Transport in cellsOxford AQA IGCSE Biology: Revision notes
Section 1
What is diffusion and how does it work?
Diffusion is the net movement of particles from a region of high concentration to a region of low concentration. It is a passive process, meaning no energy from respiration is required.
Diffusion occurs because particles move randomly in all directions. When there is a concentration gradient (a difference in concentration between two areas), there are more particles moving from the high concentration region to the low concentration region than in the opposite direction. Over time, particles spread out and become evenly distributed.
Key points:
- Diffusion works for gases, liquids, and dissolved substances
- It continues until concentration is equal throughout (equilibrium)
- It is a passive process – no ATP energy is used
- The net movement is always down the concentration gradient
Examiners often test whether you understand that diffusion is about net movement, not that particles only move one way. Always say 'net movement' in your answers to show you understand particles move randomly but there is an overall movement down the gradient.
Imagine a room where people are scattered unevenly – more crowded in one corner than another. Even though people move randomly in all directions, more will leave the crowded corner than enter it, so gradually the crowd spreads out evenly.
Section 2
What factors affect the rate of diffusion?
The rate of diffusion is the speed at which particles move from high to low concentration. Four key factors affect this rate:
| Factor | Effect on diffusion rate | Explanation |
|---|---|---|
| Concentration gradient | Larger gradient = faster diffusion | The greater the difference in concentration, the more particles move down the gradient |
| Temperature | Higher temperature = faster diffusion | Higher temperature gives particles more kinetic energy, so they move faster and collide more frequently |
| Surface area | Larger surface area = faster diffusion | More area allows more particles to cross the partially permeable membrane simultaneously |
| Distance | Shorter distance = faster diffusion | Particles have less distance to travel, so they reach equilibrium more quickly |
How to apply this in exams:
- If you are asked to explain why something diffuses faster, identify which factor has changed and explain the mechanism
- Remember that all four factors work together – the rate depends on the combination of conditions
- In questions about living organisms (e.g. leaf cells absorbing CO₂), think about how these factors apply in real biological systems
A cube of sugar in cold water dissolves more slowly than in hot water. Why? Higher temperature increases the kinetic energy of water and sugar molecules, making them move faster. This increases collision frequency and the rate at which sugar particles spread through the water.
Students often say 'more surface area means faster diffusion because more particles can diffuse'. Be precise: larger surface area allows more particles to cross the membrane simultaneously, so the total amount diffusing per unit time is greater.
Section 3
What is osmosis and how does it differ from diffusion?
Osmosis is the net movement of water molecules through a partially permeable membrane from a region of high water potential to a region of low water potential. Alternatively, it can be described as the net movement of water from a dilute to a more concentrated solution through a partially permeable membrane.
Key differences from diffusion:
| Feature | Diffusion | Osmosis |
|---|---|---|
| Particles moving | Any dissolved substance or gas | Water molecules only |
| Membrane | Not always involved; can occur across any boundary | Must involve a partially permeable membrane |
| Gradient | Along concentration gradient (of dissolved substance) | Along water potential gradient |
| Direction | From high to low concentration | From dilute (high water potential) to concentrated (low water potential) |
Why osmosis occurs:
- In a dilute solution, there are more free water molecules
- In a concentrated solution, dissolved particles are surrounded by and bound to water molecules, reducing the number of free water molecules
- Water molecules move randomly but there is a net movement of free water molecules towards the concentrated solution
- This continues until water potential is equal on both sides of the membrane
Examiners distinguish between students who say 'osmosis is water moving' (vague) and those who say 'osmosis is water moving through a partially permeable membrane' (correct). Always mention the membrane – it is essential to the definition.
Imagine a nightclub (dilute) and a VIP room (concentrated solution). The VIP room has fewer free people because some are occupied with celebrities (solute particles). More people drift from the nightclub into the VIP room than vice versa, so gradually people accumulate in the VIP room.
Section 4
What are the effects of osmosis on plant and animal cells?
Osmosis causes water to move into or out of cells, depending on whether the surrounding solution is dilute or concentrated. This has different effects on plant and animal cells because plant cells have rigid cell walls.
Effects on plant cells:
-
Dilute external solution (hypotonic): Water enters the cell by osmosis
- Cell becomes turgid (firm and rigid)
- Cell wall pushes inward on the cytoplasm, creating turgor pressure
- Plant remains rigid and upright
- This is the normal, healthy state
-
Concentrated external solution (hypertonic): Water leaves the cell by osmosis
- Cell becomes flaccid (limp and soft)
- Cytoplasm pulls away from the cell wall (plasmolysis occurs)
- Cell becomes plasmolysed – the cell membrane separates from the cell wall
- Plant wilts and droops
- If water re-enters, the cell becomes turgid again (deplasmolysis)
Effects on animal cells:
Animal cells have no cell wall, so they respond differently:
-
Dilute external solution (hypotonic): Water enters the cell
- Cell swells and bursts (lyses)
- This state is called lysed or haemolysis (in red blood cells)
- Cell contents spill out – this is irreversible damage
-
Concentrated external solution (hypertonic): Water leaves the cell
- Cell shrinks and shrivels
- Cell membrane pinches inward, creating a spiky appearance
- This is called crenation (in red blood cells)
- Cell may recover if placed in isotonic solution
Comparison table:
| Cell type | Dilute solution | Concentrated solution |
|---|---|---|
| Plant | Turgid (normal) | Flaccid and plasmolysed (wilted) |
| Animal | Lysed/burst (damage) | Crenated/shrivelled (may recover) |
A plant cell in pure water (dilute/hypotonic): water enters by osmosis → cell becomes turgid → plant is rigid. A plant cell in salt solution (concentrated/hypertonic): water leaves by osmosis → cell becomes flaccid and plasmolysed → plant wilts. A red blood cell in pure water: water enters → cell lyses (bursts irreversibly). A red blood cell in salt solution: water leaves → cell becomes crenated (shrivelled, may recover).
Students often confuse 'flaccid' with 'plasmolysed'. Flaccid means the cell is limp; plasmolysed means the cell membrane has separated from the cell wall. A plasmolysed cell is always flaccid, but a cell can be slightly flaccid without being fully plasmolysed.
Section 5
What is active transport and how does it work?
Active transport is the movement of substances against a concentration gradient using energy from respiration. It is the opposite of diffusion and osmosis, which are passive processes.
Key features of active transport:
- Substances move from low to high concentration (against the gradient)
- Requires ATP energy produced by respiration
- Involves carrier proteins in the cell membrane that actively transport particles
- Occurs in living cells that are respiring actively
- Can transport substances that cannot diffuse (e.g. some ions and glucose in certain contexts)
- Does not require a concentration gradient to work
Why is active transport important?
Active transport allows cells to accumulate substances they need, even when the external concentration is lower. This is essential for survival and growth.
Examples of active transport:
-
Absorption of mineral ions by root hair cells:
- Soil water often has a lower concentration of mineral ions than root cells
- Root hair cells actively transport mineral ions (e.g. nitrate, phosphate, potassium) from soil into the cell
- This requires energy because ions move against the concentration gradient
- These ions are essential for plant growth (e.g. nitrogen from nitrate for protein synthesis)
-
Glucose absorption in the gut:
- Although some glucose can diffuse across the intestinal epithelium, much is absorbed by active transport
- Glucose is transported from the gut lumen (lower concentration) into gut epithelial cells (higher concentration)
- This requires ATP energy from respiration
- Active transport ensures maximum glucose uptake, even when dietary glucose is low
- Glucose then moves into the bloodstream by diffusion
Comparison of transport mechanisms:
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Energy required? | No | No | Yes (ATP from respiration) |
| Direction of movement | High → low concentration | High → low water potential | Low → high concentration |
| Membrane required? | Not always | Yes (partially permeable) | Yes (carrier proteins) |
| Rate affected by temperature? | Yes | Yes | Yes (affects respiration rate) |
| Can move against gradient? | No | No | Yes |
When explaining active transport, always mention three key points: (1) it moves substances against the concentration gradient, (2) it requires ATP from respiration, and (3) it involves carrier proteins. Examiners expect all three for full marks.
Root hair cell and nitrate ions: Soil water has low nitrate concentration, but the root cell needs nitrate for protein synthesis. Carrier proteins in the root cell membrane use ATP energy to actively transport nitrate ions from the soil into the cell, moving them against the concentration gradient. This accumulation of nitrate is impossible by diffusion alone.
Must Know
- Diffusion is the net movement of particles from high to low concentration without energy; affected by concentration gradient, temperature, surface area, and distance
- Osmosis is the net movement of water only through a partially permeable membrane from high to low water potential (or dilute to concentrated solution)
- Plant cells in dilute solutions become turgid (firm); in concentrated solutions become flaccid and plasmolysed (wilted)
- Animal cells in dilute solutions become lysed (burst irreversibly); in concentrated solutions become crenated (shrivelled, may recover)
- Active transport moves substances against the concentration gradient using ATP energy from respiration; examples include mineral ion absorption in roots and glucose absorption in the gut
- Active transport requires carrier proteins and living, respiring cells; it is the only mechanism that can accumulate substances from a dilute external solution
That's the notes covered.
Carry on to the next subtopic.