Lab 6

Lab 6: Diffusion

Concepts to explore:

      Diffusion Rate of diffusion Direction of diffusion Concentration gradient Membrane permeability Dialysis

Introduction

Molecules are constantly in motion due to the kinetic energy present in every atom. This energy results in the net movement of molecules from areas of high concentration to areas of low concentration, or diffu‐ sion (Figure 1). If uninhibited, this movement will continue until equilibrium is reached and the mole‐ cules are uniformly distributed. The rate of diffusion depends on the medium used, size of the molecule, and polarity of molecule. Be‐ cause the medium will not change in a biological sys‐ tem, the diffusion rate is usually dictated by molecu‐ lar characteristics. Small, non‐polar molecules ex‐ hibit a higher rate of diffusion than large, charged ones.

Figure 1 Diffusion through a semi‐permeable membrane (lipid bilayer)

The direction of diffusion depends on concentration gradients, heat and pressure. The concentration gradient is the change of molecular density over a given area. Temperature and pressure typically remain constant in biological systems, making the con‐ centration gradient the best indicator of directionality. In general, molecules will move towards areas of lower concentrations. A major determinant of diffusion within a biological system is membrane permeability. Cells, as well as organelles within the cell, are surrounded by selective and differentially permeable membranes. These membranes control the interaction of the cell and its surrounding environment. Acting as a living gate‐ keeper, the membrane allows, slows, or denies access into the cell. Cellular membranes are composed of two layers of hydrophobic lipids. This lipid bilayer selects for molecules that can dissolve into the lipid environment and against those that cannot. The ability of a

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Lab 6: Diffusion

molecule to cross the membrane is also determined by its size. Small, uncharged...