Diffusion and Selectively Permeable Membranes

In a laboratory setting, we use a dialysis membrane to model these biological properties. A dialysis membrane is a synthetic material that mimics the selective permeability of a cell membrane based primarily on the physical size of the molecules attempting to pass through its microscopic pores.

Biological Membrane Composition

While a dialysis membrane is a simple model, a real biological plasma membrane is a complex structure primarily composed of phospholipids. These are amphipathic molecules, containing both:

• Hydrophilic heads: Negatively charged phosphate groups that attract water.
• Hydrophobic tails: Long fatty acid chains that repel water.

Figure 1 - Structure of phospholipid and phospholipid bilayer.

Factors Affecting Diffusion Rate

The speed and success of diffusion across a membrane are not uniform; they are influenced by specific physical variables.

1. Molecular Weight (MW)

The size of a molecule is one of the most significant determinants of permeability. Smaller molecules can navigate the microscopic pores of a membrane much more easily than larger ones.

In experimental practice, we can observe this by comparing different solutes:

• Methyl orange dye has a relatively low molecular weight of approximately $320 \text{ MW}$. Because of its small size, it can diffuse through a dialysis membrane into surrounding water.
• Starch-iodine solution has a much higher molecular weight of approximately $54,000 \text{ MW}$. Due to its massive size, these molecules are generally restricted from passing through the membrane pores.

2. Concentration Gradient

The concentration gradient refers to the difference in solute concentration between two sides of a membrane. The "steepness" of this gradient—how much higher the concentration is on one side compared to the other—directly impacts the rate of movement.

A higher concentration difference typically results in a faster rate of diffusion. For example, if you compare a $1.0 \text{ M}$ solution of potassium permanganate to a $0.1 \text{ M}$ solution of the same substance, the $1.0 \text{ M}$ solution will exhibit a much more rapid rate of diffusion because the gradient is steeper.

Summary of Diffusion Factors:

1. Molecular Weight: Smaller molecules = Faster/Easier diffusion.
2. Concentration Gradient: Steeper gradient = Faster diffusion.
3. Temperature: Higher temperature = Faster diffusion.

Experimental Controls: Ensuring Accuracy

When conducting experiments to observe diffusion (such as watching a dye change the color of water in a beaker), it is vital to use control beakers.

In an experimental beaker, the solute is separated from the solvent by the dialysis membrane. In a control beaker, the solute is mixed directly with the solvent. By comparing the two, researchers can confirm that any color change observed in the experimental beaker is actually due to the solute moving across the membrane, rather than a leak or an error in the setup.

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Key Terms for Review:

• Solute: The substance being dissolved and moved.
• Diffusion: Net movement from high to low concentration.
• Selectively Permeable: Allowing only certain molecules to pass.
• Molecular Weight: The mass of a molecule, determining its ability to pass through pores.
• Concentration Gradient: The difference in concentration between two areas.