Chapter 3 of 8

All 8 chapters
  1. 1Cell structure in detail, readunderstood
  2. 2Microscopes and size, readunderstood
  3. 3Diffusion, osmosis and active transport, readunderstood
  4. 4Cell division, readunderstood
  5. 5Enzymes, readunderstood
  6. 6Photosynthesis, readunderstood
  7. 7Testing photosynthesis, readunderstood
  8. 8Respiration, readunderstood

S4.1.3 Chapter 3

Diffusion, osmosis and active transport

A potato strip in plain water goes firm. In strong salt water it goes floppy. What is the water doing?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain osmosis using potato pieces in salty and plain water
  • Compare diffusion, osmosis and active transport

Everything a cell needs has to get in, and the waste has to get out. All of it crosses the cell membrane. This chapter is about the three ways it does.

A cell membrane is partially permeable. It lets some particles through and holds others back. Small particles such as water and oxygen get through. Many others, such as sugar, are held back.

The particles of a liquid or a gas are always moving and bumping about. In a crowded place more of them move out than move in. The result is a spread from a place with more of them to a place with fewer. This is diffusion.

A box divided by a membrane with gaps in it. The left side is labelled outside and the right side inside. There are many oxygen particles outside and few inside. An arrow labelled net movement points from outside to inside. A key says the dots are oxygen.outsideinsidenet movementoxygen
Particles move both ways through the gaps. More cross from the crowded side, so the net movement is from outside to inside.

Diffusion is the net movement of particles from a higher concentration to a lower concentration. The cell does not have to use energy for it. Oxygen diffuses into cells, and carbon dioxide diffuses out.

Water also moves across a membrane, in a way with its own name. Osmosis is the movement of water across a partially permeable membrane from a dilute solution to a more concentrated solution.

A dilute solution has a lot of water for each particle of solute. A concentrated solution has less. The solute particles, such as sugar or the ions of salt, cannot cross the membrane easily. The water particles can, and more of them cross towards the concentrated side.

A box divided by a membrane with gaps in it. The left side is labelled dilute and the right side concentrated. The left has 12 water particles and 2 solute particles. The right has 6 water particles and 6 solute particles. An arrow labelled net movement of water points from the dilute side to the concentrated side. A key says the open rings are water and the dots are solute.diluteconcentratednet movement of waterwatersolute
The gaps let water through but not the larger solute particles. Water moves to the concentrated side.

A potato is a good test of osmosis. Cut strips of the same length and put them in different solutions. Then measure them again.

Draw it first

Potato strips in plain water and strong salt water

  1. Two pictures of potato strips against a ruler in millimetres. In the first, the two strips have the labels plain water: 50 mm and strong salt water: 50 mm, and a dashed line marked start length 50 mm. In the second, after an hour, the labels are plain water: 53 mm and strong salt water: 46 mm, with the same line marked start length 50 mm.0102030405060mmplain water: 50 mmstrong salt water: 50 mmstart length 50 mm

    1 Two strips are cut to the same length, 50 mm. One goes into plain water and one into strong salt water.

  2. Two pictures of potato strips against a ruler in millimetres. In the first, the two strips have the labels plain water: 50 mm and strong salt water: 50 mm, and a dashed line marked start length 50 mm. In the second, after an hour, the labels are plain water: 53 mm and strong salt water: 46 mm, with the same line marked start length 50 mm.0102030405060mmplain water: 53 mmstrong salt water: 46 mmstart length 50 mm

    2 After an hour the strip in plain water is longer, 53 mm. The strip in strong salt water is shorter, 46 mm.

In plain water the liquid outside the cells is more dilute than the liquid inside them. Water moves in by osmosis. The cells swell and press against their walls. The strip becomes longer and firm.

In strong salt water the liquid outside is more concentrated. Water moves out of the cells by osmosis. The cells shrink, and the strip becomes shorter and floppy. Its mass falls. The wall of a plant cell stops it bursting in plain water.

Scientists often compare the percentage change. Percentage change = (end value − start value) ÷ start value × 100. A gain gives a plus and a loss gives a minus.

Worked example

A potato strip is 50 mm long. After an hour in plain water it is 53 mm long. What is the percentage change in length?

  1. Change in length = 53 − 50 = 3 mm.
  2. Percentage change = change ÷ start length × 100.
  3. 3 ÷ 50 × 100 = 6, so the strip is 6% longer.

Answer: 6% longer

Worked example

A potato piece has a mass of 10.0 g. After an hour in strong salt water it has a mass of 9.1 g. What is the percentage change in mass?

  1. Change in mass = 9.1 − 10.0 = −0.9 g, a loss of 0.9 g.
  2. Percentage change = change ÷ start mass × 100.
  3. 0.9 ÷ 10.0 × 100 = 9.
  4. The piece has lost 9% of its mass.

Answer: 9% lighter

Try a range of salt concentrations. Each piece starts at 10.0 g. The percentage change goes from a gain, to nothing, to a loss. Where it is nothing, the liquid outside has the same concentration as the liquid inside the cells.

A table with the columns Salt, End mass and Change. Every piece started at 10.0 g. Salt 0 g: 11.0 g, +10%. Salt 0.5 g: 10.5 g, +5%. Salt 1 g: 9.9 g, −1%. Salt 1.5 g: 9.5 g, −5%. Salt 2 g: 9.1 g, −9%.SaltEnd massChange0 g11.0 g+10%0.5 g10.5 g+5%1 g9.9 g−1%1.5 g9.5 g−5%2 g9.1 g−9%
Salt is in grams for every 100 mL of water. The mass changes from a gain to a loss as the salt goes up.

Diffusion and osmosis move things down the gradient. The third way goes the other way. Active transport moves particles from a place where they are at a lower concentration to a place where it is higher. This is against the concentration gradient.

That takes energy, which comes from respiration. A carrier protein in the membrane picks up the particle and carries it across. Root hair cells take up mineral ions from the soil in this way. The gut takes up glucose into the blood when there is more in the blood than in the gut.

A box divided by a solid membrane with a carrier in it. The left side is labelled soil water and the right side root hair cell. There are 4 mineral ion particles in the soil water and 14 in the cell. An arrow labelled net movement points from the soil water to the cell, with the labels carrier protein and energy from respiration. A key says the dots are mineral ions.soil waterroot hair cellnet movementcarrier proteinenergy from respirationmineral ion
The ions move from fewer to more, so the cell must use energy. Cells that do a lot of active transport have many mitochondria.
A table with the columns Kind, Goes and Energy. Diffusion: down, no. Osmosis: down, no. Active: up, yes.KindGoesEnergydiffusiondownnoosmosisdownnoactiveupyes
Down is from a higher to a lower concentration. Up is from a lower to a higher one. Active is active transport. Osmosis is for water, and the other two are for any particles.

Try it

Potato strips in plain and salty water

You need: a potato, two cups, water, 3 spoonfuls of salt, a ruler with millimetres, a table knife and a chopping board, paper towel, a clock.

Be safe: An adult cuts the potato. Or use a table knife on a board, with your other hand well away from the blade. Do not eat or taste the potato or the water afterwards.

  1. Ask an adult to cut two strips from the potato, each 50 mm long and about as thick as a finger. Cut the strips from the same potato.
  2. Fill one cup with plain water. Fill the other with water and stir in the salt until it is gone. This is strong salt water.
  3. Put one strip in each cup. Wait one hour.
  4. Take the strips out, dry them lightly on the paper towel and measure each one. Bend each strip gently.
  5. Work out the percentage change in length of each strip.

What you will see: The strip in plain water is a little longer and firm. The strip in strong salt water is shorter and bends easily. The results of the strips in the picture were 53 mm and 46 mm.

If you cannot try it, this is what happens.
Two potato strips against a ruler in millimetres, with the labels plain water: 53 mm and strong salt water: 46 mm. A dashed line is marked start length 50 mm.0102030405060mmplain water: 53 mmstrong salt water: 46 mmstart length 50 mm

A wrong idea many people have

Water moves to wherever there is more water.

Why it feels right: It sounds as if water should gather where it is plentiful, like a crowd moving towards a crowd.

The test that breaks it: Put a potato strip in strong salt water. The salt water has less water for each particle of solute than the potato cells do. The strip shrinks, so the water has left the potato.

What is true: Water moves from the more dilute solution, where there is more water, to the more concentrated one, where there is less.

Key words

partially permeable
letting some particles through and not others, as a cell membrane does.
diffusion
the net movement of particles from a higher to a lower concentration.
osmosis
the movement of water across a partially permeable membrane from a dilute to a more concentrated solution.
active transport
moving particles against a concentration gradient, using energy from respiration.
concentration gradient
the difference in concentration between two places.
percentage change
the change as a percentage of the start value.

Back to your guess

The answer is B: Water moves into the potato in plain water and out of it in strong salt water.

Water moves across the membranes of the potato cells. In plain water it goes in, and the strip gains water and firmness. In strong salt water it goes out, and the strip loses water and goes floppy.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. A potato strip is put into strong salt water. What happens to it?

    Tap your answer.

    1. Not this one. That happens in plain water. In strong salt water the liquid outside is more concentrated, so water leaves the cells. Try another.

    2. Yes, that is right. The salt water is more concentrated than the liquid in the cells. Water moves out of the cells by osmosis.

    3. Not this one. Salt cannot cross easily, but water can. Water moves out by osmosis, so the strip changes. Try another.

    See the answer

    It loses water and gets shorter and floppy.

    The salt water is more concentrated than the liquid in the cells. Water moves out of the cells by osmosis.

    • If you chose A: That happens in plain water. In strong salt water the liquid outside is more concentrated, so water leaves the cells.
    • If you chose C: Salt cannot cross easily, but water can. Water moves out by osmosis, so the strip changes.

  2. Which statement describes osmosis?

    Tap your answer.

    1. Not this one. That describes active transport, which needs energy. Try another.

    2. Not this one. In osmosis it is the water that crosses. The solute particles stay behind. Try another.

    3. Yes, that is right. Osmosis is the movement of water, across a partially permeable membrane, from a dilute to a more concentrated solution.

    See the answer

    Water crosses a partially permeable membrane from a dilute solution to a more concentrated solution.

    Osmosis is the movement of water, across a partially permeable membrane, from a dilute to a more concentrated solution.

    • If you chose A: That describes active transport, which needs energy.
    • If you chose B: In osmosis it is the water that crosses. The solute particles stay behind.

  3. Why does active transport need energy from respiration?

    Tap your answer.

    1. Yes, that is right. Particles do not move from a lower to a higher concentration by themselves. The cell uses energy from respiration to carry them.

    2. Not this one. Moving down the gradient is what diffusion does, and it needs no energy. Try another.

    3. Not this one. Moving water is osmosis, which needs no energy. Try another.

    See the answer

    It moves particles against the concentration gradient.

    Particles do not move from a lower to a higher concentration by themselves. The cell uses energy from respiration to carry them.

    • If you chose B: Moving down the gradient is what diffusion does, and it needs no energy.
    • If you chose C: Moving water is osmosis, which needs no energy.

  4. There are fewer mineral ions in the soil water than in a root hair cell. How do the ions get into the cell?

    Tap your answer.

    1. Not this one. Osmosis is the movement of water, not of mineral ions. Try another.

    2. Yes, that is right. The ions go from a lower to a higher concentration, so the cell uses energy to carry them in. A root hair cell has many mitochondria for this.

    3. Not this one. Diffusion would move the ions out of the cell, down the gradient. They would never build up inside. Try another.

    See the answer

    By active transport

    The ions go from a lower to a higher concentration, so the cell uses energy to carry them in. A root hair cell has many mitochondria for this.

    • If you chose A: Osmosis is the movement of water, not of mineral ions.
    • If you chose C: Diffusion would move the ions out of the cell, down the gradient. They would never build up inside.

  5. A potato strip 40 mm long is 36 mm long after an hour in salt water. By what percentage did its length change?

    A potato strip against a ruler in millimetres, with the label salt water: 36 mm. A dashed line is marked start length 40 mm.010203040mmsalt water: 36 mmstart length 40 mm
    Tap your answer.

    1. Yes, that is right. Change = 40 − 36 = 4 mm. 4 ÷ 40 × 100 = 10, so the strip is 10% shorter.

    2. Not this one. The strip is 4 mm shorter, but that is 4 mm, not 4%. Divide the change by the start length, then multiply by 100. Try another.

    3. Not this one. You divided by the end length, 36. Divide by the start length, 40. Try another.

    See the answer

    10% shorter

    Change = 40 − 36 = 4 mm. 4 ÷ 40 × 100 = 10, so the strip is 10% shorter.

    • If you chose B: The strip is 4 mm shorter, but that is 4 mm, not 4%. Divide the change by the start length, then multiply by 100.
    • If you chose C: You divided by the end length, 36. Divide by the start length, 40.

  6. Which is an example of diffusion?

    Tap your answer.

    1. Not this one. That is osmosis, which is the movement of water across a membrane. Try another.

    2. Not this one. The ions go from a lower to a higher concentration. That is active transport. Try another.

    3. Not this one. Against the gradient means active transport, not diffusion. Go to S3.3.2. Try another.

    4. Yes, that is right. The carbon dioxide goes from a higher to a lower concentration, with no energy needed. That is diffusion.

    See the answer

    Carbon dioxide leaving a cell for the blood, where there is less of it

    The carbon dioxide goes from a higher to a lower concentration, with no energy needed. That is diffusion.

    • If you chose A: That is osmosis, which is the movement of water across a membrane.
    • If you chose B: The ions go from a lower to a higher concentration. That is active transport.
    • If you chose C: Against the gradient means active transport, not diffusion. Go to S3.3.2.

  7. Every piece in the table started at 10.0 g. Between which two salt amounts does the mass stay about the same?

    A table with the columns Salt, End mass and Change. Salt 0 g: 11.0 g, +10%. Salt 0.5 g: 10.5 g, +5%. Salt 1 g: 9.9 g, −1%. Salt 1.5 g: 9.5 g, −5%. Salt 2 g: 9.1 g, −9%.SaltEnd massChange0 g11.0 g+10%0.5 g10.5 g+5%1 g9.9 g−1%1.5 g9.5 g−5%2 g9.1 g−9%
    Tap your answer.

    1. Not this one. There the pieces still gain 10% and 5%. The change is not yet near 0. Try another.

    2. Not this one. There the pieces lose 5% and 9%. The change is not near 0 any more. Try another.

    3. Yes, that is right. The change is +5% at 0.5 g and −1% at 1 g. It goes through 0 between them, where the liquid outside matches the liquid in the cells.

    See the answer

    Between 0.5 g and 1 g

    The change is +5% at 0.5 g and −1% at 1 g. It goes through 0 between them, where the liquid outside matches the liquid in the cells.

    • If you chose A: There the pieces still gain 10% and 5%. The change is not yet near 0.
    • If you chose B: There the pieces lose 5% and 9%. The change is not near 0 any more.

  8. From an earlier chapter

    A cell is 80 µm long. In a photograph it is 40 mm long. What is the magnification?

    Tap your answer.

    1. Not this one. You divided 40 by 80 without changing the units. Turn mm into µm first. Go to S4.1.2. Try another.

    2. Yes, that is right. 40 mm = 40 000 µm. 40 000 ÷ 80 = 500.

    3. Not this one. You multiplied 40 by 80. Magnification is image size divided by real size. Go to S4.1.2. Try another.

    See the answer

    ×500

    40 mm = 40 000 µm. 40 000 ÷ 80 = 500.

    • If you chose A: You divided 40 by 80 without changing the units. Turn mm into µm first. Go to S4.1.2.
    • If you chose C: You multiplied 40 by 80. Magnification is image size divided by real size. Go to S4.1.2.

  9. A potato strip is 40 mm long at the start and 44 mm long at the end. What is the percentage change in length?

    See the answer

    10% longer

    44 − 40 = 4 mm. 4 ÷ 40 × 100 = 10, so the strip is 10% longer.

  10. A potato piece has a mass of 20 g at the start and 17 g at the end. What is the percentage change in mass?

    See the answer

    15% lighter

    20 − 17 = 3 g. 3 ÷ 20 × 100 = 15, so the piece is 15% lighter.

Keep it

Copy this into your exercise book.

Diffusion: any particles, high to low, no energy. Osmosis: water, dilute to concentrated, through a partially permeable membrane. Active transport: low to high, uses energy from respiration. Percentage change = change ÷ start × 100.

Draw: Three boxes with a membrane in each, an arrow for the net movement, and the words energy on the third.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.