IGCSE Biology 0610 · Common mistakes
Common mistakes in diffusion and osmosis
Updated · by the Fahmazing team
Diffusion and osmosis look simple, but the definitions are strict and the potato-chip calculations are full of traps. Here are the slips we see most often, and how to fix them.
Leaving "net" out of the definition
Define diffusion
✗ Wrong: Particles move from a high concentration to a low concentration using energy.
✓ Right: Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random movement.
Tip: Diffusion is passive. The energy comes from the kinetic energy of the particles, not from respiration. "Energy from respiration" is the answer for active transport.
Free lessons: Diffusion · Active transport
Saying the sugar moves in osmosis
Explain why a potato chip in concentrated sucrose loses mass
✗ Wrong: Sucrose moves into the potato cells.
✓ Right: Water moves out of the potato cells by osmosis, from a dilute solution to a more concentrated solution, through the partially permeable cell membrane.
Tip: Always say WATER moves, say which way, and name the membrane. Sucrose molecules are too big to pass through.
Free lesson: Osmosis
Dividing by the final mass
A chip goes from 2.50 g to 2.00 g. Calculate the percentage change.
✗ Wrong: 0.50 ÷ 2.00 × 100 = 25%
✓ Right: (2.00 − 2.50) ÷ 2.50 × 100 = −20%
Tip: Percentage change always compares with where you started. Keep the minus sign for a loss in mass.
Free lesson: Osmosis
Saying plant cells burst
What happens to a plant cell in pure water?
✗ Wrong: Water enters and the cell bursts.
✓ Right: Water enters by osmosis and the cell becomes turgid. The cell wall stops it bursting. Animal cells, such as red blood cells, can burst.
Free lesson: Osmosis
Getting surface area to volume upside down
Which agar cube has the larger surface area to volume ratio?
✗ Wrong: The big cube, because it has more surface.
✓ Right: The small cube. Work out surface area ÷ volume: smaller cubes have the bigger ratio, so diffusion into them is faster.
Tip: Rate of diffusion = distance ÷ time. Dividing time by distance gives an upside-down answer.
Free lesson: Diffusion
Questions students ask
Is osmosis a type of diffusion?
Yes. Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to lower water potential (concentrated solution) through a partially permeable membrane.
Does diffusion need energy from respiration?
No. Diffusion is passive: the particles move because of their own random movement. Active transport is the process that needs energy from respiration.
How do I find where there is no change in mass on an osmosis graph?
Find where the line crosses 0% change. That concentration matches the concentration inside the cells. Do not just pick the lowest value.