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IGCSE Biology 0610 · Chapter 3 notes · ~12 min read

Movement into & out of cells

How substances get across cell membranes: diffusion, osmosis (with water potential) and active transport — including surface area to volume, turgor and plasmolysis, and the potato practical.

DiffusionOsmosisWater potentialTurgorActive transport
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Diffusion: net movement of particles from a high to a low concentration, down a concentration gradient. No energy needed from the cell.
  • Osmosis: net movement of water through a partially permeable membrane from a region of higher water potential to lower water potential.
  • Active transport: movement of particles against the concentration gradient, using energy from respiration.

By the end you can

  • Define diffusion, osmosis and active transport and give an example of each.
  • Describe the factors that affect the rate of diffusion.
  • Explain what happens to animal and plant cells in different solutions.
  • Describe and interpret the potato-cylinder investigation.
  • Define water potential, and use the terms turgid, flaccid and plasmolysis. EXT
  • Explain how carrier proteins use energy in active transport. EXT

1Diffusion

What is diffusion?

starthigh concentrationon the lefta little laterspreading outmuch laterevenly spread: no netmovement (they still move!)net movement: from HIGH to LOW concentrationPassive: no energy needed. Particles move randomly (kinetic energy).Rate depends on the concentration gradient, temperature, surface area and distance.
Diffusion spreads particles out until they are evenly mixed. Individual particles keep moving after that, but there is no net movement.

Diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.

  • Particles in gases and liquids are always moving. More particles leave a crowded region than arrive, so there is a net movement to the less crowded region.
  • The energy for diffusion comes from the kinetic energy of the particles. The cell does not use energy. Diffusion is a passive process.
  • Diffusion works well for small molecules that can pass through the cell membrane: oxygen, carbon dioxide, water, urea, glucose (in some cases).
WhereWhat diffuses
alveoli in the lungsoxygen into the blood, carbon dioxide out
leafcarbon dioxide into the leaf cells, oxygen out
small intestinedigested food (partly by diffusion) into the blood
body cellsoxygen in, carbon dioxide and urea out
Common mistake"Diffusion needs energy." The particles' own energy (kinetic energy) drives it. The cell supplies no energy.
Common mistake"Diffusion stops when the concentrations are equal." Particles keep moving. There is just no net movement.

What affects the rate of diffusion?

hotcoldsmalllargethinthicksteeper gradienthigher temperaturelarger surface areashorter distancebigger differencein concentrationparticles havemore energy: move fastermore room forparticles to crossless far to travel= fasterAll four make diffusion FASTER(red arrows = diffusion; shorter arrow = slower movement)
Four factors that make diffusion faster.
  • Concentration gradient — the steeper (bigger difference), the faster.
  • Temperature — particles gain kinetic energy, move faster, collide more: the higher, the faster.
  • Surface area — a larger exchange surface gives more room for particles to cross.
  • Distance — the shorter the distance, the faster. This is why exchange surfaces are thin.
Exam tipExam trick: for "explain how X is adapted for diffusion", use these four: large surface area, thin, steep gradient (blood supply / ventilation).

Surface area to volume ratio

1 cm cube2 cm cube3 cm cubeSurface area6 cm²24 cm²54 cm²Volume1 cm³8 cm³27 cm³SA : V6 : 13 : 12 : 1Bigger object → smaller surface area : volume ratiosmall cells exchange faster; big organisms need exchange surfaces
As a cube gets bigger, its volume grows faster than its surface area. Large objects have a small SA:V.
  • A cube with side 1 cm has surface area 6 × 1² = 6 cm² and volume 1³ = 1 cm³. SA:V = 6 : 1.
  • A cube with side 2 cm has surface area 6 × 2² = 24 cm² and volume 2³ = 8 cm³. SA:V = 3 : 1.
  • A cube with side 3 cm has surface area 6 × 3² = 54 cm² and volume 3³ = 27 cm³. SA:V = 2 : 1.
  • Small cells have a large SA:V, so substances can diffuse in and out fast enough. Large organisms have a small SA:V. They need special exchange surfaces (lungs, villi, root hairs) and transport systems (the circulatory system).
Worked example

Calculate the surface area to volume ratio of a cube of side 4 cm.

  1. surface area = 6 × 4 × 4 = 96 cm²
  2. volume = 4 × 4 × 4 = 64 cm³
  3. ratio = 96 : 64 = 1.5 : 1

Investigating diffusion

  • Cut agar jelly containing phenolphthalein indicator and a little alkali (it is pink) into cubes of different sizes.
  • Put the cubes in dilute acid. As acid diffuses in, the pink colour disappears.
  • Measure the time to go colourless. The small cube goes colourless fastest because it has the largest SA:V.
  • Keep the same acid concentration and temperature, so only one variable (cube size) changes.

2Osmosis

What is osmosis?

STARTdilute(high water pot.)concentrated(low water pot.)LATERdilute(high water pot.)concentrated(low water pot.)watersolute (too big to pass)LATER: more water on the concentrated side, so its level risesWater moves from HIGH to LOW water potential through a partially permeable membrane
Water moves across the membrane towards the more concentrated solution. Solute particles are too big to pass.

Osmosis — the diffusion of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

  • Water is the solvent in cells. A dilute solution has a high proportion of water. A concentrated solution has a low proportion of water.
  • A partially permeable membrane lets small molecules like water through but stops larger solute molecules.
  • Dialysis (Visking) tubing acts as a partially permeable membrane. A bag of sugar solution in a beaker of water gains mass as water enters by osmosis.
Picture itWater behaves like a crowd leaving a packed room through a door that only small people fit through: they move to where there are fewer of them.

Water potentialEXTENDED

pure waterHIGHEST water potentialdilute solutionlower water potentialconcentrated solutionLOWEST water potentialwater moveswater movesWater potential falls as more solute is dissolvedOsmosis: water moves DOWN the water-potential gradient(high → low) through a partially permeable membrane
Pure water has the highest water potential. Dissolving more solute lowers it. Water moves down the water-potential gradient.

Water potential — the tendency of water molecules to move from one region to another. Pure water has the highest water potential. Adding solute lowers it.

  • Water moves from a higher water potential (dilute) to a lower water potential (concentrated).
  • So "osmosis" and "diffusion of water down a water-potential gradient" mean the same thing.
Common mistakeMixing up "high water potential" with "concentrated". A concentrated solution has a low water potential, because it has less free water.

Osmosis in animal cells

In dilute solutionIn same concentrationIn concentrated solutionwater enters(osmosis):cell swellsand may burstno net movementof water:cell staysthe samewater leaves(osmosis):cell shrinksand shrivelsAnimal cells have NO cell wall, so they cannot resist swelling(blue arrows = water in · red arrows = water out)
An animal cell has no cell wall, so it bursts or shrinks depending on the solution.
  • In a dilute solution (higher water potential than the cell), water enters by osmosis. The cell swells and may burst.
  • In a solution with the same concentration, there is no net movement of water.
  • In a concentrated solution (lower water potential than the cell), water leaves by osmosis. The cell shrinks and shrivels.

Osmosis in plant cells

In dilute solutionIn same concentrationIn concentrated solutionTURGIDwater in: vacuole swellsand pushes on the wall= firm, supports plantFLACCIDno net movement:no pressure on the wall= soft, limpPLASMOLYSEDwater out: membranepulls away from the wall(gap fills with solution)Turgor pressure keeps stems firm; flaccid cells = a wilting plant(blue arrows = water in · red arrows = water out)
A plant cell in a dilute solution becomes turgid. In a concentrated solution it becomes plasmolysed.

Turgid — a plant cell that is full of water, with the vacuole pushing the cell membrane against the cell wall.

Turgor pressure — the pressure of the cell contents pushing on the cell wall. It supports non-woody plants and keeps leaves firm.

Flaccid — a cell that has lost water and is no longer pressing on the wall: soft and limp. EXT

Plasmolysis — the cell membrane pulls away from the cell wall because water has left the cell by osmosis. EXT

  • The strong cellulose cell wall stops a plant cell from bursting, so it stays turgid in water.
  • When a plant does not get enough water, its cells lose turgor, become flaccid and the plant wilts.
Exam tipTurgor is the reason celery goes crisp in water and limp in salt solution.
Common mistake"Plant cells burst in pure water." The cell wall stops them bursting. They just become turgid.

Investigation: potato cylinders

Method1 cut potato cylinders of the same size2 blot dry, weigh each3 soak 30 min in sugar solutions (0 → 0.8 M)4 blot dry, weigh again% change in mass= (final − start) ÷ start× 1000.00.20.40.60.8sugar concentration (mol/dm³)+–% change in massno change at ≈ 0.32 M:same water potentialas the potato cellsgains waterloses water
Potato cylinders gain mass in dilute solutions and lose mass in concentrated ones. Where the line crosses zero, the solution matches the potato.
  • Method: cut potato into cylinders of the same size. Blot and weigh each. Put one in each sugar solution (for example 0, 0.2, 0.4, 0.6, 0.8 mol/dm³) for about 30 minutes. Blot dry and weigh again.
  • Control variables: same size and type of potato, same time, same volume of solution, same temperature.
% change in mass = (final mass − starting mass) ÷ starting mass × 100
positive = gained water, negative = lost water
Worked example

A potato cylinder has a mass of 3.00 g. After soaking in 0.2 mol/dm³ sugar solution it weighs 3.27 g. Calculate the percentage change.

  1. change = 3.27 − 3.00 = 0.27 g
  2. 0.27 ÷ 3.00 = 0.09
  3. × 100 = +9.0% (it gained water)
Worked example

Another cylinder of 3.00 g weighs 2.46 g after soaking in 0.6 mol/dm³ sugar solution. Calculate the percentage change.

  1. change = 2.46 − 3.00 = −0.54 g
  2. −0.54 ÷ 3.00 = −0.18
  3. × 100 = −18% (it lost water)
  • The graph crosses 0% change at about 0.32 mol/dm³. At this concentration the sugar solution has the same water potential as the cell sap, so there is no net movement of water.
Common mistakeUsing the final mass only. Always calculate the percentage change so that the cylinders can be compared fairly.

3Active transport

What is active transport?

outside:few ionsinside:many ions1 ion bindsto carrier proteinoutside:few ionsinside:many ions2 energy changesthe carrier’s shapeoutside:few ionsinside:many ions3 ion releasedinside the cellATP → ADPMoves ions AGAINST the concentration gradient (low → high): needs energy from respiration
A carrier protein in the membrane uses energy from respiration to pump ions across, against the concentration gradient.

Active transport — the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration.

  • Root hair cells take up mineral ions (for example nitrate) from the soil, even though the soil water holds less of them than the cell. Active transport pulls them in.
  • Glucose is absorbed in the small intestine and kidney by active transport when there is less in the gut than in the blood.
  • EXT Carrier proteins in the membrane pick up a particle, change shape using energy released by respiration, and release the particle on the other side.
  • Cells that do a lot of active transport have many mitochondria. If a cell is poisoned so it cannot respire, active transport stops.

Comparing the three processes

DiffusionOsmosisActive transport
what movesany small particleswater onlyions and molecules
directionhigh → low concentrationhigh → low water potentiallow → high concentration
energy from respiration?nonoyes
membrane protein needed?nonoyes (carriers) EXT
Exam tip"Against the gradient" = active transport. "Down the gradient" = diffusion or osmosis.

4Test yourself

Test yourself — cover the answers first

Quick check

  1. Define diffusion.
  2. Does diffusion need energy from respiration?
  3. Name two factors that increase the rate of diffusion.
  4. What type of membrane does osmosis need?
  5. What happens to an animal cell placed in pure water?
  6. What stops a plant cell bursting in water?
  7. Name a substance taken up by root hair cells by active transport.
  8. EXT What do you call a plant cell whose membrane has pulled away from the wall?

Exam-style

  1. A cube has a side of 2 cm. Calculate its surface area to volume ratio. [3]
  2. Explain why a large animal needs a transport system but a single-celled organism does not. [3]
  3. Describe what happens to a plant cell placed in a concentrated sugar solution and explain why. [3]
  4. Explain how a root hair cell takes up nitrate ions from soil that has a low nitrate concentration. [3]
Show answers
  1. net movement of particles from higher to lower concentration, down a concentration gradient
  2. no (passive)
  3. larger surface area, higher temperature, steeper concentration gradient, shorter distance
  4. partially permeable
  5. water enters by osmosis; it swells and may burst
  6. the cellulose cell wall
  7. mineral ions (e.g. nitrate)
  8. plasmolysed
  9. surface area = 6 × 2 × 2 = 24 cm²
    volume = 8 cm³
    ratio = 3 : 1
  10. large animals have a small surface area to volume ratio
    diffusion is too slow to supply all cells
    single-celled organisms have a large SA:V and a short diffusion distance
  11. water leaves the cell by osmosis
    because the solution has a lower water potential than the cell
    the cell shrinks and the membrane pulls away from the wall (plasmolysis)
  12. active transport
    using energy from respiration
    against the concentration gradient, with carrier proteins
Practise this chapter
DiffusionOsmosisActive transport
Next chapterBiological molecules & enzymes →
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