Diffusion and Osmosis Worksheet- Practice Problems
What You Actually Need to Know About Diffusion and Osmosis
Diffusion and osmosis are two of the most fundamental concepts in biology. They're also two of the most misunderstood. Students memorize definitions without understanding the mechanics, then bomb exams when questions get slightly tricky.
This guide cuts through the confusion. You'll get real practice problems, clear explanations, and the common mistakes that cost students points.
Quick Definitions (The Short Version)
Diffusion is the movement of particles from an area of high concentration to an area of low concentration. No energy required—this is passive transport.
Osmosis is diffusion of water across a selectively permeable membrane. Water moves toward the solution with more solutes (less water concentration).
That's it. Everything else builds from these two ideas.
Key Vocabulary You Must Know
- Selectively permeable membrane – Lets some substances through, blocks others. Usually lets water through but blocks larger molecules.
- Hypotonic solution – Lower solute concentration outside the cell. Water moves INTO the cell.
- Hypertonic solution – Higher solute concentration outside the cell. Water moves OUT of the cell.
- Isotonic solution – Equal solute concentration inside and outside. No net water movement.
- Tonicity – The ability of a solution to cause water to move in or out of a cell.
The Tonicity Table (Memorize This)
| Solution Type | Outside vs Inside | Water Movement | Cell Effect |
|---|---|---|---|
| Hypotonic | Less solutes outside | Water moves IN | Cell swells (may burst) |
| Hypertonic | More solutes outside | Water moves OUT | Cell shrinks |
| Isotonic | Equal solutes | No net movement | No change |
Practice Problems with Answers
Problem 1: Basic Diffusion
A drop of food coloring is placed in a glass of still water. What happens?
Answer: The dye spreads from the high concentration point throughout the water until concentration is equal everywhere. No membrane needed—diffusion works in open systems.
Problem 2: Osmosis in Plant Cells
A plant cell is placed in a hypertonic salt solution. What happens?
Answer: Water moves out of the cell. The cytoplasm shrinks away from the cell wall—this is called plasmolysis. The cell becomes flaccid (limp).
Problem 3: Osmosis in Animal Cells
A red blood cell is placed in a hypotonic solution. What happens?
Answer: Water moves into the cell. Animal cells have no cell wall, so they swell and can burst—this is called hemolysis.
Problem 4: Identifying Solution Types
A cell placed in solution A shriveled up. A cell placed in solution B swelled and burst. What can you conclude?
Answer: Solution A is hypertonic (water left the cell). Solution B is hypotonic (water entered the cell).
Problem 5: Predicting Direction of Water Movement
Side A contains 30% sugar. Side B contains 15% sugar. A permeable membrane allows water but not sugar through. Which direction does water move?
Answer: Water moves from Side B (lower solute, higher water concentration) to Side A (higher solute, lower water concentration).
Common Mistakes That Lose Points
- Mixing up diffusion and osmosis – Diffusion moves any particle. Osmosis specifically moves water.
- Forgetting that water moves toward solutes – Water moves to where there's more "stuff" dissolved, not away from it.
- Confusing the prefixes – Hypo means "low," hyper means "high." Students often reverse these.
- Ignoring the cell type – Plant cells behave differently than animal cells because of the rigid cell wall.
- Thinking isotonic means no movement – Water molecules still move both ways, but the rates are equal. Net movement is zero.
How to Use This Worksheet Effectively
Step 1: Cover the answers. Try every problem without looking.
Step 2: Check your work. If you got it wrong, figure out why before moving on.
Step 3: Draw diagrams. Sketching cells in different solutions forces you to visualize the process. This matters on exams.
Step 4: Create your own problems. If you can write a question about tonicity, you understand it.
Quick Study Guide
- Diffusion = particles spread out from high to low concentration
- Osmosis = water moves across a membrane toward higher solute concentration
- Hypotonic = water IN = swelling
- Hypertonic = water OUT = shriveling
- Isotonic = no net movement = stable
- Plant cells = tolerate more water (cell wall prevents bursting)
- Animal cells = no wall = can burst or shrivel
Final Note
Understanding diffusion and osmosis isn't optional. These concepts show up in every biology course, on the AP exam, and in real-world applications like kidney function, plant water uptake, and food preservation. Memorize the table above. Practice the problems until you can solve them cold. That's all you need.