Osmosis

Water moves in and out of cells constantly, and getting the water balance right is a matter of life and death for a cell. The special name for the diffusion of water across a cell membrane is osmosis. This lesson, part of Topic B2 of OCR Gateway Science A, defines osmosis, explains what happens to animal and plant cells in solutions of different strengths, and works carefully through the required practical that investigates osmosis in potato (plant) tissue — including the key percentage change in mass calculation that examiners love to set.

By the end of this lesson you should be able to define osmosis, describe its effect on animal and plant cells, calculate a percentage change in mass, and describe the required practical investigating osmosis in plant tissue.

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What Is Osmosis?

Osmosis is the net movement of water molecules across a partially permeable membrane, from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution).

Two phrases need unpacking:

• A partially permeable membrane lets small water molecules through but not larger solute molecules (such as sugar). The cell membrane is partially permeable.
• Higher water concentration means a dilute solution (lots of water, little solute); lower water concentration means a concentrated solution (less water, more solute). Water moves from where there is more water to where there is less.

So osmosis is really just diffusion of water — the net movement of water down its own concentration gradient, through a membrane that holds the solute back. Like diffusion, it is passive and needs no energy.

In the diagram, the large brown circles are solute molecules, too big to cross the membrane; the small blue circles are water molecules, which do cross. The net flow of water is from the dilute side towards the concentrated side.

Exam Tip: A full definition of osmosis must include all three ideas: water molecules, a partially permeable membrane, and movement from high water concentration (dilute) to low water concentration (concentrated). Saying only "water moves across a membrane" loses marks.

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Osmosis and Animal Cells

Animal cells have no cell wall, so they are vulnerable to gaining or losing too much water:

Surrounding solution	Water movement	Effect on an animal cell (e.g. red blood cell)
More dilute than the cell (lots of water outside)	Water moves in by osmosis	The cell swells and may burst (lyse)
More concentrated than the cell (little water outside)	Water moves out by osmosis	The cell shrinks / shrivels (crenates)
Same concentration as the cell (isotonic)	No net movement	The cell stays the same

This is why body fluids must be kept at a steady concentration — a red blood cell placed in pure water would take in water and burst.

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Osmosis and Plant Cells

Plant cells do have a strong cell wall, which changes the picture completely — the wall stops the cell bursting and gives plants their support.

Surrounding solution	Water movement	Effect on a plant cell
More dilute than the cell (e.g. pure water)	Water moves in	The cell becomes turgid (firm); the wall stops it bursting
More concentrated than the cell	Water moves out	The cell becomes flaccid (floppy), then plasmolysed (membrane pulls away from the wall)
Same concentration (isotonic)	No net movement	The cell stays the same

• Turgid: the vacuole fills with water and pushes the cell contents against the cell wall, making the cell firm. Many turgid cells together keep a non-woody plant upright and its leaves held out.
• Flaccid: the cell has lost water and is floppy; if many cells are flaccid the plant wilts.
• Plasmolysed: so much water has left that the cell membrane pulls away from the cell wall. This happens only in very concentrated solutions.

Exam Tip: Know the three plant-cell states and the order they occur in as a cell loses water: turgid → flaccid → plasmolysed. The plant cell wall is the reason a plant cell becomes turgid rather than bursting like an animal cell.

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Required Practical: Investigating Osmosis in Plant Tissue

A core practical of Topic B2 investigates osmosis using pieces of potato (plant tissue) placed in sugar or salt solutions of different concentrations. By measuring how the mass of each piece changes, you can tell which way water has moved.

Method

flowchart TD
  A["Cut several potato cylinders<br/>of equal size using a cork borer"] --> B["Blot dry and measure the<br/>starting mass of each on a balance"]
  B --> C["Place each piece in a different<br/>concentration of sugar solution"]
  C --> D["Leave for a set time<br/>(e.g. 30 minutes)"]
  D --> E["Remove, blot dry the same way,<br/>and measure the final mass"]
  E --> F["Calculate the percentage change<br/>in mass for each concentration"]

Variables

Variable type	In this experiment
Independent (what you change)	The concentration of the sugar solution
Dependent (what you measure)	The change in mass of the potato pieces
Control (kept the same)	Size of potato pieces, temperature, time in solution, type of potato, how they are blotted dry

A few method points the exam rewards:

• Blot the pieces dry before each weighing (so surface water does not affect the mass) — and blot them the same way each time for a fair test.
• Use pieces of equal size so the comparison is fair.
• Because pieces are different starting sizes in practice, you calculate the percentage change in mass (not just the change in grams), so the results can be compared fairly.

Results and the percentage-change calculation

The mass change tells you which way water moved by osmosis:

• If the potato gains mass, water moved into the cells — the solution was more dilute than the cell contents.
• If the potato loses mass, water moved out of the cells — the solution was more concentrated than the cell contents.
• If the mass stays the same, the solution was at the same concentration as the cell contents (the isotonic point) — no net movement.

The equation for percentage change in mass is:

$\text{percentage change in mass} = \frac{\text{change in mass}}{\text{starting mass}} \times 100$percentage change in mass=starting masschange in mass​×100

Worked Example — Percentage change in mass

A potato cylinder has a starting mass of $4.0 \text{ g}$4.0 g. After 30 minutes in dilute sugar solution its mass is $4.6 \text{ g}$4.6 g. Calculate the percentage change in mass.

Step 1 — find the change in mass (final − starting):

$\text{change in mass} = 4.6 \text{ g} - 4.0 \text{ g} = 0.6 \text{ g}$change in mass=4.6 g−4.0 g=0.6 g

Step 2 — apply the equation:

$\text{percentage change} = \frac{\text{change in mass}}{\text{starting mass}} \times 100 = \frac{0.6 \text{ g}}{4.0 \text{ g}} \times 100 = 15\%$percentage change=starting masschange in mass​×100=4.0 g0.6 g​×100=15%

Answer: a $+15\%$+15% change in mass. The mass increased, so water moved into the potato by osmosis — the solution was more dilute than the cell contents.