MadSci Network: General Biology

Re: what exactly is osmosis? how does it effect plant cells?

Date: Thu Dec 10 11:52:18 1998
Posted By: Rolf Marteijn, Grad Student, Bioprocesengineering and Virology, Wageningen Agricultural University
Area of science: General Biology
ID: 913035740.Gb


Osmosis is the process by which a liquid passes through a thin piece of solid substance.
This is what the dictionary tells us.

In nature all processes and systems try to get to a stable situation (a balance). Inside a plant cell, there is a relatively high concentration of sugars and salts. Usually, on the outside of the cell there is a lower concentration of salts and sugars than the inside, except where plants are growing in sea-water.

Since nature tries to reach a balance, water will move (diffuse) into the cells to try to lower the concentration of salts and sugars on the inside.

This will result in an increase in the plantcell volume. Since plant cells have a wood-like cell-wall around them, they can not increase in volume indefinitely, as is the case with mammalian cells. The cells will push against the cell-wall and build up tension (turgor pressure).

You can compare this with inflating a bike-tire. The inner tire is filled with air (water in plantcells) and pushes against the outer tire, which will not expand much if at all.

The effect of osmosis can be seen in plants that are short and sit in water (the cells are 'flat'). As soon as water is added, the cells start increasing in size and after some time (water has to be transported to e.g. the leaves), the 'hanging' leaves start to stand more upright due to this pressure increase.

Hope this helps you,
Rolf Marteijn Wageningen University and Researchcentre

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