Example 1: percentage change
A potato cylinder goes from 3.00 g to 3.24 g. Find the percentage change in mass.
- Change = 3.24 − 3.00 = 0.24 g.
- Percentage change = 0.24 ÷ 3.00 × 100 = +8.0%.
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IB DemystifiedMYP Sciences
Diffusion, osmosis and active transport
Every second, oxygen enters your blood, water moves in and out of your cells, and roots pull minerals out of the soil. Cells constantly exchange substances with their surroundings through their membranes, using three processes: diffusion, osmosis and active transport.
Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Comparing the three processes
- Transport in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- explain diffusion and the factors that affect its rate
- explain osmosis and its effects on plant and animal cells
- explain active transport and compare it with diffusion
- relate surface area to volume ratio to exchange in organisms
- process osmosis data and design fair experiments
- discuss oral rehydration and salty soils
Before you start
You will use these skills. If any feel shaky, review them first.
- cell structure (see Cells and microscopy)
- particles and states of matter (see States of matter)
- percentages and ratios
Key vocabulary
- Diffusion
- The net movement of particles from a higher to a lower concentration.
- Concentration gradient
- The difference in concentration between two regions.
- Osmosis
- The movement of water through a partially permeable membrane from a dilute to a more concentrated solution.
- Partially permeable membrane
- A membrane that lets small molecules through but not large ones.
- Active transport
- Moving substances from a lower to a higher concentration, using energy from respiration.
- Surface area to volume ratio
- Surface area divided by volume; it falls as organisms get bigger.
Understanding the ideas
-
Diffusion
Particles are always moving, so they spread from where they are concentrated to where they are less concentrated. This is diffusion. It is faster when the gradient is steep, the temperature is high, the surface area is large and the distance is short.
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Osmosis
Osmosis is a special case: water moves through a partially permeable membrane from a dilute solution to a more concentrated one. Plant cells gain water and become firm (turgid) or lose it and become soft (flaccid); animal cells can burst or shrink because they have no cell wall.
-
Active transport
Active transport moves substances against the concentration gradient, for example mineral ions into root hairs or glucose into gut cells. It uses carrier proteins and energy from respiration.
-
Why does it matter?
Large organisms have a small surface area to volume ratio, so they need lungs, gills, villi and transport systems. These ideas also explain food preservation with salt, oral rehydration therapy and the problems of salty soils.
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What does it connect to?
This topic links to cells, respiration, digestion, breathing, circulation and transport in plants.
Comparing the three processes
- Diffusion: any small particles; high → low concentration; no energy needed (passive).
- Osmosis: water only; dilute → concentrated solution across a partially permeable membrane; passive.
- Active transport: ions and molecules; low → high concentration; needs energy from respiration and carrier proteins.
- Faster diffusion: steeper gradient, higher temperature, larger surface area, shorter distance.
Transport in the real world
Oral rehydration solution uses glucose and salt so that water is absorbed by osmosis, saving the lives of children with diarrhoea. Salting and sugaring preserve food by drawing water out of microbes. Farmers in irrigated areas struggle with salty soils that stop roots taking up water.
Worked examples
Example 2: surface area to volume
Find the SA:V ratio of a 4 cm cube.
- Surface area = 6 × 4 × 4 = 96 cm².
- Volume = 4 × 4 × 4 = 64 cm³.
- Ratio = 96 ÷ 64 = 1.5.
Assessment tips
Questions on this topic often use potato or agar data and diagrams of cells. Expect to:
- Explain movements using concentration gradients and correct terms.
- Calculate percentage changes and SA:V ratios.
- Interpret osmosis graphs, including the no-change point.
- Design fair experiments with appropriate controls.
Common mistakes: saying water moves from concentrated to dilute solution; saying sugar moves in osmosis; forgetting that active transport needs energy from respiration; and dividing volume by surface area.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: the naked egg
Partially guided investigation · about 3 days, 20 minutes a day · pairs
- Research question
- What happens to the mass of an egg without its shell when it is placed in water compared with thick sugar syrup?
- Scientific background
- Soaking an egg in vinegar dissolves its shell, leaving the egg inside a thin partially permeable membrane. Water can then move in or out by osmosis.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Two raw eggs, vinegar, two jars, water, thick sugar syrup (or golden syrup), balance, spoon, paper towels.
- Method
- Soak the eggs in vinegar for 1–2 days until the shells dissolve; rinse gently.
- Weigh each egg, then place one in water and one in syrup.
- After 24 hours, lift each out, rinse and blot gently, and weigh again.
- Calculate the percentage change in mass for each egg.
Safety. Handle raw eggs carefully and wash hands afterwards; do not eat the eggs. Wipe up any spills.
Then evaluate: which way did water move for each egg, and how could you make the experiment more reliable?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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