Osmosis is a process by which water moves through a semi-permeable membrane to try to equalize the concentration of a solute (such as salt or sugar) on both sides of the membrane.

What does this mean?

Let's explain this with pictures.

Here we have two solutions.

Each one has the same amount of water.

However, the solution on the left has a lot of sugar dissolved in it.

The solution on the right has only a little sugar in it.

Is this fair?

The permeable membrane is like a sieve.

Water can go through it, but the big sugar molecules cannot.

The water molecules go to where there is lots of sugar!    Yum yum!!!!

This will be fair when there is the same amount of  sugar for each water molecule!

Now,  see how many of the water molecules have gone through the permeable membrane to the sugar- rich side.

There is less water on the sugar-poor side now.

Sometimes osmosis can make the two sides have an equal concentration, but sometimes other factors prevent that.

For example, osmosis would not lift the water up so high as the water level that you see in the diagram

What is important here is that you understand the main idea.

Let's see why understanding this is important for understanding a little about how cells work.

Try this next example.

	Here is a cell in the early oceans of the world.  There is salt  in the water outside the cell.  There is salt inside the cell.  The salinity or saltiness is the same on both sides of the cell membrane.
The water outside the cell and the water inside the cell are equally salty.  This is a balanced situation. Molecules may pass through the cell membrane, but about the same number go in as go out.

	Here we have a higher concentration of salt inside the cell than in the water outside,  See what happens when you put a salt water fish in fresh water. The salt cannot get out of the cell, so water flows in to try to equalize the saltiness.
The water moves toward where the salt is.  There is more saltiness inside the cell than there is outside it.  Water moves into the cell to try to even this out.  Eventually there will be balance -- or the  cell will burst -- and die.

	Here the cell finds itself in very salty water.  The salt concentration is greater outside the cell than it is inside the cell. Water leaves the cell to try to dilute the salt solution.
The water moves toward the salt.  As it moves out of the cell, the cell grows smaller.  It shrivels.  Is is becoming dehydrated. Humans use salt to to preserve food by having the water in the food go out to the salt, for example, in salted fish. Also, natron is a salt used to dry out mummies.

     Natron is a kind of salt.  The Egyptians  used to pack the bodies of dead pharaohs in natron (a kind of salt)  in order to dry out their tissues and make mummies of their remains.

The first living cells had to stabilize their interior chemicals despite the changes in the physical world around them.  Only those that succeeded lived and were able to reproduce themselves..

     Now how does this work with plants and animals?

Plants

A plant cell has a cell membrane that is inside a cell wall.  If the cell takes in a lot of water, the membrane winds up pushing against the stiff cell wall.  The cell wall stops the cell from absorbing any more water.  

When plant cells are full of water, we say they are turgid.   It is this turgidity that makes the grass stand up and the leaves spread out in the sunshine.

When there is a shortage of water, water may be released through the cell membrane.  The plant's leaves start to wilt.  The cell membrane pulls away from the cell wall, but it is still protected by it and so often can take up water again when water becomes available.   Plant cells have a structure called a vacuole which is like a bubble.  When water is withdrawn from the cells, the vacuoloe expands and helps the cell structure to remain intact.

Animals

Animals are more vulnerable than plants to changes of water in their cells.  If they take in too much water, the cell membrane just breaks, as there is no cell wall to support it.  Animals have kidneys to keep the water and salts in their bodies in balance with the normal state of their cells.  

I have described what happens with salt solutions, but the same thing happens with sugar solutions.   Life is a chemical balancing act.  The animals and plants evolve ways to keep their internal environments functioning well.  Changes in the external environment in which we all live pose continuing challenges to living organisms.