How Does Diffusion Actually Move Substances Without Energy?
Definition
Diffusion
The net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient, as a result of random movement.
Diffusion is a passive process, it doesn't require energy input from the cell (no ATP needed).
It works because of the concentration gradient:
More particles on one side = more random movement on that side.
More movement = more particles moving outward.
Fewer particles on the other side = fewer particles moving back.
Net movement is therefore from high to low concentration.
This will continue until equilibrium is reached (equal concentration everywhere).
Once equilibrium is reached, the concentration gradient will no longer exist.
Tip
Diffusion doesn't mean all particles move in one direction.
Individual particles will still move randomly in all directions.
It's just the case that more will move from high to low than the reverse.
Analogy
Imagine two rooms connected by a door.
Room A has 100 people, Room B has 10 people.
People walk randomly through the door in both directions, but because Room A is more crowded, more people happen to walk from A to B than from B to A.
Eventually, both rooms have 55 people.
No one forced anyone to move - it just happened through random movement.
Common Mistake
"Passive" doesn't mean slow.
Diffusion can be very fast over short distances - oxygen diffuses from your lungs to your blood in less than a second.
Why Is Diffusion Fast in Some Places and Slow in Others?
Several factors affect how fast diffusion occurs:
Concentration gradient
Steeper gradient = faster diffusion:
Large difference in concentration = more net movement.
Small difference in concentration = less net movement.
As diffusion proceeds, the gradient decreases:
The difference gets smaller over time.
Diffusion slows down as it approaches equilibrium.
Example
If you sprayed a lot of perfume (steep gradient), you'll smell it across the room quickly.
If you only sprayed a tiny amount (shallow gradient) , it'll take longer to spread.
Temperature
Higher temperature = faster diffusion:
Particles have more kinetic energy.
They move faster and collide more frequently.
More collisions = faster spreading.
Lower temperature = slower diffusion:
Particles have less kinetic energy.
They move slower with fewer collisions.
Example
This is why it's quicker to make your tea with hot water versus cold water
Surface Area
Larger surface area = faster diffusion:
More space for particles to cross.
More particles can move simultaneously.
Cells and organisms increase surface area for diffusion:
Lungs have millions of tiny air sacs (alveoli) - massive surface area.
Small intestine has millions of tiny projections (villi) - increases surface area for nutrient absorption.
Root hair cells have long projections - more surface area for water absorption.
Example
A crushed sugar cube dissolves faster than a whole cube even if it's the same amount of sugar.
But since crushed sugar has much more surface area exposed to water, diffusion will occur much faster.
Distance
Shorter distance = faster diffusion:
Particles reach their destination quicker.
Diffusion is only efficient over short distances (typically less than 1mm).
Longer distance = much slower diffusion:
Diffusion time increases exponentially with distance.
This is why large organisms need transport systems (blood, xylem, phloem) - diffusion alone is too slow.
Example
Capillary walls are only one cell thick (about 1 micrometer).
Oxygen diffuses from blood to tissues in a fraction of a second.
If the wall were thicker, diffusion would be too slow to keep tissues alive.
Size of Molecules
Smaller molecules = faster diffusion:
Less mass means easier to move.
Can slip between other particles more easily.
Larger molecules = slower diffusion:
More mass means more inertia.
More likely to collide and bounce back.
Tip
This is why cells are small.
If cells were huge, diffusion would be too slow to supply the center with oxygen and nutrients.
Most cells are 10-30 micrometers across - the perfect size for efficient diffusion.
Example
Oxygen molecules (O₂) are tiny and diffuse very quickly through cell membranes.
Glucose molecules are much larger and diffuse more slowly, often needing transport proteins to help them cross.
Exam technique
The eAssessment questions will often ask you to explain adaptations for diffusion.
Make sure to always link structure to function using the factors.
Active recall
Why does diffusion not require energy (ATP)?
List three factors that increase the rate of diffusion.
Why does a concentration gradient get smaller over time during diffusion?
Explain why single-celled organisms can rely on diffusion but humans cannot.
What Is Diffusion?
Have you ever noticed how the scent of perfume spreads across a room, or how a drop of ink slowly spreads out in water?
This process is called diffusion.
This happens naturally because all particles are in constant random motion.
Definition
Diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
Analogy
Consider a crowded room where people are standing very close together.
If the doors open to an empty hallway, people will naturally walk out and spread into the free space.
Over time, the people become more evenly spread out.
This spreading out is similar to diffusion.
The Particle Model and Diffusion
The particle model helps us understand why diffusion occurs in different states of matter.
Note
Diffusion can be described using the particle model: particles are always moving, and over time their random motion causes them to spread out and mix.
These properties of gases, liquids, and solids perfectly describe diffusion:
Gases:
Particles are far apart, move very quickly and collide frequently.
This allows gases to diffuse rapidly.
Liquids:
Particles are closer together than in gases, but they still have enough energy to move around each other.
Diffusion still happens, but usually more slowly than in gases.
Solids:
Particles are packed very closely and can only vibrate around fixed positions.
Diffusion in solids is therefore extremely slow and often negligible at everyday temperatures.
Common Mistake
Try not to confuse diffusion with sedimentation.
Diffusion: Particles move in all directions due to random motion, spreading out from high to low concentration.
Sedimentation: Particles settle to the bottom of a container because of gravity, usually when they are large or dense.
Both processes can happen at the same time, but they are not the same.
Particles that are diffusing are still moving randomly through the fluid, while sedimented particles have mostly collected at the bottom and are no longer spreading through the fluid.
Example
Ammonium Chloride Ring
A classic experiment demonstrates diffusion in gases using ammonia gas (NH₃) and hydrogen chloride gas (HCl).
The two gases are released at opposite ends of a long glass tube.
As they diffuse along the tube, they meet and react to form a white solid, ammonium chloride (NH₄Cl).
The white ring forms closer to the HCl end because NH₃ molecules have a lower relative mass and diffuse faster than HCl molecules.
Potassium Permanganate in Water
Diffusion in liquids can be seen using potassium permanganate (KMnO₄):
A small crystal of KMnO₄ is placed gently in a beaker of water.
At first, the purple colour is concentrated near the crystal.
Over time, the colour spreads throughout the water as KMnO₄ particles diffuse between the water particles.
Why Diffusion Is Slower in Liquids than in Gases
At the same temperature, diffusion is usually much faster in gases than in liquids.
This is mainly because:
In gases, particles are much further apart, so they can travel longer distances between collisions.
This allows them to spread out quickly.
In liquids, particles are much closer together and attract each other more strongly.
Their movement is hindered, so they move more slowly through the liquid and diffusion is slower.
Analogy
This is why the scent of perfume in a room spreads quickly to other people, whereas it takes much longer for milk added to coffee to mix evenly if you do not stir it.
Note
Diffusion in a liquid will still occur even if:
the diffusing substance has a higher density or larger molecular mass than the liquid, or
the liquid is cold.
However, under these conditions the movement of particles is slower, so diffusion takes longer and may be less obvious to see.
Diffusion in Solids
Many ecosystems rely on the diffusion of gases through soil, which is a mixture of solid particles, air and water.
Example
Oxygen Diffusion Through Soil
Most organisms in soil need oxygen for aerobic respiration and release carbon dioxide as a waste gas.
Oxygen from the air diffuses into the air spaces (pores) between soil particles.
Carbon dioxide produced by microorganisms and plant roots diffuses out of the soil into the atmosphere.
Diffusion through these air spaces is essential for the survival of roots and many soil organisms.
When soil becomes very compact or waterlogged, the air spaces fill with water and gases cannot diffuse easily:
Oxygen cannot diffuse into the soil effectively.
Carbon dioxide can build up in the pores.
Roots and aerobic microorganisms may not receive enough oxygen for respiration.
If soil oxygen levels fall significantly:
Plant roots may be damaged or die because they cannot respire properly.
Aerobic microorganisms decrease, and anaerobic microorganisms (which do not need oxygen) become more common.
Anaerobic respiration produces substances such as alcohols, methane, sulfides and nitrogen-containing compounds (e.g. ammonium and nitrites), some of which can be harmful to plants and can disrupt nutrient cycles such as the nitrogen cycle.
This is why groundskeepers and turf managers sometimes aerate sports fields by poking small holes in the grass: it allows air to diffuse more easily into the soil and helps maintain healthy root systems.
Factors Affecting the Rate of Diffusion
The rate of diffusion is how fast particles spread from an area of higher concentration to an area of lower concentration. Several factors affect this rate:
Temperature
At higher temperatures, particles have more kinetic energy.
This makes them move faster, so they diffuse more quickly.
Example
Food coloring spreads much faster in hot water than in cold water because the water particles (and the dye particles) move more quickly at higher temperatures.
Particle Mass
Lighter particles move faster (on average) than heavier ones at the same temperature.
Because they move faster, lighter particles diffuse more quickly.
Example
Hydrogen gas (H₂) molecules are lighter than carbon dioxide (CO₂) molecules.
At the same temperature, hydrogen molecules move faster, so hydrogen diffuses more rapidly than carbon dioxide.
Concentration Gradient
The concentration gradient is the difference in concentration between two regions.
A steeper concentration gradient (a bigger difference between high and low concentration) makes diffusion faster.
As diffusion continues and concentrations become more similar, the rate of diffusion gradually decreases.
Particle Size
In mixtures, substances made of smaller particles tend to diffuse faster than substances made of larger particles.
Smaller particles can move more easily between other particles.
Diffusion Medium
As discussed earlier, diffusion happens fastest in gases, more slowly in liquids, and is usually very slow in solids.
In gases:
Particles are far apart and move very quickly.
In liquids:
Particles are closer together and their movement is more restricted.
In solids:
Particles only vibrate in fixed positions, so diffusion is extremely slow.
Investigating Diffusion Experimentally
We can observe diffusion in simple experiments with gases and liquids.
Example
Diffusion in Gases
Set up a long glass tube with a plug of cotton wool soaked in aqueous ammonia (NH₃ solution) at one end and another plug soaked in concentrated hydrochloric acid (HCl) at the other end.
Both substances give off gas particles: ammonia gas (NH₃) and hydrogen chloride gas (HCl).
The gases diffuse along the tube and eventually meet.
Where they meet, they react to form a white solid, ammonium chloride (NH₄Cl), which appears as a white ring inside the tube.
The white ring forms closer to the HCl end because NH₃ molecules are lighter and diffuse faster than HCl molecules.
Example
Diffusion in Liquids
Fill a beaker with water.
Carefully add a few drops of potassium permanganate (KMnO₄) solution (or a small crystal of KMnO₄) to the water.
At first, the purple colour is concentrated near where the solution was added.
Over time, the purple colour spreads throughout the water as KMnO₄ particles diffuse between the water particles.
To investigate the effect of temperature:
Repeat the experiment using cold water and warm/hot water.
Compare how quickly the purple colour spreads.
You should observe that diffusion is faster in warm water because particles move faster at higher temperatures.
Real-Life Applications of Diffusion
Diffusion is not just seen in the laboratory. It is essential in many everyday and biological processes.
Example
Perfume and Air Pollution
When someone sprays perfume, tiny gas molecules spread out by diffusion from the area of high concentration (near the spray) into the rest of the room.
Eventually, they reach your nose and you can smell them.
In a similar way, pollutant gases such as nitrogen dioxide or methane can diffuse through the atmosphere, affecting air quality over a wide area.
Example
Perfume and Air Pollution
When someone sprays perfume, tiny gas molecules spread out by diffusion from the area of high concentration (near the spray) into the rest of the room.
Eventually, they reach your nose and you can smell them.
In a similar way, pollutant gases such as nitrogen dioxide or methane can diffuse through the atmosphere, affecting air quality over a wide area.