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#diffusion

12 public questions tagged with this topic.

Induction Morphogen -ll

Which signaling type involves diffusion of molecules to nearby cells?

Paracrine signaling involves secretion of soluble ligands that diffuse locally through extracellular matrix to affect neighboring cells within several cell diameters. Ligands like FGF, Hedgehog, BMP and EGF bind cognate receptor tyrosine kinases or serine-threonine kinases activating MAPK or Smad cascades. Concentration declines with distance, enabling gradient formation and threshold responses. Juxtacrine requires direct membrane contact via Notch-Delta, autocrine targets same producing cell, synaptic is specialized neuronal transmission via neurotransmitters. Paracrine underlies embryonic induction, wound healing, lateral inhibition and is primary mode of morphogen dispersal in developing epithelia and mesenchyme.

Ref: Alberts, Molecular Biology of the Cell, 6th ed., Chapter 15: Paracrine Signaling Mechanisms.

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Membrane Transportation

Which of the following is not an example of facilitated diffusion?

Facilitated diffusion denotes protein mediated passive transport operating down electrochemical gradient, characterized by saturation kinetics and competitive inhibition but lacking ATP hydrolysis. Several protein families exemplify it. GLUT uniporters possess twelve transmembrane helices organized into two bundles that alternate between outward open and inward open via rocker switch, transporting glucose along blood tissue gradients. Ion channels such as voltage gated sodium and potassium channels form aqueous pores with selectivity filters containing carbonyl oxygens and charged rings permitting millions of ions per second. Aquaporins are tetramers where each monomer forms single file water pore with NPA motifs and aromatic arginine constriction blocking protons while allowing water at billion per second rates. All operate spontaneously without nucleotide. In contrast Na+/K+ ATPase is primary active transporter of P-type family hydrolyzing ATP at DKTGT aspartate forming phosphoenzyme E1P to E2P, extruding three sodium against gradient and importing two potassium, maintained by ouabain sensitivity. ABC and V-ATPases also hydrolyze ATP. Due to direct ATP dependence and uphill ion movement, Na+/K+ ATPase does not represent facilitated diffusion.

Ref: Cooper, The Cell, 8th ed., Chapter: Facilitated Diffusion vs Active Transport - Na+/K+ ATPase Distinction.

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Membrane Transportation

Which of the following conditions describes osmotic pressure?

Osmotic pressure is a colligative property describing tendency of water to move across semipermeable membranes that allow solvent but restrict solute permeation. When compartment containing dilute solution is separated from concentrated solution, water chemical potential is higher where solute activity is lower. Spontaneous diffusion drives water toward higher solute until chemical potentials equalize, causing volume increase and hydrostatic pressure rise on concentrated side. The additional hydrostatic pressure required to stop net influx and hold volumes constant at equilibrium defines osmotic pressure. Thermodynamically it can be approximated by van't Hoff equation pi equals i times M times R times T, where i is van't Hoff factor for dissociation, M molarity, R gas constant, T absolute temperature. It depends only on total number of solute particles, not identity. This concept explains cellular swelling, water absorption in roots, and clinical use of isotonic solutions. Reverse osmosis applies pressure exceeding pi to invert flow. It is not force exerted by water passing through membrane nor pressure generated by ion channels, but prevention pressure balancing osmotic tendency.

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Osmotic Pressure Definition and van't Hoff Equation.

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Membrane Transportation

Which of the following best defines Fick’s law of diffusion?

Fick's laws provide quantitative framework for diffusion in solutions and across biological membranes integrating concentration, distance and membrane properties. First law states net diffusive flux J, amount per area per time, is proportional to concentration difference divided by distance, expressed mathematically as J equals minus D diffusion coefficient times dC/dx concentration gradient, where negative sign denotes direction from high to low. D depends on temperature increasing Brownian motion, solute Stokes radius inversely via Stokes-Einstein relation, viscosity of medium and partition coefficient for membranes. Within membrane context, permeability P equals D times partition coefficient K divided by thickness delta. Factors accelerating diffusion include larger gradient, higher temperature, smaller molecular weight, greater lipophilicity, larger surface area and thinner barrier. Fick's law predicts linear relationship between gradient and flux until transporter saturation intervenes, applicable to alveolar gas exchange, glucose entry via simple diffusion at very high concentrations, and morphogen gradient formation in development. Unlike carrier-mediated transport, Fickian diffusion shows no saturation, no stereospecificity beyond partition, no competition and cannot move solute against gradient without energy input, providing baseline against which facilitated processes are compared for selectivity advantage.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 11: Fick's Law of Diffusion and Concentration Gradient.

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Membrane Transportation

Which membrane transport process is energy-independent?

Transport classification distinguishes energy-dependent versus independent processes based on need for free energy beyond ambient thermal energy and existing gradients. Simplest criterion examines whether net flux can occur spontaneously down electrochemical potential or requires coupling to external energy store. Simple diffusion of hydrophobic molecules like steroid hormones and non-polar gases occurs directly across lipid without protein and without ATP, driven by entropy. Facilitated diffusion remains energy-independent but overcomes barrier for polar solutes using integral membrane proteins to provide aqueous pore or carrier binding site; flux occurs strictly down gradient without nucleotide hydrolysis, though proteins confer selectivity for glucose versus fructose and saturability following Michaelis-Menten kinetics. Examples include GLUT1 glucose uniport, aquaporin-1 water channel, and CFTR chloride channel. These systems can be regulated by gating, phosphorylation or vesicular trafficking such as vasopressin induced AQP2 insertion, but regulation controls availability not thermodynamic drive. In contrast primary active transport like Na+/K+ ATPase, V-ATPase acidifying lysosomes, F-ATP synthase operating reverse, and ABC exporters hydrolyze ATP directly, while secondary active uses ion gradients. Facilitated diffusion therefore occupies middle ground as protein-assisted passive route enabling efficient nutrient uptake without energy cost.

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 11: Energy-Independent Transport – Facilitated Diffusion.

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Membrane Transportation

Which of the following statements about osmosis is incorrect?

Osmosis describes net water movement across selectively permeable membrane driven by difference in solute concentration and water chemical potential, not by ATP. Plasma membranes and artificial phospholipid bilayers act as semi-permeable barriers allowing water to partition and diffuse but restricting most solute diffusion due to polarity. Water molecules move randomly but statistically from compartment with lower osmolarity, higher water mole fraction and higher chemical potential, to compartment with higher solute concentration, lower water potential, until hydrostatic pressure compensates or concentrations equalize, generating osmotic pressure described by van't Hoff equation pi equals MRT times total osmolarity. This thermodynamic process is spontaneous and does not consume ATP directly; energy originates from mixing entropy. Although active maintenance of ion gradients by Na+/K+ ATPase and NKCC cotransporter indirectly influences overall tonicity and cell volume, osmosis itself is passive. Aquaporins increase hydraulic conductivity in erythrocytes, kidney proximal tubule and collecting duct but are not obligatory for osmosis to occur, as lipid bilayer alone permits basal water flux albeit slower with permeability coefficient about 10^-3 cm/s. Claiming ATP requirement misinterprets distinction between primary establishment of solute gradient and passive water equilibration following isosmotically.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 11: Osmosis and Water Movement.

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Membrane Transportation

Which of the following statements about membrane transport is true?

Plasma membrane selectively controls solute entry through distinct physico-chemical mechanisms dictated by molecular properties. Small non-polar gases such as O2, CO2, N2 and NO partition readily into hydrophobic core and cross by simple diffusion governed by Fick's law, without assistance, explaining lung alveolar exchange and mitochondrial oxygen supply. However many essential nutrients are hydrophilic or charged and face high energetic barrier. Facilitated diffusion provides protein-assisted route down concentration gradient without direct energy coupling, accelerating equilibration thousandsfold. Carriers such as GLUT family for glucose and channels such as aquaporins for water or ion channels for K+ increase effective permeability by lowering activation energy through transient binding or aqueous pore formation. Both display saturability, specificity, competitive inhibition and regulation by gating or translocation, distinguishing them from simple diffusion which is linear, non-saturable and non-selective beyond partition coefficient. Active transport moves solutes against gradients requiring energy, either primary via ATP hydrolysis as with Na+/K+ ATPase and V-ATPase or secondary using coupled ion gradients such as Na+/glucose symport. Osmosis represents water diffusion across semipermeable membranes, also energy independent. Therefore hallmark of facilitated diffusion is requirement for integral membrane protein while remaining passive and driven by gradient.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 11: Membrane Transport – Facilitated Diffusion Principles.

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Cell Communication

Which junction permits rapid diffusion of small molecules? (June 2018)

Gap junctions, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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BASICS of PLANT+PYQS

At cellular level, the most dominant mode of transport for small molecules is:

That points to B: Diffusion. Compared with the other options, Diffusion is the one that correctly describes the BASICS of PLANT concept. The wrong ones are A) Active transport; C) Mass flow; D) Endocytosis. If you’re stuck, eliminate anything that contradicts a basic fact you already know for this topic.

Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.

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