Osmosis

Navigate the knowledge tree: 🌿 Biology ➡ NCEA Level 2 Biology ➡ 2.1 Investigation ➡ Lesson 2: Osmosis

Learning Intentions

By the end of this topic, you should be able to:

Describe what osmosis is and how water moves across a semi-permeable membrane.
Explain how osmosis can change the size and shape of plant and animal cells.
Predict what will happen to cells when placed in different solution concentrations.

Glossary

Key terms and definitions.

cell membrane: A semi-permeable barrier around a cell that controls what enters and leaves.

cell wall: Rigid structural organelle that surrounds the cell membrane and provides support to the plant cell.

concentrated solution: A solution that has lots of solute and less water.

dilute solution: A solution that has little solute and lots of water.

equilibrium: When particles (or water) are balanced and there is no net movement.

flaccid: A plant cell that has lost water and feels soft or floppy; the state of a plant cell in a hypertonic solution.

hypertonic: Solution that has a higher concentration of solute than the cell. Causes water to move out of the cell. Cells shrink.

hypotonic: Solution that has a lower concentration of solute than the cell. Causes water to move into the cell. Plant cells become turgid, animal cells may burst.

isotonic: Solution that has an equal concentration of solute compared with the cell. There is no net movement of water, and cells stay the same size.

osmosis: Movement of water molecules from an area of high concentration to an area of low concentration through a semi-permeable membrane.

osmotic potential: How strongly a solution pulls water toward it because of dissolved solutes.

plasmolysis: Collapse of a plant cell's cytoplasm due to a lack of water.

pressure potential: The effect of physical pressure (wall pressure and turgor pressure) on water inside a cell.

semi-permeable: Ability of cell membranes to allow some substances to pass it but not others.

solute: The substance that is dissolved in a solution.

solvent: The liquid that dissolves something.

tonicity: Difference in relative concentration of solutes of two solutions, which determines the direction of osmosis.

turgid: A plant cell that is full of water and firm. This helps plants stand upright.

turgor pressure: The pressure created by water and cell contents pushing up against the cell wall.

wall pressure: The resisting force from the cell wall pushing back against turgor pressure. This stops the plant cell from bursting when in hypotonic solutions.

water potential: A way of describing how likely water is to move by osmosis.

What is osmosis?

Osmosis is a special type of diffusion — but it only involves water.

Osmosis is the movement of water molecules from a region where there is more water (dilute solution with a lower solute concentration) to a region where there is less water (concentrated solution with a higher solute concentration) through a semi-permeable membrane.

(Remember from the lesson on diffusion that a semi-permeable membrane allows some particles (like water) to pass through, but not others (like large or charged molecules). A cell membrane is a semi-permeable membrane.)

Think of it like this:

Dilute solution = lots of water (solvent), little solute (e.g. little salt/sugar)
Concentrated solution = less water, lots of solute

Water moves to balance both sides out, until solute concentrations become more similar. An equilibrium is reached when the net movement of water is zero.

It's important to remember that osmosis is a special type of diffusion, therefore cells don't spend energy for osmosis — water moves passively because of concentration differences.

How to describe the forces relating to osmosis

Water potential

The water potential of a solution describes the tendency for water molecules to enter or leave a solution by osmosis. The greater the movement of water molecules, the higher the water potential.

The presence of solutes (e.g. salt or water) lowers water potential because the solutes restrict the movement of water molecules.

Pure water has the highest water potential (zero). Dissolving any solute (like salt or sugar) in water lowers the water potential (makes it more negative). Water always diffuses from regions of higher (less negative) water potential to lower (more negative) water potential.

Osmotic potential (osmotic pressure)

When substances dissolve in water (like salt or sugar), they pull water towards them. The more dissolved solute a solution has, the stronger its pull on water. This is called osmotic potential (or osmotic pressure).

For example:

Fresh water = few solutes = weak pull = low osmotic potential
Salt water = many solutes = strong pull = high osmotic potential

Therefore, water moves into salt water because it has a greater osmotic potential.

Pressure potential (wall pressure)

Pressure potential is the tendency of water to move in response to pressure.

Turgor pressure is the outward force from water inside the plant cell pushing against the cell wall, while wall pressure is the equal and opposite inward force exerted by the rigid cell wall resisting this expansion.

Turgor pressure provides rigidity to the cell, and without it the cell becomes flaccid and the plant wilts. Wall pressure balances out turgor pressure and prevents the plant cell from bursting and maintains structure.

What effect does osmosis have on plant cells?

Plant cells and turgor

When water enters a plant cell, the cell contents push against the cell wall to create turgor (tightness). Turgor gives support for the plant body and helps the plant keep upright. When cells lose water, there is a loss of turgor and the cell becomes soft or flaccid, causing the plant to wilt. Because the cell wall is rigid, when the water leaves the cell, the cell membrane shrinks away from the cell wall and the cell becomes flaccid. When too much water leaves the plant cell, the cell membrane pulls away from the cell wall. This is called plasmolysis and is irreversible because the cell cannot recover by taking up water.

Plant cells in different solution concentrations

Tonicity is the measure of the difference osmotic potential (gradient) between two solutions - in this case, that's the solution outside of the plant cell and the solution inside the plant cell.

Solutions are usually categorised as isotonic, hypotonic or hypertonic, depending on the relative solute concentration across a cell membrane.

A hypotonic solution (more water outside) has a higher water potential than the cell.

Water moves into the plant cell. Cell contents and the cell membrane push against the cell wall to create turgor.

The cell doesn't rupture because the cell wall is rigid. Cells in this state are turgid and the plant stands upright.

An isotonic solution has the same water potential as the cell.

There is no net movement of water because there is no osmotic potential gradient between the two solutions.

As a result, there is no change in the cell size or shape.

A hypertonic solution (less water outside) has a lower water potential than the cell.

Water moves out of the plant cell, and the volume of the cell decreases, causing the cell membrane to shrink away from the cell wall.

With less pressure potential (wall pressure and turgor pressure), the cell becomes flaccid and the plant wilts. If too much water is lost, the cell becomes irreversibly plasmolysed.

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