Lesson focus: diffusionThe net movement of particles from an area of high concentration to an area of low concentration, like the smell of perfume spreading across a room. is net movement caused by random particle motion and a concentration difference.
Explain diffusion, equilibriumA balance where things are still moving or changing both ways, but there is no overall change. and surface-area-to-volume effects.
Build on earlier learning
Plant and animal cellsRevisit the cell membrane as the exchange boundary before explaining particle movement across it.
You will be successful when you can…
- Define diffusion using net movement.
- Explain dynamic equilibrium.
- Predict effects of gradient, temperature, area and distance.
- Calculate and interpret simple surface-area-to-volume ratios.
Words that carry the science
- diffusion
- Net movement of particles from higher to lower concentration due to random motion.
- concentration gradient
- Difference in concentration across a distance.
- equilibrium
- State with no net change because opposing rates are equal.
- surface-area-to-volume ratio
- Exchange area compared with the volume requiring exchange.
- diffusion distance
- Distance particles must travel across a barrier or medium.
Move from random motion to net exchange
01Particles move both ways
Particles move randomly. If more start on one side, more cross from high to low concentration than the reverse, creating net movementThe overall movement once you take away the movement going the other way, like the final score in a tug of war..
At equilibriumA balance where things are still moving or changing both ways, but there is no overall change., particles still cross both ways, but equal average rates give no net change.
02Rate depends on conditions
A steeper concentration gradientA difference in concentration between two areas; particles diffuse from the high concentration to the low concentration., higher temperature and larger surface area can increase diffusionThe net movement of particles from an area of high concentration to an area of low concentration, like the smell of perfume spreading across a room. rate. A longer diffusion distanceHow far particles have to travel to get from one place to another, such as from the air in the lungs into the blood. slows exchange.
These factors change collision frequency, particle motion or available pathway; they do not give particles a goal.
03Size creates an exchange problem
As a cube grows, volume increases faster than surface area, so surface-area-to-volume ratioHow much outside surface something has compared with how big it is inside; small things have more surface for their size. falls. Large organisms need specialised exchange surfaces and transport systems because diffusion alone over long distances is too slow. What does this mean? surface areaThe total area of the outside of an object, where things can touch or pass through. is the outside of the cell, where substances get in and out. volumeHow much space something takes up, measured in cubic centimetres (cm³) or millilitres (mL). is the inside, which needs the substances. A small cell has a lot of outside compared with its inside, so it can take in enough. Substances also do not have far to travel to reach the middle. A very big cell has too little outside for its inside, and the middle is too far away.
Diffusion across a cell membrane
Oxygen particles move randomly. More cross from the side where they are concentrated, so there is a net movement into the cell, down the concentration gradient.
Controls
Compare cube exchange ratios
Compare cubes with side lengths 1 cm and 2 cm.
- Surface area = 6 × side²; volume = side³.
- 1 cm cube: area 6 cm², volume 1 cm³, ratio 6:1.
- 2 cm cube: area 24 cm², volume 8 cm³, ratio 3:1.
- The larger cube has less exchange area per unit volume.
How to make the reasoning visible: Doubling length does not double volume; the ratio falls because volume grows faster.
Cube exchange data
Calculate ratios and infer which cube exchanges most effectively per unit volume.
| Side / cm | Surface area / cm² | Volume / cm³ | SA:V |
|---|---|---|---|
| 1 | 6 | 1 | 6:1 |
| 2 | 24 | 8 | 3:1 |
| 3 | 54 | 27 | 2:1 |
| 4 | 96 | 64 | 1.5:1 |
Which cube has the greatest exchange area per unit volume?
Why can a large organism not rely only on diffusion across its outer surface?
Use the table. What happens to the surface area to volume ratio as the cube gets bigger?
Check your understanding
Answer from memory, then check against the suggested answer.
What direction is net diffusion?
Do particles stop at equilibrium?
What happens to SA:V as a cube grows?
What is diffusion?
Name one factor that makes diffusion faster.
Why does a thin exchange surface increase diffusion rate?
Explain why a folded surface can support exchange without increasing total organism volume greatly.
Why does a drop of food colouring spread faster in warm water than in cold water?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Small cells can take in and get rid of substances better than very large cells. Answer the parts to explain why. (The surface area is the outside of the cell. The volume is the inside.)
Compared with its volume, does a small cell have a lot of surface area or a little?
(1)Where do substances get into and out of a cell?
(1)Does the small inside of a small cell need a lot of substances or a little?
(1)How far do substances travel to reach the middle of a small cell?
(1)Total for this question: 4 marks
Command word: Use the words outside (surface area) and inside (volume) in your answer.