Grade by Grade
B. Life Systems · Cells

Diffusion: How Substances Move Into and Out of Cells

B2.4 A1.2 A1.5About 55 minutes
Learning goals
  • I can explain diffusion using the particle theory and the idea of concentration.
  • I can explain how a selectively permeable cell membrane lets some particles diffuse into and out of a cell.
  • I can predict which way a substance will diffuse.
  • I can plan and carry out a fair test on a factor that affects the rate of diffusion.

Words to know

diffusion
The movement of particles from an area where they are more concentrated to an area where they are less concentrated, until they are evenly spread.
concentration
The amount of a substance in a certain volume. A lot of sugar in a little water is a high concentration.
concentration gradient
A difference in concentration between two areas. Particles diffuse "down" the gradient, from high to low.
selectively permeable
Letting some substances pass through but not others. Cell membranes are selectively permeable.
equilibrium
When particles are evenly spread, so there is no overall movement in one direction (although particles keep moving).
rate
How fast something happens.

Remember from Grade 7

In Grade 7 you learned the particle theory of matter: all matter is made of tiny particles that are always moving, and they move faster when they are warmer. You also learned that particles spreading out through another substance is called diffusion. That's why the smell of popcorn spreads from the kitchen to every room in the house.

This lesson asks a new question: how does diffusion help cells get what they need?

High to low

Think of a crowded school hallway at the end of the day. As soon as the doors open, students spread out into the empty schoolyard. Nobody tells them to go from crowded to less crowded; it just happens as everyone moves around.

Particles do the same thing. Where particles of a substance are crowded, we say the concentration is high. Because the particles are always moving randomly, more of them move from the high concentration area into the low concentration area than the other way. Over time they become evenly spread. That's diffusion.

When they are evenly spread, we reach equilibrium. The particles don't stop moving; they just move equally in all directions, so nothing changes overall.

Blue food colouring spreading through a clear glass of water.
The colouring starts dark and concentrated, then spreads into pale clouds. Its particles move from where they are crowded to where there are fewer of them. AI-generated image
BeforeLatermanyparticlesfewparticlesspread out evenly
Diffusion across a membrane (dashed line). Before: many particles on the left and few on the right. Later: the particles have spread out until there are about the same number on each side.

Diffusion in cells

The cell membrane is selectively permeable: it lets some particles pass and blocks others. Small particles such as oxygen, carbon dioxide and water can pass through it. Large particles, such as starch and proteins, can't diffuse through.

  • Oxygen in. Mitochondria use oxygen all the time, so the concentration of oxygen inside a cell is lower than in the blood around it. Oxygen diffuses into the cell.
  • Carbon dioxide out. Cellular respiration makes carbon dioxide, so its concentration is higher inside the cell. It diffuses out into the blood, which carries it to the lungs.
  • In your lungs, oxygen diffuses from the air in tiny air sacs into the blood, and carbon dioxide diffuses from the blood into the air sacs, to be breathed out.
  • In a leaf, carbon dioxide diffuses in through tiny openings, and the oxygen made by photosynthesis diffuses out.

Diffusion is passive: the cell doesn't use any energy for it. The movement of the particles does all the work.

FactorEffect on the rate of diffusionWhy (particle theory)
Higher temperatureFasterParticles move faster when warmer.
Bigger difference in concentrationFasterMany more particles move from the crowded side than come back.
Smaller particlesFasterSmall particles move faster and fit through the membrane more easily.
Shorter distanceFasterParticles don't have as far to travel. That is one reason cells are so small.

Coming up in high school

Diffusion always goes from high to low concentration. But cells can also use energy to pump particles from low to high, for example when root cells take in minerals from soil where there are very few. This is called active transport, and you will study it in later science courses.

Worked example

Let's predict the direction of diffusion

In a muscle cell during a hockey game, oxygen concentration is low and carbon dioxide concentration is high. In the blood beside it, oxygen is high and carbon dioxide is low. Which way does each gas diffuse? Why does the cell need both movements?

  1. Oxygen: high in blood → low in cell, so oxygen diffuses into the cell.
  2. Carbon dioxide: high in cell → low in blood, so carbon dioxide diffuses out of the cell.
  3. The cell needs oxygen for cellular respiration to release energy for skating.
  4. Carbon dioxide is a waste; if it built up, it would harm the cell.

Oxygen diffuses in and carbon dioxide diffuses out, each moving from high to low concentration. The harder the muscle works, the faster it uses oxygen, keeping the gradient steep.

BloodBody cellmitochondrionoxygen inCO₂ outoxygencarbon dioxide (CO₂)Each gas moves from high to low.
Oxygen (circles) is crowded in the blood, so it diffuses into the cell. Carbon dioxide (squares) is crowded in the cell, where mitochondria make it, so it diffuses out.
Worked example

Let's analyse a diffusion experiment

A student put one drop of food colouring in three clear cups of water at different temperatures and timed how long the colour took to spread evenly. Cold water (5 °C): 9 min. Room temperature (20 °C): 5 min. Warm water (40 °C): 2 min. What do the results show?

  1. Independent variable (what was changed): water temperature.
  2. Dependent variable (what was measured): time to spread evenly.
  3. Controlled variables: same amount of water, same cup, same size of drop, same colouring, not stirred.
  4. Pattern: as temperature went up, time went down: 9, 5, 2 minutes.
  5. Explanation: warmer particles move faster, so diffusion is faster.
  6. Improve: repeat each temperature 3 times and average, because one drop can be a slightly different size.

The higher the temperature, the faster the diffusion. The results fit the particle theory.

Just for fun

Why can't you open a bag of nacho chips quietly in a test?

Diffusion. Ten seconds later, the whole class has lost its concentration, and the nacho smell has found a new one.

Hands-on activity

Fair test: does temperature affect the rate of diffusion?

You need: 3 identical clear glasses or jars, cold water from the fridge, room-temperature water, warm tap water, food colouring, a measuring cup, a kitchen thermometer if you have one, a timer, a white sheet of paper.

  1. Write a hypothesis: If the water is warmer, then the colour will spread (faster / slower), because…
  2. Pour 250 mL of each water into its own glass. Measure the temperatures if you can. Place the glasses on the white paper.
  3. Wait 1 minute for the water to become still.
  4. Add one drop of food colouring to the centre of each glass, at the same time if possible (a helper is useful). Don't stir or bump them.
  5. Time how long until each glass looks evenly coloured. Also sketch what you see after 1 minute.
  6. Record your results in a table and make a bar graph.
  7. Write a conclusion, and explain your results using the particle theory.

Safety: a grown-up runs the hot tap; use warm water only, never boiling. Food colouring stains clothes and counters.

What to look for: a hypothesis in "If…, then…, because…" form; the colour spreads fastest in warm water; the explanation says warm particles move faster. The "evenly coloured" moment is hard to judge, so a good student notes this as a source of error.

Your turn

1

Define diffusion.

2

What does selectively permeable mean?

3

Name two substances that diffuse through a cell membrane.

4

Why does oxygen diffuse into body cells?

5

Does the cell use energy for diffusion?

6

Name two factors that make diffusion faster.

7

At equilibrium, have the particles stopped moving? Explain.

More practice: warm-up

1

Particles diffuse from ___ concentration to ___ concentration.

2

Does warming speed up or slow down diffusion?

3

What gas diffuses out of your cells as a waste?

4

Can starch diffuse through a cell membrane?

5

Name an example of diffusion at home.

More practice: on your own

1

A tea bag is placed in hot water and another in cold water. Predict and explain which colours its water first.

2

Explain why a cell with a very thick, large body would have trouble getting oxygen to its centre.

3

In a leaf in sunlight, which gas diffuses in and which diffuses out? Explain.

4

In the food colouring test, why must the glasses not be stirred?

5

In the lungs, oxygen moves from air sacs into the blood. Explain why it keeps moving that way even though we keep breathing.

Challenge

1

A student says, "Diffusion stops when the concentrations are equal." Correct this statement using the particle theory.

2

Design a fair test to find out whether the size of particles affects the rate of diffusion. (Hint: compare two substances.) Name the variables.

3

Why do you think doctors give some patients extra oxygen to breathe? Use diffusion in your answer.

Check your understanding

Quiz

1.

During diffusion, particles move from…

2.

Which gas diffuses out of a body cell?

3.

Which change would make diffusion faster?

4.

A cell membrane that lets oxygen in but keeps proteins out is…

5.

At equilibrium, particles…

6.

How much energy does a cell use for diffusion?

For parents and teachers

Big idea: particles always move, and they spread from where they are more concentrated to where they are less concentrated. Cells rely on this to get oxygen in and carbon dioxide out, through a selectively permeable membrane.

How to help at home:

  • Run the food colouring fair test together. Be the timer and ask: What did you keep the same?
  • Point out diffusion in daily life: cooking smells, tea brewing, perfume.

Safety: adults handle the hot tap; warm water only. Food colouring stains.

Common difficulties:

  • Thinking particles stop at equilibrium.
  • Getting the direction backwards. Always ask: Where is there more? It moves from there.
  • Mixing up diffusion (any particles) with osmosis (water through a membrane), which is next lesson.