Diffusion principles in biological systems are fundamental for B. Pharm students studying drug absorption, distribution, and membrane transport. This concise guide explains passive diffusion, Fick’s laws, diffusion coefficient, permeability, concentration gradients, osmosis, and factors like membrane thickness, surface area, lipophilicity and ionization that govern molecular movement across membranes. Understanding how temperature, molecular size, and partition coefficients affect flux helps predict drug bioavailability and design formulations. Clinical applications include transdermal delivery, GI absorption, blood-brain barrier penetration, and dialysis. Clear grasp of these diffusion concepts supports pharmacokinetics, formulation development and therapeutic optimization. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which law quantitatively describes steady-state diffusion across a membrane?
- Henderson-Hasselbalch equation
- Fick’s first law
- Stokes-Einstein relation
- Michaelis-Menten equation
Correct Answer: Fick’s first law
Q2. In Fick’s first law J = -D (dC/dx), what does D represent?
- Permeability coefficient
- Diffusion coefficient
- Partition coefficient
- Flux density
Correct Answer: Diffusion coefficient
Q3. Which factor does NOT directly increase passive diffusion flux across a membrane?
- Higher concentration gradient
- Greater membrane surface area
- Increased membrane thickness
- Higher diffusion coefficient
Correct Answer: Increased membrane thickness
Q4. Fick’s second law is primarily used to describe which situation?
- Steady-state diffusion where concentration does not change with time
- Non-steady-state diffusion where concentration varies with time
- Active transport with energy input
- Partitioning between two immiscible phases
Correct Answer: Non-steady-state diffusion where concentration varies with time
Q5. Which equation represents Fick’s second law in one dimension?
- dC/dt = -D dC/dx
- dC/dt = D d²C/dx²
- J = -D dC/dx
- J = P (C1 – C2)
Correct Answer: dC/dt = D d²C/dx²
Q6. What effect does increasing temperature have on the diffusion coefficient (D) for small molecules in solution?
- D decreases with rising temperature
- D is independent of temperature
- D increases with rising temperature
- D becomes negative at high temperature
Correct Answer: D increases with rising temperature
Q7. The Stokes-Einstein equation relates diffusion coefficient to which of the following?
- Membrane thickness and surface area
- Particle radius, temperature, and viscosity
- Concentration gradient and flux
- Partition coefficient and pKa
Correct Answer: Particle radius, temperature, and viscosity
Q8. Which parameter describes the ease with which a drug partitions into the lipid membrane from aqueous phase?
- Diffusion coefficient
- Partition coefficient (K)
- Permeability coefficient (P)
- Flux (J)
Correct Answer: Partition coefficient (K)
Q9. Permeability coefficient (P) is best described as:
- The product of diffusion coefficient and partition coefficient divided by membrane thickness
- The concentration difference across membrane
- Flux per unit time only
- Membrane porosity times tortuosity
Correct Answer: The product of diffusion coefficient and partition coefficient divided by membrane thickness
Q10. Which factor reduces effective diffusion in porous biological tissue?
- Increased porosity
- Reduced tortuosity
- Higher tortuosity
- Lower viscosity
Correct Answer: Higher tortuosity
Q11. Which transport process requires metabolic energy and is not diffusion?
- Osmosis
- Facilitated diffusion
- Active transport
- Simple diffusion
Correct Answer: Active transport
Q12. According to pH partition theory, which drug will diffuse more readily across a lipid membrane?
- The ionized form
- The unionized form
- Either form equally
- Only large macromolecules
Correct Answer: The unionized form
Q13. Which equation is used to estimate the fraction of ionized drug at a given pH?
- Fick’s first law
- Henderson-Hasselbalch equation
- Stokes-Einstein equation
- Arrhenius equation
Correct Answer: Henderson-Hasselbalch equation
Q14. What is the main driving force for passive diffusion across a membrane?
- Concentration gradient
- Electrical potential only
- ATP hydrolysis
- Vesicular transport
Correct Answer: Concentration gradient
Q15. Which structure provides a selective barrier and major resistance to diffusion in cells?
- Glycocalyx
- Cell membrane lipid bilayer
- Cytoskeleton
- Mitochondrial matrix
Correct Answer: Cell membrane lipid bilayer
Q16. Aquaporins facilitate diffusion of which substance across membranes?
- Glucose
- Ions like Na+
- Water molecules
- Large peptides
Correct Answer: Water molecules
Q17. Which of the following increases drug flux through skin in transdermal delivery?
- Increasing skin thickness
- Decreasing partition coefficient into stratum corneum
- Using penetration enhancers to increase skin partitioning
- Lowering drug lipophilicity drastically
Correct Answer: Using penetration enhancers to increase skin partitioning
Q18. Mean squared displacement in diffusion experiments is proportional to:
- Distance only
- Time squared
- Time
- Velocity
Correct Answer: Time
Q19. Which unit is typically used for diffusion coefficient in biological systems?
- mol/L
- cm/s
- cm²/s
- moles
Correct Answer: cm²/s
Q20. In a two-compartment model, diffusion-limited distribution implies which rate-limiting step?
- Metabolism in tissue
- Perfusion of tissue
- Membrane permeability/diffusion into tissue
- Renal excretion
Correct Answer: Membrane permeability/diffusion into tissue
Q21. Which phenomenon describes solvent movement across a semipermeable membrane driven by solute concentration differences?
- Simple diffusion
- Osmosis
- Filtration
- Active transport
Correct Answer: Osmosis
Q22. Donnan equilibrium affects diffusion by:
- Equalizing concentrations of all ions
- Creating unequal ion distribution due to impermeant charged species
- Removing all charged species from one side
- Preventing water movement
Correct Answer: Creating unequal ion distribution due to impermeant charged species
Q23. Which pathway is most relevant for small hydrophilic drugs crossing epithelia?
- Transcellular lipid pathway
- Paracellular aqueous pathway through tight junctions
- Endocytosis only
- Passive diffusion through cell nucleus
Correct Answer: Paracellular aqueous pathway through tight junctions
Q24. What is the effect of increased viscosity of the medium on diffusion coefficient?
- Diffusion coefficient increases
- Diffusion coefficient decreases
- No effect
- Diffusion coefficient becomes negative
Correct Answer: Diffusion coefficient decreases
Q25. Which term describes flux normalized to concentration difference across a barrier?
- Diffusion coefficient
- Partition coefficient
- Permeability coefficient
- Tortuosity
Correct Answer: Permeability coefficient
Q26. Which is TRUE for facilitated diffusion compared to simple diffusion?
- It requires ATP
- It shows saturation kinetics with increasing substrate
- It is independent of carrier saturation
- It transports molecules against a concentration gradient without energy
Correct Answer: It shows saturation kinetics with increasing substrate
Q27. Which property of a drug most increases its transcellular membrane permeability?
- High molecular weight and high polarity
- Low lipophilicity and high ionization
- Moderate lipophilicity and low ionization
- Large size and positive charge
Correct Answer: Moderate lipophilicity and low ionization
Q28. What does the term “unstirred water layer” refer to in absorption studies?
- A static layer of solvent near a membrane that limits mass transfer
- An area of turbulent flow enhancing diffusion
- The bulk solution where rapid mixing occurs
- Solid tissue adjacent to the membrane
Correct Answer: A static layer of solvent near a membrane that limits mass transfer
Q29. Which experimental method directly measures diffusion coefficient in solutions?
- HPLC assay only
- Dialysis and membrane permeation studies
- Radiotracer diffusion or FRAP (fluorescence recovery after photobleaching)
- pH titration
Correct Answer: Radiotracer diffusion or FRAP (fluorescence recovery after photobleaching)
Q30. How does molecular size generally affect diffusion coefficient in a fluid?
- Larger molecules have higher diffusion coefficients
- Molecular size has no effect
- Smaller molecules diffuse faster (higher D)
- Only shape matters, not size
Correct Answer: Smaller molecules diffuse faster (higher D)
Q31. Which of the following best describes flux (J)?
- Mass transfer per unit area per unit time
- Total mass transferred over infinite area
- Concentration divided by time
- Partition coefficient times diffusion coefficient
Correct Answer: Mass transfer per unit area per unit time
Q32. In membrane diffusion, which change would most increase permeability for a weak acid drug across acidic stomach mucosa?
- Increasing drug ionization
- Lowering lipophilicity
- Formulating drug as unionized prodrug
- Adding a charged counterion
Correct Answer: Formulating drug as unionized prodrug
Q33. Which barrier strongly limits penetration of many drugs into the brain?
- Skin barrier
- Blood-brain barrier with tight junctions and efflux transporters
- Renal glomerulus
- Gastric mucosa
Correct Answer: Blood-brain barrier with tight junctions and efflux transporters
Q34. Which process describes movement of substances by bulk fluid flow rather than diffusion?
- Osmosis
- Convection
- Facilitated diffusion
- Passive diffusion
Correct Answer: Convection
Q35. Which factor is most important for drug diffusion through tight junctions (paracellular path)?
- Lipophilicity of drug
- Molecular radius and hydrophilicity
- Drug pKa only
- Binding to plasma proteins
Correct Answer: Molecular radius and hydrophilicity
Q36. Which of these increases apparent diffusion distance and slows diffusion in tissues?
- High capillary density
- Lower extracellular matrix density
- Edema increasing interstitial distance
- Thin epithelial layers
Correct Answer: Edema increasing interstitial distance
Q37. Which transport system can become saturated at high substrate concentrations?
- Simple diffusion
- Facilitated diffusion via carrier proteins
- Passive paracellular diffusion
- Osmosis
Correct Answer: Facilitated diffusion via carrier proteins
Q38. Which experimental condition would most likely increase observed membrane flux in vitro?
- Decrease donor concentration
- Increase membrane thickness
- Increase surface area of membrane
- Decrease temperature
Correct Answer: Increase surface area of membrane
Q39. Which descriptor quantifies resistance of a membrane to diffusion?
- Permeability (P) is low
- High diffusion coefficient (D)
- Low partition coefficient (K)
- High surface area
Correct Answer: Permeability (P) is low
Q40. Which mechanism explains why charged drugs have reduced membrane permeability?
- Charged drugs have higher lipophilicity
- Charged species cannot cross lipid bilayer easily due to hydrophobic core
- Charged drugs are smaller and diffuse rapidly
- Charged drugs increase partition coefficient into lipids
Correct Answer: Charged species cannot cross lipid bilayer easily due to hydrophobic core
Q41. Which is an example of a carrier-mediated facilitated diffusion transporter?
- GLUT glucose transporter
- Aquaporin water channel (no substrate binding)
- Na+/K+ ATPase
- Simple pore allowing any solute
Correct Answer: GLUT glucose transporter
Q42. Which parameter would you change to increase drug uptake by increasing its unionized fraction?
- Alter pH of environment relative to drug pKa
- Increase drug molecular weight
- Decrease temperature drastically
- Reduce surface area of absorption site
Correct Answer: Alter pH of environment relative to drug pKa
Q43. Which statement about diffusion through pores is TRUE?
- Pore size is irrelevant to solute size
- Smaller pores restrict larger solutes more
- Pores never allow hydrophilic molecules
- Tortuosity shortens diffusion path
Correct Answer: Smaller pores restrict larger solutes more
Q44. What role does the partition coefficient play in transmembrane diffusion?
- It determines aqueous solubility only
- It governs how much drug enters membrane from aqueous phase
- It equals the diffusion coefficient
- It is irrelevant for lipophilic drugs
Correct Answer: It governs how much drug enters membrane from aqueous phase
Q45. Which phenomenon explains accumulation of weak bases inside acidic cellular organelles?
- pH partitioning leading to ion trapping
- Active transport by pumps not related to pH
- Increased lipid solubility in acidic environments
- Enhanced diffusion due to low viscosity
Correct Answer: pH partitioning leading to ion trapping
Q46. Which factor most directly affects the diffusion coefficient of a solute in water?
- Membrane potential
- Viscosity of water and solute size
- pKa of the solute only
- Electrical conductivity
Correct Answer: Viscosity of water and solute size
Q47. Which technique models non-steady-state diffusion numerically for formulation design?
- Henderson-Hasselbalch plotting
- Finite difference or finite element methods solving Fick’s second law
- Simple linear regression
- Chromatography
Correct Answer: Finite difference or finite element methods solving Fick’s second law
Q48. Which is TRUE about efflux transporters like P-glycoprotein in relation to diffusion?
- They increase passive diffusion into cells
- They actively pump substrates out, reducing net intracellular accumulation
- They are passive pores without energy requirement
- They only transport water
Correct Answer: They actively pump substrates out, reducing net intracellular accumulation
Q49. In dialysis, what main principle allows removal of small solutes from blood?
- Active transport across membrane driven by ATP
- Convective transport only
- Diffusion down concentration gradients across a semipermeable membrane
- Endocytosis of solutes by membrane vesicles
Correct Answer: Diffusion down concentration gradients across a semipermeable membrane
Q50. Which strategy can be used in formulation to enhance diffusion of a poorly permeable drug?
- Decrease drug lipophilicity further
- Use prodrugs, permeation enhancers, or nanoparticles to improve partitioning and D
- Increase the molecular weight of the drug
- Prevent any interaction with lipid membranes
Correct Answer: Use prodrugs, permeation enhancers, or nanoparticles to improve partitioning and D
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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