Movement Into and Out of Cells
Summary: Substances move across cell membranes by three main processes: diffusion (net movement down a concentration gradient, passive), osmosis (net movement of water through a partially permeable membrane from high to low water potential, passive), and active transport (movement against a concentration gradient, requires energy from respiration). Tags: igcse biology diffusion osmosis active-transport cell-membrane Created: 2026-07-16 Last Updated: 2026-07-16
1. Diffusion
Definition: Diffusion is 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.
Key points:
- Diffusion is a passive process — it does NOT require energy (ATP) from respiration
- The particles move down the concentration gradient (from high to low)
- Diffusion continues until the particles are evenly distributed (equilibrium is reached), but particles continue to move randomly in both directions — there is just no net movement
- Diffusion occurs in liquids and gases (but not solids, where particles are fixed in position)
Examples of diffusion in living organisms:
- Oxygen diffuses from the alveoli (high O2 concentration) into the blood in capillaries (low O2 concentration)
- Carbon dioxide diffuses from the blood (high CO2 concentration) into the alveoli (low CO2 concentration)
- Digested food (e.g. glucose, amino acids) diffuses from the small intestine into the blood
- Carbon dioxide diffuses from the air spaces in a leaf into the leaf cells during photosynthesis (through open stomata)
- Oxygen in a plant diffuses out of leaf cells after photosynthesis when O2 concentration is higher inside the leaf
2. Factors Affecting the Rate of Diffusion
| Factor | How it Affects Rate | Explanation |
|---|---|---|
| Temperature | Higher temperature → faster diffusion | Particles have more kinetic energy and move faster, so they spread out more quickly |
| Concentration gradient | Steeper gradient → faster diffusion | A larger difference in concentration means more net movement per unit time |
| Surface area | Larger surface area → faster diffusion | More particles can cross the exchange surface at the same time |
| Distance (thickness) | Shorter distance → faster diffusion | Particles don’t have to travel as far to cross the exchange surface; this is why exchange surfaces are thin (e.g. alveolar walls are one cell thick) |
| Size of particles/molecules | Smaller particles → faster diffusion | Smaller, lighter particles diffuse more quickly than larger, heavier ones |
3. Osmosis
Definition: Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane.
Key points:
- Osmosis is a special case of diffusion — only for water molecules, through a partially permeable membrane
- Osmosis is also a passive process — no energy required
- Water potential = a measure of how freely water molecules can move. Pure water has the highest water potential
- Adding solute lowers the water potential (water molecules are bound to solute particles and move less freely)
- Water always moves from high water potential to low water potential — i.e. from a dilute solution to a concentrated solution
Water potential can be thought of as:
Pure water → very high water potential (0 kPa) Dilute solution → high water potential (slightly negative kPa) Concentrated solution → low water potential (more negative kPa)
4. Effects of Osmosis on Cells
Effects on Animal Cells
| Type of Solution | Water Potential Comparison | Effect on Cell | Explanation |
|---|---|---|---|
| Hypotonic (more dilute than cell contents) | External water potential > internal water potential | Cell swells and may burst (lysis) | Water enters cell by osmosis; animal cells have no cell wall to prevent bursting |
| Hypertonic (more concentrated than cell contents) | External water potential < internal water potential | Cell shrinks/shrivels (crenation) | Water leaves the cell by osmosis |
| Isotonic (same concentration as cell contents) | External water potential = internal water potential | No net movement; cell stays the same | Water enters and leaves at equal rates |
Effects on Plant Cells
| Type of Solution | Effect on Cell | Explanation | What you observe |
|---|---|---|---|
| Hypotonic (more dilute) | Cell becomes turgid (swollen and firm) | Water enters by osmosis. The cell swells, pushing the cytoplasm against the cell wall. The cell wall prevents bursting — the cell is now firm and rigid. | Cell is full and firm; this is the normal, healthy state for plant cells. The cell wall prevents bursting. |
| Hypertonic (more concentrated) | Cell becomes flaccid and then plasmolysed | Water leaves by osmosis. The vacuole shrinks, the cytoplasm pulls away from the cell wall (plasmolysis). | Cell becomes soft/wilted. Under a microscope, the cell membrane is seen pulling away from the cell wall. |
| Isotonic (same concentration) | No net movement; cell is flaccid (limp, not turgid) | Water enters and leaves at equal rates. | Cell is not firm (flaccid) — plants wilt when cells are not turgid |
Key terms for plants:
- Turgid: The cell is full of water, cytoplasm pushes tightly against cell wall → plant stands upright (healthy)
- Flaccid: The cell has lost water but the cytoplasm has not yet pulled away from the cell wall → plant wilts
- Plasmolysed: The cytoplasm has pulled away from the cell wall due to severe water loss → plant severely wilts
5. Active Transport
Definition: Active transport is the movement of particles against a concentration gradient (from a region of lower concentration to a region of higher concentration), using energy released from respiration.
Key points:
- Active transport requires energy (ATP) from respiration
- It uses carrier proteins (also called protein pumps) in the cell membrane
- The carrier proteins bind to specific particles and change shape to move them across the membrane
- It can only happen in living cells (because dead cells cannot respire)
Examples of active transport:
- Root hair cells taking up mineral ions (e.g. nitrate, magnesium) from the soil — the concentration of mineral ions in the soil is lower than inside the root cells, so they must be actively transported against the gradient
- Glucose reabsorption in the kidney nephrons — glucose is actively transported from the filtrate back into the blood, as all glucose should be reabsorbed
- Sodium-potassium pump in nerve cells (neurones) — actively transports Na+ out and K+ in to set up resting potential
6. Surface Area to Volume Ratio (SA:V)
The surface area to volume ratio is crucial for determining how organisms exchange substances with their environment.
| Organism Size | SA:V Ratio | What This Means |
|---|---|---|
| Small (e.g. amoeba, bacteria) | Large SA:V | Surface area is large enough relative to volume for diffusion to meet all of the organism’s needs. No specialised exchange system needed. |
| Large (e.g. mammals) | Small SA:V | Surface area is not large enough relative to volume for diffusion alone to supply all cells. Large organisms need specialised exchange surfaces (lungs, gills) and transport systems (circulatory system). |
Why SA:V matters:
- As an organism gets larger, its volume increases faster than its surface area (cube vs square relationship)
- A small organism can rely on diffusion alone for gas exchange and nutrient supply
- A large organism needs specialised systems — lungs (large SA for gas exchange), circulatory system (transport over large distances), kidneys (excretion)
Adaptations that increase surface area:
- Alveoli in lungs — millions of tiny air sacs → enormous total surface area
- Villi in small intestine — finger-like projections → increased SA for absorption
- Root hair cells — long extensions → increased SA for water/mineral uptake
- Flattened shapes — e.g. red blood cells (biconcave) → increased SA for oxygen diffusion
7. Summary Comparison
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| What moves? | Any small particles (O2, CO2, glucose, etc.) | Water molecules only | Ions, glucose, amino acids |
| Direction of movement | Down concentration gradient (high → low) | From high water potential to low water potential (dilute → concentrated) | Against concentration gradient (low → high) |
| Membrane needed? | No (but often occurs across one) | Yes — partially permeable membrane | Yes — cell membrane with carrier proteins |
| Energy (ATP) needed? | No (passive) | No (passive) | Yes (needs energy from respiration) |
| Carrier proteins? | No | No | Yes |
Sources
- BBC Bitesize GCSE Biology — Movement across cell membranes, BBC (free educational resource)
- OpenStax Biology 2e — Ch. 5 Structure and Function of Plasma Membranes, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Syllabus 3: Movement into and out of cells, Cambridge Assessment International Education
- CK-12 Biology for High School — Cell Transport, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Cell Structure and Organisation — Structure of the cell membrane and organelles
- Biological Molecules — The substances that move across membranes (water, glucose, ions)
- Transport in Plants — Xylem, phloem, transpiration, and root uptake
- Gas Exchange in Humans — Diffusion of O2 and CO2 at the alveoli
- IGCSE-Bio-Index — Full IGCSE Biology index
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Diffusion stops once equilibrium is reached” | Particles continue to move in both directions; there is just no net movement — it is a dynamic equilibrium |
| ”Osmosis is the movement of any substance through a membrane” | Osmosis specifically refers to the movement of water molecules through a partially permeable membrane |
| ”Active transport doesn’t need a membrane” | Active transport always involves carrier proteins in the cell membrane |
| ”If a plant cell is turgid, water is still moving into it” | A turgid cell is at equilibrium — water enters and leaves at the same rate (no net movement); the cell wall prevents further expansion |
| ”A plant cell placed in pure water will burst” | Plant cells have a cell wall that prevents them from bursting. They become turgid, which is the normal healthy state |
| ”Animal cells have a cell wall too” | Animal cells do NOT have a cell wall — only plant cells do. Animal cells burst in hypotonic solutions because there is no cell wall to restrict swelling |