Re: Why would a 0.5% suspension of starch cause the cytoplasm to shrink away?

Hello Dan,

To understand what you have observed, you need to know what OSMOSIS is (not to be confused with DIFFUSION). It is defined as:

"the net movement of water molecules from a less concentrated solution to a more concentrated solution across a semi-permeable membrane..." [1]

Whoa! Let's look at this carefully.

• A "SEMI-PERMEABLE MEMBRANE" is something that physically separates two volumes (in this case, water solution) and allows only certain (semi) molecules to pass (permeable).
• "NET MOVEMENT OF WATER MOLECULES" implies that water can flow across (i.e. through) a membrane in both directions. Osmosis occurs when the overall result (net movement) is water molecules accumulating on one side.
• The rest of the definition says that in osmosis, water molecules tend to flow to the more concentrated solution. Why? Because this brings the concentrations of the two solutions closer, a position that is less energetic. The potential for a net osmosis always exists if the concentrations are not equal.

In a plant cell, the cell wall and membrane are "semi-permeable membranes" that are permeable to water but not to a large molecule like starch. Your 0.5% suspension of starch had a higher solute (dissolved material) concentration than that of the inside of the cell. (Cells, are after all, mostly water.) So, OSMOSIS occurred, and the water inside the cell travelled across the membrane to the starch solution.

Now, plant cell walls are fairly rigid, but the cell membranes are not. As the water exits the cell, something has to give: the cell membrane. The membrane shrinks away from the cell wall and this is called PLASMOLYSIS.

What happens when you place plant cells in distilled (pure) water? Can you explain why using osmosis?

This was a good question...it touches on some fundamental stuff. Stay curious!

Sincerely,
Raymond

Check out these references for more:

[1] http ://www.biology.demon.co.uk/Biology/mod1/osmosis/osmosis.htm
[2] http://tidepoo l.st.usm.edu/crswr/pagesf/isotonic.html
[3] htt p://www.madsci.org/posts/archives/dec97/877728329.Cb.q.html