DIFFUSION is the net movement of particles (molecules or ions) from a region of HIGHER concentration to a region of LOWER concentration — down the concentration gradient.
It is a PASSIVE process — no energy (ATP) is required. Particles move due to their own random kinetic energy.
Diffusion continues until concentrations are equal on both sides — this is called EQUILIBRIUM. Even at equilibrium, particles are still moving randomly — there's just no NET movement.
Factors that increase the rate of diffusion:
• STEEPER concentration gradient — bigger difference in concentration = faster net movement
• HIGHER temperature — more kinetic energy → particles move faster
• LARGER surface area — more space available for diffusion to occur across
• O₂ diffuses from alveoli (high [O₂]) → into blood (lower [O₂]) → into muscle cells (even lower [O₂])
• CO₂ diffuses from respiring cells (high [CO₂]) → into blood → into alveoli (low [CO₂])
• Glucose diffuses from the small intestine (high [glucose] after digestion) → into blood
• Urea diffuses from liver cells (where it is made) → into blood → into kidney tubules → excreted in urine
Osmosis
OSMOSIS is a special type of diffusion — it is the net movement of WATER MOLECULES ONLY across a PARTIALLY PERMEABLE MEMBRANE, from a DILUTE solution (more water molecules, high water potential) to a MORE CONCENTRATED solution (fewer water molecules, low water potential).
A partially permeable membrane has tiny pores that allow small water molecules through but block larger solute molecules.
Osmosis is also PASSIVE — no ATP energy is required.
In PLANT CELLS:
• Cell placed in DILUTE solution: water enters by osmosis → vacuole swells → cell membrane pushes against rigid cell wall → cell becomes TURGID (firm). Turgid cells provide structural support to the plant.
• Cell placed in CONCENTRATED solution: water leaves by osmosis → vacuole and cytoplasm shrink → cell membrane pulls away from cell wall → PLASMOLYSIS. Plant wilts.
In ANIMAL CELLS (no cell wall to limit swelling):
• Cell in DILUTE solution: water enters by osmosis → cell SWELLS → may BURST (LYSIS) if too much water enters — e.g. red blood cells burst in pure water
• Cell in CONCENTRATED solution: water leaves by osmosis → cell SHRINKS (CRENATION)
This is why the body carefully controls the concentration of blood and body fluids — deviations cause serious damage to cells.
Active Transport
ACTIVE TRANSPORT is the movement of substances from a region of LOWER concentration to a region of HIGHER concentration — AGAINST the concentration gradient.
This requires:
1. ENERGY from ATP (produced by aerobic respiration)
2. CARRIER PROTEINS embedded in the cell membrane
Because it requires ATP, active transport STOPS immediately if respiration is blocked (e.g. by cyanide, which blocks aerobic respiration, or by removing oxygen).
Examples in biology:
• ROOTS absorbing minerals: the concentration of mineral ions (e.g. nitrates, magnesium) inside root hair cells is ALREADY HIGHER than in the soil water. Yet plants need even more. Active transport pumps them in against the gradient.
• SMALL INTESTINE absorbing glucose: once most glucose has been absorbed by diffusion, the remaining glucose must be moved from the gut (low [glucose]) into the blood (higher [glucose]) by active transport — ensuring all available glucose is absorbed.
Exchange Surfaces in Multicellular Organisms
Single-celled organisms have a high surface area to volume ratio — simple diffusion is sufficient.
Large multicellular organisms have a much LOWER surface area to volume ratio — the distance from surface to interior cells is too great for diffusion alone.
They need SPECIALISED EXCHANGE SURFACES and TRANSPORT SYSTEMS.
Features of an efficient exchange surface:
• LARGE SURFACE AREA: more area for particles to cross per unit time
• THIN MEMBRANE: short diffusion path
• STEEP CONCENTRATION GRADIENT: maintained by good blood supply (or ventilation for gas exchange)
Examples:
• ALVEOLI (lungs): surrounded by dense capillary network, one cell thick, highly folded — for O₂/CO₂ exchange
• VILLI (small intestine): finger-like projections with microvilli (brush border), rich blood supply, one cell thick walls — for nutrient absorption
• ROOT HAIR CELLS: long projections increase surface area enormously — for water and mineral absorption from soil
⚠️ Common Mistake
Osmosis involves WATER ONLY — through a PARTIALLY PERMEABLE MEMBRANE. Glucose, ions and other molecules do NOT move by osmosis. Active transport goes AGAINST the gradient and requires ATP energy — without ATP (no respiration) it stops. Diffusion and osmosis are both PASSIVE — no ATP needed. Never say 'diffusion requires energy'.
📌 Key Note
Diffusion: particles, high → low, passive. Osmosis: water only, dilute → concentrated, partially permeable membrane, passive. Active transport: low → high, against gradient, requires ATP + carrier proteins.
🎯 Matching Activity — Match the Transport Process
Match each description to diffusion, osmosis or active transport. — drag the symbols on the right to match the component names on the left.
Diffusion
Drop here
Osmosis
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Active transport
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Diffusion
Drop here
Osmosis
Drop here
Active transport
Drop here
Mineral ions absorbed into root hair cells from dilute soil water — against the gradient
Oxygen moving from alveoli into the blood — no ATP required
Movement of water molecules only across a partially permeable membrane — passive
Net movement of particles from high to low concentration — no energy needed
Movement against the concentration gradient — requires ATP and carrier proteins
Water entering a root hair cell from the soil — the soil water is more dilute than cell contents
⚽ FIFA Worked Examples
Osmosis — Percentage Change in Mass
A potato cylinder has a mass of 4.0 g before being placed in a concentrated sugar solution. After 30 minutes it has a mass of 3.4 g. Calculate the percentage change in mass.
F
% change = (final mass − initial mass) ÷ initial mass × 100
I
% change = (3.4 − 4.0) ÷ 4.0 × 100
F
= (−0.6) ÷ 4.0 × 100 = −15%
A
% change = −15% (a decrease — water left by osmosis into the concentrated solution)
⭐ Higher Tier Only
Calculating percentage change in mass in osmosis: % change = (change in mass ÷ original mass) × 100. The isotonic point — where a plant tissue neither gains nor loses mass — is where the external solution concentration matches the cell's water potential. Exchange surfaces are adapted with large surface area, thin walls, good blood supply/ventilation. Students should be able to explain and calculate surface area to volume ratios and explain why large organisms need exchange systems.
🧪 Required Practical
🔬 RP2 — Investigate osmosis: place potato cylinders in different concentrations of sucrose solution. Measure mass before and after. Calculate % change in mass.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 5
1. Which process requires ATP energy to move substances across a cell membrane?
2. A potato cylinder is placed in pure water. What happens and why?
3. Why does active transport stop if oxygen is removed from the environment of a cell?
4. A plant cell is placed in a very concentrated salt solution. What will happen?
5. What is the definition of osmosis?
⭐ How Well Do You Understand This Topic?
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