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#concentration gradient

5 public questions tagged with this topic.

Membrane Transportation

In a U-tube experiment, if Side A has 4M NaCl and Side B has 10M NaCl, what will be the movement of water?

Demonstration using U-tube separated by membrane permeable only to water nicely illustrates colligative water movement. Osmotic pressure approximated by van't Hoff law pi equals i times C times R times T proportional to total particle concentration. Solution A holding 4 molar NaCl dissociates into sodium and chloride giving factor near two, so effective osmolarity roughly 8 osmoles, solution B 10 molar near 20 osmoles, markedly higher. Chemical potential of water mu equals mu naught plus RT ln a water minus V bar P, and a water declines with solute. Therefore water potential higher in dilute arm and lower in concentrated arm. Spontaneous flux proceeds from high potential to low potential, meaning from dilute 4 molar side toward concentrated 10 molar side, attempting to equalize activities. Volume rises on high solute side, falls on low solute side until hydrostatic pressure difference compensates. Sodium chloride itself does not cross because membrane excludes ions. Analogous processes regulate cell volume. Hence in this scenario water moves from Side A containing 4M NaCl to Side B containing 10M NaCl following osmotic gradient toward higher solute concentration.

Ref: Nelson & Cox, Lehninger Principles of Biochemistry, Chapter 11: Osmosis and Water Movement Between Compartments.

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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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BASICS of PLANT+PYQS

According to Fick’s law, diffusion flux is directly proportional to:

Answer: C) Concentration gradient. For BASICS of PLANT, remember the key idea and you’ll land on this option; the distractors usually swap cause and effect or confuse similar terms. Avoid: A) Distance travelled; B) Temperature only; D) Molecular weight. Name the process first, then pick the option that describes it — don’t reverse cause and effect.

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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