Osmosis and Water Potential Explained

0:34Water potential: the idea

Water molecules are always moving. Water potential is a measure of how freely they can move, in other words the tendency of water to move from one place to another. It is measured in kilopascals. Pure water has the highest water potential possible, and that is zero. Not one. Zero. Every other solution has a water potential below zero, so it is a negative number.

Why? Dissolved solutes attract water molecules and hold them, so they move less freely. More solute means a more negative water potential.

1:13Water potential: the gradient

Now osmosis. Take two solutions separated by a membrane. On the left, minus two hundred kilopascals. On the right, minus eight hundred. Minus two hundred is the higher water potential, because it is closer to zero. Osmosis is the net movement of water from a region of higher water potential to a region of lower water potential, across a partially permeable membrane.

So here, water moves left to right, down the water potential gradient. Notice the word net. Water moves both ways, but more goes one way than the other. And the steeper the water potential gradient, the faster the rate of osmosis.

1:55The partially permeable membrane

A quick word about that membrane. It lets water through, but it does not let all solutes through. The word is partially permeable. Avoid semi-permeable. AQA says partially permeable, so use that.

2:10Plant cell in pure water

Now a plant cell. Put it in a solution with a higher water potential than the cell, such as pure water. Water enters by osmosis. The vacuole swells, and the cell contents push against the cell wall. The cell becomes turgid. The strong cell wall resists the pressure, so the cell does not burst. Turgid cells hold up soft plant tissue, which is why a watered plant stands upright.

2:37Plant cell in strong solution

Now the reverse. Put the cell in a solution with a lower water potential, like a strong sugar solution. Water leaves by osmosis. The vacuole shrinks, and the volume of the cytoplasm falls. The membrane pulls away from the cell wall. The cell is now plasmolysed.

Between the two extremes is the point where the cell and the solution have the same water potential. Water moves in and out equally, there is no net movement, and the cell is neither swollen nor shrunk.

3:11Turgid, isotonic and plasmolysed

Whatever the question, do the same thing. Compare the water potential outside with the water potential inside. Outside higher, water goes in, and the cell is turgid. Outside lower, water goes out, and the cell becomes plasmolysed. Equal, no net movement.

4:02Examiner trap and recap

The traps. Never say water moves from a high concentration of water to a low one, and water concentration is no good in a gradient statement either. Use water potential. And pure water is zero, not one. A water potential of zero does not mean no water. It just means the highest value possible.

Recap. Water moves from higher to lower water potential across a partially permeable membrane. Next time, what happens when a cell needs to move things against a gradient, using active transport and ATP.