Osmosis (Mario Martinez)

Title: Observing Osmosis Using Celery (and/or Gummy Bears)

Principle(s) Investigated:

• Passive Transport
• Osmosis
• Water Potential
• Turgor Pressure
• Tonicity
• Surface Area (relative to diffusion efficiency)

Standards :

LS1.A: Structure and Function; Energy and Matter; HS-LS1-2

Materials:

• Celery (and/or gummy bears)
• Knife
• Water
• Salt
• Tablespoons
• 6 small-sized cups or 50-100 ml beakers per group, 9 if doing both gummy bears and celery.
• Glass rods (or other stirring device) for each group
• Scales
• Rulers (if incorporating volume)
• Masking tape
• Refrigerator (recommended, but not required)

Procedure:

Pre-lab Preparation

(This could be given to the student groups to do, but for the sake of safety it may be better to do ahead of time)

1. Cut celery into equal sized pieces that will fit into the cups (about 1-2 inches in length). Each group will need 6 pieces of celery.
2. For each group, take 3 pieces of celery and cut slits on both sides. Make sure that the cuts are not so deep that the celery may be broken during the experiment or that structural integrity is lost. It is suggested that you make shallow horizontal slits on the outer side and shallow vertical slits on the inner side as shown below.

Student prior knowledge:

Students should be familiar with the concept of passive transport prior to participating in this demonstration.

Explanation:

Osmosis is the passive diffusion of water across a semi-permeable membrane in an effort to achieve equilibrium. To do so, water will typically flow (unless affected by other forces such as pressure) from an area that is less concentrated with solute (or hypotonic) to an area that is more concentrated with solute (or hypertonic) in order to balance the concentrations on both sides of the membrane. This can also be thought of as water moving from an area where it is proportionally more concentrated to an area where it is proportionally less concentrated in order to keep with the concept of moving down a concentration gradient.

This lab allows students to view the effects of osmosis by observing the water uptake/loss in celery stalks and/or gummy bears while simultaneously being able to see the effects of surface area on diffusion. They will be able to compare the weights, stiffness, and size of their celery and gummy bears both before and after the experiment, thereby allowing them to tangibly experience osmosis. In regards to the gummy bears, the students will be able to easily see the change in size between the three different scenarios, as the gummies in hypotonic solution will noticeably swell while the ones in hypertonic solution will noticeably shrivel. As for the celery, students will be able to kinesthetically see the effects of osmosis as the celery pieces in salt water will become much more pliable while the celery pieces in pure water will stiffen. At the same time, they will also be able to compare the relative stiffness of the celery pieces with or without slits in order to see the effects of greater exposed surface area on the rate of diffusion. The teacher should be aware, though, of the likely possibility that students may attribute the greater degree of pliability to the slits themselves, and should be prepared to spin this suggestion into a class discussion that will guide students towards understanding of the effects of surface area on diffusion efficiency.

The experiential learning that this lab provides students can then be expanded upon through use of time lapse videos (some of which have been linked below) in order to show students a more drastic view of osmosis. Furthermore, the teacher can then discuss with students the everyday applications of osmosis in order to help them understand the practicality and accessibility of what they are learning.

Questions & Answers:

• Do you think that the concentration of solute in the hypertonic solution affects the rate of osmosis? Why or why not?
  • Yes, the concentration of the solute in the hypertonic solution does affect the rate of osmosis. The diffusion of water across a membrane is determined by the net water potential which is part affected by the relative concentration of solutes on either side of the semipermeable membrane, or the solute potential. The greater the difference between these two concentrations, the greater the solute potential, which in turn results in a faster rate of osmosis as the system tries to progress towards equilibrium.
• Is it possible for water to travel through a semipermeable membrane going from the hypertonic side to the hypotonic side? Why would this be useful?
  • Yes, it is possible to overcome the osmotic pressure created by the concentrations of solute on either side of the membrane through use of applied pressure, there by causing water to flow from the area of the membrane that is hypertonic to the area that is hypotonic. This process, known as reverse osmosis, is used in some filtration systems, such as when purifying water for drinking.
• Why could it be bad for a cell to be placed into an environment that is exceedingly hypotonic or hypertonic?
  • The cell might experience a net movement of water that is too drastic for it to survive. For instance, if placed into an exceedingly hypotonic environment, too much water might passively move into the cell which could cause it to rupture. Likewise, placing a cell into an exceedingly hypertonic environment may cause it to lose too much water, also potentially resulting in its death.

Applications to Everyday Life:

• Osmosis is one of the main reasons that salt water and fresh water fish must be raised in their respective environments. If, for instance, fresh water fish are placed into a salt water environment, then the differences in tonicity between the fish and the environment will cause a rapid loss of water from the fish, causing it to die. Likewise, placing a salt water fish into a fresh water environment will cause it to rapidly absorb water due to osmosis, also causing it to die.
• For those of you who wear contact lenses, osmosis and tonicity play a role in the development of contact solutions. The contact solutions must be produced to be isotonic (or in osmotic equilibrium) with your eyes in order for the contacts to not cause discomfort due to osmotic movement. You may have noticed this discomfort before if you have ever had to put a contact lens back in if it has fallen out after only rinsing it with tap water.
• Osmosis plays a huge role in the urinary system of animals. Manipulation of this pressure allows the body to recycle or remove water as necessary in order to help maintain homeostasis. This is particularly important for animals who live in areas with little water who need to be that much more efficient with their water use.
• Using pressure to overcome osmotic pressure and cause water to flow from a hypertonic to a hypotonic area, or reverse osmosis, is widely used as a filtration method, such as in the production of drinkable water.
• Placing flowers in water helps to keep them from wilting thanks to osmotic pressure.

Photographs:

Photographs have been used throughout this page.

Videos:

These time lapse videos can be used to provide a more dramatic look at the effects of osmosis over time for students.