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 depends on the size of the molecule and the temperature of the environment.

• Molecular Size: Smaller molecules diffuse faster than larger molecules due to less resistance in the medium.

• Temperature: Higher temperatures increase the kinetic energy of molecules, resulting in faster diffusion rates.

Example: In the experiment, Potassium Permanganate (294 Daltons) and Janus Green (511 Daltons) were placed on agar plates at room temperature and 37°C. The diameter of the dye rings was measured to assess diffusion.

Equation:

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

Experimental Results Table

Additional info: Higher temperature and smaller molecular size result in greater diffusion distance.

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: Used as a model for a semi-permeable membrane.

• Solute Concentration: Determines the direction and rate of water movement.

Example: Three dialysis bags were set up with different combinations of sucrose and water to observe changes in mass due to osmosis.

Experimental Results Table

Key Points:

• Bag 1: Water moves into the bag (hypotonic environment).

• Bag 2: Water moves out of the bag (hypertonic environment).

• Bag 3: No net movement (isotonic environment).

Scientific Method:

• Independent Variable: Solute concentration in the beaker.

• Dependent Variable: Change in mass of the dialysis bag.

Additional info: Osmosis is governed by the concentration gradient across the membrane.

Selective Permeability of Membranes

Principles of Selective Permeability

Selective permeability refers to the ability of a membrane to allow certain molecules to pass through while blocking others, based on size, charge, or solubility.

• Dialysis Tubing: Used to test which molecules can diffuse across the membrane.

• Detection Assays: Various reagents are used to test for the presence of ions and molecules in the beaker after incubation.

Experimental Results Table

Key Points:

• Small molecules (e.g., ions, glucose) can diffuse through the membrane.

• Larger molecules (e.g., starch, protein) are blocked by the membrane.

Example: Positive results for chloride ions, sulfate ions, and reducing sugars indicate permeability; negative results for starch and protein indicate impermeability.

Additional info: The property demonstrated is selective permeability.

Effect of Variations in Tonicity on Animal Cells

Tonicity and Cellular Morphology

Tonicity describes the relative concentration of solutes in the solution outside the cell compared to inside the cell, affecting water movement and cell shape.

• Isotonic: Equal solute concentration; no net water movement; cells retain normal shape.

• Hypertonic: Higher solute concentration outside; water leaves the cell; cells shrink (crenation).

• Hypotonic: Lower solute concentration outside; water enters the cell; cells swell and may burst (lysis).

Experimental Results Table

Example: Sheep erythrocytes in isotonic solution remain normal; in hypertonic solution shrink; in hypotonic solution swell.

Additional info: Animal cells lack cell walls and are susceptible to osmotic changes.

Osmosis with Plant Cells (Cells with Cell Walls)

Turgor Pressure and Plant Cell Response

Plant cells possess a rigid cell wall, allowing them to withstand osmotic pressure and maintain turgidity. Turgor pressure is the force exerted by water inside the cell against the cell wall, providing structural support.

• In Distilled Water: Water enters the cell, increasing turgor pressure; cells become turgid.

• In 20% NaCl Solution: Water leaves the cell, decreasing turgor pressure; cells undergo plasmolysis (cytoplasm shrinks away from cell wall).

Example: Elodea leaves in distilled water show turgid cells; in NaCl solution, cells show plasmolysis.

Additional info: Osmosis occurs in plant cells, but the cell wall prevents bursting.

Discussion Points

Key Concepts and Applications

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

• Direction of Water Movement: Determined by relative solute concentrations (hypotonic, hypertonic, isotonic).

• Permeability: Membrane permeability depends on molecular size and properties.

• Mixture Effects: Presence of multiple molecules does not necessarily affect individual diffusion unless interactions occur.

• Osmosis in Plant Cells: Plant cells undergo osmosis; cell walls prevent lysis and allow turgor pressure.

Summary Table: Tonicity Effects on Cells

Key Terms

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

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

• Tonicity: Relative solute concentration affecting cell volume.

• Turgor Pressure: Pressure exerted by water inside plant cells.

• Selective Permeability: Membrane property allowing certain molecules to pass.