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.

Experimental Design

• Two diffusion plates with holes are used, each containing drops of potassium permanganate (294 Daltons) and Janus Green (511 Daltons).

• One plate is incubated at 37°C, the other at room temperature.

• The diameter of the dye spread is measured at 0 and 30 minutes.

Key Points

• Temperature Effect: Higher temperature increases diffusion rate, resulting in larger dye diameters.

• Molecular Mass Effect: Potassium permanganate (smaller molecule) diffuses faster than Janus Green (larger molecule).

• Example: After 30 minutes, the dye ring is larger on the plate at 37°C, and potassium permanganate spreads more than Janus Green.

Table: Diffusion Results

Additional info: Actual values would show greater distances for dyes at higher temperature and for smaller molecules.

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) to lower water concentration (higher solute).

• Dialysis tubing acts as a model for a semi-permeable membrane.

Experimental Design

• Three dialysis bags are prepared:

• Bag 1: 10 ml of 10% sucrose in distilled water

• Bag 2: 10 ml of distilled water in 10% sucrose

• Bag 3: 10 ml of 10% sucrose in 10% sucrose

• Bags are weighed at 0 and 40 minutes to determine mass change.

Key Points

• Independent Variable: Solute concentration in beaker

• Dependent Variable: Change in mass of dialysis bag

• Process Governing Movement: Osmosis

• Example: Water moves into Bag 1 (hypotonic environment), out of Bag 2 (hypertonic environment), and no net movement in Bag 3 (isotonic environment).

Table: Osmosis Results

Additional info: Bag 1 gains mass, Bag 2 loses mass, Bag 3 remains unchanged.

Selective Permeability of Membranes

Principles of Selective Permeability

• Cell membranes allow certain molecules to pass while restricting others based on size, charge, and solubility.

• Dialysis tubing models selective permeability.

Experimental Design

• Dialysis bag filled with a mixture: sodium chloride, sodium sulfate, glucose, starch, protein.

• Bag placed in distilled water for 40 minutes.

• Beaker solution tested for presence of each molecule using specific reagents.

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).

Table: Selective Permeability Results

Additional info: Small molecules (ions, glucose) typically diffuse through; large molecules (starch, protein) do not.

Effect of Variations in Tonicity on Animal Cells

Principles of Tonicity

• Tonicity describes the relative concentration of solutes in solution compared to the cell.

• Isotonic: Equal solute concentration; no net water movement.

• Hypertonic: Higher solute outside; water leaves cell, causing shrinkage (crenation).

• Hypotonic: Lower solute outside; water enters cell, causing swelling and possible lysis.

Experimental Design

• Sheep blood mixed with three salt solutions:

• Tube A: 0.9% NaCl (isotonic)

• Tube B: 10% NaCl (hypertonic)

• Tube C: 0.45% NaCl (hypotonic)

• Microscopic examination of cell morphology.

Table: Tonicity Effects on Animal Cells

Additional info: Tube A: normal cells; Tube B: shrunken cells; Tube C: swollen/lysed cells.

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 plants rigidity.

• In hypotonic solutions, plant cells become turgid; in hypertonic solutions, they undergo plasmolysis (cell membrane pulls away from cell wall).

Experimental Design

• Elodea leaves are soaked in distilled water (hypotonic) and 20% NaCl (hypertonic).

• Microscopic examination reveals differences in cell appearance.

Key Points

• Distilled Water: Cells are turgid, chloroplasts pressed against cell wall.

• 20% NaCl: Cells undergo plasmolysis, chloroplasts cluster in center.

• Example: Plant cells maintain turgor in hypotonic environments, but lose water and shrink in hypertonic environments.

Points for Discussion

1. Temperature and Diffusion

• At 0°C, diffusion of Janus Green and Potassium Permanganate would be slower due to reduced molecular motion.

2. Direction of Water and Sucrose Movement in Dialysis Bags

• Bag 1: Water moves into the bag; sucrose would move out if permeable.

• Bag 2: Water moves out of the bag; sucrose would move in if permeable.

• Bag 3: No net movement; equilibrium.

3. Tonicity of Solutions in Osmosis Experiment

• Bag 1: Hypotonic solution

• Bag 2: Hypertonic solution

• Bag 3: Isotonic solution

4. Permeability of Dialysis Bag

• Permeability depends on molecular size; small molecules pass, large molecules are restricted.

5. Effect of Molecular Mixture on Diffusion

• Mixture does not significantly affect individual molecule diffusion unless interactions occur.

6. Osmosis in Plant Cells

• Osmosis occurs in plant cells; cell walls allow turgor pressure, preventing lysis.

• Cells without walls (animal cells) are more susceptible to osmotic lysis.

Key Definitions and Equations

• Diffusion: Passive movement of molecules from high to low concentration.

• Osmosis: Diffusion 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.

Equation: Fick's Law of Diffusion

Where: J = flux (rate of diffusion) D = diffusion coefficient dC/dx = concentration gradient

Equation: Osmotic Pressure

Where: \Pi = osmotic pressure i = van 't Hoff factor M = molarity R = gas constant T = temperature (Kelvin)

Additional info: These equations are fundamental for understanding the quantitative aspects of diffusion and osmosis.