By the end of this lesson
- Explain transport need in multicellular organisms.
- Distinguish exchange surfaces from transport systems.
- Relate mass flow to maintained gradients.
BIG QUESTION
Why can a large organism not simply let oxygen diffuse from its skin to every cell?
Low surface-area-to-volume ratio, long distances and high demand make diffusion alone too slow.
Most cells are far from the environment
In a large organism, many cells are internal and have no direct contact with air, water or soil.
Long diffusion distances make delivery and waste removal too slow.
Demand rises with active volume
A lower surface-area-to-volume ratio provides less external area per unit tissue. Active organs may also have high oxygen, nutrient and temperature-regulation demands.
Specialised exchange surfaceA surface across which substances pass between an organism and its surroundings, adapted by having a large surface area, a short diffusion distance and a transport system that keeps gradients steep. load substances into a transport medium.
Mass transport links body regions
mass transportBulk movement of substances in a flowing liquid, such as blood or xylem sap, over long distances in an organism. (bulk flow) in blood, xylem or phloem moves materials rapidly over long distances. Diffusion then acts over short distances at tissues.
Transport also maintains steep gradients by carrying absorbed substances away and delivering fresh supplies.
Exchange and transport roles
The systems cooperate rather than replace one another.
| Process | Distance | Example |
|---|---|---|
| Diffusion | Short. | Capillary to cell. |
| Ventilation | Bulk flow to exchange surface. | Air to alveoli. |
| Circulation | Long-distance mass flow. | Blood between organs. |
| Plant vascular transport | Long-distance mass flow. | Water or sucrose through stems. |
Language that earns precision
- mass transport
- Bulk movement of materials in a transport medium.
- exchange surface
- Specialised boundary for movement into or out of an organism.
- circulatory system
- A system moving a transport fluid through vessels.
Common misconceptions
Claim: Transport systems replace diffusion.
Correction: They move materials over long distances; diffusion still crosses short tissue boundaries.
Claim: Only animals require transport systems.
Correction: Large plants use xylem and phloem.
Match the answer to the command word
Use low ratio, long distance and high metabolic demand.
Name both the exchange surface and transport medium.
Recall, apply, then justify
Explain why a large multicellular organism needs a transport system.
Hint
Use geometry and speed.
Suggested marking points
- It has a relatively low surface-area-to-volume ratio.
- Many cells are far from the external environment.
- Diffusion over long distances is too slow.
- Mass transport supplies active cells and removes wastes rapidly.
Explain how circulation maintains diffusion at tissues.
Hint
Deliver and remove.
Suggested marking points
- Blood supplies substances such as oxygen and removes products such as carbon dioxide.
- This maintains concentration gradients between capillaries and cells.
Evaluate whether a transport system alone is enough for a large active organism.
Planning hint
Consider loading, movement and unloading.
What a strong design includes
- A transport system moves materials long distances.
- Specialised exchange surfaces are needed to load oxygen/nutrients and remove wastes.
- A pump or pressure gradient may be needed for mass flow.
- Thin capillary or tissue barriers are needed for local diffusion.
- Ventilation may maintain external gradients.
- An integrated set of systems is therefore required.