Diffusion and Osmosis

Introduction

Diffusion and osmosis are fundamental processes in biology that govern the movement of molecules and water across cell membranes. These processes are essential for maintaining cellular homeostasis and are influenced by factors such as temperature, molecular size, solute concentration, and membrane permeability.

Effect of Size and Temperature on Diffusion

Principles of Diffusion

• Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration.

• The rate of diffusion is affected by temperature and molecular size.

• Higher temperatures increase molecular kinetic energy, resulting in faster diffusion.

• Smaller molecules diffuse more rapidly than larger molecules.

Example: Diffusion of Dyes

• Potassium permanganate (294 Daltons) and Janus Green (511 Daltons) were used to observe diffusion in agar plates at room temperature and 37°C.

• The diameter of the dye ring increases with temperature and decreases with molecular mass.

Key Equations

• Fick's Law of Diffusion: Where: J = flux, D = diffusion coefficient, dC/dx = concentration gradient

Effect of Solute Concentration on Osmosis

Principles of Osmosis

• Osmosis is the diffusion of water across a semi-permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration).

• Dialysis tubing acts as a semi-permeable membrane, allowing water but not all solutes to pass through.

Experimental Setup

• Three dialysis bags were prepared with different internal and external solutions:

• Bag 1: 10% sucrose inside, distilled water outside

• Bag 2: distilled water inside, 10% sucrose outside

• Bag 3: 10% sucrose inside, 10% sucrose outside

Variables

• Independent variable: Solute concentration in the beaker

• Dependent variable: Change in mass of the dialysis bag

Results Table

Key Points

• Water moves into Bag 1 (hypotonic environment), increasing its mass.

• Water moves out of Bag 2 (hypertonic environment), decreasing its mass.

• Bag 3 (isotonic environment) shows little or no change in mass.

Example

• Osmosis governs water movement in dialysis bags, demonstrating the effect of solute concentration gradients.

Selective Permeability of Membranes

Principles of Selective Permeability

• Cell membranes and dialysis tubing are selectively permeable, allowing certain molecules to pass while restricting others based on size, charge, or solubility.

• Small ions and molecules (e.g., chloride, sulfate, glucose) can diffuse through the membrane, while larger molecules (e.g., starch, protein) are restricted.

Detection Assays

• Silver Nitrate: Detects chloride ions (milky-white precipitate).

• Barium Chloride: Detects sulfate ions (white precipitate).

• Benedict’s Reagent: Detects reducing sugars (color change upon boiling).

• Lugol’s Iodine: Detects starch (color change).

• Biuret Reagent: Detects protein (color change).

Results Table

Key Points

• Selective permeability is demonstrated by the ability of some molecules to diffuse through the membrane while others cannot.

• Molecular size and structure are primary determinants of permeability.

Effect of Variations in Tonicity on Animal Cells

Principles of Tonicity

• Tonicity refers to the relative concentration of solutes in the solution outside the cell compared to inside the cell.

• Isotonic: Equal solute concentration; cells retain normal shape.

• Hypertonic: Higher solute concentration outside; cells lose water and shrink (crenation).

• Hypotonic: Lower solute concentration outside; cells gain water and may burst (lysis).

Results Table

Key Points

• Animal cells in isotonic solutions remain unchanged.

• In hypertonic solutions, cells shrink due to water loss.

• In hypotonic solutions, cells swell and may burst due to water influx.

Example

• Sheep erythrocytes demonstrate morphological changes in response to different tonicities.

Osmosis with Plant Cells (Cells with Cell Walls)

Principles of Osmosis in Plant Cells

• Plant cells have rigid cell walls that allow them to withstand osmotic pressure.

• Turgor pressure is the force exerted by water inside the cell against the cell wall, giving the plant rigidity.

• In hypotonic solutions, plant cells become turgid (firm).

• In hypertonic solutions, plant cells undergo plasmolysis (cell membrane pulls away from the wall).

Example

• Elodea leaves in distilled water show turgid cells; in 20% NaCl, cells show plasmolysis.

Key Equation

• Osmotic Pressure: Where: \Pi = osmotic pressure, i = van 't Hoff factor, M = molarity, R = gas constant, T = temperature

Discussion Points and Applications

Summary of Key Concepts

• Temperature increases diffusion rate; lower temperatures slow diffusion.

• Direction of water movement in osmosis depends on relative solute concentrations (hypotonic, hypertonic, isotonic).

• Selective permeability is determined by molecular size and membrane properties.

• Mixtures of molecules do not necessarily affect individual diffusion rates unless interactions occur.

• Osmosis occurs in both plant and animal cells, but plant cells resist bursting due to cell walls.

Comparative Table: Animal vs. Plant Cell Response to Osmosis

Definitions

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Movement of water across a semi-permeable membrane.

• Tonicity: Relative solute concentration of two solutions.

• Selective Permeability: Property of membranes to allow some substances to pass while restricting others.

• Turgor Pressure: Pressure exerted by water inside plant cells against the cell wall.

Applications

• Understanding diffusion and osmosis is critical for explaining nutrient uptake, waste removal, and cellular responses to environmental changes.

• Laboratory techniques such as dialysis tubing experiments and indicator assays are used to study membrane permeability and solute movement.

Additional info: The notes expand on the brief lab instructions and hypotheses by providing academic context, definitions, and relevant equations. Tables are recreated for clarity, and comparisons between animal and plant cell responses are included for exam preparation.