What are the main types of movement of substances into and out of cells?

The main types of movement of substances into and out of cells include diffusion, osmosis, and active transport. Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration. Osmosis is the diffusion of water molecules through a selectively permeable membrane. Active transport requires energy to move substances against their concentration gradient, from an area of lower concentration to an area of higher concentration.

How does diffusion differ from active transport in cells?

Diffusion is a passive process that does not require energy, where substances move from an area of higher concentration to an area of lower concentration. In contrast, active transport is an energy-dependent process that moves substances against their concentration gradient, from an area of lower concentration to an area of higher concentration, using cellular energy in the form of ATP.

Why is osmosis important for cells?

Osmosis is crucial for maintaining cell turgor pressure, which helps keep cells rigid and supports plant structure. It also regulates the balance of water between the cell and its environment, ensuring that cells do not become dehydrated or burst due to excessive water intake. This balance is vital for the proper functioning of cells and the overall health of the organism.

What role do cell membranes play in the movement of substances?

Cell membranes are selectively permeable, meaning they allow certain substances to pass through while blocking others. This selective permeability is crucial for maintaining homeostasis within the cell. The membrane's structure, composed of a phospholipid bilayer with embedded proteins, facilitates the movement of substances via diffusion, osmosis, and active transport, ensuring that essential nutrients enter the cell and waste products are expelled.

How is the process of active transport significant in biological systems?

Active transport is significant because it allows cells to maintain concentration gradients that differ from their surroundings, which is essential for various cellular functions. For example, active transport enables the uptake of essential ions and nutrients that are in lower concentrations outside the cell. It also plays a critical role in nerve impulse transmission and muscle contraction, making it vital for the functioning of complex biological systems.

Why is "Forgetting 'partially permeable membrane' in the osmosis definition" a common mistake in Movement of Substances into and out of Cells?

Why it happens: Students remember 'water moves from dilute to concentrated' but skip the membrane requirement. How to avoid it: Write the FULL definition: 'net movement of water molecules from higher to lower water concentration, **through a partially permeable membrane**'. Without the membrane clause it's just diffusion. Source: 4BI1 Examiner Reports 2022-2024

Why is "Saying active transport 'uses energy' without naming ATP and respiration" a common mistake in Movement of Substances into and out of Cells?

Why it happens: 'Uses energy' feels good enough. How to avoid it: Always write '**ATP from respiration**'. Edexcel mark schemes explicitly require the link to respiration; just 'energy' loses the mark. Source: 4BI1 Examiner Reports 2023-2024

Why is "Writing 'plasmolysed cell shrinks' without saying the membrane pulls away" a common mistake in Movement of Substances into and out of Cells?

Why it happens: Shrinking is the visible outcome students remember. How to avoid it: Plasmolysis is specifically the **cell-surface membrane (and cytoplasm) PULLING AWAY from the cell wall**. The wall keeps its shape; the contents shrink. Source: 4BI1 Examiner Reports 2024

Why is "Saying red blood cells become 'turgid' in pure water" a common mistake in Movement of Substances into and out of Cells?

Why it happens: Confusing animal and plant cell responses. How to avoid it: Animal cells have **NO cell wall** → they cannot become turgid. They **BURST (haemolyse)** in pure water. Only PLANT cells go turgid.

Why is "Writing 'water moves from high to low CONCENTRATION' (without saying water concentration)" a common mistake in Movement of Substances into and out of Cells?

Why it happens: Loose phrasing — students mean 'water concentration' but skip the word. How to avoid it: Always specify **water concentration** (or 'water potential', or 'from dilute to concentrated SOLUTION'). 'High to low concentration' is ambiguous and loses the mark. Source: 4BI1 Examiner Reports 2023

Why is "Forgetting to blot potato strips dry in the osmosis practical" a common mistake in Movement of Substances into and out of Cells?

Why it happens: Method seems intuitive — students focus on weighing. How to avoid it: **Blot dry** before BOTH weighings — initial and final. Surface water inflates the apparent mass and ruins the data. Source: 4BI1 Examiner Reports 2024

Structure and Functions in Living OrganismsEdexcel IGCSE Biology (4BI1)

Movement of Substances into and out of Cells

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Movement of Substances into and out of Cells — Pearson Edexcel International GCSE Biology 4BI1 Study Notes (2026 onwards, Spec Issue 4)

Three modes of transport: diffusion (passive, down gradient), osmosis (water through a partially permeable membrane), active transport (against gradient, ATP-dependent). Factors affecting rate; plant cells (turgid → flaccid → plasmolysed) and animal cells (crenate / haemolyse) in different solutions. Required practical on osmosis with potato strips. Covers spec 2.11-2.16.

What you’ll learn

Mapped to the Cambridge IGCSE 4BI1 syllabus (2026 onwards).

2.11 — Understand diffusion as the net movement of particles from a region of higher to lower concentration; recognise that this is passive (no ATP required).
2.12 — Understand osmosis as the net movement of water molecules through a partially permeable membrane from a dilute to a concentrated solution.
2.13 — Practical: investigate osmosis in living tissue (e.g. potato cylinders in sucrose solutions of different concentration).
2.14 — Understand active transport as the movement of substances against a concentration gradient using energy from respiration (ATP).
2.15 — Know examples in living organisms: gas exchange in lungs/leaves (diffusion), water uptake by root hair cells (osmosis), mineral ion uptake by root hair cells and glucose absorption in the gut (active transport).
2.16 — Understand the factors affecting the rate of movement of substances: surface area, temperature, concentration gradient, distance/diffusion path.

Diffusion — net movement down the gradient (spec 2.11)

Passive movement of particles from high to low concentration.

Diffusion is the NET movement of particles (atoms, ions or molecules) from a region of HIGH concentration to a region of LOW concentration. Particles move in all directions because of their random kinetic energy, but the NET (overall) direction is always down the concentration gradient.

Diffusion is passive — it does NOT require energy from respiration. Eventually the particles are evenly spread (equilibrium); there is no NET movement after that, although the particles continue to move randomly.

Examples in living organisms:

Oxygen diffuses INTO red blood cells in the alveoli (high O₂ in alveoli, low O₂ in blood).
Carbon dioxide diffuses OUT of cells into the blood (high CO₂ in cells, low CO₂ in blood).
Oxygen diffuses INTO leaves through the stomata (during the day, in cells that aren't photosynthesising, e.g. roots).
Digested food molecules (e.g. amino acids, glucose) diffuse from the gut lumen into the bloodstream when their concentration in the gut is HIGHER than in the blood.

Edexcel keyword note. The phrase 'NET MOVEMENT' is essential — random motion is happening in both directions all the time; only the net flow is one-way. Examiner reports flag students who omit 'net'.

Diffusion = net movement HIGH → LOW concentration.
Passive — no ATP required.
Examples: O₂ into RBCs, CO₂ out of cells.
Continues until equilibrium.

See the full worked example for movement of substances into and out of cells →

Osmosis — special case of diffusion (spec 2.12)

Water through a partially permeable membrane.

Osmosis is the NET movement of WATER molecules from a region of HIGHER water concentration (DILUTE solution) to a region of LOWER water concentration (CONCENTRATED solution), through a partially permeable membrane.

Three keywords MUST appear in your osmosis answers:

WATER molecules (NOT 'solute' or 'particles' — osmosis is water-specific).
Partially permeable membrane (lets water through but not larger solute molecules).
NET movement (random water motion is bidirectional; only the net flow is one way).

Like diffusion, osmosis is passive — no ATP required.

Three useful comparisons:

Pure water has the HIGHEST possible water concentration.
Concentrated sucrose solution has a LOW water concentration (solute molecules take up "space").
The cytoplasm of a typical cell is intermediate.

Examples in living organisms:

Water absorption by root hair cells — soil water has higher water concentration than the cell cytoplasm, so water enters by osmosis.
Water absorption from the gut into the bloodstream after a meal.
Water movement between plant cells that maintains turgor pressure.

Edexcel pitfall. Don't write 'water moves from high to low CONCENTRATION' — that's ambiguous. Write 'from higher to lower WATER concentration' or 'from a dilute to a concentrated SOLUTION'.

Osmosis = movement of WATER through a partially permeable membrane.
Direction: dilute solution → concentrated solution (= high water → low water).
Passive — no ATP.
Must include 'partially permeable membrane' in the definition.

See the full worked example for movement of substances into and out of cells →

Plant cells in different solutions (spec 2.13)

Turgid (pure water) → flaccid (mild loss) → plasmolysed (severe loss).

Plant cells respond to changes in surrounding water concentration in three steps:

1. Turgid — in pure water (or dilute solution).

Higher water concentration OUTSIDE the cell.
Water enters by osmosis → cytoplasm + vacuole expand → push outward against the cellulose cell wall.
The wall is rigid and inelastic, so it STOPS the cell bursting.
The cell becomes turgid (firm and full) — turgor pressure is the pressure of the contents against the wall.
Turgor keeps non-woody plants UPRIGHT. Lose turgor → plant wilts.

2. Flaccid — in a slightly concentrated solution.

Slightly lower water concentration outside → small net loss of water.
Cell loses some turgor pressure → soft/limp.
Cytoplasm still in contact with the cell wall.

3. Plasmolysed — in a strongly concentrated solution.

Much lower water concentration outside → large net loss of water.
Vacuole and cytoplasm shrink so much that the cell-surface membrane PULLS AWAY from the cell wall.
The wall keeps its shape; the cell contents shrink inside it.
If prolonged, the cell dies.

Edexcel keyword check. Plasmolysis is specifically the membrane and cytoplasm pulling away from the wall — NOT just 'the cell shrinks'. Examiner reports mark down vague answers.

Demonstration in school. Place a strip of red onion epidermis on a microscope slide. Add salt solution. Within minutes, you can see the purple-stained vacuole + cytoplasm shrinking and pulling away from the cell wall — classic plasmolysis. Rinsing with pure water reverses it (cells re-turgid) provided plasmolysis is not yet permanent.

Turgid = full, firm, pushing on the wall (in dilute solution).
Flaccid = soft, still touching the wall (small water loss).
Plasmolysed = membrane PULLED AWAY from wall (severe water loss).
Cellulose wall stops plants bursting in pure water.

See the full worked example for movement of substances into and out of cells →

Animal cells in different solutions

No cell wall — burst in pure water, shrivel in salt.

Animal cells have NO cell wall. They are only enclosed by the cell-surface membrane, which is flexible but does not provide structural support. Their response to changes in surrounding water concentration is therefore very different from plant cells.

1. In pure water (or solution more dilute than cytoplasm):

Water enters by osmosis.
Membrane stretches as the cell swells.
Cell BURSTS (a process called HAEMOLYSIS in red blood cells, or LYSIS more generally).

2. In isotonic solution (same water concentration as cytoplasm):

No net water movement → cell stays the same size and shape.
This is why intravenous fluids are made up as 0.9% NaCl (saline) — it is isotonic to blood plasma.

3. In concentrated salt solution (lower water concentration than cytoplasm):

Water LEAVES by osmosis.
The cell shrinks and the membrane wrinkles inward — the cell is CRENATED (sometimes called 'shrivelled' or 'crenated').
No cell wall, so no fixed outline — just a shrivelled blob.

Why the difference between plant and animal cells? The cellulose cell wall in plants gives them physical support. It stops them bursting in pure water (because the wall can resist the outward pressure). It also keeps the cell's overall shape even when the cytoplasm shrinks. Animals have no wall, so swelling = bursting and shrinking = shrivelling.

Edexcel pitfall. Don't say RBCs become 'turgid' or 'plasmolysed' — those terms are for plant cells. Use lysed / haemolysed and crenated for animal cells.

Animal cells have NO cell wall.
Pure water → cell BURSTS (haemolysis / lysis).
Isotonic → no change (e.g. 0.9% saline).
Concentrated salt → cell SHRIVELS (crenated).

See the full worked example for movement of substances into and out of cells →

Active transport — against the gradient (spec 2.14)

Carrier proteins + ATP from respiration.

Active transport is the movement of particles AGAINST a concentration gradient — from LOW to HIGH concentration. It is the OPPOSITE direction to diffusion.

Because particles are being moved against their natural tendency, energy from respiration in the form of ATP is required. Active transport also needs specific carrier proteins in the cell-surface membrane:

The carrier protein binds the substance on the LOW-concentration side.
ATP is used to change the carrier's shape.
The substance is released on the HIGH-concentration side.

Examples in living organisms:

Mineral ion uptake by root hair cells. Soil water has very low concentrations of nitrate and other ions; cell cytoplasm has higher concentrations. The plant needs the ions to make amino acids and other molecules → active transport powered by ATP from respiration.
Glucose absorption from the gut. When food has been digested and most glucose has already been absorbed, the gut lumen has LOW glucose, but the blood (downstream) has HIGHER. Active transport moves the last glucose against the gradient so very little is lost in faeces.
Glucose reabsorption from the kidney filtrate back into the blood.

Adaptations of root hair cells for active transport:

Many mitochondria → release lots of ATP.
Long hair-like extension → large surface area for more carrier proteins.
Thin cell wall and cell-surface membrane → short path for ions and water.

Practical evidence for active transport. Adding a respiratory inhibitor (e.g. cyanide) to roots stops mineral-ion uptake but NOT water uptake. This shows that ion uptake is ATP-dependent (active transport) while water uptake is passive (osmosis).

Active transport = AGAINST the gradient.
Needs ATP from respiration and carrier proteins.
Examples: mineral ion uptake by roots; glucose absorption in gut.
Root hair cells have many mitochondria for ATP.
Respiratory inhibitors stop active transport but not osmosis.

See the full worked example for movement of substances into and out of cells →

Factors affecting the rate of movement (spec 2.16)

Surface area, gradient, distance, temperature.

The rate at which substances move into or out of cells (whether by diffusion, osmosis or active transport) is affected by several factors:

1. Surface area — the larger the area of membrane available, the more particles can cross per unit time. Many specialised cells have a HUGE surface area:

Alveoli in the lungs → fast O₂ / CO₂ exchange.
Villi and microvilli in the small intestine → fast nutrient absorption.
Root hair cells → fast water and ion absorption.

2. Concentration gradient — the bigger the difference in concentration across the membrane, the faster the rate of diffusion/osmosis. (Active transport can move particles even when the gradient is 'wrong'.)

3. Distance (diffusion path) — the SHORTER the distance to be crossed, the faster the rate. Many exchange surfaces are only ONE CELL THICK to minimise the diffusion path:

Alveolus wall (one epithelial cell).
Capillary wall (one endothelial cell).

4. Temperature — higher temperature → more kinetic energy → particles move faster → faster diffusion. Active transport speeds up with temperature too (more ATP from respiration, more enzyme activity) but only up to the optimum.

5. Size of particle — smaller particles diffuse faster than larger ones at the same temperature.

Edexcel question style. A typical Paper 1 question asks you to list THREE factors and explain HOW each increases rate. Use the template 'factor X, SO that rate is faster' — examiners reject lists without the link.

Surface area ↑ → rate ↑ (alveoli, villi, root hairs).
Concentration gradient ↑ → rate ↑.
Diffusion distance ↓ → rate ↑ (one-cell-thick membranes).
Temperature ↑ → rate ↑ (more KE).
Particle size ↓ → rate ↑.

See the full worked example for movement of substances into and out of cells →

Required practical — osmosis in potato strips (spec 2.13)

% mass change in sucrose solutions of different concentration.

Edexcel names this as a required practical. Examiners can ask about it on both papers.

Method:

Use a cork borer to cut several potato cylinders of the same diameter from ONE potato (controls tissue type).
Trim each to the same length (e.g. 3 cm). Blot dry with paper towel.
Weigh each cylinder on a balance and record the initial mass $m_i$ .
Prepare a range of sucrose solutions of known concentration — e.g. 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol dm⁻³ (the independent variable).
Place one cylinder in each solution. Leave for a fixed time (e.g. 30 min) at room temperature.
Remove each cylinder, blot dry again, and reweigh. Record the final mass $m_f$ .
Calculate percentage change in mass: $\%\,\Delta m = (m_f - m_i)/m_i \times 100$ .
Repeat × 3 and calculate the mean % change at each concentration.
Plot % change in mass (y-axis) against sucrose concentration (x-axis).

Variables:

IV: sucrose concentration.
DV: % change in mass.
CVs: time, temperature, potato source, dimensions of cylinder, volume of sucrose, blotting technique.

Why % change rather than absolute change? Even with careful cutting, cylinders have slightly different starting masses. Calculating the % of the original mass normalises for this so all data points are directly comparable.

Expected results:

In dilute sucrose (0.0 - ~0.3 mol dm⁻³): water moves IN → mass INCREASES → POSITIVE % change.
In concentrated sucrose (~0.5 - 1.0 mol dm⁻³): water moves OUT → mass DECREASES → NEGATIVE % change.
The line crosses zero at the isotonic point — where the sucrose has the SAME water concentration as the potato cell sap.

Edexcel mark-scheme tip. Examiner reports note that 'blot dry' is a frequent omission that loses 1-2 marks. Surface water on the cylinder inflates the apparent mass and corrupts the data.

Cut cylinders equal size from ONE potato.
Blot DRY before AND after weighing.
Calculate % mass change, not absolute change.
Repeat × 3 and take the mean.
Isotonic point = where % change crosses zero.

See the full worked example for movement of substances into and out of cells →

How it’s examined

Movement of substances generates 8-12 marks per paper — one of the highest-yield Topic-2 areas. Paper 1: define the three modes (4-6 marks), 'explain why X happens in pure water' (4 marks), factors-affecting-rate listing (5 marks). Paper 2 (4BI1/2B): full required-practical method on potato + sucrose (6-8 marks), plus application questions (e.g. 'why do root hair cells contain many mitochondria'). Mark scheme spelling keywords: 'NET movement', 'partially permeable membrane', 'AGAINST the gradient', 'ATP from respiration', 'membrane PULLS AWAY from cell wall'.

Sources: Pearson Edexcel International GCSE Biology 4BI1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4BI1 Examiner Reports 2022-2024; 4BI1/1B May/Jun 2024 question paper and mark scheme; 4BI1/1B January 2024 question paper and mark scheme; 4BI1/2B May/Jun 2024 question paper and mark scheme; 4BI1/2B January 2024 question paper and mark scheme; 4BI1/1BR May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-12.

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Step-by-step worked examples — Movement of Substances into and out of Cells

Step-by-step solutions to past-paper-style questions on movement of substances into and out of cells, written exactly the way a tutor would explain them at the board.

1Defining diffusion, osmosis and active transport (6 marks, Paper 1)
Core• Adapted from 4BI1/1B May/Jun 2024• diffusion, osmosis, active transport, spec-2.11, spec-2.12, spec-2.14
Question
Define (a) diffusion, (b) osmosis and (c) active transport. (6 marks)
Step-by-step solution
1. Step 1
  (a) Diffusion (2 marks). The net movement of particles from a region of HIGH concentration to a region of LOW concentration (1 mark) — i.e. down a concentration gradient — as a result of the random movement of particles; passive (no energy from respiration required) (1 mark).
2. Step 2
  (b) Osmosis (2 marks). The net movement of WATER molecules from a region of HIGHER water concentration (dilute solution) to a region of LOWER water concentration (concentrated solution) (1 mark), through a partially permeable membrane (1 mark).
3. Step 3
  (c) Active transport (2 marks). The movement of particles AGAINST a concentration gradient (from low to high) (1 mark) — requires energy from respiration (ATP) (1 mark). Carrier proteins in the membrane move the particle across.
Answer
(a) Diffusion = net movement DOWN the concentration gradient; passive. (b) Osmosis = net movement of WATER across a partially permeable membrane from dilute to concentrated solution. (c) Active transport = movement AGAINST the gradient using ATP from respiration.
Examiner tip
Edexcel mark schemes split each definition into 2 marks: 1 for the DIRECTION + 1 for the ENERGY requirement (or membrane type for osmosis). Examiner reports flag candidates who forget the words 'NET MOVEMENT' (the random motion of particles in both directions is happening at all times — only the NET flow is one-way).
2Investigating osmosis with potato strips (6 marks, Paper 2)
Extended• Adapted from 4BI1/2B May/Jun 2024• required practical, osmosis, spec-2.13, Paper 2
Question
Describe how a student could use potato strips and sucrose solutions to investigate the effect of solute concentration on the mass change of plant tissue due to osmosis. (6 marks)
Step-by-step solution
1. Step 1
  Cut potato strips of equal dimensions (1 mark). Use a cork borer to cut several cylinders of the same diameter, then trim each to the same length (e.g. 3 cm) with a scalpel. Blot dry with paper towel.
2. Step 2
  Record the initial mass of each strip on a balance (1 mark). Use the same balance and same number of decimal places throughout.
3. Step 3
  Prepare a range of sucrose solutions (1 mark). e.g. 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol dm⁻³ — the independent variable. Use a known volume of each in separate boiling tubes.
4. Step 4
  Place one potato strip in each solution (1 mark). Leave for the same fixed time (e.g. 30 minutes) at the same temperature (controlled variables). Use a labelled tube for each concentration.
5. Step 5
  Remove the strips, blot dry and reweigh (1 mark). Calculate percentage change in mass = (final mass − initial mass) / initial mass × 100. % change is used because it normalises for any slight variation in starting mass.
6. Step 6
  Plot % mass change on y-axis vs sucrose concentration on x-axis (1 mark). The concentration where the line crosses zero (no change) is the concentration isotonic to the potato cell sap. Repeat 3 times and take the mean for reliability.
Answer
Cut equal-size strips → record initial mass → place one in each sucrose concentration (e.g. 0-1 mol dm⁻³) → leave fixed time → blot and reweigh → calculate % mass change → plot vs concentration → identify the concentration of zero change (isotonic point) → repeat × 3 and average.
Examiner tip
AVP up to 6. Edexcel mark schemes specifically credit: equal-size strips; blot dry (otherwise extra surface water inflates mass); use of % change rather than absolute change; fixed temperature/time; repeats and mean. Examiner reports often flag students who forget the blotting step — a major source of error.
3Turgid, flaccid, plasmolysed plant cells (5 marks, Paper 1)
Core• Adapted from 4BI1/1B January 2024• osmosis, plant cell, turgor, spec-2.13
Question
Plant cells placed in different sucrose solutions can become turgid, flaccid or plasmolysed. Describe what each term means and explain how it arises. (5 marks)
Step-by-step solution
1. Step 1
  Turgid (B1). Cell placed in a DILUTE solution (or pure water) — water has higher concentration outside.
2. Step 2
  Water enters by osmosis (B1). Through the partially permeable cell-surface membrane → cytoplasm and vacuole expand → push outward against the cell wall.
3. Step 3
  The cellulose cell wall stops the cell bursting (B1). It can stretch only slightly. The pressure of the contents on the wall is turgor pressure — keeps the plant firm and upright.
4. Step 4
  Flaccid (B1). Cell placed in a solution slightly more concentrated than the cell sap → small net loss of water → the cell loses turgor pressure → goes soft. Cytoplasm still touching cell wall.
5. Step 5
  Plasmolysed (B1). Cell placed in a much more concentrated solution → large net loss of water → cytoplasm and vacuole shrink → cell-surface membrane PULLS AWAY from the cell wall. If prolonged, the cell DIES.
Answer
Turgid = in dilute solution; water enters; full and firm against cell wall. Flaccid = water loss; soft; cytoplasm still touching wall. Plasmolysed = severe water loss; cytoplasm pulled away from wall; cell may die.
Examiner tip
Edexcel mark scheme grants 1 mark per term + 1 for the osmosis mechanism. Examiner reports specifically credit 'cell-surface MEMBRANE pulls away from CELL WALL' — students who say 'the cell shrinks' lose the plasmolysis mark.
4Animal cells in different solutions (4 marks, Paper 1)
Core• Adapted from 4BI1/1B May/Jun 2024• osmosis, red blood cell, spec-2.13
Question
Red blood cells placed in pure water swell and burst. The same cells placed in concentrated salt solution shrivel. Explain. (4 marks)
Step-by-step solution
1. Step 1
  In pure water — higher water concentration OUTSIDE than INSIDE (B1). Water enters the red blood cell by osmosis through the partially permeable cell-surface membrane.
2. Step 2
  Cell swells and bursts (B1). Animal cells have NO cell wall to prevent expansion. The membrane stretches until it ruptures — HAEMOLYSIS.
3. Step 3
  In concentrated salt solution — lower water concentration OUTSIDE than INSIDE (B1). Water leaves the cell by osmosis.
4. Step 4
  Cell shrinks and shrivels (B1). The membrane wrinkles inward — the cell is CRENATED. No cell wall, so no fixed shape.
Answer
Pure water → water enters by osmosis → no cell wall → cell bursts (haemolysis). Concentrated salt → water leaves → cell shrivels (crenated). Animal cells, unlike plant cells, have no cell wall to protect them.
Examiner tip
Edexcel awards M/A pairs: each state-water-direction earns M1; the consequence (burst / shrivel) earns A1. The phrase 'no cell wall' is the discriminating point between plant and animal water relations — examiner reports note that students who miss this lose easy marks.
5Factors affecting diffusion rate (5 marks, Paper 1)
Core• Adapted from 4BI1/1B January 2024• diffusion, factors, spec-2.11, spec-2.16
Question
State THREE factors that affect the rate of diffusion across a membrane and explain how each one increases the rate. (5 marks)
Step-by-step solution
1. Step 1
  Concentration gradient (B1). The STEEPER the gradient (bigger difference in concentration across the membrane), the FASTER the rate of diffusion.
2. Step 2
  Surface area (B1). A LARGER surface area means more particles can cross at the same time → faster rate. (e.g. villi in the small intestine; alveoli in lungs; root hair cells in roots.)
3. Step 3
  Distance / diffusion path (B1). A SHORTER diffusion distance (e.g. thin membrane, single cell layer) means particles travel a shorter way → faster rate. (e.g. one-cell-thick alveolar epithelium.)
4. Step 4
  Temperature (B1). Higher temperature → more kinetic energy of particles → particles move faster → faster diffusion.
5. Step 5
  Size of particle (B1). Smaller particles diffuse faster than larger ones (same KE → faster speed).
Answer
Any THREE: steeper concentration gradient → faster; larger surface area → faster; shorter diffusion distance → faster; higher temperature → faster (more KE); smaller particle → faster.
Examiner tip
Edexcel mark schemes typically award B1 for naming the factor + B1 for explaining 'how' (so 3 factors × 1.67 → 5 max). Examiner reports criticise students who list factors without explaining the link — always write 'larger surface area, SO faster rate'.

Model Answers — Movement of Substances into and out of Cells

High-scoring sample answers for movement of substances into and out of cells on the Pearson Edexcel IGCSE 4BI1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question

Adapted from 4BI1/2B May/Jun 2024 Q610 marks

Compare and contrast diffusion, osmosis and active transport as ways in which substances move into and out of cells, giving named examples of each in living organisms. (10 marks)

Model answer

Diffusion is the net movement of particles (gases or dissolved solutes) from a region of HIGH concentration to a region of LOW concentration — down a concentration gradient — as a result of the random motion of particles. It is passive: no energy from respiration is needed.

Named examples: oxygen diffusing INTO red blood cells in the alveoli; carbon dioxide diffusing OUT of cells into the bloodstream; oxygen diffusing INTO leaves through stomata for respiration.

Osmosis is a special case of diffusion involving water. It is the net movement of WATER molecules from a region of HIGHER water concentration (dilute solution) to a region of LOWER water concentration (concentrated solution) through a partially permeable membrane. Also passive — no ATP needed.

Named examples: water absorbed by root hair cells from the soil; water moving from the gut lumen into bloodstream after a meal; water moving between cells of a plant maintaining turgor.

Active transport is the movement of particles AGAINST a concentration gradient (from LOW to HIGH concentration). It requires energy from respiration in the form of ATP and is carried out by carrier proteins in the cell-surface membrane.

Named examples: mineral ion uptake (e.g. nitrate, phosphate) by root hair cells from soil where ion concentrations are LOW; glucose absorption from the gut in the small intestine when blood glucose is HIGHER than gut glucose (preventing simple diffusion); reabsorption of glucose from the kidney filtrate.

Comparison table:

Feature	Diffusion	Osmosis	Active transport
Particles moved	Solutes / gases	Water only	Solutes (ions, glucose)
Direction	High → low	High water → low water	Low → high (against gradient)
Energy from ATP?	NO (passive)	NO (passive)	YES (active)
Membrane needed?	Often	Partially permeable	Yes — needs carrier protein
Examples	O₂ into RBCs, CO₂ out of cells	Water into root hair cells	Mineral ions into root hair cells; glucose absorption in gut

Why this scores

Mark scheme: definition of each process with key details (3 × 2 = 6 marks). Named examples (1 mark each, max 3). At least one contrast point (e.g. ATP requirement, direction). Max 10. Edexcel mark schemes are strict on the keyword 'NET MOVEMENT' for diffusion / osmosis, and 'AGAINST the gradient + ATP' for active transport. Common mark-loser: forgetting 'partially permeable membrane' in osmosis definition.

Question
Adapted from 4BI1/2B January 2024 Q410 marks
Describe an experiment using potato cylinders to investigate the effect of sucrose concentration on the mass of plant tissue, and explain why the % change in mass should be calculated rather than the absolute change. Include a sketch of the expected results. (10 marks)
Model answer
Apparatus. 5-6 boiling tubes, sucrose solutions of known concentration (0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³), cork borer, scalpel, ruler, balance (to 2 d.p.), paper towels, marker pen.

Method:

Use a cork borer to cut several cylinders of potato of the same diameter from one potato (uses same tissue → controlled variable).

Trim each cylinder to the same length (e.g. 3 cm) with a scalpel.

Blot dry each cylinder with paper towel to remove surface water and weigh on a balance to the same precision. Record the initial mass $m_i$ .

Place one cylinder into each boiling tube, each containing 20 cm³ of a different sucrose solution. Label each tube clearly.

Leave for a fixed time (e.g. 30 minutes) at room temperature.

Remove each cylinder, blot dry with the same care as step 3, and reweigh. Record the final mass $m_f$ .

Calculate the percentage change in mass: $\%\,\Delta m = \dfrac{m_f - m_i}{m_i} \times 100$ .

Repeat the experiment 3 times and calculate the mean % change for each concentration.

Independent variable: sucrose concentration. Dependent variable: % change in mass. Control variables: potato source, dimensions, time, temperature, volume of sucrose, blotting technique.

Why use % change and not absolute change? Even when cylinders are cut to the same dimensions, they will have slightly different starting masses. Calculating the percentage of the original mass that was gained or lost normalises for this variation, so the values are directly comparable between cylinders. It also makes the results comparable with other students' data or with potato tissue of a different starting mass.

Expected results:

In DILUTE sucrose (0.0 - ~0.3 mol dm⁻³): water moves IN by osmosis → cylinder GAINS mass → POSITIVE % change.

In CONCENTRATED sucrose (~0.5 - 1.0 mol dm⁻³): water moves OUT → cylinder LOSES mass → NEGATIVE % change.

The line crosses zero at the concentration where there is no net water movement — this concentration is isotonic with the potato cell sap (typically around 0.3 mol dm⁻³).

A sketch graph: x-axis = sucrose concentration; y-axis = % change in mass; line slopes downward from positive on the left to negative on the right, crossing zero at the isotonic point.
Why this scores
Mark scheme: method points 1-6 (B1 each, max 4); IV/DV/CVs (B1 each, max 3); % change calculation + reason (B1 × 2); shape of expected curve (B1). Max 10. Edexcel mark schemes are strict on 'blot dry' (otherwise surface water inflates the mass) and '% change' (rather than absolute change). The isotonic-point interpretation is the credit-keyword for the upper bands.
Question
Adapted from 4BI1/2B May/Jun 2024 Q5(b)8 marks
Compare what happens to red blood cells and plant cells when placed in (a) pure water and (b) a concentrated salt solution. Explain why the responses are different. (8 marks)
Model answer
(a) In pure water: the water concentration outside the cell is HIGHER than inside. Water moves into both cells by osmosis through their partially permeable cell-surface membranes.

Red blood cell — has NO cell wall. As water enters, the cell swells, the cell-surface membrane stretches and eventually BURSTS (a process called haemolysis). Animal cells in a hospital setting are kept in isotonic saline (~0.9% NaCl) for this reason.

Plant cell — has a cellulose cell wall outside the cell-surface membrane. Water enters; the cytoplasm and vacuole expand; the contents push outward against the cell wall. The wall is rigid and inelastic — it stops the cell bursting. The cell becomes TURGID (firm and full) with high turgor pressure. Turgor keeps the plant upright.

(b) In concentrated salt solution: water concentration outside is LOWER than inside the cell. Water leaves both cells by osmosis.

Red blood cell — shrinks and shrivels; the membrane wrinkles inward — the cell is CRENATED. No cell wall means no fixed shape; just a wrinkled blob.

Plant cell — small water loss → cell becomes FLACCID (soft, no turgor). Large water loss → cytoplasm and vacuole shrink so much that the cell-surface membrane PULLS AWAY from the cell wall; the cell is PLASMOLYSED. The cell wall keeps its shape, but the cytoplasm separates from it. Prolonged plasmolysis kills the cell.

Why the difference? The presence of a rigid cellulose cell wall in plant cells is the key. It limits how much the cell can swell (so plants don't burst in pure water — they become firm and turgid instead) and it keeps the cell's outer shape even when the cytoplasm shrinks (so plants 'wilt' rather than shrivel out of recognition).

This is why plants wilt when not watered — their cells go from turgid to flaccid to plasmolysed. Watering reverses the process by allowing water to re-enter.
Why this scores
Mark scheme: pure water + plant cell (1 B-mark direction, 1 B-mark turgid + wall stops bursting); pure water + RBC (1 B-mark direction, 1 B-mark haemolysis); salt + plant cell (1 B-mark direction, 1 B-mark plasmolysis with membrane pulled from wall); salt + RBC (1 B-mark direction, 1 B-mark crenation); 'cell wall is the difference' explicitly stated (1 B-mark). Max 8. Examiner reports specifically award the 'cell-surface membrane PULLS AWAY' phrase for plasmolysis.
Question
Adapted from 4BI1/2B January 2024 Q5(b)6 marks
Explain how root hair cells absorb mineral ions from the soil, and why this process is described as active transport. (6 marks)
Model answer
Mineral ions (e.g. nitrate, phosphate, magnesium) are present in soil water at very low concentration, yet plants need them at much higher concentration inside cells (e.g. nitrate for making amino acids).

The concentration of mineral ions INSIDE the root hair cell is HIGHER than in the soil water. This means simple diffusion CANNOT move the ions inward — they would naturally diffuse OUT of the cell, not in.

The ions are absorbed by active transport:

Carrier proteins in the root hair cell-surface membrane bind specific ions on the outside.

ATP from respiration is used to change the shape of the carrier protein.

The protein releases the ion on the INSIDE of the cell — i.e. the ion has been moved from low to high concentration, against the concentration gradient.

Adaptations of root hair cells for active transport:

Long, thin extension (the 'hair') → very large surface area in contact with soil water, so more carrier proteins are exposed.

MANY MITOCHONDRIA → release a large amount of ATP to power the carrier proteins.

Thin cell wall and cell-surface membrane → short diffusion path for water (which enters by osmosis simultaneously).

This is described as active transport because:

The ions move AGAINST a concentration gradient.

Energy (ATP from respiration) is required.

Specific carrier proteins are needed.

Practical evidence. When a respiratory inhibitor (e.g. cyanide) is added to roots, mineral-ion uptake stops while water uptake continues — confirming that ion uptake is active (ATP-dependent) but water uptake (osmosis) is passive.
Why this scores
Mark scheme: ions move against gradient (B1); carrier proteins (B1); ATP from respiration (B1); root hair cell adaptations — many mitochondria for ATP (B1) AND large surface area (B1); 'why active transport' explicitly linked back to ATP + against-gradient (B1); AVP includes respiratory inhibitor evidence (B1). Max 6. Examiner reports flag students who write 'active transport uses energy' without specifying ATP and respiration as the source.

Key Definitions and Keywords — Movement of Substances into and out of Cells

Definitions to memorise and the exact keywords mark schemes credit for movement of substances into and out of cells answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4BI1 sitting.

Diffusion
Examiner keyword
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 the random motion of particles. Passive — does NOT require ATP.
Osmosis
Examiner keyword
The net movement of WATER molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution), through a partially permeable membrane. Passive.
Active transport
Examiner keyword
The movement of particles AGAINST a concentration gradient (from low to high concentration), using carrier proteins in the membrane and requiring energy from respiration in the form of ATP.
Partially permeable membrane
Examiner keyword
A membrane that allows some molecules (e.g. water) to pass through but not others (e.g. large sugars, ions). The cell-surface membrane of every cell is partially permeable.
Turgid
Examiner keyword
A plant cell that has gained water by osmosis until its cytoplasm pushes firmly against the cell wall. The pressure inside is turgor pressure. Keeps non-woody plants upright.
Flaccid
Examiner keyword
A plant cell that has lost some water by osmosis so the cytoplasm no longer pushes hard against the cell wall. The cell becomes soft. Cytoplasm is still in contact with the wall.
Plasmolysed
Examiner keyword
A plant cell that has lost so much water by osmosis that the cell-surface membrane and cytoplasm have pulled away from the cell wall. Severe and usually fatal if prolonged.
Crenated
An animal cell (e.g. red blood cell) placed in a solution that is more concentrated than its cytoplasm. Water leaves by osmosis; the cell shrivels and the cell-surface membrane wrinkles inward.
Haemolysed / lysed
A red blood cell (or other animal cell) placed in a solution that is less concentrated than its cytoplasm. Water enters by osmosis; the cell swells and bursts because it has no cell wall.
Isotonic solution
A solution with the same water concentration (same solute concentration) as a cell's contents. No NET water movement in or out — the cell stays the same size.

Common Mistakes and Misconceptions — Movement of Substances into and out of Cells

The traps other students keep falling into on movement of substances into and out of cells questions — taken from recent Pearson Edexcel IGCSE 4BI1 examiner reports and mark schemes — and how to avoid them.

✕Forgetting 'partially permeable membrane' in the osmosis definition
4BI1 Examiner Reports 2022-2024
Why it happens
Students remember 'water moves from dilute to concentrated' but skip the membrane requirement.
How to avoid it
Write the FULL definition: 'net movement of water molecules from higher to lower water concentration, through a partially permeable membrane'. Without the membrane clause it's just diffusion.
✕Saying active transport 'uses energy' without naming ATP and respiration
4BI1 Examiner Reports 2023-2024
Why it happens
'Uses energy' feels good enough.
How to avoid it
Always write 'ATP from respiration'. Edexcel mark schemes explicitly require the link to respiration; just 'energy' loses the mark.
✕Writing 'plasmolysed cell shrinks' without saying the membrane pulls away
4BI1 Examiner Reports 2024
Why it happens
Shrinking is the visible outcome students remember.
How to avoid it
Plasmolysis is specifically the cell-surface membrane (and cytoplasm) PULLING AWAY from the cell wall. The wall keeps its shape; the contents shrink.
✕Saying red blood cells become 'turgid' in pure water
Why it happens
Confusing animal and plant cell responses.
How to avoid it
Animal cells have NO cell wall → they cannot become turgid. They BURST (haemolyse) in pure water. Only PLANT cells go turgid.
✕Writing 'water moves from high to low CONCENTRATION' (without saying water concentration)
4BI1 Examiner Reports 2023
Why it happens
Loose phrasing — students mean 'water concentration' but skip the word.
How to avoid it
Always specify water concentration (or 'water potential', or 'from dilute to concentrated SOLUTION'). 'High to low concentration' is ambiguous and loses the mark.
✕Forgetting to blot potato strips dry in the osmosis practical
4BI1 Examiner Reports 2024
Why it happens
Method seems intuitive — students focus on weighing.
How to avoid it
Blot dry before BOTH weighings — initial and final. Surface water inflates the apparent mass and ruins the data.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Movement of Substances into and out of Cells — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Movement of Substances into and out of Cells

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Defining diffusion, osmosis and active transport (6 marks, Paper 1)

2Investigating osmosis with potato strips (6 marks, Paper 2)

3Turgid, flaccid, plasmolysed plant cells (5 marks, Paper 1)

4Animal cells in different solutions (4 marks, Paper 1)

5Factors affecting diffusion rate (5 marks, Paper 1)

Diffusion

Osmosis

Active transport

Partially permeable membrane

Turgid

Flaccid

Plasmolysed

Crenated

Haemolysed / lysed

Isotonic solution

✕Forgetting 'partially permeable membrane' in the osmosis definition

✕Saying active transport 'uses energy' without naming ATP and respiration

✕Writing 'plasmolysed cell shrinks' without saying the membrane pulls away

✕Saying red blood cells become 'turgid' in pure water

✕Writing 'water moves from high to low CONCENTRATION' (without saying water concentration)

✕Forgetting to blot potato strips dry in the osmosis practical

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Movement of Substances into and out of Cells — frequently asked questions