Title: Diffusion and Osmosis
Principle(s) Investigated: Materials move across membranes because of concentration differences
Standards : Cell Structure and Function
1a. students know cells are enclosed within semipermeable membranes that regulate their interaction
with their surrounding.
Materials: Red Vinega
Table Salt and water
Concrete and Garage floor paint
clear plastic cups
large size ziplock bags
5 to 10 raw eggs (depending on the number of cooperative groups)
playtex nitrile gloves
Procedure: (In Student friendly language):
1. Choose 2 solutions you want to test in your experiment.
2. Determine whether they are hypotonic, isotonic or hypertonic relative to the chicken egg.
3. a. Identify what will change in the experiment:
example: The things you are going to measure-- these are your variables
Independent Variable: Time
Dependent Variable: weight of the increase or decreases or may remain the same.
b. Identify what will not change or remain the same during your experiment-- these are your constants.
Constant(s): amount of solution used for each egg.
4. Use the data table to organize your results.
5.. Throughly rinse each of your eggs, weigh them and record their masses in the table.
6. Label each cup with your solution's name
7.. Place each egg in a separate plastic cup.
Student prior knowledge: Differences between solute and solvent.
Students picture a cup of water and a powder mix. Teacher creates a solution by mixing the two. Teacher asks,
which is the solute? Teacher asks students, What is being dissolved? Students answer: The powder mix.
Teacher asks, What is dissolving it? students answer, " the water ".
Teacher asks, What happens to the concentration of each if you continue to add to add the powder mix?
Students answer: The solute becomes more concentrated and the solvent less concentrated. Teacher tells
students that concentration is what affects what happens across a cell membrane.
If you were to drop a pinch of sugar (the solute) into a beaker of water (the solvent), the resulting
solution would, after a time, have a uniform sweetness. The uniform sweetness would result fro the constant state of motion of all of the solute and solvent molecules in the solution, producing a net movement of solute molecules from
regions in the the solution of higher concentration to regions in the solution of lower concentration. This net
movement is known as diffusion.
A molecule may move into or out of a cell by diffusion if
1. a difference in the concentration of that molecule exists between the intracellular and extracellular
2. the cell membrane will allow the passage of that molecule.
The movement of a molecule across the cell membrane by diffusion is called passive transport. Now, imagine a
solution divided into two compartments by a membrane. If the membrane is completely permeable to solute and solvent
molecules, these molecules will be able to diffuse across it, so that the solute and solvent concentrations will
be the same on both sides of the membrane. However, if the membrane is permeable to the solvent but not to the solute.
And if the solvent is water, the water will diffuse from the region where the solute and solvent ratio is lower (relatively
more water) to the region where the solute and solvent ratio is higher (relatively less water), until the solute
and solvent ratio is the same on both sides of the membrane. The net diffusion of water across a membrane is
called osmosis. (Human Physiology, Fox 1999).
The more highly concentrated solution is said to have a greater osmotic pressure than the less concentrated
solution. The osmotic pressure is a mesure of the ability of a solution to "pull in" water from another solution that is
separated from it by a semipermeable membrane. In more simple scientific terms, ismosis is the simple diffusion of water through a membrane, water moves into the more concentrated solution as a result of the higher to lower water concentration gradient. (Human Physiology, Fox 1999).
Questions & Answers: 1. What effect would eating too much salt have on the body?
Problems with toxic elimination and blood circulation (http://www.ehow.com).
2. When a person becomes dehydrated, why do medical personnel infuse a saline solution?
(water and salts) into the bloodstream rather than water?
Water would be hypotonic relative to the blood cells and could cause cells to burst. The
saline solution is isotonic relative to the cell contents. (McDougal Littell 2008)
3. Why are some animals and single-celled organisms able to survive in hypotonic
Their cells have adaptations for removing excess water. In plants, the rigid cell wall
prevents the membrane from expanding too much, and provides structural support for
each cell,and for the plant as a whole. ((McDougal Littell 2008).
Applications to Everyday Life:
One instance, can be observed by the preservation of meat by American pioneers, by salting it. the excess concentration of salt would dried out decomposers, such as bacteria and fungi.
The sensation of thirst is stimulated by receptors located in a part of the brain called the hypothalamus. which sends signals to receptors, called osmoreceptors. They are stimulated by an increase in blood osmolality.
Long distance running, requires that the body loose water through the process of osmosis. The body releases excess heat through the process of sweating.
Photographs: included later.
Osmosis lab: www. youtube.com
Human Physiology, Laboratory, Guide Stuart Ira Fox
McDougal Littell Biology Lab Binder, Chapter Investigation.
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