Diffusion, Osmosis Lab 7

Diffusion and Brownian Movement

Introduction to Diffusion

Diffusion is a fundamental process in biology, describing the passive movement of molecules from regions of higher concentration to regions of lower concentration. This process is driven by the random thermal motion of molecules and is essential for many physiological functions in living organisms.

• Definition: Diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration, resulting from their random motion.

• Temperature Effect: The rate of diffusion increases with temperature, as molecules move more rapidly at higher kinetic energies.

• Biological Importance: Diffusion is crucial for processes such as gas exchange, nutrient uptake, and waste removal in cells.

Brownian Movement

Brownian movement refers to the random, erratic motion of small particles suspended in a fluid, resulting from collisions with the much smaller, fast-moving molecules of the fluid.

• Observation: Under a microscope, small particles (such as carmine) in water can be seen moving in random paths due to Brownian motion.

• Significance: Brownian movement provides visual evidence of the constant motion of molecules that underlies diffusion.

Demonstration Example

• Experiment: Place a drop of carmine suspension on a microscope slide and observe the movement of particles under high power. The random movement is due to collisions with water molecules.

• Question: Is the movement of a carmine particle due to its own kinetic energy or the bombardment by water molecules? Answer: The movement is primarily due to the bombardment by water molecules.

Diffusion in Solutions

Types of Mixtures

When substances dissolve in water, they form mixtures that can be classified as solutions or colloids.

Type	Description	Properties
Solution	Homogeneous mixture where solute molecules are small and completely dissolved	Transparent, stable, does not scatter light
Colloid	Mixture with larger particles suspended in solvent	Cloudy appearance, scatters light, particles remain suspended
Suspension	Mixture with very large particles that settle out	Particles visible, settle over time

Diffusion in Gases, Liquids, and Solids

• Gases: Molecules move freely and rapidly, leading to fast diffusion.

• Liquids: Molecules move more slowly than in gases, but diffusion still occurs.

• Solids: Diffusion is very slow due to limited molecular movement.

Osmosis

Definition and Mechanism

Osmosis is a special type of diffusion involving the movement of water molecules across a selectively permeable membrane from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential).

• Semipermeable Membrane: Allows passage of water but restricts solute movement.

• Driving Force: Water moves to balance solute concentrations on both sides of the membrane.

Osmotic Potential and Pressure

• Osmotic Potential (Ψs): The potential of water to move from one area to another due to solute concentration. Pure water has the highest (zero) osmotic potential; adding solute lowers it (more negative).

• Osmotic Pressure: The pressure required to prevent water from moving across the membrane by osmosis.

• Solution Types:

  • Hyperosmotic (Hypertonic): Higher solute concentration than another solution.

  • Hypoosmotic (Hypotonic): Lower solute concentration than another solution.

  • Isosmotic (Isotonic): Equal solute concentration as another solution.

Illustration of Osmosis

The following diagrams show osmosis across a semipermeable membrane separating distilled water and a sucrose solution:

• Water moves from the side with distilled water (higher water potential) to the side with 1 M sucrose (lower water potential).

• As water moves, the level of solution rises on the sucrose side until hydrostatic pressure balances osmotic potential, reaching equilibrium.

Measuring Water Potential in Plant Cells

Water Potential in Plants

Plant cells use osmotic potentials to maintain turgor pressure, which is essential for structural support. The water potential (Ψ) of a cell is determined by its solute concentration and pressure potential.

• Water Potential Equation:

$\Psi = \Psi_s + \Psi_p$

• $\Psi$ = total water potential

• $\Psi_s$ = solute (osmotic) potential (always negative or zero)

• $\Psi_p$ = pressure potential (can be positive, negative, or zero)

Experiment: Determining Water Potential Using Potato Cylinders

This experiment measures the water potential of potato cells by soaking potato cylinders in solutions of varying sucrose concentrations and measuring changes in mass.

1. Prepare solutions of different sucrose concentrations (e.g., 0, 0.2, 0.4, 0.6, 0.8, 1.0 M).

2. Cut potato cylinders and record their initial masses.

3. Soak cylinders in solutions for 90-120 minutes.

4. Remove, blot dry, and record final masses.

5. Calculate percent change in mass for each cylinder:

$\%\ \text{Change in Mass} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100$

6. Plot percent change in mass versus sucrose concentration.

7. The point where the line crosses zero percent change indicates the isotonic concentration (where water potential inside and outside the cells is equal).

Dialysis and Differential Permeability

Dialysis Tubing Experiments

Dialysis tubing acts as a selectively permeable membrane, allowing small molecules (like water) to pass while restricting larger molecules (like sucrose or starch). These experiments demonstrate the principles of osmosis and diffusion.

• Setup: Fill dialysis bags with different solutions (e.g., distilled water, sucrose, or maple syrup) and submerge in beakers containing various solutions.

• Observation: Monitor changes in bag volume or mass, and test for the presence of specific molecules to determine which substances can cross the membrane.

Key Questions for Analysis

• Draw diagrams to show the initial setup and direction of net water movement.

• Explain which molecules or ions were able to diffuse through the membrane and why.

• Discuss how to determine if the dialysis bag leaked during the experiment.

Summary Table: Types of Solutions and Water Movement

Solution Type	Relative Solute Concentration	Water Movement (relative to cell)
Hypotonic	Lower outside than inside	Into the cell
Isotonic	Equal inside and outside	No net movement
Hypertonic	Higher outside than inside	Out of the cell

Additional info:

• These notes expand on the brief points in the original material, providing definitions, equations, and context for key biological processes related to diffusion and osmosis.

• Examples and experiment procedures are included to illustrate practical applications of these concepts in laboratory settings.