** * **Cholera Toxin Mechanism:** *Vibrio cholerae* toxin causes permanent activation of Gs proteins, leading to constitutively high cAMP levels. This results in the overactivation of CFTR channels, causing the characteristic "rice-water" diarrhea due to massive Cl- and water loss. * **Cystic Fibrosis:** This condition is caused by a mutation in the CFTR gene. The lack of functional cAMP-responsive Cl- channels leads to thick, dehydrated secretions in the lungs and pancreas. * **Other cAMP-mediated effects:** Beyond ion transport, cAMP is involved in bronchodilation (via β2 receptors) and increased heart rate/contractility (via β1 receptors).

Q: Active transport across the cell membrane is mediated by which of the following mechanisms?

Carrier protein. **Explanation:** **Mechanism of Active Transport:** Active transport is the movement of substances against their electrochemical gradient (from low to high concentration), requiring energy (ATP). This process **must** be mediated by **Carrier Proteins**. These proteins undergo conformational changes to "pump" molecules across the membrane. Unlike simple diffusion, carrier-mediated transport is saturable (shows $V_{max}$) and specific. **Analysis of Options:** * **Carrier Protein (Correct):** All forms of active transport (Primary and Secondary) require carrier proteins. These proteins have specific binding sites and change their shape to transport solutes. * **Na+-K+ ATPase (Incorrect):** While this is the classic example of a primary active transporter, it is a *specific type* of carrier protein. In MCQ patterns, "Carrier Protein" is the broader, more fundamental mechanism that encompasses all active transport pumps (including Ca²⁺ ATPase and H⁺-K⁺ ATPase). * **Channel Protein (Incorrect):** Channels facilitate **passive transport** (facilitated diffusion). They form open pores that allow ions to flow *down* their gradient at very high speeds. They cannot move substances against a gradient. * **G-proteins (Incorrect):** These are membrane-associated proteins involved in **signal transduction** (second messenger systems), not the direct transmembrane transport of solutes. **High-Yield NEET-PG Pearls:** 1. **Primary vs. Secondary:** Primary active transport uses direct ATP hydrolysis (e.g., Na+-K+ pump). Secondary active transport uses the energy stored in the Na+ gradient (e.g., SGLT-1 for glucose). 2. **Kinetics:** Carrier-mediated transport (both active and facilitated diffusion) exhibits **stereospecificity, saturation, and competitive inhibition.** 3. **Na+-K+ ATPase:** It pumps **3 Na+ OUT and 2 K+ IN**, making it electrogenic and essential for maintaining resting membrane potential.

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

It requires a carrier protein.. **Explanation:** Facilitated diffusion is a form of **carrier-mediated passive transport**. Unlike simple diffusion, where molecules pass directly through the lipid bilayer, facilitated diffusion requires specific **integral membrane proteins (carriers)** to move large or polar molecules across the cell membrane. 1. **Why Option B is Correct:** Facilitated diffusion relies on carrier proteins that undergo conformational changes to move solutes. A classic example is the **GLUT (Glucose Transporter)** family, which moves glucose into cells. 2. **Why Option A & D are Incorrect:** Facilitated diffusion is **passive**, meaning it moves solutes *down* their electrochemical gradient. Therefore, it does not require metabolic energy in the form of **ATP** or **Creatine Phosphate** (which is an energy reserve for ATP regeneration). 3. **Why Option C is Incorrect:** Unlike simple diffusion (which is linear), facilitated diffusion exhibits **saturation kinetics**. As the concentration gradient increases, the rate of transport increases only until all available carrier proteins are occupied (**Vmax**). Thus, the rate is not indefinitely proportionate to the gradient. **High-Yield NEET-PG Pearls:** * **Characteristics of Carrier-Mediated Transport:** Saturation (Vmax), Stereospecificity (e.g., D-glucose is transported, but L-glucose is not), and Competitive Inhibition. * **GLUT-4:** The only insulin-dependent glucose transporter, found in skeletal muscle and adipose tissue. * **SGLT (Sodium-Glucose Linked Transporter):** This is **Secondary Active Transport**, not facilitated diffusion, as it uses the sodium gradient established by the Na+/K+ ATPase.

Question 1

All of the following statements about cadherins are true, EXCEPT:

Accepted Answer

Cadherins interact with components of the basement membrane.

Answer

Adhesion is Ca++-dependent and homophilic.

Answer

When mixed together, cells expressing different cadherins sort out into homotypic cell aggregates.

Answer

Cadherins associate with the actin cytoskeleton.

Question 2

What do tyrosine kinase receptors mediate?

Accepted Answer

They mediate cellular processes involved in growth and differentiation.

Answer

They have constitutively active tyrosine kinase domains.

Answer

They phosphorylate and activate ras directly.

Answer

They are not phosphorylated upon activation.

Question 3

True about caveolin?

Accepted Answer

All of the above

Answer

Involved in transcytosis

Answer

Involved in cholesterol regulation

Answer

Involved in signal transduction

Question 4

If a cell membrane were permeable only to K+, what would be the effect of reducing the extracellular K+ concentration from 5 mM to 2.5 mM on the membrane potential, given the following intracellular and extracellular ion concentrations: Intracellular (mM): K+ 140, Na+ 12, Cl- 5, Ca++ 0.0001; Extracellular (mM): K+ 5, Na+ 145, Cl- 125, Ca++ 5?

Accepted Answer

19 millivolts hyperpolarization

Answer

19 millivolts depolarization

Answer

38 millivolts depolarization

Answer

38 millivolts hyperpolarization

Question 5

The magnitude of the electrical potential difference that exists across a membrane can be determined by which of the following equations?

Accepted Answer

Nernst equation

Answer

Gibbs-Donnan equation

Answer

Henderson-Hasselbalch equation

Answer

Fick's law

Question 6

The transition temperature of lipid bilayers of a cell membrane is increased by which of the following?

Accepted Answer

Saturated fatty acids

Answer

Cholesterol

Answer

Hydrocarbons

Answer

Unsaturated fatty acids

Question 7

Transfer of solutes between cells occurs via which of the following structures?

Accepted Answer

Gap Junctions

Answer

Desmosomes

Answer

Fascia adherens

Answer

Zonula occludens

Question 8

cAMP acts by:

Accepted Answer

Increasing Cl- secretion

Answer

Increasing Na+ and K+ secretion

Answer

Increasing K+ secretion

Answer

Increasing Na+ secretion

Question 9

Active transport across the cell membrane is mediated by which of the following mechanisms?

Accepted Answer

Carrier protein

Answer

G-proteins

Answer

Na+-K+ ATPase

Answer

Channel protein

Question 10

Which of the following statements about facilitated diffusion is true?

Accepted Answer

It requires a carrier protein.

Answer

It is a form of active transport.

Answer

Rate of transport is proportionate to the concentration gradient.

Answer

Requires creatine phosphate.

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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