GCSE Biology - AQA
3.2.3 - Osmosis
Osmosis is a special way that water in a solution can move across a membrane.
Osmosis is defined as follows:
Osmosis is the net movement of water through a partially permeable membrane from a more dilute solution to a more concentrated solution.
(Note: whenever the word 'solution' is used on this page, it refers to a solution in which water is the solvent. Since almost all solutions in biology have water as the solvent, it is common to simply use the word 'solution' rather than stating every time that the solvent is water).
To understand the definition of osmosis, we first need to know what is meant by the term 'partially permeable membrane'.
Something is described as permeable if it allows substances to pass through it. For example, a sponge is permeable to water - it allows water to pass through it.
A sponge is permeable to water - it allows water to pass through it. Image: Pascua Theus alias Körnerbrötchen, white background by Amada44 via Wikimedia Commons (GNU Free Documentation License - https://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License,_version_1.2)
Cell membranes allow some substances to pass through them, but not others - therefore, they are described as partially permeable.
The cytoplasm of a cell is made up of water with many different solutes dissolved in it.
Cell membranes are permeable to water and to some of solutes dissolved in it, however there are some solutes that cell membranes are not permeable to.
A partially permeable membrane is one which is permeable to some substances but not others. Cell membranes are partially permeable because they allow water and some solutes through but they do not allow other solutes through.
Osmosis will only happen in situations where all of the following are true:
If all of these conditions are in place, then osmosis will happen.
The conditions for osmosis.
It is very important that the membrane is not permeable to the solute. If the solute was able to pass through the membrane, it would simply diffuse down its concentration gradient - from the side with the higher concentration to the side with the lower concentration. Because the solute cannot pass through the membrane, it is trapped where it is.
Instead of the solute moving by diffusion, the water moves by osmosis. As we will see below, the water actually moves in the opposite direction to the one that the solute would diffuse in if it was able to pass through the membrane.
During osmosis, water always moves from the side with the lower solute concentration to the side with the higher solute concentration.
Another way of saying this is to say that during osmosis, water always moves from the more dilute solution to the more concentrated solution.
The movement of water by osmosis.
Note that when we talk about 'dilute' and 'concentrated', we are talking about the concentration of the solute, not the concentration of the water. Even though water is the substance that is moving, the direction that it moves in depends on the concentration of the solute on each side of the membrane.
In order to predict which way water will move by osmosis, we simply need to look at the solute concentration on each side of the membrane. The side which has the lower solute concentration is the side that water will move away from. The side which has the higher solute concentration is the side that water will move towards.
Imagine a unicellular organism floating in a pond. The organism's cytoplasm contains a high concentration of a particular solute. The pond water contains a lower concentration of the solute. The organism's cell membrane is permeable to water, but not to the solute.
Because the solute concentration is higher inside the cell than outside the cell, water will move into the cell by osmosis.
If the solute concentration is higher inside the cell than outside, water will move into the cell by osmosis.
If, on the other hand, the solute concentration was higher outside the cell than inside, then water would move out of the cell by osmosis.
If the solute concentration is higher outside the cell than inside, water will move out of the cell by osmosis.
The diagram below shows two cells in contact with each other. The cell on the right has a higher solute concentration than the cell on the left. The cell membranes of the two cells are permeable to water but not to the solute.
The cell on the right has a higher solute concentration than the cell on the left.
Water will move by osmosis from the cell with the lower solute concentration to the cell with the higher solute concentration.
Water moves by osmosis into the cell with the higher solute concentration.
As water leaves the cell on left, its volume decreases (since it has less water in it).
Similarly, as the water enters the cell on the right, its volume increases.
The volume of the cell on the left has decreased. The volume of the cell on the right has increased.
Since the cell on the left now has a smaller volume, its solute concentration has increased. This is because the same amount of solute particles are now in a smaller volume and therefore they are more closely packed together.
Similarly, since the cell on the right now has a larger volume, its solute concentration has decreased. This is because the same amount of solute particles are now in a larger volume and therefore they are more spread out.
Therefore, osmosis causes the difference in solute concentration between the two sides of the membrane to decrease. As osmosis continues, the solute concentrations get closer and closer together. If the solute concentrations become equal to each other, osmosis will stop (since osmosis requires a difference in solute concentration).
For your GCSE exams, you do not need to be able to explain why osmosis happens. All you need to know is the information explained above.
Some textbooks and videos give an explanation for osmosis which involves water concentration. This explanation is incorrect.
The problem with this common explanation is given on the next page, along with some other advanced information about osmosis. None of the information on the next page is needed for your exams, so feel free to skip it unless you are curious.
Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.
What is osmosis?
What is a partially permeable membrane?
Why are cell membranes described as partially permeable?
3.2.4 - Beyond The Syllabus: Advanced Osmosis Information
3.2.2 - Factors That Affect the Rate of Diffusion
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