Lesson focus: diffusionThe net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, without needing energy from respiration. is the net result of random particle movement and is affected by gradient, temperature, exchange area and 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..
Explain diffusion in living systems and use surface areaThe total area of the outside of an object, where things can touch or pass through., distance, temperature and concentration gradientThe difference in concentration between two regions; the steeper the gradient, the faster the rate of diffusion. to predict its rate.
Build on earlier learning
Plant and animal cellsYear 7 introduced diffusion across cell membranes. This bridge lesson makes the particle model quantitative and applies rate factors.
You will be successful when you can…
- Define net diffusion precisely.
- Apply four rate factors.
- Explain exchange-surface adaptations.
- Distinguish random motion from net movement.
Words that carry the science
- diffusion
- Net movement of particles from higher to lower concentration due to random motion.
- concentration gradient
- A difference in concentration across a distance.
- net movement
- The overall movement after motion in both directions is considered.
- surface area
- The total exposed area available for exchange.
- diffusion distance
- The distance particles must travel through an exchange barrier.
- dynamic equilibrium
- Particles continue moving in both directions at equal rates, giving no net change.
Random motion produces net diffusion
01Random motion produces net diffusion
Particles move randomly in liquids and gases. If one region contains more particles per unit volume, more particles cross away from that region each second than cross towards it. The resulting net movementThe overall movement once you take away the movement going the other way, like the final score in a tug of war. is down the concentration gradientThe difference in concentration between two regions; the steeper the gradient, the faster the rate of diffusion..
At dynamic equilibriumWhen particles keep moving both ways at the same rate, so overall nothing seems to change., particles have not stopped. Equal movement in both directions produces no net change.
02Four factors change diffusion rate
A steeper concentration gradient increases the imbalance in crossings. Higher temperature increases particle kinetic energy and random motion. Greater surface areaThe total area of the outside of an object, where things can touch or pass through. provides more crossing locations, while a shorter 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. reduces the journey through the barrier.
When comparing rates, change one factor at a time or acknowledge interacting variables. Thinness is an advantage only if the exchange surface remains intact.
03Living exchange surfaces maintain gradients
Alveoli have a large surface, thin walls and blood flow that removes oxygen and supplies carbon dioxide. Villi provide a large absorptive surface and transport absorbed substances away. Leaves use broad internal surfaces, short gas pathways and stomata.
Small unicellular organisms can often rely on diffusion because every cell region is close to the surface. Larger organisms require ventilation and transport to maintain gradients and overcome long distances.
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
Predict a rate change
A membrane is made twice as thick while area, temperature and concentration difference remain constant.
- Identify diffusion distance as the changed variable.
- A thicker membrane gives particles a longer route.
- Longer routes take more time on average.
- Therefore the rate of net diffusion decreases.
How to make the reasoning visible: State the control variables so the causal claim is limited to membrane thickness.
Analyse diffusion evidence
Agar cubes containing indicator are placed in acid for ten minutes. Acid decolourises the agar it reaches.
| Cube side / mm | Surface area / mm² | Volume / mm³ | SA:V |
|---|---|---|---|
| 2 | 24 | 8 | 3.0 |
| 4 | 96 | 64 | 1.5 |
| 6 | 216 | 216 | 1.0 |
| 8 | 384 | 512 | 0.75 |
Which cube should have the greatest percentage decolourised?
Why is this model not identical to a living cell?
Check your understanding
Answer from memory, then check against the suggested answer.
Define diffusion.
What happens at dynamic equilibrium?
Give two adaptations for rapid exchange.
Why can pneumonia reduce oxygen uptake?
Why can blood flow increase diffusion without changing membrane thickness?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Oxygen passes from the air in the alveoli into the blood.
Explain how oxygen moves into the blood and how the alveoli are adapted.
(3)Explain how a steep concentration gradient is maintained.
(3)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., name diffusion and link each adaptation to a specific rate factor.