Osmosis | IGCSE Biology Revision Notes

1. Overview

Osmosis is a specific type of diffusion focusing exclusively on the movement of water. It is a fundamental process in biology that determines how cells maintain their shape, how plants stay upright, and how nutrients and waste products are transported in solution throughout an organism.

Key Definitions

Osmosis: The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
Partially Permeable Membrane: A barrier (like the cell membrane) that allows small molecules like water to pass through but prevents larger solute molecules (like sugar or salt) from crossing.
Solvent: A substance (usually a liquid) in which other materials dissolve to form a solution. Water is the "universal solvent" in biological systems.
Water Potential: A measure of the "free" water molecules in a solution; pure water has the highest water potential.
Turgid: A term used to describe a plant cell that is swollen and firm due to high internal water pressure.
Plasmolysis: The process where the cell membrane pulls away from the cell wall because the cell has lost too much water.

Core Content

The Role of Water as a Solvent

Water is essential for life because it acts as a solvent in:

Digestion: Food molecules must be dissolved in water to be broken down by enzymes and absorbed into the blood.
Excretion: Waste products like urea and excess salts are dissolved in water to form urine, allowing them to be removed from the body.
Transport: In animals, blood plasma (mostly water) transports glucose and CO2. In plants, water in the xylem and phloem transports minerals and sucrose.

Mechanism of Osmosis

Water molecules move randomly due to kinetic energy.
When a membrane separates two solutions of different concentrations, more water molecules move from the dilute side to the concentrated side than vice-versa.
This results in a net movement of water.
📊Two compartments separated by a dashed line (membrane). Left side has many blue dots (water) and few red circles (sugar). Right side has few blue dots and many red circles. An arrow shows the net flow of blue dots from left to right.

Investigating Osmosis: Dialysis (Visking) Tubing

Dialysis tubing is an artificial partially permeable membrane used to model the cell membrane.

Fill tubing with a concentrated sugar solution.
Place the tubing in a beaker of pure water.
Observation: The tubing becomes firm and increases in volume/mass as water moves in by osmosis.

Osmosis in Plant Tissues

When plant tissues (like potato cylinders) are immersed in solutions:

In Pure Water: Water enters the cells. The cells increase in mass and length.
In Concentrated Sugar/Salt Solution: Water leaves the cells. The cells decrease in mass and length; the tissue becomes "soft" or "floppy."

Support in Plants

Plants do not have a skeleton. They rely on turgor pressure.

When water enters a plant cell, it pushes the cytoplasm against the rigid cell wall.
This internal pressure keeps the cells firm (turgid).
These turgid cells press against each other, providing structural support for the stem and leaves.

Extended Content (Extended Only)

Water Potential and Movement

Instead of "concentration," use the term Water Potential ($\Psi$).

High Water Potential: A dilute solution (lots of water, little solute).
Low Water Potential: A concentrated solution (little water, lots of solute).
Water always moves down a water potential gradient.

Effects on Plant Cells: Step-by-Step

Turgid (Hypotonic solution): The external solution has a higher water potential than the cell sap. Water enters by osmosis. The vacuole increases in size, pushing the cell membrane against the cell wall.
Flaccid (Isotonic solution): The water potential is equal inside and outside. There is no net movement of water. The cell is not firm.
Plasmolysed (Hypertonic solution): The external solution has a lower water potential than the cell sap. Water leaves the cell. The vacuole shrinks, and the cell membrane pulls away from the cell wall (plasmolysis).

📊Three plant cells. 1. Turgid: Rectangular, large vacuole, membrane tight against wall. 2. Flaccid: Slightly rounded corners, vacuole smaller. 3. Plasmolysed: Clear gaps between the cell wall and the shrunken cell membrane/contents.

Importance of Water Potential in Organisms

Root Hair Cells: These cells maintain a lower water potential than the soil water (by pumping in ions), ensuring water enters the plant via osmosis.
Animal Cells: Unlike plants, animal cells lack a cell wall. If placed in pure water (high water potential), they will take in so much water that the cell membrane bursts (lysis). This is why blood plasma concentration must be strictly controlled.

Key Equations

Percentage Change in Mass This is the most common calculation in Osmosis exam questions.

$$\text{Percentage Change} = \frac{\text{Change in Mass}}{\text{Initial Mass}} \times 100$$

Change in Mass = Final Mass - Initial Mass
Units: Percent (%)
Note: If the final mass is lower than the initial mass, the percentage change will be a negative number (indicating mass loss).

Common Mistakes to Avoid

❌ Wrong: Water moves from a concentrated solution to a dilute solution.
✅ Right: Water moves from a dilute solution (high water potential) to a concentrated solution (low water potential).
❌ Wrong: Saying the plant cell "bursts" when placed in water.
✅ Right: Plant cells become turgid; the cell wall prevents them from bursting. (Only animal cells burst).
❌ Wrong: Using "amount" of water.
✅ Right: Use "water potential" or "concentration of water molecules."

Exam Tips

Command Word - "Explain": If asked to explain why a potato lost mass, you must mention: 1. Water potential gradient (higher inside, lower outside), 2. Direction of movement (out of cell), 3. The process (osmosis), and 4. The membrane (partially permeable).
Typical Numerical Values: In experiments, concentrations are often given in mol/dm³. 0.0 mol/dm³ is pure water; values like 0.5 to 1.5 mol/dm³ are common for sugar/salt solutions.
The X-Intercept: In graphs showing percentage change in mass, the point where the line crosses the X-axis (0% change) indicates the concentration of the cell sap inside the tissue.
Contexts: Expect questions involving Visking tubing, potato cylinders, or red blood cells. Always identify where the water potential is highest first.