Diffusion and Osmosis

Introduction

Diffusion and osmosis are fundamental processes in biology, governing 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, leading to faster diffusion.

• Smaller molecules diffuse more rapidly than larger molecules.

Experimental Example

• Two dyes, Potassium Permanganate (294 Daltons) and Janus Green (511 Daltons), are placed on agar plates at room temperature and 37°C.

• The diameter of the diffused dye ring is measured after 30 minutes.

Key Points

• Temperature Effect: The dye diffuses faster at higher temperatures.

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

Equation

Where: D = diffusion coefficient k = Boltzmann constant T = temperature η = viscosity r = radius of molecule

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 lower water concentration (higher solute concentration).

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

Experimental Setup

• Three dialysis bags are 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 and outside

• Mass change is measured after 40 minutes to assess water movement.

Key Points

• Independent Variable: Solute concentration in the beaker

• Dependent Variable: Change in mass of the dialysis bag

• Process Governing Movement: Osmosis

Table: Osmosis Experiment Results

Additional info: Values are to be filled in based on experimental data.

Selective Permeability of Membranes

Principles of Selective Permeability

• Selective permeability refers to the ability of a membrane to allow certain molecules to pass while restricting others.

• Dialysis tubing is used to test which molecules can diffuse through the membrane.

Experimental Setup

• A mixture containing sodium chloride, sodium sulfate, glucose, starch, and protein is placed in a dialysis bag.

• After incubation, the beaker solution is tested for the 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: Results depend on which molecules are small enough to pass through the dialysis membrane.

The Effect of Variations in Tonicity on Animal Cells

Principles of Tonicity

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

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

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

• Hypotonic: Lower solute concentration outside; water moves in, causing cell swelling or lysis.

Experimental Setup

• Sheep’s blood is placed in three solutions: 0.9% NaCl (isotonic), 10% NaCl (hypertonic), 0.45% NaCl (hypotonic).

• Microscopic appearance of cells is observed.

Table: Tonicity Effects on Animal 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 the plant rigidity.

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

Experimental Setup

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

• Microscopic examination reveals differences in cell appearance.

Key Points

• In distilled water, Elodea cells are turgid and healthy.

• In 20% NaCl, Elodea cells show plasmolysis.

Discussion Points

• Temperature and Diffusion: Lower temperatures (e.g., 0°C) slow diffusion due to reduced molecular movement.

• Direction of Water Movement: Water moves into the bag in hypotonic conditions, out in hypertonic, and equally in isotonic.

• Solution Conditions: Bag 1: hypotonic; Bag 2: hypertonic; Bag 3: isotonic.

• Permeability: Only small molecules (e.g., ions, glucose) pass through dialysis membrane; large molecules (starch, protein) do not.

• Mixture Effects: The presence of multiple molecules does not significantly affect individual diffusion unless there is competition or interaction.

• Osmosis in Plant Cells: Osmosis occurs in plant cells; cell walls prevent bursting and allow turgor pressure.

Key Terms and Definitions

• 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 blocking others.

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

• Plasmolysis: Shrinking of cytoplasm away from cell wall in hypertonic solution.

Additional info: These experiments illustrate core concepts in cell biology, including membrane transport, homeostasis, and the physical properties of cells in different environments.