By the end of this lesson
- Explain diffusion in gas exchange.
- Apply concentration gradients to oxygen and carbon dioxide.
- Relate surface properties to diffusion rate.
BIG QUESTION
How can gas exchange occur continuously without a pump moving each molecule?
Random molecular motion produces net movement down concentration gradients; biological structures maintain those gradients.
Random motion produces net exchange
diffusionThe net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. is the net movement of particles from higher to lower concentration due to random motion. Molecules move both ways, but more cross in the direction of the gradient.
No ATP is required for diffusion itself. Cells may use energy to maintain conditions that preserve a gradient.
Cell activity creates gradients
Respiring cells consume oxygen and produce carbon dioxide, lowering oxygen and raising carbon-dioxide concentration near them. Photosynthesising cells consume carbon dioxide and produce oxygen.
The direction and steepness of each gradient therefore depend on cellular rates and the surrounding air or blood.
Exchange surfaces accelerate diffusion
Large surface area permits many molecules to cross simultaneously. A thin barrier shortens diffusion distance, while ventilation or blood flow renews material on each side.
Moist surfaces allow gases to dissolve before crossing cell membranes, but excessive fluid thickness would lengthen the pathway.
Factors affecting gas diffusion
Each adaptation changes a different part of the diffusion problem.
| Factor | Change | Effect |
|---|---|---|
| Concentration gradient | Steeper difference. | Greater net movement per unit time. |
| Surface area | Larger exchange area. | More particles cross simultaneously. |
| Diffusion distance | Thinner barrier. | Shorter route and faster exchange. |
| Medium renewal | Ventilation or circulation. | Maintains gradients. |
Language that earns precision
- diffusion
- Net movement of particles down a concentration gradient due to random motion.
- concentration gradient
- A concentration difference across a distance.
- gas exchange
- Movement of respiratory or photosynthetic gases between an organism and its environment.
- exchange surface
- A specialised boundary across which substances move.
Common misconceptions
Claim: Diffusion means all molecules move in one direction.
Correction: Molecules move randomly both ways; only the net movement follows the gradient.
Claim: Cells spend ATP to push oxygen across alveoli.
Correction: Oxygen crosses mainly by diffusion; ventilation and circulation maintain gradients.
Claim: A concentration gradient is a physical slope.
Correction: It is a difference in concentration across distance.
Match the answer to the command word
Name the gas, higher-concentration side and lower-concentration side.
Use the process changing concentration, such as respiration or photosynthesis.
Keep surface area, distance and gradient as separate mechanisms.
Recall, apply, then justify
Explain why oxygen diffuses from an alveolus into blood.
Hint
Compare oxygen concentration on each side.
Suggested marking points
- Ventilated alveolar air has a higher oxygen concentration than incoming blood.
- Oxygen molecules move randomly across the thin surface.
- There is net diffusion into blood down the concentration gradient.
Explain two ways an exchange surface can increase diffusion rate.
Hint
Give feature and mechanism twice.
Suggested marking points
- Large surface area allows more molecules to cross at once.
- This increases total transfer per unit time.
- A thin barrier gives a short diffusion distance.
- This reduces time needed to cross.
Explain why ventilation can increase diffusion without changing the exchange membrane itself.
Planning hint
Track the air-side concentration.
What a strong design includes
- Diffusion tends to reduce concentration differences.
- Ventilation replaces used air with fresh air.
- Fresh air restores high oxygen and low carbon-dioxide concentrations.
- This maintains steep gradients across the membrane.
- The membrane structure can remain unchanged while net flux increases.