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Detailed notes on Cells for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Three processes move substances across cell membranes: diffusion (passive, any particles), osmosis (passive, water only through partially permeable membrane), and active transport (against gradient, needs ATP). These underpin every other biology topic — from gas exchange to kidney function.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
NET movement, high → low, random movement, no energy. All three keywords required for full marks.
Cambridge-approved definition (memorise verbatim):
"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."
Three exam keywords Cambridge deducts marks for missing:
Diffusion is PASSIVE — no energy (ATP) from respiration is needed.
Factors affecting rate of diffusion:
| Factor | ↑ Factor → Rate? | Why |
|---|---|---|
| Concentration gradient | Increases | More particles moving in one direction |
| Surface area | Increases | More space for particles to cross |
| Diffusion distance | Decreases with shorter | Shorter path = faster |
| Temperature | Increases | More kinetic energy = faster random movement |
| Molecular size | Smaller = faster | Smaller molecules pass through more easily |
Fick's Law (Extended): Rate of diffusion∝diffusion distancesurface area×concentration difference
Biological examples:
Osmosis is a special case of diffusion — only water molecules move, only through a partially permeable membrane.
Cambridge-approved definition:
"The NET movement of WATER molecules from a region of LOWER solute concentration (higher water potential) to a region of HIGHER solute concentration (lower water potential), through a PARTIALLY PERMEABLE MEMBRANE."
Key distinctions from diffusion:
Water potential (Extended):
In plant cells:
| Condition | Direction of water | Result |
|---|---|---|
| Cell in dilute solution (lower solute than cell) | Water ENTERS by osmosis | Cell becomes turgid — swells, firm, rigid |
| Cell in concentrated solution (higher solute than cell) | Water LEAVES by osmosis | Cell becomes flaccid → then plasmolysed (membrane pulls away from cell wall) |
In animal cells (NO cell wall):
| Condition | What Happens | Term |
|---|---|---|
| Dilute solution (hypotonic) | Water enters → cell swells → bursts | Lysis (haemolysis in red blood cells) |
| Concentrated solution (hypertonic) | Water leaves → cell shrinks | Crenation |
| Equal concentration (isotonic) | No net movement | Equilibrium |
Demonstrating osmosis experimentally:
Active transport moves substances from LOW to HIGH concentration — uphill. It needs energy from respiration.
Cambridge-approved definition:
"The movement of molecules or ions through a cell membrane, from a region of LOWER concentration to a region of HIGHER concentration (against the concentration gradient), using ENERGY from RESPIRATION."
Comparing the three processes:
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Direction | High → low | High → low water potential | LOW → HIGH (against gradient) |
| Energy (ATP) | No | No | YES — from respiration |
| Carrier proteins | Not always | No | Always required |
| Substances | Any dissolved | Water only | Specific ions/molecules |
Why active transport needs aerobic respiration:
Biological examples:
Verbatim phrases and definitions Cambridge mark schemes credit.
This is one of the highest-frequency topics in both Paper 2 and Paper 4. Expect: (1) 'Define osmosis' (must include water, partially permeable membrane, and direction); (2) 'Explain why glucose is absorbed by active transport in the ileum' (link to against-gradient + ATP); (3) graph questions showing % change in mass of potato chips at different sucrose concentrations — identify the isotonic point where mass = 0% change. Always include 'net' in diffusion definitions.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B3); 0654/42 May/Jun 2024 — Q3 (osmosis); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on movement in and out of cells, written exactly the way a tutor would explain them at the board.
Question
Explain why oxygen moves from the alveoli into the blood.
Step-by-step solution
Step 1
Identify the process: diffusion — the net movement of particles from a region of high concentration to a region of low concentration.
Step 2
State the concentration gradient: oxygen concentration in the alveoli (after breathing in) is higher than in the blood arriving at the lungs (which has been delivering oxygen to body tissues).
Step 3
Therefore oxygen diffuses down its concentration gradient from the alveoli into the blood. No energy is required (passive process).
Answer
Oxygen diffuses from the alveoli into the blood because its concentration is higher in the alveoli than in the blood; substances move by diffusion from high to low concentration (down the concentration gradient). No energy is required.
Question
A piece of potato (mass 5.2 g) is placed in a concentrated salt solution for 30 minutes and its mass falls to 4.6 g. Explain this change in mass.
Step-by-step solution
Step 1
Identify the process: osmosis — the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Step 2
The concentrated salt solution has a lower water potential (more solutes) than the potato cells.
Step 3
Water molecules therefore move out of the potato cells into the salt solution by osmosis, down the water potential gradient.
Step 4
The potato loses water → its mass decreases from 5.2 g to 4.6 g. The cells become flaccid (and may become plasmolysed if the solution is very concentrated).
Answer
Water moves out of the potato cells by osmosis into the concentrated salt solution, because the salt solution has a lower water potential than the potato cells. The potato loses water and its mass decreases.
Question
Explain how mineral ions are absorbed from the soil into root hair cells even when their concentration in the soil is lower than in the root hair cell.
Step-by-step solution
Step 1
Diffusion cannot account for this — diffusion only moves substances from high to low concentration. Here, ions move from low (soil) to high (cell) concentration.
Step 2
The process is active transport — the movement of substances against their concentration gradient across a membrane, using energy.
Step 3
Energy is released by aerobic respiration (as ATP). Carrier proteins in the cell membrane use this energy to move mineral ions into the cell against the gradient.
Answer
Mineral ions are absorbed by active transport. This requires energy (ATP) released by respiration, and carrier proteins in the cell membrane move ions from low concentration (soil) to high concentration (root hair cell) — against the concentration gradient.
Examiner tip
Always state that energy (from respiration/ATP) is required for active transport. Simply saying 'active transport' without explaining it requires energy earns partial credit only.
Question
State what is meant by osmosis.
Step-by-step solution
Step 1
Osmosis involves only water molecules (not dissolved solutes).
Step 2
Movement is across a partially permeable membrane (allows water but not large solute molecules).
Step 3
Direction: from higher water potential to lower water potential (i.e. from a dilute solution to a more concentrated solution).
Answer
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Examiner tip
Cambridge mark schemes require 'water molecules', 'partially permeable membrane', and 'water potential' (or equivalent). Missing any one of these loses marks.
Definitions to memorise and the exact keywords mark schemes credit for movement in and out of cells answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration (down a concentration gradient). A passive process — no energy required.
Example
Oxygen diffusing from the alveoli into the blood.
The net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Example
Water moving into a root hair cell from soil water.
The movement of substances against a concentration gradient (from low to high concentration) across a membrane, using energy (ATP) released by respiration and carrier proteins.
Example
Absorption of mineral ions into root hair cells; reabsorption of glucose in kidney tubules.
The difference in concentration of a substance between two regions. Particles diffuse down a concentration gradient (from high to low).
A membrane that allows small molecules (such as water) to pass through but not large molecules (such as sucrose or starch).
The pressure that the cell contents exert on the cell wall when a plant cell takes in water by osmosis. Turgid cells provide support to plants.
Example
A well-watered plant is held upright because its cells are turgid.
The shrinkage of the cell membrane away from the cell wall in a plant cell placed in a solution of lower water potential (concentrated solution), due to water loss by osmosis.
The traps other students keep falling into on movement in and out of cells questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
Why it happens
Students confuse osmosis with diffusion and use the same generic language.
How to avoid it
Osmosis always and only refers to the movement of water molecules. Solutes do not move by osmosis.
Why it happens
Students know active transport is against the gradient but forget the energy requirement is key to the definition.
How to avoid it
Always include: 'requires energy from respiration (ATP)' — without this the answer is incomplete.
Why it happens
Students assume all transport across membranes needs energy.
How to avoid it
Diffusion (and osmosis) are passive processes — particles move spontaneously down their gradient. No energy (ATP) is needed.
Why it happens
Students mix up the direction — high solute = low water potential confuses the gradient direction.
How to avoid it
Water always moves from high water potential (dilute solution) to low water potential (concentrated solution). Think: water moves toward where there are more solutes.
The things students keep getting wrong in this sub-topic, answered.