Featured in PASCO’s Advanced Biology through Inquiry Teacher Guide
This manual features 22 challenging, standards-based, inquiry lab activities for AP, IB, and honors programs.
Students use a pH sensor to investigate the diffusion of hydrogen ions through a semipermeable membrane, comparing the rates of diffusion for two solutions that differ in their acidity.
Students investigate the diffusion of ions through a semipermeable membrane using apple cider vinegar and a pH sensor. The vinegar is added to a dialysis bag to represent the intracellular solution of a model cell. Distilled water in a beaker represents extracellular fluid and students monitor the pH of the water over time to measure the rate of diffusion out of the model cell.
The color of the apple cider vinegar provides evidence of the semipermeable nature of the membrane; hydrogen ions easily diffuse through the membrane while pigments molecules do not. Student-designed experiments that test factors affecting the rate of diffusion can be accomplished with minimal materials and within the time frame of a single class period.
NOTE: Since hydrogen ions form bonds with water molecules, students are actually determining the rate of diffusion of hydronium ions: H3O+(aq).
Diffusion within biological systems is affected by several factors. The most notable is the concentration gradient across a membrane. A large concentration gradient across the membrane speeds up diffusion. Other factors affecting diffusion rates within biological systems include temperature, distance between solute and membrane, surface area-to-volume ratios, and solute size. All these factors can be manipulated in student-centered investigations.
This experiment may require software and an interface for data collection.