Learn · Biology · O Level

Movement In & Out of Cells

Diffusion, osmosis and active transport, three processes with one comparison table, and the potato experiment the practical paper keeps returning to.

7 parts
Scroll to begin ↓
📘 DefinitionThe three processes, in the wording that scoresPart 1 of 7
turgidin pure waternormalin equal solutionplasmolysedin strong sugar solutionWater moves by osmosis; the wall never movesThe membrane pulls away from the wall in a plasmolysed cell — the outline stays the same.An animal cell has no wall, so in pure water it bursts and in strong solution it shrinks.
The wall protects a plant cell from bursting, and stays put when the membrane pulls away.
🔍 In detailThe potato experiment, and how to handle the numbersPart 3 of 7
🌍 In the real worldSurface area to volume ratio, and why big organisms need transport systemsPart 4 of 7
⚠️ Common mistakesWhere marks goPart 5 of 7
✅ SummaryWhat to rememberPart 6 of 7
✏️ Worked exampleWatch one worked throughPart 7 of 7
1📘 DefinitionPart 1 / 7

The three processes, in the wording that scores

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 motion. Passive, no energy from respiration.

Osmosis: the net movement of water molecules from a region of higher water potential (dilute solution) to one of lower water potential (concentrated solution), through a partially permeable membrane. Also passive. The three underlined ideas (water only, water potential, partially permeable) are the three marks.

Active transport: movement of particles against the concentration gradient, using energy from respiration and carrier proteins in the membrane. Root hair cells absorbing mineral ions from dilute soil water is the standard example.

The one-line comparison: diffusion and osmosis go down the gradient and need no energy; active transport goes up it and does.

2🧩 The partsPart 2 / 7

What happens to a cell in different solutions

Plant cell in pure water: water enters by osmosis, the vacuole swells, the cytoplasm pushes against the wall, the cell becomes turgid. It cannot burst because the rigid cellulose wall resists. Turgor is what keeps a non-woody stem upright, and losing it is what wilting is.

Plant cell in concentrated solution: water leaves, the cell becomes flaccid, and if enough leaves, the membrane pulls away from the wall, plasmolysis. The cell outline does not change, because the wall does not move.

Animal cell has no wall. In pure water it takes in water and bursts (lysis); in concentrated solution it shrinks and crenates. This is why blood plasma must have a water potential close to that of the red blood cells in it.

3🔍 In detailPart 3 / 7

The potato experiment, and how to handle the numbers

Cut identical potato strips, blot them, weigh each, leave each in a different sucrose concentration for a fixed time, blot and reweigh. Blotting matters: surface liquid would be weighed as if it were absorbed.

Percentage change in mass = (change ÷ initial mass) × 100. Percentage is used, not raw change, so strips of slightly different starting mass can be compared fairly. A strip going 4.0 g → 3.4 g has changed by −0.6 ÷ 4.0 × 100 = −15%.

Plot percentage change against concentration. Gain in mass means water entered, so the solution was more dilute than the cell contents. Loss means the reverse. Where the line crosses zero, the external solution equals the cell sap concentration, and that reading is usually what the question is driving at.

Variables to control: temperature, time, volume of solution, size and surface area of the strips, and the same potato throughout.

4🌍 In the real worldPart 4 / 7

Surface area to volume ratio, and why big organisms need transport systems

Diffusion is fast enough to supply a cell only over very short distances. As an organism gets larger its volume grows faster than its surface area, so its surface area to volume ratio falls and diffusion across the outer surface can no longer supply the cells inside.

That single fact explains most of the anatomy in this syllabus. Large organisms solve it with specialised exchange surfaces and a transport system. Every exchange surface has the same three adaptations, and the paper wants all three: a large surface area, a short diffusion distance (thin walls), and a steep concentration gradient maintained by ventilation or blood flow.

Alveoli, villi, root hairs and gills are all the same design answering the same problem. Learn the trio once and reuse it.

5⚠️ Common mistakesPart 5 / 7

Where marks go

  • Osmosis defined without "partially permeable". That phrase is a mark.
  • Osmosis described as moving solute. Water moves.
  • "Water moves to where there is less water." Acceptable only if phrased as water potential; better to use the proper term.
  • Active transport without mentioning energy or respiration. Both are needed.
  • Plant cells described as bursting. The wall prevents it.
  • Raw mass change instead of percentage. Percentage allows fair comparison.
6✅ SummaryPart 6 / 7

What to remember

  • Diffusion: any particle, down the gradient, passive.
  • Osmosis: water, down a water-potential gradient, through a partially permeable membrane.
  • Active transport: against the gradient, needs energy from respiration.
  • Plant: turgid / flaccid / plasmolysed. Animal: bursts or shrinks.
  • % change = (change ÷ initial) × 100; the zero crossing gives the cell concentration.
  • Exchange surfaces: large area, short distance, steep gradient.
7✏️ Worked examplePart 7 / 7

Watch one worked through

Here is one done slowly, with the reason for every move. Then one for you.

Watch one first

A plant cell is put in pure water. Describe what happens and name the process and the final state.

  1. Compare the water concentration inside and outside.
    outside: pure water, high inside: cell sap, lower
    Always state both sides. Osmosis questions are marked on the comparison, not on the word osmosis alone.
  2. Water moves down its concentration gradient through the membrane.
    water: high → low through a partially permeable membrane = osmosis
    Osmosis is the movement of water only, and only through a partially permeable membrane. Leaving either out costs a mark.
  3. The cell takes in water and the vacuole swells.
    water in → vacuole swells → pushes on wall
    The pressure of the contents against the wall is turgor pressure. It is what holds a non-woody plant upright.
  4. The cell wall stops it bursting.
    plant cell → turgid (does not burst) animal cell → would burst (lysis)
    The rigid wall is the whole difference. The same experiment bursts a red blood cell, which has no wall.

Answer: Water enters by osmosis, the vacuole swells and pushes against the wall, and the cell becomes turgid rather than bursting

Now you try

What is a plant cell called when it has lost water and the membrane pulls away from the wall? One word.

Show the answer

plasmolysed

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