Diffusion, Osmosis & Active Transport
Revise Diffusion, Osmosis & Active Transport for Biology 0610 (O Level) — revision notes and instant AI marking.
Diffusion, Osmosis & Active Transport
Cambridge IGCSE Biology — Complete Revision Guide
Quick Summary
This chapter covers how substances move into and out of cells. You need to understand three transport mechanisms, what controls how fast they work, and why each one matters to living organisms.
Diffusion
Random movement of particles from high to low concentration. No energy needed. Happens in all directions but net movement is one way.
Osmosis
Diffusion of water specifically, across a partially permeable membrane. Water moves toward salt/sugar. No energy needed.
Active Transport
Particles move against concentration gradient (uphill). Requires energy from respiration. Cells use proteins to pump things in.
Diffusion in Biology
What is Diffusion?
Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration. This happens because all particles move randomly all the time—they bounce around. On average, more end up moving one way than the other.
Key point: Diffusion is always passive—no cell energy (ATP) is used. The particles have kinetic energy from heat, and they use that to bump around.
Diffusion Across Cell Membranes
The cell membrane is partially permeable. This means it lets some molecules through easily, but blocks others. The main rule:
- Small molecules (oxygen, carbon dioxide, glucose) can pass through.
- Large molecules (proteins) and ions (depending on charge) are blocked or move slowly.
The membrane is made of a lipid bilayer—a double layer of fat-like molecules with embedded proteins. Small molecules can slip between or through the lipids. Larger ones get stuck.
Why Diffusion Matters to Living Cells
Cells rely on diffusion for:
- Gas exchange: Oxygen diffuses in for respiration; CO₂ diffuses out as waste.
- Nutrient uptake: Glucose and amino acids diffuse into cells.
- Waste removal: Urea diffuses out of cells into the blood.
People push toward exits randomly—not organized, just each person moving independently. More people move out (from high crowd density) than in (from low crowd density). Eventually, the crowd spreads evenly through the streets. No one is "guiding" people; they just naturally spread out.
Energy Source: Brownian Motion
Brownian motion is the random, continuous movement of particles caused by their thermal energy (heat). At the atomic scale, particles never stop moving—they collide with each other constantly. This is the energy source for diffusion.
You don't need to understand the physics in detail, but remember: diffusion doesn't require the cell to use ATP; the heat energy already in the particles drives it.
Examples of Diffusion in Living Organisms
| Location | Molecule | Direction |
|---|---|---|
| Leaf (mesophyll) | Oxygen | Into mitochondria (respiration) |
| Leaf (mesophyll) | Carbon dioxide | Into chloroplasts (photosynthesis) |
| Alveoli (lungs) | Oxygen | Into blood capillaries |
| Small intestine (villi) | Digested food (glucose, amino acids) | Into blood |
Plants need oxygen for respiration at all times, even though they also photosynthesize. Don't think plants only need CO₂—they respire 24/7 and oxygen diffuses into every cell.
Factors That Influence Diffusion Rate
Diffusion happens in all living cells, but how fast it happens varies. Understanding the factors that speed it up or slow it down is crucial for explaining why cells are shaped the way they are.
1. Concentration Gradient
The concentration gradient is the difference in concentration between two areas. The bigger the difference, the faster diffusion.
Why? If one side has lots of molecules and the other has few, more collisions will happen that push molecules toward the dilute side. Think of it like this: if 100 people are trying to leave a room but only 1 is trying to enter, most will leave.
Imagine a hallway divided in half: one side packed with people (high concentration), the other nearly empty (low concentration). Even though everyone moves randomly, the net flow is toward the empty side—simply because more people are on the packed side to begin with.
2. Distance (Thickness)
The smaller the distance molecules have to travel, the faster diffusion occurs. Molecules only have to bounce their way a short distance.
Real-world example: Blood capillaries have walls only one cell thick so oxygen can diffuse quickly into surrounding tissues. If capillary walls were thick, diffusion would take much longer.
3. Temperature
Higher temperature → faster diffusion. This is because heat gives molecules more kinetic energy. Warmer molecules bounce faster and collide more frequently.
This is why reactions speed up when you heat them (in a petri dish or in an organism). At 37°C (human body temperature), diffusion is faster than at 20°C.
4. Surface Area to Volume Ratio
This is the trickiest factor. Cells with larger surface area relative to volume can diffuse substances faster.
Why? Diffusion only happens across the cell membrane (surface). The larger the surface, the more diffusion can occur per unit of cytoplasm inside.
- A small cube has a high surface area to volume ratio.
- A large cube has a low surface area to volume ratio.
Example: If you have a cube with sides of 1 cm:
- Surface area = 6 cm²
- Volume = 1 cm³
- Ratio = 6 : 1
If you double the cube's size (2 cm sides):
- Surface area = 24 cm²
- Volume = 8 cm³
- Ratio = 3 : 1 (half as much!)
As cells get bigger, their surface area to volume ratio decreases—meaning they can't absorb nutrients or remove waste as efficiently. This is why cells stay small.
Cell Adaptations to Increase Surface Area
Cells that need to absorb or transport lots of substances have evolved to increase their surface area. Here are the main examples:
- Root hair cells (plants): Long, finger-like projections increase surface area for absorbing water and mineral ions from soil.
- Intestinal epithelial cells (animals): Covered in finger-like villi, which are themselves covered in microvilli. This creates a huge absorptive surface for nutrients.
- Alveoli (lungs): Tiny air sacs maximize surface area for gas exchange.
- Fish gills: Thin, folded structures with massive surface area for oxygen absorption.
When asked "Why does this cell have this shape?", think about surface area. Root hair cells are long and thin, intestinal cells have villi—these adaptations increase surface area for diffusion.
Water as a Solvent
Why is Water So Important?
Water is called the "universal solvent" because many vital substances dissolve in it. This is one of the most important properties of water for life.
Why Cells Need Water as a Solvent
- Transport: Dissolved nutrients (glucose, amino acids) travel through plants in xylem and phloem, and through animals in blood—all dissolved in water.
- Digestion: Food molecules are broken down in aqueous solution in the alimentary canal. Water is essential for digestive enzymes to work.
- Excretion: Toxic waste (urea) and excess salts dissolve in water and are removed in urine.
- Metabolic reactions: Nearly all enzyme reactions happen in aqueous solution. Without dissolved substances, reactions can't occur.
- Temperature regulation: Water's high heat capacity helps organisms maintain stable temperatures.
Imagine dissolved glucose is a package and water is the delivery truck. Glucose alone can't move through tissues; but dissolved in water, it travels through the blood to cells that need it. Without the "truck" (water), the "packages" (nutrients) have nowhere to go.
Water makes up ~70% of cells. Most metabolic reactions occur in this watery environment. Without it, life as we know it is impossible.
Osmosis
What is Osmosis?
Osmosis is a special case of diffusion—specifically, the diffusion of water molecules across a partially permeable membrane.
Understanding Water Potential
To understand osmosis, you must understand water potential. This is a term that sounds scary but is actually simple:
- Higher water potential: Dilute solution (lots of water molecules, few solute molecules). Pure water has the highest water potential.
- Lower water potential: Concentrated solution (many dissolved solutes). The solutes "tie up" water molecules, reducing their ability to move.
Water always moves from high water potential to low water potential. Another way to say this: water moves toward the salt/sugar.
Think of water molecules as dancers. In pure water (high water potential), all dancers are free to move. In a salty solution (low water potential), some dancers are "stuck" holding hands with salt ions—they're less free. Free dancers naturally move toward the "stuck" dancers because there's less freedom on that side. The net flow is toward where more dancers are bound up.
Why the Cell Membrane is Important
The cell membrane is partially permeable:
- Water molecules are small → they pass through easily.
- Salt and sugar molecules are large → they cannot pass through.
This is why osmosis happens. Water can cross, but solutes can't. So water is "pulled" toward wherever there's more solute stuck behind the membrane.
Students often confuse osmosis with diffusion. Remember: osmosis is only about water moving across a partially permeable membrane. Diffusion is about any particles moving down a concentration gradient in any direction.
Osmosis Experiments: Plant Cells
Setup: Osmosis with Potato Cylinders
Students typically investigate osmosis by placing potato or radish cylinders in solutions of different concentrations:
- Cut cylinders of uniform length and mass.
- Place one in distilled water (dilute/high water potential).
- Place others in sucrose solutions of increasing concentration (lower water potential).
- After 20–30 minutes, dry and reweigh them.
Results Interpretation
- In distilled water: Mass increases. Water enters by osmosis. The cell becomes turgid (swollen, firm).
- In dilute sucrose (0.1 M): Small mass increase (or no change). Water potential inside and outside nearly balanced.
- In concentrated sucrose (1.0 M): Mass decreases. Water leaves the cell by osmosis. The cell becomes plasmolysed (cell membrane pulls away from the wall).
A potato is mostly water. When you put it in pure water, water soaks in (osmosis) and it gets heavier and firmer. When you put it in salty water, water leaks out and it gets lighter and limp. The salty solution "sucks out" the water because salt can't cross the cell membrane.
Osmosis Experiments: Dialysis Tubing (Visking Tubing)
Dialysis tubing is a non-living, partially permeable membrane made from cellulose. It behaves like a cell membrane:
- Small molecules (glucose, water) pass through.
- Large molecules (sucrose, proteins) do not.
Setup
- Fill dialysis tubing with concentrated sucrose solution.
- Tie the ends tightly.
- Suspend it in a beaker of distilled water.
- Measure the water level and mass over time.
Observation
Water moves into the tubing by osmosis. The tubing swells, and the water level outside decreases. This is physical, visible evidence of osmosis.
Why? Inside the tubing is a concentrated solution (low water potential). Outside is pure water (high water potential). Water moves toward the sucrose, even though the sucrose can't leave.
Osmosis in Plant Cells: Turgidity and Plasmolysis
Turgid Plant Cells (in dilute solutions)
When a plant cell is placed in distilled water or a dilute solution (high water potential outside):
- Water enters the cell by osmosis.
- The vacuole swells, pushing the cell membrane against the cell wall.
- The cell wall is rigid and resists further expansion—this creates turgor pressure.
- The cell becomes turgid (rigid, firm, full of water).
Why this matters: Plant structure depends on turgid cells. When cells lose turgor, the plant wilts. Turgor also helps leaves absorb sunlight more effectively by holding them flat.
Plasmolysed Plant Cells (in concentrated solutions)
When a plant cell is placed in a concentrated solution (low water potential outside, e.g., strong salt or sugar solution):
- Water leaves the cell by osmosis.
- The vacuole shrinks.
- The cell membrane loses contact with the cell wall and peels inward.
- The cell becomes plasmolysed (flaccid).
This is reversible—if you put the plasmolysed cell back in pure water, water re-enters and the cell becomes turgid again. But if you leave it too long, the cell dies.
Osmosis in Animal Cells: Why They're Different
Animal cells have no cell wall—only a cell membrane. This makes them vulnerable to osmotic stress:
Animal Cell in Hypotonic Solution (dilute, high water potential)
- Water enters by osmosis.
- The cell swells.
- With no cell wall to stop it, the cell keeps swelling until the membrane tears.
- The cell lyses (bursts) and dies.
Animal Cell in Hypertonic Solution (concentrated, low water potential)
- Water leaves by osmosis.
- The cell shrinks and shrivels.
- The cell membrane wrinkles inward.
- The cell becomes crenated (spiky, shriveled appearance).
Animal Cell in Isotonic Solution (balanced water potential)
- No net movement of water.
- The cell maintains its normal shape.
- This is ideal for keeping cells alive outside the body (e.g., blood transfusions use isotonic saline).
Freshwater fish have evolved special adaptations to cope with hypotonic environments. Without these, water would enter their cells by osmosis and they would burst. Saltwater fish face the opposite problem: water constantly leaves their cells, so they drink seawater and excrete excess salt.
Active Transport
What is Active Transport?
Active transport is fundamentally different from diffusion and osmosis. Here, cells use energy to move substances against the concentration gradient.
Think of it as "pumping uphill"—you're moving molecules toward an area where there are already more of them. This is opposite to diffusion.
Why Active Transport is Necessary
Some cells need to accumulate substances even when the concentration gradient is against them. Examples:
- Glucose uptake in the small intestine: Even after digestion, glucose concentration in the gut may be lower than in the intestinal cell. The cell actively transports glucose anyway—it needs it for energy.
- Mineral ion uptake in root hair cells: Soil solution often has a lower mineral concentration than the plant cell. Yet roots actively transport ions in using energy from respiration.
- Kidney tubules: Reabsorb useful glucose and ions that were filtered out, moving them back into the blood even though the gradient is against them.
How Active Transport Works: Carrier Proteins
Active transport relies on carrier proteins embedded in the cell membrane. Here's the process:
- Recognition: A specific molecule (e.g., glucose) approaches and binds to a carrier protein on the membrane.
- Energy input: ATP is broken down (respiration releases this energy). The energy causes the carrier protein to change shape.
- Transport: As the protein changes shape, it pushes the molecule through the membrane toward the higher concentration side.
- Release: The molecule is released inside the cell. The carrier protein returns to its original shape, ready to bind another molecule.
Comparing the Three Transport Methods
| Property | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Energy needed? | No (passive) | No (passive) | Yes (active) — ATP |
| Direction | High → Low conc. | High WP → Low WP | Low → High conc. |
| Mechanism | Random movement | Random movement (water) | Carrier proteins + ATP |
| Selectivity | Size-based | Water only | Highly specific |
| Examples | O₂ into mitochondria | Water into turgid cell | Glucose uptake (intestine) |
Students often think diffusion requires energy. It does not. The energy comes from the particles' thermal motion. Active transport does require ATP from respiration. The cell must be alive and respiring for active transport to work.
What to Memorise
These are the key definitions, formulas, and facts you need to be able to recall and apply in exam questions.
Diffusion Definition
Net movement of particles from high to low concentration. Passive (no energy). Driven by Brownian motion.
Osmosis Definition
Diffusion of water across a partially permeable membrane from high to low water potential.
Active Transport Definition
Movement against concentration gradient using ATP energy from respiration. Uses carrier proteins.
Water Potential
High WP = dilute solution (high water concentration). Low WP = concentrated solution (high solute concentration). Water moves from high → low.
Turgid Plant Cell
Cell full of water with turgor pressure. Occurs in dilute or pure water. Cell is rigid and firm.
Plasmolysed Plant Cell
Cell has lost water in concentrated solution. Cell membrane peels away from wall. Cell is flaccid. Reversible.
Crenated Animal Cell
Animal cell has lost water in hypertonic solution. Cell membrane wrinkles inward. Cell shrinks and dies if prolonged.
Carrier Protein
Protein in membrane that binds specific molecules. Uses ATP energy to change shape and transport molecule across membrane.
Factors Affecting Diffusion Rate
Concentration gradient, distance, temperature, surface area to volume ratio. Larger gradient, shorter distance, higher temp, larger surface area → faster diffusion.
Cell Membrane is Partially Permeable
Allows small molecules (O₂, CO₂, water, glucose) through. Blocks large molecules (proteins) and some ions.
Concepts Checklist
Tick each box as you master the concept. If you can't tick it with confidence, review that section.
Exam Tips & Common Pitfalls
Mark-Winning Strategies
- Use precise language: "Osmosis is the diffusion of water..." or "Active transport moves particles against the concentration gradient." Examiners mark harshly for vague definitions.
- Always reference the concentration gradient or water potential: Don't just say "water moves in"; say "water moves in because the external solution has higher water potential than the cell."
- For osmosis experiments: Always explain what you observe and why it happened in terms of osmosis. E.g., "The potato gained mass because water entered by osmosis because the external solution had higher water potential than the cell sap."
- Link to respiration: If asked about active transport, mention that energy comes from aerobic respiration. Examiners want to see you understand the connection.
- Be specific about which way particles move: Diffusion: high → low. Osmosis: high WP → low WP. Active transport: low → high. Get these backwards and you lose marks.
Common Exam Mistakes to Avoid
- Confusing "concentration gradient" with "water potential." Concentration gradient is the difference in number of solute particles. Water potential includes both solute and water. Use the right term for the right transport type.
- Saying diffusion requires energy. It doesn't. The cell doesn't pay for diffusion—particles have their own thermal energy. This is a very common wrong answer.
- Forgetting that osmosis needs a partially permeable membrane. If the membrane lets solutes through, it's just diffusion, not osmosis.
- Reversing plasmolysis and turgidity. Turgid = full of water (good). Plasmolysed = lost water (bad). Plant leaves are limp when plasmolysed, not when turgid.
- Saying animal cells can become plasmolysed. Only plant cells do—they have a cell wall. Animal cells become crenated or lyse.
- Not explaining why something happens. "The cell lost water" is not good enough. "The cell lost water because the external solution had lower water potential, so water moved out by osmosis" is exam-standard.
Question Types You'll See
- "Explain why..." + describe an observation: You must link the observation to the mechanism (diffusion/osmosis/active transport). Use the right scientific term.
- Practical questions: You'll need to interpret results from potato or dialysis tubing experiments. Always mention osmosis, water potential, or diffusion by name.
- Comparison questions: "Compare diffusion and osmosis" or "Compare active transport and diffusion." Make 2–3 clear, contrasting points (energy, direction, mechanism).
- Adaptation questions: "Explain why root hair cells have a large surface area." Answer: "To increase the rate of diffusion/osmosis/active transport of water and ions."
- Graph/calculation questions: Rare, but you might see data from a potato experiment (mass change vs. solution concentration). You need to interpret which solutions were hypertonic, isotonic, or hypotonic.
How to Ace Extended-Response Questions
Extended responses (6–8 mark questions) ask you to explain a scenario in detail. Here's the formula:
- State which process is happening (diffusion/osmosis/active transport) and why.
- Explain the driving force (concentration gradient, water potential, ATP).
- Describe what happens to the substance/cell.
- If it's a plant/animal cell, link to real consequences (turgidity, plasmolysis, lysis, wilting, etc.).
Example structure for "Explain how a plant cell becomes turgid in pure water":
"The pure water has a higher water potential than the cell sap. Water molecules move from high to low water potential across the partially permeable cell membrane by osmosis. Water enters the vacuole, which expands and pushes the cell membrane against the cell wall. The cell wall is rigid and resists further expansion, creating turgor pressure. The cell becomes turgid (rigid and firm). This is important for plant support—turgid cells hold the plant upright and keep leaves flat to absorb light."
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