The Osmosis Lab

The Osmosis Lab 🧪

A cell has to let some things in and keep others out — oxygen in, waste out, water balanced, minerals pulled in even when they're scarce. There are **three** ways substances cross a membrane: **diffusion**, **osmosis** and **active transport**. Tell them apart fluently and the marks are yours.

Diffusion: spreading out 🌫️

**Diffusion** is the net movement of particles from a region of **higher** concentration to a region of **lower** concentration — down the **concentration gradient**. It's **passive**: no energy from respiration is needed. Oxygen and carbon dioxide cross cell membranes this way. Diffusion is **faster** when the concentration gradient is steeper, the temperature is higher, or the surface area is larger.

Speed it up

Which change would make a substance diffuse **faster** across a membrane?

• A steeper concentration gradient
• A lower temperature
• A smaller surface area
• A thicker membrane to cross

Osmosis: water only 💧

**Osmosis** is a special case of diffusion for **water**. It is the movement of **water** across a **partially permeable membrane** from a **dilute** solution (high water concentration) to a more **concentrated** solution (lower water concentration). Like diffusion, osmosis is **passive** — no energy needed. The catch that trips students up: osmosis moves **water**, not the dissolved solute.

What actually moves?

In osmosis, what moves across the partially permeable membrane?

• Water molecules
• The dissolved sugar molecules
• Water and sugar equally
• Only salt ions

Pin down the definition

Osmosis is the movement of _____ across a _____ membrane, from a _____ solution to a more concentrated one.

RP3: the potato experiment 🥔

**Required practical 3** measures how the concentration of a sugar solution affects a plant tissue. You cut equal-sized **potato cylinders**, weigh each one, then leave each in a different sugar concentration. If water moves **in** by osmosis, the cylinder **gains** mass; if water moves **out**, it **loses** mass. To compare fairly you record the **percentage change in mass**, not the raw mass.

RP3: get the method right

An interactive activity.

Why blot the cylinders?

Why must each potato cylinder be **blotted dry** before it is weighed?

• Surface water would add to the mass and give a false reading
• To clean sugar off so it tastes better
• To remove the water from inside the cells
• To make the osmosis happen faster

Percentage change in mass

An interactive activity.

Reading the graph

On a graph of **percentage change in mass** (y-axis) against **sugar concentration** (x-axis), the line crosses **zero** at 0.3 mol/dm³. What does that crossing point tell you?

• The cell sap concentration equals 0.3 mol/dm³, so there is no net osmosis
• The solution there is pure water
• The potato cells have died
• Osmosis is happening fastest there

Active transport: uphill ⚡

Sometimes a cell needs to move a substance the **wrong** way — from a **low** concentration to a **higher** one, **against** the concentration gradient. Diffusion won't do that. **Active transport** moves substances against the gradient, so it **needs energy** released by **respiration**. Two examples: **root hair cells** absorb mineral ions from very dilute soil water, and the gut absorbs **sugar** into the blood even when blood sugar is already higher.

Sort the three

In the exam 🎓

Lab notes written up. Grade-9 habits for transport in cells: • **Diffusion** and **osmosis** are **passive** (no energy); **active transport** needs **energy** because it works **against** the gradient. • **Osmosis** moves **water** only, across a **partially permeable** membrane. • RP3: compare with **percentage change in mass** (starting masses differ), and **blot dry** first so surface water doesn't inflate the reading.