Osmotic vs. Ionic

This page builds on the previous one, Modified Seawaters.

What is osmotic pressure? Osmotic pressure occurs when water can pass a barrier, but a component dissolved in the water cannot. There will be a natural tendency for the system to come to an equilibrium with equal water concentration on both sides. Let's look at an example:

Wet cellophane is stretched over the end of a glass tube. An inch of 30% sugar solution is placed inside the tube and the tube is stuck into a beaker of distilled water. It helps to add a little food coloring to the sugar side. The system will try and equilibrate, but will be working against the force of gravity. In a few hours we will see how strong that force is by the level to which the water has risen in the tube.

Notice how the sugar water concentration is diluted as the distilled water is taken into the tube.

Sea urchin eggs are subject to osmotic pressure. The osmotic pressure is higher in seawater than in distilled water, but not as high as a salt pond. So what happens when we vary the osmotic pressure?

Procedure 1 – Is Osmotic Strength Important?

Make up normal artificial seawater by following the recipe in the Modified Seawater guide, only make it up 2 times normal strength, i.e., instead of 25 grams of sodium chloride add 50 grams and so on. OR make a commercial seawater mix in half the total volume specified.

Next let the students make a dilution series, from distilled water to 2X seawater, see Simple Dilution Lab.

Final % Normal Seawater Parts Distilled Water Parts 2X Seawater
0 10 0
20 9 1
40 8 2
60 7 3
80 6 4
100 (control) 5 5
120 4 6
140 3 7
160 2 8
180 1 9
200 0 10

To exchange normal seawater for the experimental seawater:

  1. Spawn the eggs in normal seawater and dilute to about 1% egg concentration (see Gametes)
  2. Measure out a volume of egg suspension and let the eggs settle to the bottom of the test tube.
  3. Pour off or pipet off, the seawater, leaving the eggs at the bottom.
  4. Add an equal volume of the experimental seawater.
  5. Fertilize all experimental solutions at the same time by adding one drop of sperm suspension to each tube in rapid succession.
  6. Do they fertilize? Record what happens. If they fertilize, record how far they develop.

Procedure 2 – Is Ionic Strength Important?

The above experiment showed the limits of osmotic pressure sea urchin eggs can tolerate. What about ionic strength? In the above we also varied the ionic strength along with the osmotic. How could we vary just the ionic? It turns out that sugars have very little if any ionic strength, but they are osmotic. So if we mix up a 1 molar solution of glucose, we have a sugar solution that is Iso-Osmotic (same osmotic strength) with seawater.

So here is the dilution series:

Final % Normal Ionic Stength Parts normal seawater Parts 1 M glucose
0 0 5
20 1 4
40 2 3
60 3 2
80 4 1
100 (control) 5 0

Again do a wash in the solution you will. Attempt to fertilize and watch development.