Movement into and out of Cells
The three mechanisms by which substances cross cell membranes: diffusion (including the factors that affect rate), osmosis (Core definition + Extended water potential, turgidity and plasmolysis), and active transport (energy, protein carriers, and root hair ion uptake). High-priority for Paper 5/6 practicals.
Diffusion
COREDiffusion is the process by which many useful substances enter cells and waste products leave. Understanding it precisely — including where the energy comes from and which factors speed it up — is essential for Topics 6, 9, 11, and 12.
Syllabus Definition — Learn Word for Word
Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement.
The energy for diffusion comes from the kinetic energy of the random movement of molecules and ions — no additional energy input is required.
Key Terms Unpacked
Particles move randomly in both directions across a membrane, but more move from high to low concentration than the reverse. The overall (net) result is movement down the gradient.
The difference in concentration between two regions. The steeper the gradient (bigger the difference), the faster the net rate of diffusion.
Particles move constantly in all directions due to their kinetic energy. No cellular energy (ATP) is used — diffusion is a passive process.
Diffusion requires no energy from respiration. It continues as long as a concentration gradient exists, stopping only when equilibrium is reached.
Importance of Diffusion in Living Organisms
| Example | Substance diffusing | Direction | Why it matters |
|---|---|---|---|
| Gas exchange in alveoli | O₂ and CO₂ | O₂: air → blood; CO₂: blood → air | Delivers O₂ to blood; removes CO₂ from body |
| Gas exchange in leaf | CO₂ and O₂ | CO₂: air → mesophyll cells; O₂: mesophyll → air | Supplies CO₂ for photosynthesis; removes O₂ |
| Nutrient absorption in small intestine | Glucose, amino acids | Gut lumen → blood (initially) | Gets digested food into the bloodstream |
| Excretion in lungs | CO₂ | Blood → alveolar air | Removes metabolic waste |
Factors Affecting the Rate of Diffusion
The syllabus limits investigation of factors to four. For each, be able to explain the mechanism not just state the direction of effect.
| Factor | Effect on rate | Mechanism |
|---|---|---|
| Surface area | Larger → faster | More surface available for particles to cross simultaneously. This is why alveoli, villi, and root hairs all have adaptations that maximise surface area. |
| Temperature | Higher → faster | Higher temperature gives particles more kinetic energy → they move faster and collide with the membrane more frequently → more successful crossings per second. |
| Concentration gradient | Steeper (bigger difference) → faster | A larger difference in particle numbers between the two regions means more particles are “available” to move from the high side per unit time. As the gradient flattens (equilibrium approached), rate slows to zero. |
| Distance | Shorter → faster | Particles take less time to travel across a thinner membrane or shorter diffusion pathway. This is why gas exchange surfaces (alveolar wall + capillary wall) are only one cell thick. |
Common investigations: agar blocks stained with indicator (e.g. agar with NaOH + phenolphthalein, placed in acid) — measure how far the colour front moves inward. By varying block size, you test the effect of surface area : volume ratio and distance. Larger blocks diffuse more slowly to the centre because the distance is greater. When interpreting results, always state what you are measuring and calculate the rate (distance/time or % colour change per minute).
A student places a drop of dye in a beaker of still water and observes it spreading evenly over 10 minutes. Which explanation correctly accounts for this?
- A. The dye is pushed away from the concentrated region by pressure
- B. Dye particles move randomly from a region of higher concentration to lower concentration
- C. The dye requires energy from the water to spread out
- D. Water molecules push the dye particles toward areas of lower water concentration
The alveoli in the lungs have a very large total surface area and walls only one cell thick. Explain how each of these features increases the rate of oxygen diffusion into the blood. [4 marks]
- Large surface area: more oxygen molecules can cross the membrane simultaneously [1 mark]; this increases the number of successful diffusion events per second / increases the overall rate of diffusion [1 mark]
- Thin walls (one cell thick): reduces the distance that oxygen molecules must travel [1 mark]; shorter diffusion distance means less time for each molecule to reach the blood / increases rate [1 mark]
Osmosis
CORE EXTENDEDOsmosis is a special case of diffusion — it applies only to water moving across a partially permeable membrane. Examiner reports flag osmosis as one of the most poorly answered topics year after year, especially the direction of water movement in experiments.
Core Definition — Water and Partially Permeable Membranes
Water diffuses through partially permeable membranes by osmosis. Water molecules move from a dilute solution (high water concentration) to a concentrated solution (low water concentration) through the membrane.
A partially permeable membrane allows water molecules to pass through but restricts larger solute molecules (e.g. sucrose, starch).
Plants are supported by the pressure of water inside cells pushing outwards on the cell wall (turgor). Without enough water, plants wilt.
Role of Water as a Solvent
Before osmosis moves water, that water must do biological work:
Water is the medium in which enzymes work, and a reactant in hydrolysis reactions that break down large food molecules (e.g. starch + water → glucose).
Blood plasma (mostly water) carries dissolved substances — glucose, O₂, CO₂, urea, hormones — around the body. Xylem carries water and mineral ions dissolved in it up the plant.
Urea (a toxic waste product of amino acid breakdown) is dissolved in water and excreted in urine. CO₂ dissolves in blood plasma for transport to the lungs.
Osmosis Experiments (Paper 5/6)
Setup: Fill dialysis tubing with concentrated sugar solution. Tie both ends. Weigh. Place in a beaker of distilled water. Leave for 20 minutes. Reweigh.
Result: The tubing gains mass (and may swell). Water enters the tubing by osmosis — from the higher water concentration (distilled water) to the lower water concentration inside the tubing (concentrated sugar solution).
Control: Repeat with distilled water inside tubing. Result: no change in mass. This confirms the effect is due to osmosis, not leakage.
Setup: Cut identical potato strips. Measure initial length and mass. Place each strip in a different concentration of sucrose solution (e.g. 0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³). Leave for 30 minutes. Measure final length and mass.
| Solution | Change in mass | Explanation |
|---|---|---|
| Distilled water (0 mol/dm³) | Gains mass | Water concentration outside > inside potato cells → water enters by osmosis → cells become turgid |
| Isotonic solution | No change | Water concentration equal on both sides → no net movement of water |
| Concentrated sucrose (high mol/dm³) | Loses mass | Water concentration inside potato cells > outside → water leaves by osmosis → cells become flaccid/plasmolysed |
Plotting results: Graph % change in mass (y-axis) vs. sucrose concentration (x-axis). Where the line crosses the x-axis = isotonic point = approximate solute concentration of the potato cells.
Water Potential — Extended Definition
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
Pure water has the highest possible water potential. Adding solute lowers water potential. Water always moves from higher → lower water potential.
Plant Cell States — Turgid, Flaccid, Plasmolysed
| State | What happened | Vacuole | Cell membrane | Plant effect |
|---|---|---|---|---|
| Turgid | Cell placed in dilute / pure water → water entered by osmosis | Large, fully inflated with cell sap | Pressed firmly against cell wall by turgor pressure | Plant is firm and upright (normal healthy state) |
| Flaccid | Cell placed in concentrated solution → water left by osmosis | Shrunken; cell sap more concentrated | Beginning to pull slightly inward; still in contact with wall | Plant begins to wilt; no longer firm |
| Plasmolysed | Severe water loss → cell membrane pulled away from wall | Very small | Separated from cell wall (visible gap) | Cell dead or near death; complete wilting |
Plant cells have a rigid cell wall. As water enters by osmosis, the cell wall resists further expansion — turgor pressure builds up until it counteracts osmosis. The cell becomes turgid but cannot burst.
Animal cells have no cell wall. If placed in very dilute (hypotonic) water, water enters without limit, and the cell swells until it bursts (lysis). If placed in very concentrated (hypertonic) solution, the cell shrinks (crenation).
A plant cell is placed in a concentrated sucrose solution. Which term correctly describes the state of the cell after several hours?
- A. Turgid — the cell membrane presses firmly against the cell wall
- B. Lysed — the cell membrane has burst due to water entering
- C. Plasmolysed — the cell membrane has pulled away from the cell wall
- D. Flaccid — the cell is firm but the vacuole is enlarged
A plant cell is placed in distilled water. Which of the following correctly describes the movement of water?
- A. Water moves out of the cell because the cell is already full
- B. Water moves into the cell because the cell solution is more concentrated than the water outside
- C. No water movement occurs because both sides of the membrane are equally concentrated
- D. Water moves out of the cell because distilled water has a low water concentration
❌ "Osmosis is the movement of water from dilute to concentrated" → This is backwards. Dilute = high water concentration = high water potential. Water moves FROM dilute TO concentrated.
❌ Saying substances other than water move by osmosis → Osmosis is specifically water movement only. Glucose, amino acids, and mineral ions do not move by osmosis.
❌ Forgetting "partially permeable membrane" → Without this condition, osmosis cannot occur. Always mention the membrane in your definition.
❌ Ext: Thinking high water potential = concentrated solution → Opposite is true. Concentrated solution = more dissolved solutes = fewer free water molecules = lower water potential. Pure water = highest water potential.
Active Transport
CORE EXTENDEDActive transport is unique because it moves substances against the concentration gradient — something diffusion and osmosis cannot do. This requires energy from respiration, making it the most energy-expensive membrane transport mechanism.
Core Definition
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient), using energy from respiration.
Mechanism: Protein Carriers — Extended
Protein carriers (also called transport proteins or carrier proteins) are embedded in the cell membrane. Each carrier is specific to particular molecules or ions. The carrier binds the substance on the low-concentration side, uses energy from respiration (ATP), changes shape, and releases the substance on the high-concentration side.
Moving particles against a gradient is like pushing a ball uphill — it requires work. The energy comes from ATP produced in cellular respiration (in mitochondria). Cells carrying out high levels of active transport have many mitochondria (e.g. root hair cells, intestinal epithelium cells).
Mineral ions (e.g. nitrate, potassium) in the soil are often at lower concentration in the soil water than inside the root hair cell. Diffusion would move ions out of the root. Active transport pumps ions in against the gradient, using protein carriers and energy from respiration.
A scientist measures the concentration of nitrate ions inside root hair cells and in the surrounding soil water. The concentration inside the cells is much higher than in the soil water. Explain how the root hair cells are able to absorb nitrate ions from the soil. [4 marks]
- Nitrate ions must move against a concentration gradient (from lower to higher concentration) [1 mark]
- This requires active transport (not diffusion, which only moves down the gradient) [1 mark]
- Protein carrier proteins in the cell membrane bind the nitrate ions and transport them across [1 mark]
- Energy from respiration (ATP) is required for this process [1 mark]
A cell absorbs glucose from a solution in which glucose concentration is lower than inside the cell. This process requires energy. What is the name of this process?
- A. Diffusion
- B. Osmosis
- C. Active transport
- D. Transpiration
Three Mechanisms — Side-by-Side Comparison
This table is the single most useful revision tool for Topic 3. Questions often describe a scenario and ask which mechanism is involved — use this as your decision framework.
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| What moves? | Any dissolved particles (gases, solutes) | Water molecules only | Specific molecules or ions |
| Direction | High → low concentration (down gradient) | High → low water potential / dilute → concentrated | Low → high concentration (against gradient) |
| Energy needed? | ✗ No — uses kinetic energy of particles | ✗ No — uses kinetic energy of particles | ✓ Yes — energy from respiration (ATP) |
| Membrane type | Can cross any membrane or between cells | Requires partially permeable membrane | Cell membrane with protein carriers |
| Protein carriers? | ✗ Not required | ✗ Not required (water passes through aquaporins, but not syllabus) | ✓ Required — specific to the substance |
| Classic biological example | O₂ from alveoli into blood; CO₂ out of respiring cells | Water uptake by plant roots; water into red blood cells | Mineral ion uptake by root hairs; glucose re-absorption in kidney |
| Effect of cyanide (blocks respiration)? | No effect — passive | No effect — passive | Stops — energy supply cut off |
Step 1: Is the substance being moved water? → Yes → Osmosis (if partially permeable membrane present)
Step 2: Is movement from low to high concentration (against gradient)? → Yes → Active transport
Step 3: Is movement from high to low concentration, no energy needed? → Yes → Diffusion
Comprehensive Practice Questions
Mixed questions across all of Topic 3, in the style of Cambridge IGCSE 0610 Papers 1–4.
Oxygen enters a red blood cell from the surrounding blood plasma. Which term best describes this movement?
- A. Diffusion — oxygen moves from high to low concentration down its gradient
- B. Osmosis — oxygen is dissolved in water and crosses the membrane
- C. Active transport — energy is needed to move oxygen against its gradient
- D. Transpiration — oxygen is lost from the cell surface
A student cuts five potato chips of equal length (50 mm) and mass (5.0 g). Each chip is placed in a sucrose solution of a different concentration for 30 minutes. The results are shown below:
Tube 1 (0 mol/dm³): final mass 5.8 g | Tube 2 (0.2 mol/dm³): 5.3 g | Tube 3 (0.4 mol/dm³): 5.0 g | Tube 4 (0.6 mol/dm³): 4.6 g | Tube 5 (1.0 mol/dm³): 4.1 g
(a) Explain why the chip in Tube 1 gained mass. [3 marks]
(b) Identify the isotonic concentration for this potato tissue and justify your answer. [2 marks]
(c) Predict what would happen to the potato chip if it were placed in a 1.5 mol/dm³ sucrose solution. [2 marks]
- Distilled water (0 mol/dm³) has a higher water concentration than the cell sap inside the potato cells [1 mark]
- Water entered the potato cells by osmosis [1 mark]
- Through the partially permeable cell membrane, from high to low water concentration [1 mark]
- Isotonic concentration = 0.4 mol/dm³ [1 mark]
- At 0.4 mol/dm³ the final mass equals the initial mass (5.0 g = 5.0 g) — no net water movement, so the solute concentration of the sucrose solution equals the solute concentration inside the potato cells [1 mark]
- The chip would lose further mass (more than in Tube 5) [1 mark]
- Because the sucrose solution is more concentrated than the cell sap, so water continues to leave the cells by osmosis; cells would become plasmolysed [1 mark]
Intestinal epithelial cells absorb glucose from the gut, even when the glucose concentration in the gut is lower than inside the cells. Meanwhile, water is absorbed by osmosis from the gut into the blood.
(a) Explain why glucose absorption cannot occur by diffusion in this situation. [2 marks]
(b) State the name of the process by which glucose is absorbed and give two features that distinguish it from diffusion. [3 marks]
(c) Explain why a drug that inhibits cellular respiration would reduce glucose absorption but not water absorption. [3 marks]
- Diffusion only moves substances down a concentration gradient (high → low) [1 mark]
- Glucose concentration is lower in the gut than in the cells, so diffusion would move glucose out of the cells, not into them [1 mark]
- Process: active transport [1 mark]
- Feature 1: active transport moves glucose against the concentration gradient (low → high); diffusion moves down the gradient [1 mark]
- Feature 2: active transport requires energy from respiration; diffusion is passive (no energy required) [1 mark]
- Active transport requires energy (ATP) from respiration [1 mark]
- Inhibiting respiration stops ATP production → no energy for protein carriers → active transport (glucose absorption) stops [1 mark]
- Osmosis is a passive process — it does not require energy from respiration, so it continues unaffected [1 mark]
High-Frequency Mistakes — Topic 3 Overall
- ⇌Osmosis described as "water moving from concentrated to dilute"This is backwards. Concentrated solution = low water concentration = low water potential. Water moves FROM dilute (high water concentration) TO concentrated (low water concentration). Think: water moves toward the more crowded solute particles.
- 💧Saying substances other than water move by osmosisOsmosis is exclusively the movement of water molecules through a partially permeable membrane. Glucose, amino acids, and ions do NOT move by osmosis.
- ⚡Stating that diffusion requires energyDiffusion and osmosis are both passive processes — they use only the kinetic energy of the particles themselves, not energy from cellular respiration. Only active transport requires energy from respiration.
- 🔴Forgetting "partially permeable membrane" in osmosis definitionsWithout this condition, the process is just ordinary diffusion. The partially permeable membrane is essential — it allows water through but not large solute molecules.
- 🌿Thinking plants can burst in dilute solutionPlant cells cannot burst because the cell wall resists swelling — turgor pressure builds until equilibrium. Animal cells CAN burst (lysis) because they lack a cell wall.
- 📈Stopping the osmosis experiment answer at "water moves in/out"Exam mark schemes require the full chain: state the relative concentrations, state the direction, state the mechanism (osmosis), and state the outcome (turgid/flaccid/plasmolysed). One-word answers lose most of the marks.
- ⚓Ext: "High water potential = concentrated solution"The opposite is true. Pure water has the highest water potential. Adding solute lowers water potential. Concentrated solution = lower water potential.
- 🔥Ext: Forgetting protein carriers in active transportExtended candidates must state that active transport uses specific protein carriers embedded in the cell membrane. These bind the substance and use ATP to move it against the gradient.
- 🌿Ext: "Root hairs absorb minerals by diffusion"Mineral ion concentrations are often higher inside root cells than in soil water. This means ions cannot diffuse in — they must be absorbed by active transport using protein carriers and energy from respiration.
Topic 3 is one of the highest-frequency topics across all papers. The three-way comparison table is your most powerful revision tool. Key high-yield items: osmosis definition and direction (Core), the potato chip / dialysis tubing experiment results and interpretation (Paper 5/6), turgid/plasmolysis vocabulary (Extended), and the explanation of why mineral uptake by root hairs requires active transport (Extended Paper 4). Always include “partially permeable membrane” in osmosis answers and “energy from respiration” in active transport answers.