Movement of Substances
3. Osmosis
Learning outcomes
- I can define osmosis.
- I can explain the movement of water across partially permeable membranes.
- I can describe the effects of osmosis on plant and animal cells.
- I can compare osmosis and diffusion.
- I can predict osmotic changes in different environments.
Osmosis
Osmosis is a special type of movement involving water molecules.
Cells are surrounded by cell membranes that control the movement of substances between the cell and its environment. Because water can cross these membranes, changes in the concentration of water and dissolved substances around a cell can cause water to move into or out of the cell.
Osmosis is extremely important in living organisms because cells must maintain the correct amount of water to function properly.
What Is Osmosis?
Osmosis is the net movement of water molecules across a partially permeable membrane from an area of higher water concentration to an area of lower water concentration.
Another way to describe this is:
Water moves from a dilute solution toward a more concentrated solution through a partially permeable membrane.
Osmosis is a passive process.
This means the cell does not need to provide energy for osmosis to occur.
Understanding Osmosis
Imagine two solutions separated by a partially permeable membrane.
Side A contains:
- many water molecules
- few dissolved solute particles
Side B contains:
- fewer water molecules
- many dissolved solute particles
Water shows a net movement:
Side A → Side B
The following visualization shows how changing the solute concentration outside a cell affects water movement across the membrane.
Water moves toward the side with the higher concentration of dissolved substances, because that side has a lower concentration of free water molecules.
What Is a Partially Permeable Membrane?
A partially permeable membrane allows some substances to pass through but prevents others from crossing easily.
The cell membrane is partially permeable.
Water molecules can cross the membrane, while many larger dissolved substances cannot move freely through it.
This difference allows osmosis to occur.
Water and Solute Concentration
A solute is a substance dissolved in a liquid.
Examples include:
- salt
- sugar
- mineral ions
A solution with relatively little solute is dilute.
A solution with a large amount of solute is concentrated.
Compare these two solutions:
Solution A
Lots of water + little salt
This has a relatively high water concentration.
Solution B
Less water + lots of salt
This has a relatively low water concentration.
If a partially permeable membrane separates them, water shows a net movement:
Solution A → Solution B
Osmosis and the Particle Model
Like diffusion, osmosis can be explained using the particle model.
Water molecules:
- are constantly moving
- move randomly
- collide with other particles
- can move across suitable membranes
Water molecules actually move in both directions across the membrane.
However, if one side has a higher water concentration, more water molecules move from that side than move back.
The result is a net movement of water.
Osmosis Continues Toward Equilibrium
As water moves across the membrane, the difference between the two sides becomes smaller.
Eventually, the system may approach dynamic equilibrium.
At equilibrium:
- water molecules continue moving
- water crosses in both directions
- approximately equal amounts move each way
- there is no net movement of water
The water molecules have not stopped moving.
Predicting the Direction of Osmosis
To predict osmosis, compare the solutions on either side of the membrane.
Ask:
Which side is more dilute?
Then ask:
Which side is more concentrated?
Water will show a net movement:
more dilute solution → more concentrated solution
For example:
Inside cell: 5% salt
Outside cell: 1% salt
The outside solution is more dilute.
Therefore, water moves:
outside → inside
Osmosis in Animal Cells
Animal cells are surrounded by a cell membrane but do not have a rigid cell wall.
This means that gaining or losing too much water can significantly change their size and shape.
Animal Cell in a Dilute Solution
Suppose an animal cell is placed in a solution that is more dilute than the cell contents.
The surrounding solution has:
- higher water concentration
- lower solute concentration
Water moves:
outside → inside the cell
The cell gains water and swells.
If too much water enters, the cell membrane may rupture.
This is called lysis.
For red blood cells, rupture caused by excessive water entry is called haemolysis.
Animal Cell in a Concentrated Solution
Suppose an animal cell is placed in a solution that is more concentrated than its contents.
The surrounding solution has:
- lower water concentration
- higher solute concentration
Water moves:
inside → outside the cell
The cell loses water and shrinks.
A red blood cell that loses water becomes shrivelled or crenated.
Animal Cell in an Isotonic Solution
An isotonic solution has approximately the same effective solute concentration as the cell.
Water still moves across the membrane in both directions.
However:
water entering ≈ water leaving
Therefore, there is no net movement of water.
The animal cell remains approximately its normal size.
Animal Cells Compared
| Environment | Net Water Movement | Effect on Animal Cell |
|---|---|---|
| More dilute outside | Into cell | Cell swells; may burst |
| Similar concentration | No net movement | Cell remains normal |
| More concentrated outside | Out of cell | Cell shrinks |
Osmosis in Plant Cells
Plant cells respond differently to osmosis because they have a strong cell wall outside the cell membrane.
The cell wall prevents the cell from easily bursting when water enters.
This makes osmosis particularly important for supporting plants.
Plant Cell in a Dilute Solution
When a plant cell is surrounded by a dilute solution, water enters by osmosis.
Water moves:
outside → inside
The vacuole fills with water and becomes larger.
The contents of the cell push outward against the cell wall.
The cell becomes turgid.
A turgid cell is firm and well supported.
This is usually the normal and healthy condition for many plant cells.
Turgor Pressure
As water enters a plant cell, the cell contents push against the cell wall.
This produces turgor pressure.
Turgor pressure helps:
- support leaves
- support young stems
- keep plant tissues firm
- prevent wilting
The rigid cell wall prevents the plant cell from continuing to expand until it bursts.
This is an important difference between plant and animal cells.
Plant Cell in a Concentrated Solution
If a plant cell is placed in a concentrated solution, water leaves the cell by osmosis.
Water moves:
inside → outside
The vacuole becomes smaller.
The cytoplasm shrinks.
The cell membrane may pull away from the cell wall.
This condition is called plasmolysis.
A severely plasmolysed cell has lost a significant amount of water.
Flaccid Plant Cells
When plant cells lose some water, they lose turgor pressure.
They become flaccid.
If many cells in a plant become flaccid, the plant may begin to wilt.
This is why plants often wilt when they cannot absorb enough water from the soil.
Plant Cells Compared
| Environment | Net Water Movement | Effect on Plant Cell |
|---|---|---|
| More dilute outside | Into cell | Becomes turgid |
| Similar concentration | Little/no net movement | May become flaccid |
| More concentrated outside | Out of cell | Becomes plasmolysed |
Why Plant Cells Do Not Normally Burst
Animal cells may burst if too much water enters them.
Plant cells usually do not.
This is because plant cells have a rigid cell wall.
As water enters:
- the vacuole expands
- the cytoplasm pushes outward
- pressure builds against the cell wall
- the cell wall resists further expansion
The cell becomes turgid rather than bursting.
This is a good example of how cell structure affects cell function.
Osmosis and Plant Support
Plants do not have skeletons like animals.
Instead, water inside plant cells contributes to structural support.
When cells are turgid:
- leaves remain firm
- stems remain upright
- plant tissues maintain their shape
When cells lose water:
- turgor pressure decreases
- cells become flaccid
- the plant may wilt
Comparing Osmosis and Diffusion
Osmosis and diffusion are closely related processes.
Both involve the passive movement of particles.
However, there are important differences.
| Feature | Diffusion | Osmosis |
|---|---|---|
| What moves? | Any suitable particles | Water molecules only |
| Membrane required? | Not always | Yes |
| Direction | High concentration to low concentration | High water concentration to low water concentration |
| Energy required? | No | No |
| Example | Oxygen entering a cell | Water entering a plant cell |
A useful way to remember the difference is:
Diffusion = movement of particles
Osmosis = movement of water across a partially permeable membrane
Diffusion and Osmosis Both Depend on Gradients
Both processes involve differences between two regions.
In diffusion, we usually consider the concentration gradient of the substance itself.
For example:
High oxygen concentration → low oxygen concentration
In osmosis, we consider differences in water concentration across a partially permeable membrane.
For example:
High water concentration → low water concentration
Both processes continue toward equilibrium without requiring cellular energy.
Worked Example: Animal Cell
An animal cell contains a solution that is approximately 2% dissolved substances.
It is placed in a solution containing 8% dissolved substances.
Outside: 8% solute
Inside: 2% solute
The outside solution is more concentrated.
Therefore, water moves:
inside → outside
The animal cell will:
lose water and shrink
Worked Example: Plant Cell
A plant cell contains 5% dissolved substances.
It is placed in a solution containing 1% dissolved substances.
Outside: 1% solute
Inside: 5% solute
The outside solution is more dilute.
Therefore, water moves:
outside → inside
The plant cell gains water and becomes:
turgid
Worked Example: Equal Concentrations
A cell contains a 4% solution.
It is placed in a 4% solution.
The concentrations are approximately equal.
Water molecules still move across the membrane in both directions.
However:
water entering ≈ water leaving
Therefore, there is no net movement of water.
Worked Example: Predicting Osmosis
Consider a cell surrounded by the following conditions:
Inside cell: 10% sugar
Outside cell: 3% sugar
Which direction will water move?
The outside has less sugar and therefore a higher water concentration.
The inside has more sugar and therefore a lower water concentration.
Water moves:
outside → inside
The cell will gain water.
The exact effect depends on whether it is a plant or animal cell.
If it is an animal cell, it may swell.
If it is a plant cell, it will become more turgid.
Osmosis in Root Hair Cells
Plants absorb water from the soil through root hair cells.
Root hairs provide a large surface area for water absorption.
When the soil has a higher water concentration than the root cells, water moves into the root hair cells by osmosis.
The water can then move through the root and eventually enter the plant's transport system.
This water is needed for:
- photosynthesis
- transport
- maintaining turgor pressure
- cooling through transpiration
- chemical reactions
Osmosis in Food Preservation
Osmosis also helps explain why high concentrations of salt or sugar can be used to preserve some foods.
A highly concentrated environment can cause microorganisms to lose water.
Water moves:
microorganism → surrounding concentrated solution
This makes it difficult for many microorganisms to grow and reproduce.
This is one reason why foods such as jams contain large amounts of sugar and why salt has historically been used to preserve some foods.
Osmosis Experiments
A common experiment uses pieces of potato placed in solutions of different concentrations.
Students may measure:
- initial mass
- final mass
- initial length
- final length
If a potato piece gains mass:
water entered the potato cells
If it loses mass:
water left the potato cells
If there is almost no change:
there was little or no net movement of water
This type of investigation can be used to estimate the concentration of substances inside plant cells.
Calculating Percentage Change
In osmosis experiments, percentage change is often more useful than simply comparing changes in mass.
The formula is:
Percentage change = (final value − initial value) ÷ initial value × 100
For example:
Initial potato mass = 5.0 g
Final potato mass = 5.5 g
Change = 5.5 − 5.0 = 0.5 g
Percentage change = 0.5 ÷ 5.0 × 100
Percentage change = 10%
The potato gained:
10% of its original mass
This suggests that water entered the potato cells.
Predicting Osmotic Changes
When solving an osmosis problem, use the following steps.
Step 1: Identify the membrane
Is there a partially permeable membrane separating the two solutions?
Step 2: Compare the solutions
Which side is more dilute?
Which side is more concentrated?
Step 3: Predict water movement
Water moves from:
more dilute → more concentrated
Step 4: Predict the effect
For an animal cell:
- water enters → swelling
- water leaves → shrinking
For a plant cell:
- water enters → turgid
- water leaves → flaccid or plasmolysed
Common Misconceptions
Osmosis is the movement of any substance across a membrane.
Incorrect. Osmosis specifically refers to the movement of water.
Water moves from high solute concentration to low solute concentration.
Incorrect. Water shows a net movement toward the region with the higher solute concentration.
Water molecules only move in one direction during osmosis.
Water molecules move randomly in both directions. Osmosis describes the net movement.
Plant cells burst when water enters them.
Normally they do not because the rigid cell wall resists expansion.
Animal cells have cell walls that prevent them from bursting.
Animal cells do not have cell walls.
Osmosis requires energy from the cell.
Osmosis is passive and does not require cellular energy.
At equilibrium, water molecules stop moving.
Water continues moving in both directions, but there is no net movement.
Did You Know?
Freshwater organisms face an interesting osmotic challenge.
Their surroundings can be much more dilute than their body fluids, meaning water continually tends to enter their cells.
Some single-celled freshwater organisms have structures called contractile vacuoles that collect excess water and pump it out of the cell.
Without this mechanism, too much water could accumulate inside the organism.
Key Terms
Osmosis – The net movement of water molecules across a partially permeable membrane from higher water concentration to lower water concentration.
Partially permeable membrane – A membrane that allows some substances to cross more easily than others.
Solute – A substance dissolved in a liquid.
Dilute solution – A solution containing relatively little solute.
Concentrated solution – A solution containing relatively more solute.
Isotonic – Having approximately the same effective solute concentration.
Turgid – A plant cell that is firm because it has gained water.
Turgor pressure – Pressure produced when the contents of a plant cell push against the cell wall.
Flaccid – A plant cell that has lost turgor pressure.
Plasmolysis – The condition in which a plant cell loses enough water for the cell membrane and cytoplasm to pull away from the cell wall.
Lysis – Rupture of a cell.
Passive transport – Movement that does not require cellular energy.
Key Takeaways
- Osmosis is the net movement of water molecules across a partially permeable membrane.
- Water moves from an area of higher water concentration to lower water concentration.
- This can also be described as movement from a more dilute solution toward a more concentrated solution.
- Osmosis is passive and does not require energy from the cell.
- Water molecules continue moving in both directions even when equilibrium is reached.
- Animal cells may swell and burst if too much water enters.
- Animal cells shrink when they lose water.
- Plant cells become turgid when water enters.
- Plant cells become flaccid and may become plasmolysed when water leaves.
- The plant cell wall prevents turgid cells from easily bursting.
- Osmosis helps plants absorb water through their roots and maintain structural support.
- Diffusion can involve many different particles, while osmosis specifically involves water moving across a partially permeable membrane.
- To predict osmosis, compare the concentrations on either side of the membrane and determine which side has the higher water concentration.