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Experiments​

Diffusion

Experiment #2 from Investigating Biology through Inquiry

Education Level
High School
College
Subject
Biology

Introduction

Diffusion is a process that allows ions or molecules to move from where they are more concentrated to where they are less concentrated. This process accounts for the movement of many small molecules across a cell membrane. Diffusion is one of the processes by which cells acquire food and exchange waste products. Oxygen, for instance, might diffuse in pond water for use by fish and other aquatic animals. When animals use oxygen, more oxygen will diffuse to replace it from the neighboring environment. Waste products released by aquatic animals are diluted by diffusion and dispersed throughout a pond.

One way to measure the rate of diffusion of ions is to monitor their concentration in solution over a period of time. Since ions are electrically charged, aqueous solutions containing ions will conduct electricity. A Conductivity Probe is capable of monitoring ions in solution. This probe however, will not measure the amount of electrically neutral molecules dissolved in water. Salts, such as sodium chloride, produce ions when they dissolve in water. An equation for the dissociation of sodium chloride in water can be written as follows:

\text{NaCl(s)} \longrightarrow \text{Na}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)}

If you place a salt solution in a container such as dialysis tubing, the salt can travel through the very small holes in the tubing. When dialysis tubing containing a solution of salt ions is placed into a beaker of water, the ions can diffuse out of the tubing and into the surrounding water. In this way, you will be able to measure the diffusion of salts in a solution of water and determine how concentration gradients and the presence of other particles affect the diffusion of the salt across a membrane.

Objectives

After completing the Preliminary Activity, you will first use reference sources to find out more about diffusion and diffusion through a membrane before you choose and investigate a researchable question dealing with diffusion rate. Some topics to consider in your reference search are:

  • diffusion
  • facilitated diffusion
  • concentration gradient
  • salts
  • ion channels
  • ions
  • dialysis
  • semipermeable membrane
  • active transport

Sensors and Equipment

This experiment features the following sensors and equipment. Additional equipment may be required.

Correlations

Teaching to an educational standard? This experiment supports the standards below.

International Baccalaureate (IB) 2025/Biology
B2.1.3—Simple diffusion across membranes
D2.3.2—Water movement from less concentrated to more concentrated solutions

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This experiment is #2 of Investigating Biology through Inquiry. The experiment in the book includes student instructions as well as instructor information for set up, helpful hints, and sample graphs and data.

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