Discussion: Diffusion & Osmosis

Discussion: Diffusion & Osmosis

Discussion: Diffusion & Osmosis


BIOL1408 Introductory Biology Name Lab Unit 6/7: Diffusion & Osmosis date Dr. Flo Oxley

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.


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

Diffusion Background Questions

After reading the background information about diffusion in this lab report guide and in your eSciences lab manual (posted on Blackboard, Lab Exercises, Lab 6, Introduction), answer the following questions.

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1. What causes molecules to be in constant motion?

2. What are three general factors that affect diffusion rates of a solute in a solvent? Thinking about how diffusion is important in the uptake of nutrients and the discharge of waste products by cells, which of the factors affecting diffusion rates is the most operative in the health of cells in your body?

3. Describe the molecular components of the membrane that surrounds all living cells, and discuss how this membrane limits the rate of diffusion of nutrients into the cell and the diffusion of waste products out of the cell.

4. What are two chemical properties of a solute that can prevent it from freely diffusing across a biological membrane?

5. What is the one chemical property of a solute that can prevent it from freely diffusing across a dialysis membrane?

6. Go to http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/ animation__how_diffusion_works.html to watch an animation about diffusion, and answer the following questions: a. At what point does the diffusion of a solute come to a complete stop?

b. Watching the diffusing molecules in this animation, are they moving in a random or in a directional manner? (Try this: pick one specific molecule and watch its motion: is it moving in one direction or randomly in all directions?)


Osmosis is similar to diffusion in that it is the random movement of molecules that has a net movement down its concentration gradient. But osmosis differs from diffusion in these ways:

1. Osmosis has to do with movement of solute molecules (water) instead of solute molecules. 2. Osmosis has to do with movement across a semipermeable membrane – one that is permeable to

the solvent molecules (water) and not the solute molecules. a. Biological membranes are permeable to water molecules, but is not permeable to large

and/or polar solute molecules. b. Dialysis membranes are permeable to all molecules that are smaller than the pore sizes of

the dialysis membrane.

In osmosis, water molecules cross the semipermeable membranes until the water content become equalized on both sides of the membrane. At this equilibrium point, the solutions on either side of the membrane are said to be “isotonic” or “isosmotic”.

Until equilibrium is reached, the solution with the greater solute content is said to be “hypertonic” or “hyperosmotic”. Conversely, the solution with the lesser solute content is said to be “Hypotonic” or “hypo-osmotic”. In the meantime, water molecules have net movement in a hypotonic to a hypertonic direction until equilibrium is met.

If you have ever soaked a twisted ankle in a solution of Epsom’s Salt, the same principles applied: the hypertonic Epsom’s Salt solution drew down your inflammation by osmosis. The rate of osmosis is increased when the concentration difference is high. Similarly, if you have ever noticed that the skin on your fingers pucker up after along swim, this is due to osmosis, too. The skin cells took on water because they were hypertonic to the swim

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