Diffusion/Osmosis

In this lab unit, you will follow your eSciences ACC Lab Manual (posted in Blackboard: “Lab Manual”) to learn about diffusion, osmosis, and how these processes work inside cells to support life.

This document will serve as your guide, sending you to the relevant lab activities and introductory information found in the eSciences lab manual (pages for this unit are 68 – 81), or in the online replication of the eScience lab manual uploaded to Blackboard for those of you who prefer to follow along online.

NOTE: I recommend that you read from this lab guide & report document first, going to the eSciences manual materials only as directed. Students previously using the lab manual have found that the additional background information provided in this document and the step-by-step guidance through the eSciences lab materials to be beneficial.

BACKGROUND INFORMATION: DIFFUSION

Diffusion is the net movement of a solute away from an area of high concentration towards an area of lower concentration. If you have ever watched tea diffusing from a tea bag, you are familiar with the process of diffusion. You have watched the brown molecules leaving the tea bag until eventually the tea become uniformly brown. This is sometimes referred to as solute molecules moving down their concentration gradient.

Notice that I said that it is NET movement of a solute. This means that like all molecules in liquid and gas phases, solute molecules move randomly in all directions. There is no driving force for sending a solute molecule down its concentration gradient other than there is no way to prevent the random movement of molecules. Eventually, the solute molecules will become totally randomized in their distribution throughout the solvent.

What does diffusion have to do with biology? Virtually all movements of molecules into and out of, and around the interior of the cell relies on diffusion of solutes. The removal of waste products from the cell relies on the random movement of these molecules from the inside of the cell to the outside of the cell. Similarly, the uptake of vital nutrients relies on their diffusion from the outside to the inside of the cell. The circulatory system speeds these processes up by sweeping nutrients into the vicinity of cells and sweeping away waste products from the cells to be excreted elsewhere from the body.

Note that the diffusion of oxygen (a vital nutrient) and carbon dioxide (an ever-present waste product of cellular respiration) are a part of this story. Diffusion is critical in the process of providing nutrients and oxygen circulating in the bloodstream to cells. A cell must be close to a capillary, within100 microns from a capillary, in order for these metabolites to diffuse to the cell quickly enough. If it is farther than that from the cell, it will not receive the vital nutrients or be able to relieve its metabolic waste back into the blood stream.

An especially larger cell has a bigger problem with exchanging nutrients and waste products than does a smaller cell, simply because these molecules must diffuse farther in order in infuse the cell’s interior. It is believed that this is the reason that all living cells are microscopically small – they can exchange molecules more quickly with their environment if they are small, or at least have a very narrow diameter that nutrients need to traverse.

To better understand how diffusion works and how it is required to feed cells and to help cells to eliminate wastes, you may want to watch this animations on diffusion: http://www.wisc-online.com/Objects/ViewObject.aspx?ID=AP1903